DESIGN OF A FLOATING, PRODUCTION, STORAGE, AND OFFLOADING VESSEL FOR OPERATION IN THE SOUTH CHINA SEA

Transcription

1 DESIGN OF A FLOATING, PRODUCTION, STORAGE, AND OFFLOADING VESSEL FOR OPERATION IN THE SOUTH CHINA SEA Curt Haveman Jeffrey Parliament Jeremy Sokol Joseph Swenson Timothy Wagner OCEN Design of Ocean Engineering Facilities Ocean Engineering Program Texas A&M University 15 May 2006

2 Table of Contents List of Figures...iii List of Tables... v Acknowledgements... vi Nomenclature... vii Abstract...viii Executive Summary... ix 1 Introduction FPSO Background Objective Industry Day Environment Design Criteria Gantt Chart Regulatory Compliance General Arrangements Storage and Slop Tanks Accommodations Spill Containment Fire Safety Lifesaving Appliances and Equipment Flood Survival Requirements Helicopter Deck Environmental/Global Loading and General Strength of Hull Position Mooring System Stability General Arrangement and Hull/System Design Option One Option Two Selected Design and General Layout Lifeboats Accommodations and Helipad Weight, Buoyancy, and Stability Weight Calculations Buoyancy Calculations Stability Intact Stability Damage Stability Local and Global Loading General Strength and Structural Design Wind and Current Loading Mooring/Station Keeping Hydrodynamics of Motions and Loading Cost Analysis Summary and Conclusions i

3 12 References Appendix A: Environmental Data and Loading Spreadsheets Appendix B: Mimosa Input/Output Files Appendix C: StabCAD Input/Output Files ii

4 List of Figures Figure 1: Vessel Location... 2 Figure 2: Annual Wind Profile... 3 Figure 3: JONSWAP Spectrum for South China Sea... 3 Figure 4: Most Probable Significant Wave Height by Month... 4 Figure 5: Current Profile... 4 Figure 6: Wave Height Annual Probability... 5 Figure 7: Team Design Gantt Chart... 7 Figure 8: Internal Mooring Design Figure 9: External Mooring Design Figure 10: FPSO Tank Layout with Internal Mooring Figure 11: FPSO Wing Ballast Tank Layout Figure 12: Permanent Water Tank Figure 13: Accommodations and Helipad Dimensions Figure 14: Rendered View from StabCAD Figure 15: ABS MODU 2005 Intact Stability Curve Figure 16: Intact Stability Curve Figure 17: Intact Cross Curves Plot Figure 18: Curve of Static Stability at Draft Figure 19: ABS MODU 2005 Damage Stability Curve Figure 20: Damaged Ballast Tanks - 2 total Figure 21: Damage Stability Curve Figure 22: Local Loading Figure 23: Global Loading Figure 24: Evenly Distributed Buoyancy Case Figure 25: Evenly Distributed Buoyancy Case Deflection Figure 26: Maximum Hog Buoyancy Case Figure 27: Maximum Hog Buoyancy Case Deflection Figure 28: Maximum Sag Buoyancy Case Figure 29: Maximum Sag Buoyancy Case Deflection Figure 30: Wave Induced Shear Figure 31: Wave Induced Moment Figure 32: AutoCAD Area Distribution Figure 33: Internal and External Turret Mooring Systems Figure 34: Horizontal Layout of Mooring Lines Figure 35: Vertical View of Mooring Lines Figure 36: Offsets for Survival Conditions Figure 37: Offsets for Damaged Conditions Figure 38: Stevpris Mk. 5 Anchor Figure 39: Heave Amplitude Response (0 deg) Figure 40: Heave Response (22.5 deg) Figure 41: Vessel Pitch Response (0 deg) Figure 42: Total Vessel Cost by Shipyard Figure 43: South China Sea Wave Conditions By Month Figure 44: JONSWAP Spectrum for the South China Sea iii

5 Figure 45: Significant Wave Height Design Criteria Figure 46: Annual Cumulative Wave Probability Curve Figure 47: Annual Wind Profile Rosette Figure 48: South China Sea Current Profile iv

6 List of Tables Table 1: Design Criteria for South China Sea... 5 Table 2: Factor of Safety for Anchoring Lines (ABS 2005) Table 3: Damage Assumptions Table 4: Ship Hull Ratios Table 5: Hull Weight Calculation Table 6: Weight Characteristics Table 7: Center of Gravities Table 8: Draft Under Different Load Conditions Table 9: Required Buoyancy for Varying Load Condition Table 10: Center of Buoyancy Table 11: 100% Loaded Specifics Table 12: Downflooding Points Height Above Water Table 13: Intact Stability Parameters Table 14: Damage Conditions to Satisfy Table 15: ABS 2005 Regulations Table 16: Area Calculations of Ship Shape Table 17: Wind, Current, & Significant Wave Height Implementation Table 18: Total Environmental Forces on Vessel Table 19: Environmental Conditions Considered Table 20: Environmental Load Comparison Table 21: Weighted Mooring System Selection Chart Table 22: Mooring Line Properties Table 23: Intact Survival Analysis Table 24: Damaged Survival Analysis Table 25: FPSO Parameters Table 26: FPSO Natural Periods Table 27: Vessel Response Amplitudes Table 28: Non Shipyard Associated Costs Table 29: Shipyard Associated Costs Table 30: Return Period Wind Profiles Table 31: Current Profile for South China Sea Table 32: Wave Height Annual Cumulative Probability Distribution Table 33: MIMOSA Vessel File Table 34: MIMOSA SIF File Table 35: MIMOSA Line Characteristics Table 36: Sample MIMOSA Output Table 37: StabCAD Input File Table 38: StabCAD Output File v

7 Acknowledgements Team South China Sea would like to thank the following individuals and companies, without whom the project would not have been completed, for their assistance and guidance throughout the course of the project. Dr. Robert Randall, TAMU Rodney King, ConocoPhillips Nick Heather, ConocoPhillips Barbara Stone, Sea Engineering Sergio Gutierrez, TAMU Chuck Steube, ConocoPhillips Chris Desmond, Lloyd s Register Yong Luo, SBM-IMODCO Vidar Aanesland, APL, Inc. Tom Fulton, Intermoor Dave Walters, 2H Offshore G. Liu, Technip Engineering Dynamics Incorporated Det Norske Veritas vi

8 Nomenclature FPSO Floating, Production, Storage and Offloading ABS American Bureau of Shipping API American Petroleum Institute FOS Factor of Safety Hs Significant Wave Height Tp Peak Period ρ Seawater Density F Force S Section Modulus M Moment of Inertia A Area V Velocity α Wind Velocity Time Factor φ Direction of Approaching Oblique Seas D Draft CC Current Coefficient CH Height Coefficient CS Shape Coefficient CB Block Coefficient CW Waterplane Area Coefficient KG Center of Gravity from Keel RAO Response Amplication Operator VCG Vertical Center of Gravity LCG Longitudinal Center of Gravity TCG Transverse Center of Gravity WBT Wing Ballast Tank P Port S Starboard DOST Diesel Oil Storage Tank PROD Produced Water Tank COFT Crude Fuel Oil Tank COT Crude Oil Tank OFF-SPEC COT Off-spec Product Tank SLOP Slop Tank vii

9 Abstract The objective of this design team is to analyze, research, model, and design a Floating, Production, Storage, and Offloading vessel (FPSO) capable of surviving the weather conditions found in the South China Sea. The team will have to analyze the environmental data from the site, and find the wind, wave, and current forces that the ship will have to endure both in regular and extreme cases. The ship will have to be able to keep production capability in conditions of the one year storm and must be able to survive the 100 year storm conditions. Once the environment data is complete the team will have to design all aspects of the ship, including hull size, shape, displacement, empty and loaded draft, as well as accounting for the natural heave, pitch, and roll periods. The vessel will be required to fulfill all ABS requirements for a steel vessel carrying natural gas and oil. Once the ship is designed the team will also carry out an analysis on several different mooring systems. This design project has specific conditions which must be satisfied by the International Student Offshore Design Competition (ISODC). These include safe and efficient hull design, matching production, storage, and offloading, recognizing the impact of hull deflections, and matching the mooring system to motions and loads. In order for this project to be considered by the ISODC as a design entry, it should address eight areas of competency. Five fundamental competencies must be addressed while the remaining three may be selected from the more specialized competencies. The fundamental competencies which will be covered include: Fundamental Competencies: General Arrangement and Overall Hull or System Design Weight, Buoyancy and Stability Local and Global Loading Strength and Structural Design General Cost Specialized Competencies Floating Structures Hydrodynamics of Motions and Loading Wind and Current Loading Mooring/Station Keeping (or propulsion, tendon design) The FPSO facility is located in the South China Sea in 100 meters of water at North Latitude, East Longitude. The topside information, such as weights, arrangements of components and size of components for the FPSO were provided by Mr. King with ConocoPhillips. The team s job is to design the hull characteristics, tank arrangement below deck, mooring systems as well as perform ship hull, structural and mooring system analysis. Certain conditions have to be considered when designing the FPSO. The FPSO must remain online and operating during a 1 year design storm for the area. Production is possible up to a roll of 2. Once the vessel rolls past 2 offloading operations and plant operations must cease. During a 100 year design storm, the vessel must remain on location and attached to the mooring system. Within the requirements, a basic hull design is going to be provided to the team for the basis of the design; however, the development of the stability curves for the hull design and loading specifications will be required. Determining the best arrangement and number of storage tanks and ballast tanks below the deck of the FPSO vessel will also be analyzed for maximum operation. The ideal mooring system will be designed based on factors such as water depth and environmental conditions. The type of mooring system selected will be based on the operating depth of the FPSO. viii

10 Executive Summary Introduction Team South China Sea was asked to develop a Floating, Production, Storage, and Offloading vessel that has the capability of storing 1.6 million barrels of crude oil and withstand the harsh environment found in the South China Sea. This facility is to be internally turret-moored which allows the capability of a free floating, fully weathervaning hull design. The FPSO must operate in 100 meters of water and maintain an overall 95% operating availability. All aspects of the vessel will be regulated by the American Petroleum Institute (API) and the American Bureau of Shipping (ABS) guidelines. The eight competency areas addressed are (1) general arrangement and overall hull/system design, (2) weight, buoyancy, and stability, (3) local and global loading, (4) general strength and structural design, (5) wind and current loading, (6) mooring/station keeping, (7) hydrodynamics of motions and loading, and (8) cost analysis. General Arrangement and Overall Hull/System Design The team considered two design alternatives. The chosen design option required the lengthening of the ship to incorporate an internal disconnectable turret. Length between perpendiculars (LBP) 326 m Breadth 58 m Molded depth 30.4 m There are 16 stabilized product tanks along with 2 off-spec product tanks within our double-hull design. These 18 tanks are arranged longitudinally with 14 L-shaped ballast tanks located in between the outer hull and the crude oil tanks. A double-hull layout was a direct effect of these ballast configurations, which aided in stability performance ix

11 as well as comply with ABS steel vessel design guidelines. Since this vessel s hull was extended to incorporate the internal turret, the longitudinal center of buoyancy and longitudinal center of gravity were misaligned. A large permanent ballast tank was placed in the bow following in depth research and input by industry professionals. This response involved converting several of the bow compartments to permanent water tanks. Safety restrictions and lifeboat regulations were also taken into account for the design and arrangement of the vessel. Weight, Buoyancy and Stability In order to determine the weight of steel required by the vessel an existing ship, the Nanhai Endeavour, was scaled up to the size of the design vessel. After completing the scaling process it was determined that 48,750 mt of steel will be required. Next, the hull lightship weight was added to the weight of the fully loaded product tanks in order to find the total displacement of the ship. This displacement was found to be 432,050 mt which lead to a displaced volume of seawater equal to 421,510 m 3. The VCG was found to be m above the keel, the TCG was 0.03 m port of the centerline, and the LCG was m forward of the stern in the fully loaded condition. Using these numbers, a stability analysis was conducted using commercial software known as StabCAD. The intact and damaged stability of a 100% loaded vessel passed the ABS rules and regulations. For an intact hull, the range of stability was found to be 15.1 degrees. The stability curve calculated a downflooding angle at degrees. The range of stability for the damaged condition was found to be 3.51 degrees. For a x

12 draft of meters, the input vertical center of gravity of meters was less than the maximum allowable KG of 19.2 meters. The design and calculated vertical center of gravity was less than the maximum allowable KG as dictated by ABS. For the damaged stability, the minimum ABS MODU requirements were met as well. Local and Global Loading The loading on a ship must be determined in order to ensure that the vessel can withstand the effects of these loads as well as those of the wind, waves, and current. The local loading takes the loading of several conditions every 15 meters. In order to increase this accuracy the global loading profile is found. This profile shows the weight on the ship for the entire length of the vessel instead of incrementally. Once this weight distribution is found, it can then be put into a structural analysis program, in this case Visual Analysis, to find important factors such as amount of sag or hog, shear force acting on the beam or the bending moment that must be resisted. By modeling the ship as a simply supported beam with a moment of inertia of 2400 m 4, the deflection of the ship was kept to less than 0.5 meters. The shear and moments fulfilled the requirements given by ABS rules. General Strength and Structural Design Following regulations set forth by ABS, the maximum wave induced shear and moment envelopes were calculated. Using the visual analysis program the vessel topsides, steel weight, permanent loads, and tanks were model as combinations of distributed and point loads. Next, three buoyant force configurations were modeled, one evenly xi

13 distributed to show the still-water condition, one representing the maximum sag condition, and the other the maximum hog condition. The shear and moment results from the still-water condition were then subtracted from the same results for both the hog and sag condition in order to determine the wave induced shear and wave induced moment generated. These shear and moments were compared with their corresponding envelopes and it was determined that the vessel fulfills the ABS requirements. Wind and Current Loading Once the loaded draft was established, the vessel could then be divided into sections which are exposed to the corresponding environmental loads. The hand calculated environmental loads were determined, using the coefficients which relate to an average drill ship. The loads for bow sea conditions consist of a wind force of kn, current force of 10.8 kn, and a mean wave drift force of 77.7 kn. The loads for the beam sea conditions include a wind force of kn, current force of kn, and a mean drift force of kn. The calculated bow sea forces were then compared to the results established by the mooring software which were given as a wind force of kn, current force of kn, and wave force of kn. The cause of this considerable incongruity of results, ties back in the usage of the drill ship model and the failure of these coefficients to accurately portray the FPSO. xii

14 Mooring/Station Keeping With the aid of a weighted objectives chart, the mooring system that was specified for the South China Sea in 100 m of water was an internal disconnectable turret type mooring system. The mooring system consists of 12 lines placed in groups of 3 (5 spacing between lines of the same group) with a group in each of the 4 cardinal directions. The legs of the system consist of 500 meters of 142 mm R4 grade studlink chain with a 12 metric ton Stevpris Mk. 5 anchor securing the system to the sea bottom. In this mooring system, with null environmental forces, there is 136 meters of chain resting on the sea floor which leaves 364 meters of chain suspended in the water column, which is approximately 1930 metric tons. The turret selected will house up to twenty-five risers, which are required to be flexible. Three different environmental load combinations were used in the analysis of the mooring system to check for survival in a 1yr typhoon storm and production in a 1 yr non-typhoon storm. When analyzing the system for intact survivability, the factors of safety found were 1.82, 1.67 and 1.86 with accompanying offsets of 8.19 m, 7.51 m and 7.38 m, which represent, respectively the co-linear, non co-linear case 1 and case 2 environmental load combinations. When one line was damaged, the factors of safety achieved were 1.57, 1.33 and 1.62 with accompanying offsets of 9.2 m, m and 9.4 m, with respect to co-linear environment and non co-linear cases 1 and 2. All of these values were within the values required by API and pass the survivability analysis. The operational conditions offsets were checked and determined to be adequate. The survival conditions offsets satisfied the required operational offsets. xiii

15 Hydrodynamics of Motions and Loading For the one year max typhoon conditions, the heave displacement for the vessel was found to be 0.67 meters for bow on seas, and 1.1 meters for a 22.5 degree sea. The pitch response was found to be degrees, which is below the stated limit of one degree and the roll response was found to be negligible. These values were computed for the ninety-five percent operating capability of significant wave height of 3.8 meters. The wave conditions are governed by the JONSWAP spectrum with a peak period of eight seconds. Thus the vessel can operate ninety five percent of the time it is on station. Because of the hydrodynamics of the vessel and the environment within which it operates, the offloading will be done in a tandem configuration. Under this configuration offloading can be maintained by using the vessels capability to weathervane into the prevailing conditions. Cost Analysis The cost of the Floating, Production, Storage, and Offloading (FPSO) is estimated using data provided by ConocoPhillips. The data gives the cost of the steel and the outfitting of the vessel, for three shipyards in Japan, Korea, and China. The data also included costs that are not associated with the shipyard costs. This includes things such as the turret, mooring chain, risers, anchors, topsides, accommodations. Using the data provided, the vessel will be built in China, because of the lower cost and relative location to where the vessel will operate. The estimated total cost is one billion dollars. xiv

16 1 Introduction 1.1 FPSO Background With growth of the offshore oil industry in the second half of the twentieth century, the idea of floating storage vessels became a possibility. The first floating storage vessels were installed to reduce the cost of transporting the oil to shore, and storing it, before shipping it elsewhere. These first floating storage units (FSU) were tankers that stayed moored for a few days to a few weeks. These vessels were made possible by the development of the single point mooring system. This mooring system allows for the vessel to be positioned so environmental responses are minimized. Vessel operators began to look into vessels that would remain on station for periods of months to years. This type of vessel would have to be offloaded by a shuttle tanker. The logical progression was to convert mid-size tankers into the floating storage and offloading vessels (FSO). These vessels however, still did not produce the oil thus it had to be processed on a platform. Companies saw removing the platform as a way to reduce the cost of production. This led to the idea of putting production topsides on the FSO vessels. These developed into floating production, storage, and offloading vessels. The early vessels were tanker conversions, however as the available tanker fleet has been diminished, new built FPSOs are being designed. These new FPSOs are bringing easier oil production to areas where field development is minimal. Unlike the Gulf of Mexico, where a massive pipeline network exists, most areas in the world do not have the infrastructure required for fixed platform production. The FPSO vessel brings the flexibility to produce oil for an extended period of time, and allow for ease of transportation to refineries. With the development of disconnectable turret mooring systems, the FPSO is now able to be put into areas of the world s oceans where cyclonic events are prevalent. The FPSO vessel on average produces 140,000 barrels of oil per day, while the average capacity is 1.2 million barrels of oil. This storage capacity allows for an offloading schedule of every ten days to two weeks. When it comes time to offload a shuttle tanker will offload in either a side by side or tandem configuration. The side by side offloading technique requires the offloading vessel to connect itself parallel to the FPSO. This technique is very susceptible to environmental conditions. The tandem offloading technique is much safer as it allows the offloading vessel to remain a safe distance from the FPSO. The oil is transferred through a hose extended out behind the FPSO. The floating production, storage, and offloading vessel will be a mainstay in the oil company fleets for many years to come. They provide the flexibility and sound economics of producing and storing at the offshore well site. Thus the oil does not have to touch land until it reaches the refinery. 1.2 Objective The purpose of this project is to design a Floating, Production, Storage, and Offloading (FPSO) vessel to accommodate a 1.6 million barrel storage facility. This facility is self-contained and processes, stores and offloads crude oil products. The scope of this task is limited to designing a stable, weathervaning hull, and turret-mooring system for 100 meters of open water off the coast of China. API and ABS guidelines provide structural minimal requirements and safe mooring sizing. 1.3 Industry Day ConocoPhillips hosted an industry conference day at their headquarters in Houston, Texas on February 10, The main purpose of the industry conference day was to provide the senior design teams with important topics detailing aspects, concepts, and technologies pertaining to the design of the FPSO facility. The topics, presented by various representatives from different companies, were as follows in chronological order: FPSO Project Drivers by Chuck Steube, ConocoPhillips Mooring Classifications by Chris Desmond, Lloyd s Register Riser Design by Yong Luo, SBM-IMODCO Disconnectable Turret Systems by Vidar Aanesland, APL, Inc. 1

17 Mooring/Loading Systems by Tom Fulton and Dave Walters, Inter Mor and 2H Offshore All given presentations were of important value to the members of Team South China Sea. However, the presentations that were of particular interest were: FPSO Project Drivers by Chuck Steube with ConocoPhillips, Mooring Classifications by Chris Desmond with Lloyd s Register and Disconnectable Turret Systems by Vidar Aanesland with APL, Inc. The FPSO Project Drivers presentation by Chuck Steube provided Team South China Sea with a clear understanding of the detailed tasks and project management of a large scope project, such as the one being encountered by the design group. This presentation allowed the members of Team South China Sea to make a clear and confident design plan to further the exploration and understanding of offshore development. Lloyd s Register details were provided by Chris Desmond aided in the team s design choice and analysis of the mooring system that was being evaluated. Lloyd s Register also provided all teams in attendance with a Lloyd s Register disk containing information about design specifications. The last presentation by Vidar Aanesland provided Team South China Sea much insight into disconnectable turret moored systems. He was able to answer many questions from the teams about the feasibility and efficiency about turret moored systems. The Industry Conference Day at ConocoPhillips helped the Spring 2006 Texas A&M University senior design teams broaden their scope and consider the many offshore projects and processes which make up the design of the FPSO facility. Team South China Sea would like to thank all the visiting organizations representatives for their time, effort, and dedication in providing their knowledge and expertise about their specific fields of interest. 1.4 Environment Located at North Latitude and East Longitude, the FPSO is located in some of the most volatile metocean conditions. The location, shown in Figure 1, is approximately 140 kilometers south of Hong Kong in roughly 100 meters of water. An analysis has been done on metocean data provided by ConocoPhillips. The data from the analysis will be discussed within this section. Vessel Location 100 meter water depth Figure 1: Vessel Location The general weather pattern for this location is that of a monsoon, where there is a rainy season and a dry season. During the monsoon the directionality of the environment changes and heavier rainfall 2

18 occurs. Coupled with the monsoon environment, is the presence of typhoons throughout the year. On average the location receives 17 typhoons per year, with sixty percent moving in from the Northwest Pacific Ocean and forty percent forming directly over the South China Sea. The winter months produce fairly large sea states, as cold fronts continuously move across the area. The temperature range is from - 5 C to 38 C, and the average humidity is over eighty percent. The rainy season is from May to September, with 1.5 meters of rain fall a year, and the dry season is from November to April. The environmental change with season is illustrated in Figure 2, in which the wind profile is shown for the year. Annual Wind Profile JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC Figure 2: Annual Wind Profile The wave conditions for this site are driven by the JONSWAP spectrum shown in Figure 3. This spectrum is perfect for this region, because it models developing seas which are driven by wind. It can be seen that the highest amount of energy is located in waves with a frequency of roughly 1.2 radians per second. This is equal to a yearly average mean zero crossing period of 5.3 seconds. JONSWAP Energy Density m^2 s Frequency rad/s Figure 3: JONSWAP Spectrum for South China Sea 3

19 The spectrum shown above, really only gives a distribution of the wave energy on a yearly basis. To develop a design that is safe and cost effective, one must know the wave heights by month. In the South China Sea, the wave heights are variable in amplitude throughout the year, and this can be seen below in Figure 4 with a graph of the most probable wave heights by month. Wave conditions by month meters Jan Feb Mar Mar April May May June Month July Aug Sept Oct Nov Dec Significant wave height Figure 4: Most Probable Significant Wave Height by Month As can be seen from this graph, the highest wave heights occur during the winter months. This is due to the wind coming from the northeast, which is the North Pacific. This reflects that the wind has a longer fetch over the open ocean, creating a higher wave spectrum. One can also see that in the summer when the wind is blowing offshore from the southwest, the wave heights are much lower. However, there is some deception in the values for the summer and even into October. These months are when the typhoons occur in the region, and with the typhoons come waves up to and exceeding eight meters. This cyclonic activity will become a major consideration within the design, due to the wave action that accompanies the unpredictable environment. The current profile is characterized by a roughly linear change over depth. The average expected current is 1.1 meters per second, while the highest current expected for a typhoon is 1.72 meters per second. These values both drop to 0.5 meters per second near the bottom. The profile can be seen in Figure 5. Current Profile Depth (m) Velocity (m/s) yr non-typhoon 1 yr max typhoon Mooring Enviroment Figure 5: Current Profile 4

20 In order to produce a safe and financially feasible FPSO design, environmental design criteria needs to be defined. For safety and survivability a one year return period typhoon event and one hundred year non-typhoon event have been chosen as the criteria. In order to generate maximum income, a one year non-typhoon event was chosen as the operating conditions. Surviving a one year typhoon means that the vessel must stay connected to the moorings and riser systems. In order to do the analyses of the various components required to meet the survivability condition, values must be found for significant wave height, wind speed, and current velocity. The significant wave height was found to be 8.0 meters, the wind speed is 35.7 meters per second over a one minute gust, and the current at the water s surface is 1.72 meters per second. The operating conditions set for the vessel state that the vessel must be able to produce oil and offload ninety-five percent of the year, under a one year non-typhoon storm event. The one year return period wave height was found to be 2.7 meters. When this value is correlated to the one year probability of wave height, it equates to eighty-five percent operability. The one year probability can be seen below in Figure 6 with the solid black line showing the ninety-five percent condition, and the dashed line showing the one year non-typhoon event. This change is partly affected by the typhoon occurrences in the summer as well as the larger wave heights during the winter months. However, each month should be evaluated separately. Used to show the variability throughout the year, Figure 6 also gives a good insight into what the most probable wave heights are. This means that most waves will fall below that number unless there is a storm event. Thus it can be seen that unless there is a typhoon or other significant storm event, operating conditions will prevail in all but two months, February and December. The winds are much easier to define, because the one year non-typhoon event produces wind velocities higher than any seen on average throughout the year. This wind velocity is 21.9 meters per second. The current velocity for the operating condition is 0.57 meters per second. A table showing the values for both the operating criteria as well as the survivability criteria may be found in Table 1. Table 1: Design Criteria for South China Sea Design Criteria Survivability Operating Hs (m) Ts (s) Wind Speed (m/s) Current Velocity (m/s) Annual Cumulative Probability Curve Percentage of occurance Wave Height (m/s) Annual Cumulative Probability Curve Black line corresponds to the 95% occurance of wave height. Dashed line corresponds to one year non-typhoon event. Figure 6: Wave Height Annual Probability 5

21 With the design criteria set this information will be used continuously throughout the design process. This will become evident throughout the report, as choices will have to be made based on the criteria being used. Some of the major components impacted, are the mooring design, the structural design for wave action, as well as the response of the vessel to the environment. 1.5 Design Criteria This hull design must meet the following requirements and constraints with a minimal cost. Durability, stability, and cost are paramount in all considerations. Functional Requirements: Hull design suitable to store 1.6 million barrels of crude oil 95% availability for off take systems Stable work base for a motion sensitive process Deck area to accommodate large topsides Cargo containment systems tolerant of slack fill conditions Constraints: 20 year design life Remain operational in a 1-year return period No greater than 2 max roll in 1-year return period storm Structural constraints due to double hull design Storage of production approximately 10 days 1.6 Gantt Chart A gantt chart is defined as a chart that depicts progress in relation to time, often used in planning and tracking a project. Gantt charts were developed at the beginning of the design process to keep the members of Team South China Sea on schedule and working in a timely fashion. The Gantt chart showing the team s progress and organization are presented below in Figure 7. The breakdown of individual task assignments is as follows: Curt Haveman Graphics, Mooring Analysis Jeffrey Parliament Mooring Analysis, Loading Conditions Jeremy Sokol Vessel Stability Analysis, Graphics Joseph Swenson Structural Analysis, Vessel Stability Analysis Timothy Wagner Environmental Loading, Quality Control 6

22 Figure 7: Team Design Gantt Chart 7

23 2 Regulatory Compliance This design must meet the requirements of several agencies worldwide. The American Bureau of Shipping (ABS) is the primary agency used in determining regulations and design constraints for this particular project. The major categories for which regulations will govern include general considerations, containment tanks, accommodation, facility arrangements, cargo transfer methods, fire safety, personnel protection, flood survival requirements, helicopter deck, environmental/global loading, general strength of hull, and the position mooring system. The following regulations are taken from the ABS Rules for Building and Classing Steel Vessels, 2005 (ABS BCSV), ABS Guide for Building and Classing Floating Production Installations, 2004 (ABS BCFPI, ABS Guide for Building and Classing Facilities on Offshore Installations, 2000 (ABS BCFOI). 2.1 General Arrangements Machinery and equipment are to be arranged in groups or areas in accordance with API RPI14J. Equipment items that could become fuel sources in the event of a fire are to be separated from potential ignition sources by space separation, firewalls, or protective walls. In case of a fire onboard a subject unit, the means of escape is to permit the safe evacuation of all occupants to a safe area, even when the structure they occupy can be considered lost in a conflagration. With safety spacing, protective firewalls, and equipment groupings, a possible fire from a classified location is not to impede the safe exit of personnel from the danger source to the lifeboat embarkation zone or any place of refuge. (ABS BCFOI Section 3.3 / 5.1) 2.2 Storage and Slop Tanks Supported storage tanks for crude oil or other flammable liquids are to be located as far as possible from wellheads. In addition, they are to be located far from potential ignition sources such as gas and diesel engines, fired vessels, and buildings designated as unclassified areas, or areas used as workshops, or welding locations. For crude storage tanks, slop tanks, and low flash point flammable liquid storage tanks are to be separated from machinery spaces, service spaces, and other similar source of ignition spaces by cofferdams for of at least 0.76 m (30 in.) wide. From ABS Guide Building and Classing Facilities on Offshore Installations (ABS BCFOI Section 3.3 / 5.7) 2.3 Accommodations Accommodation spaces or living quarters are to be located outside of hazardous areas and may not be located above or below crude oil storage tanks or process areas. H-60 ratings are required for the bulkheads of permanent living quarters and normally manned modules that face areas such as wellheads, oil storage tanks, fired vessels (heaters), crude oil processing vessels, and other similar hazards. If such bulkhead is more than 33m (100ft) from this source, then this can be relaxed to an H-0 rating. (ABS BCFOI Section 3.3 / 5.3), 2.4 Spill Containment Spill containment is to be provided in areas subject to hydrocarbon liquid or chemical spills, such as areas around process vessels and storage tanks with drain or sample connections, pumps, compressors, engines, glycol systems, oil metering units, and chemical storage and dispensing areas. (ABS BCFOI Section 3.3 / ) Spill containment is to utilize curbing or drip edges at deck level, recessed drip pans, containment by floor gutters, firewalls or protective walls, or equivalent means to prevent spread of discharged liquids to other areas and spillover to lower levels. A minimum of 150 mm (6 in.) coaming is to be provided 8

24 A spill containment with less than 150 mm (6 in.) coaming arrangement is subject to special consideration. (ABS BCFOI Section 3.3 / ) 2.5 Fire Safety Fixed water fire fighting systems are to be provided as follows. Water fire fighting systems are to be capable of maintaining a continuous supply in the event of damage to water piping. Piping is to be arranged so that the supply of water could be from two different sources. Isolation valves are to be provided such that damage to any part of the system would result in the loss in use of the least possible number of hydrants, water spray branches, or foam water supplies. Materials rendered ineffective by heat are not to be used in firewater piping systems. The firewater distribution system may be maintained in a charged or dry condition. The distribution system is to be maintained such that internal and external corrosion of the piping is minimized. (ABS BCFOI Section 3.8 / 5.1) 2.6 Lifesaving Appliances and Equipment Lifeboats of an approved type are to be provided, with a total capacity to accommodate twice the total number of people onboard the subject unit. They are required to be installed on at least two side of the vessel, in safe areas in which there will be accommodation for 100%, in case one of the stations becomes inoperable. Inflatable life rafts are to be provided with a total capacity equal to that of the total number of people on the vessel. They are to be placed near areas where personnel may be working and with a sufficient quantity to hold the number of people working in the area. At least four life buoys with floating water lights are to be provided. At least one life jacket is to be provided for each person on a manned facility. They are to be stored in an easily accessible location. In addition, life jackets numbering the same as the max aggregate capacity of each life boat station must be stored next to the lifeboat station. When personnel baskets are used to transfer personnel a work vest must be provided. For operations involving hydrogen sulfide, each person is expected on the facility is to be provided a self-contained breathing apparatus for escape purposes. There must be a minimum of 30 minutes air supply in the apparatus. (ABS BCFOI Section 3.8 / 15.5) 2.7 Flood Survival Requirements In any stage of flooding, taking into account sinkage, heel and trim, should be below the lower edge of any opening through which progressive flooding or downflooding may take place. This includes air pipes, weathertight doors, and hatch covers (5-8-2/9.1.1a). The maximum angle of heel due to unsymmetrical flooding should not exceed 30 (ABS BCFPI Section 5-8-2/9.1.2a). At final equilibrium after flooding, the emergency source of power should be capable of operating (ABS BCFPI Section 5-8-2/9.2.2a). 2.8 Helicopter Deck A minimum distributed loading of 2010 N/m 2 is to be taken over the entire helicopter deck (ABS BCSV Section /11.3.1). The structure supporting helicopter decks is to withstand the loads resulting from the motions of the unit (ABS BCSV Section /11.3.4). 2.9 Environmental/Global Loading and General Strength of Hull The Design Environmental Condition (DEC) is to be the following: 100-year wind with associated waves and current (ABS BCFPI Section 3-3/1.1) The current force, F current, on the submerged part of any structure is calculated as the drag force 9

25 by the following equation (ABS BCFPI Section 3-4/5c): F current (kn) = 0.5 * ρ water * C D * A current * u c * u c where ρ water = density of sea water, tonnes/m 3 C d = drag coefficient, in steady flow (dimensionless) u c = current velocity vector normal to the plane of projected area, in m/s A current = projected area exposed to current, in m 2 Wind pressure, P wind, on a particular windage of a floating vessel may be calculated as drag forces using the following equations (ABS BCFPI Section 3-4/7.1): P wind (N/m 2 2 ) = 0.610*C s *C h *V ref where C s = shape coefficient (dimensionless) C h = height coefficient (dimensionless) The corresponding wind force, F wind, on the windage is: F wind = P wind * A wind where A wind = projected area of windage on a plane normal to the direction of the wind, in m2 The wave bending moment, expressed in kn-m may be obtained from the following equations (ABS BCFPI Section 3-2-1/3.5.1). Mws = - k1c1l 2 B(Cb ) 10-3 Sagging Moment Mwh = + k2c1l 2 BCb 10-3 Hogging Moment where k1 = 110 (11.22, 1.026) L = length of vessel, in m (ft) B = breadth of vessel, in m (ft) Cb = block coefficient, but is not to be taken less than 0.6 The envelopes of maximum shearing forces induced by waves, Fw, may be obtained from the following equations (ABS BCFPI Section /3.5.3): Fwp = + kf1c1l B (Cb + 0.7) 10-2 Fwn = - kf2c1l B (Cb + 0.7) 10-2 For positive shear force For negative shear force where Fwp, Fwn = maximum shearing force induced by wave, in kn L = length of vessel, in m B = breadth of vessel, in m C1 = as defined in 3-2-1/3.5 Cb = block coefficient, but not to be taken less than 0.6 k = 30 (3.059, ) F1 = distribution factor F2 = distribution factor The minimum hull girder section modulus amidships is not to be less than obtained from the following equation (ABS BCFPI Section 3-2-1/3.7.1): where C1 = as defined in 3-2-1/3.5 C2 = 0.01 (0.01, ) L = length of vessel, in m SM = C1C2L 2 B (Cb + 0.7) cm 2 -m (in 2 -ft) 10

26 B = breadth of vessel, in m Cb = block coefficient, but is not to be taken less than 0.6 The hull girder moment of inertia, I, amidships, is to be not less than (ABS BCFPI Section /3.7.2): I = L SM / 33.3 cm 2 -m 2 (in 2 -ft 2 ) where L = length of vessel, in m SM = required hull girder section modulus, in cm 2 -m 2.10 Position Mooring System Intact Design A condition with all components of the system intact and exposed to an environment as described by the design environmental condition (DEC) (ABS BCFPI 5-1/1.1). Damaged Case with One Broken Mooring Line A condition with any one mooring line broken at the design environmental condition (DEC) that would cause maximum mooring line load for the system. The mooring line subjected to the maximum load in intact extreme conditions when broken might not lead to the worst broken mooring line case. The designer should determine the worst case by analyzing several cases of broken mooring line, including lead line broken and adjacent line broken cases (ABS BCFPI 5-1/1.3). Transient Condition with One Broken Mooring Line A condition with one mooring line broken (usually the lead line) in which the moored vessel exhibits transient motions (overshooting) before it settles at a new equilibrium position. The transient condition can be an important consideration when proper clearance is to be maintained between the moored vessel and nearby structures. An analysis for this condition under the design environmental condition (DEC) is required. The effect of increased line tensions due to overshoot upon failure of one mooring line (or thruster or propeller if mooring is power-assisted) should also be considered (ABS BCFPI 5-1/1.5). In the structural design of terminals, the interface between the positioning mooring system and the hull structure are to be considered and a finite element method analysis is to be submitted for review. For a fore end, external turret mooring, the minimum extent of the model is from the fore end of the vessel, including the turret structure and its attachment to the hull, to a transverse plane after the aft end of the foremost cargo oil tank in the vessel. The model can be considered fixed at the aft end of the model. The loads modeled are to correspond to the worst-case tank loads, seakeeping loads as determined for both the transit case and the on-site case, ancillary structure loads, and, for the on-site case, mooring loads. The mean tension in a mooring line corresponds to the mean offset and equilibrium heading of the vessel. The design (maximum) mooring line tension, T max, is to be determined as shown below (ABS BCFPI Section 5-1/3.5): T max = T mean + T lf(max) + T wf(sig) ; or T max = T mean + T lf(sig) + T wf(max) ; whichever is greater. where Tmean = mean mooring line tension due to wind, current and mean (steady) drift force. T lf(sig) = significant single amplitude low frequency tension. T wf(sig) = significant single amplitude wave frequency tension. The mooring designer may divide the environmental effects into three categories of response (3-4/9.3): First Order Motions Low Frequency Motions Steady (Mean) Drift The fatigue life of mooring lines is to be assessed using the T-N approach, using a T-N curve that gives the number of cycles, N, to failure for a specific tension range, T. The fatigue damage ratio, Di, for a particular sea state, i, is estimated in accordance with the Miner s Rule, as follows: 11

27 where n i = number of cycles within the tension range interval, i, for a given sea state. N i = number of cycles to failure at tension range, i, as given by the appropriate T - N curve. The cumulative fatigue damage, D, for all of the expected number of sea states, NN (identified in a wave scatter diagram), is to be calculated as follows: D is not to exceed unity for the design life, which is the field service life multiplied by a factor of safety, as specified in Table 2 (ABS BCFPI Section 5-1/3.7). Table 2: Factor of Safety for Anchoring Lines (ABS 2005) Factor of safety All Intact Dynamic Analysis (DEC) 1.67 Quasi-Static (DEC) 2.00 One broken Line (at New Equilibrium Position) Dynamic Analysis (DEC) 1.25 Quasi-Static (DEC) 1.43 One broken Line (Transient) Dynamic Analysis (DEC) 1.05 Quasi-Static (DEC) 1.18 Moorning Component Fatigue Life w.r.t. Design Service Life Inspectable areas 3.00 Non-inspectable and Critical Areas Where Floating Installations are equipped with thrusters to assist the mooring system, the thrusters are subject to approval in accordance with Section of the Steel Vessel Rules. The contribution of the thrusters in the mooring system design will be reviewed on a case-by-case basis (ABS BCFPI 5-1/11). For a mooring system with drag anchors, the mooring line length should be sufficiently long such that there is no angle between the mooring line and the seabed at any design condition (ABS BCFPI 5-2/1). The maximum load at anchor, Fanchor, is to be calculated, in consistent units, as follows: F anchor = P line W sub WD F friction F friction = f sl L bed W sub where P line = maximum mooring line tension WD = water depth f sl = frictional coefficient of mooring line on sea bed at sliding L bed = length of mooring line on seabed at the design storm condition, not to exceed 20 percent of the total length of a mooring line W sub = submerged unit weight of mooring line The coefficient of friction, f sl, depends on the soil condition and the type of mooring line. For the 4.5 inch oil rig quality chain used in this project, the representative values for the coefficient of 12

28 friction at start, f st, and the coefficient of friction at sliding, f sl, are 1.00 and 0.70, respectively (ABS BCFPI 5-2/1). The factors of safety for anchor holding capacity in the design of drag anchors are specified above in Table 2. The required ultimate holding capacity should be determined based on mooring line loads derived from a dynamic analysis to account for mooring line dynamics Stability Intact Stability Criteria For all units, except column-stabilized units, the area under the righting moment curve at or before the second intercept angle or the down-flooding angle, whichever is less, is to reach a value of not less than 40% in excess of the area under the overturning moment curve to the same limiting angle. In all cases, the righting moment curve is to be positive over the entire range of angles from upright to the second intercept angle (ABS BCFPI 3-3-1/3.3.1). Surface Type Drilling Units For surface-type drilling units, the following extent of damage is to be assumed to occur between effective watertight bulkheads. (ABS BCFPI 3-3-1/7.7.5). i) Horizontal depth of penetration of 1.5 m (5ft). ii) Vertical extent of damage from the bottom shell upwards Damage Assumptions (Table 3) Table 3: Damage Assumptions Side damage.1.1 Longitudinal Extent 1/3L^2/3 or 14.5 m whichever is less.1.2 Transverse Extent B/5 or 11.5 m, (measure inboard from the ship's side at right angles to whichever is less the centerline at the level of the summer load line).1.3 Vertical Extent upwards without (from the moulded line of the bottom shell plating at centerline) limit 13

29 3 General Arrangement and Hull/System Design In determining the general arrangement and hull design, one driving factor which plays a major role is the environment. Located in the South China Sea, the vessel will have a high probability of coming into contact with typhoons which are numerously generated each season. This aspect must be taken into consideration when configuring the structural arrangement along with the design of the hull itself. An agreement was reached, and two different options for the mooring layout were finalized. These two specific alternatives are shown and discussed further below. 3.1 Option One The first design thought of after analyzing the environmental conditions and researching similar vessels in the region was a FPSO with an internal disconnecting mooring system. This design will increase the total hull length to accommodate the internal mooring station and the needed permanent bow water tank. The proposed internal turret location is seen in Figure 8. Figure 8: Internal Mooring Design 3.2 Option Two The second, but less desired design that was proposed was a FPSO with an external disconnecting turret system. This specific type of mooring would allow the initial vessel length to remain unchanged at 308 meters. With this design, there would still need to be some type of a propulsion system to allow the vessel to escape quickly from environmental hazards. This representation is shown in Figure 9. Figure 9: External Mooring Design 3.3 Selected Design and General Layout The internal mooring system was the final agreed upon choice for the design of the Floating Production Storage and Offloading vessel. The simple fact of the extreme environmental conditions and the high risk of oncoming typhoons supported the final decision making process in choosing the internal disconnectable mooring system. As mentioned above, the general layout will slightly be altered to lodge the internal turret itself. This is taken into account by extending the length of the ship to a total of 333 meters. The breadth and depth of the vessel are 58 meters and 30.4 meters at the centerline, 14

30 respectively. The ship hull ratios were then calculated to make certain the vessel was within reason and tabulated to be shown in Table 4. The topsides and tanks were arranged according to specs provided by ConocoPhillips and are depicted below in Figure 10 and Figure 11. L/B B/D L/D Table 4: Ship Hull Ratios Ship Hull Ratios Figure 10: FPSO Tank Layout with Internal Mooring Figure 11: FPSO Wing Ballast Tank Layout *Refer to Nomenclature for descriptions of abbreviations Since this vessel s hull was extended without rearranging the placement of the crude oil tanks, the longitudinal center of buoyancy and longitudinal center of gravity differed dramatically from one another. This situation is not feasible for the stability of the FPSO. A solution was proposed following undergone research and accepted advice by industry professionals. This response involved converting portions of the bow compartments to permanent water ballast tanks as depicted below in Figure 12. This action was able to counter the conflicting stability parameters and allow them to realign with one another to ensure proper stability. 15

31 Figure 12: Permanent Water Tank 3.4 Lifeboats According to the ABS guide for Building and Classing Facilities on Offshore Installations, a vessel is required to have lifeboats that are capable of holding twice the maximum number of people on board. For our crew, of no more than 100, four 58 person capacity lifeboats are to be placed on the vessel to be easily accessible. There will be two lifeboats located on the starboard side and the remaining two will be found on the port side. Both sets of lifeboats are to be positioned near the bow alongside the living quarter compartments of the FPSO. 3.5 Accommodations and Helipad As found in ABS Guide for Building and Classing Facilities on Offshore Installations (Section 3.3 / 5.3), accommodation spaces or living quarters are to be located outside of hazardous areas and may not be located above or below crude oil storage tanks or process areas. For this specific vessel design and layout, both accommodations and helipad were chosen to be located at the bow. For the helipad, certain requirements must be in place including a minimum distributed loading of 2010 N/m 2 over the entire helicopter deck (3-2-11/11.3.1). The actual structure supporting the helicopter deck, which implies the accommodation structure, is to also withstand the loads resulting from the motions of the unit ( /11.3.4). The layout and dimensions of the helipad and accommodations are expressed in Figure

32 Figure 13: Accommodations and Helipad Dimensions 17

33

34 4 Weight, Buoyancy, and Stability 4.1 Weight Calculations An estimation of this vessel s steel weight was made by locating and scaling up a comparative vessel in the same location. The Nanhai Endeavour FPSO currently operating in the South China Sea was chosen as a relative vessel. The dimensions and loaded weight of the Endeavour FPSO are as follows: Length 245 m Breadth 45 m Depth 27 m Lightship Weight 42,425 metric tons Further research of the Endeavour led to the number of bulkheads. With this data the surface area of the steel used in the Endeavour was determined as well as the surface area of the steel used in the design vessel. By finding the ratio of the surface areas of the two vessels the lightship weight of the Endeavour was scaled up to a total of 79,526 metric tons. Once this lightship weight was found, the topsides and other miscellaneous weights were determined. These values were subtracted from the lightship weight to find the total steel weight. This can be seen in Table 5. Table 5: Hull Weight Calculation Hull Weight Calculation Existing Ship in the Region Design Ship length 245 m 333 m breadth 45 m 58 m depth 27 m 30.4 m Lightship mt Transverse Bulkheads 8 10 Longitudinal Bulkheads 3 4 Deck and Keel Bulkheads 2 2 Steel Area m^ m^3 scale ratio 1.87 Scale Lightship Topsides and Misc. Weight Scale Steel Weight mt mt mt With the weight of steel present in the vessel determined, the ships fully loaded displacement could be found. Then by taking the displacement and dividing it by the density of sea water (1.025 kn/m^3) the volume of water displaced by the ship was calculated. Finally, by subtracting the lightship weight from the total displacement, the fully loaded deadweight could be found. The vessel values for each of these parameters can be seen below in Table 6. Table 6: Weight Characteristics Weight Charcteristics Displacement Fully Loaded mt Displaced Volume m^3 Deadweight Fully Loaded mt 18

35 The center of gravities and weights for all tanks and modules were located and tabulated for the vessel s total center of gravity computation. This spreadsheet was also designed to modify different loading conditions. The three loading conditions which were critical were the 100%, 50%, 20%. These stabilized product loads changed the center of gravity of the vessel as depicted in Table 7. Table 7: Center of Gravities Loading Condition VCG (m) LCG (m) TCG (m) 100% % % LCG taken from the AP of FPSO with + vector Forward TCG taken from the centerline of the FPSO with + vector to Port VCG taken from the keel of the FPSO with + vector upwards Originally, the vessel s longitudinal center of gravity was a large distance (almost 20m) behind the center of buoyancy, due to the lengthening of the vessel. In order to counteract the large trim that this would have caused, the large permanent trim tanks shown previously were added in the empty space in the bow. The addition of these tanks ensures that at any loading condition the ship will remain level with only a minimum change in ballast levels. As the amount of product present on the vessel changes, the draft of the vessel changes as well. The drafts under the load conditions were tabulated and can be seen below in Table 8 Table 8: Draft Under Different Load Conditions Draft Full Load 22.57m 50% Load 20.45m 20% Load 15.22m Finally, the amount of ballast required to keep the ship level under each of these loading conditions was found. It can be seen in Table 9 that the amount of ballast required in the 50% loaded condition is actually more than that required in the 20% condition. This is due to the fact that with the large permanent tank in the bow of the ship, some of the forward ballast needs to be removed in order to level the vessel. Table 9: Required Buoyancy for Varying Load Condition Ballast Required Full Load 9120 mt 50% Load mt 20% Load mt 4.2 Buoyancy Calculations The draft was determined earlier by using the entire loaded ship weight. This loaded ship weight was equal to how much water it will displace. Therefore the loaded draft of the FPSO was figured to be 21.6 meters measured from the keel. This waterline was modeled in AutoCAD to determine the center of buoyancy, assuming simple block shape. These centers of buoyancy values are tabulated below. 19

36 Table 10: Center of Buoyancy VCB (m) LCB (m) TCB (m) LCG taken from the AP of FPSO with + vector Forward TCG taken from the centerline of the FPSO with + vector to Port VCG taken from the keel of the FPSO with + vector upwards 4.3 Stability In addition to the weight and buoyancy calculations, the stability of the FPSO needs to analyzed. Utilizing StabCAD, a general purpose computer program for designing and analyzing the stability of any floating body, the entire structure can be analyzed according to ABS MODU regulations Intact Stability For intact stability, ABS rules govern the allowable vessel response and conditions to meet. The requirements for a floating, drilling, or production vessels are: Design wind speed for all topside modules is 100 knots The angle of inclination should be no greater than 25 degrees The Righting moment must be forty percent greater than the heeling moment The designed KG must not exceed the minimum allowable KG The draft was entered in at m and a KG of m. From these configurations, the vessel was designed in StabCAD. Figure 14 shows a rendered view of the vessel that was input into StabCAD. Figure 14: Rendered View from StabCAD As stated in Section 2.12, the stability criteria set forth by ABS MODU 2005 regulations, the area under the righting moment curve to the second intercept or downflooding angle, whichever is less, shall be greater than 40% in excess of the area under the wind-heeling moment curve to the same limiting angle. This is displayed in Figure 15 as governed by ABS rules. 20

37 Figure 15: ABS MODU 2005 Intact Stability Curve As seen in Figure 16, the computed intact stability curve satisfies the ABS MODU regulations. It can be seen that the downflooding angle is 15.1 degrees and the allowable KG is from the input value. Figure 16: Intact Stability Curve 21

38 By comparing Figure 15 to the intact stability curve for the designed FPSO generated by StabCAD in Figure 16, it can be seen that the area under A and B is greater than 1.4 times the area under B and C. This successfully satisfies the conditions set forth by ABS MODU Also, the blue line that represents the downflooding angle is past the first intercept of 0.91 degrees. Table 11 shows the specifics for 100% loaded conditions at the draft of m. The values of longitudinal center of buoyancy, transverse center of buoyancy, and vertical center of buoyancy can be derived as , 0, m respectively. Table 12 shows the maximum heel angles before downflooding begins. Draft Table 11: 100% Loaded Specifics Center of Buoyancy Center of Floatation Water Plane Submerged AFT FWD Disp TPI LCB TCB VCB LCF TCF Area Volume ( M.) ( M.) (M.Tons) (MT/Cm) ( M.) ( M.) ( M.) ( M.) ( M.) (S.Meter) (M^3) Table 12: Downflooding Points Height Above Water Downflooding Points Height Above Water (M) Downflooding Angle 15.1 AFT STARBOARD POINT Weathertight Angle 15.1 AFT STARBOARD POINT From the input in StabCAD, the cross curves of stability are given in Figure 17. The cross curves show the righting arm versus draft at the heel angles. 22

39 Figure 17: Intact Cross Curves Plot This graphs shows the range of stability that is dictated by the righting arm for each draft. It can be concluded that the design vessel creates a large enough righting arm to remain stable at each of the corresponding drafts. From the cross curves graph, the righting arm and angle of heel can be taken for a draft of m and graphed to analyze the maximum righting arm at a certain degree of heel for 100% loaded conditions as seen in Figure

40 Curve of Static Stability at m Draft Righting Arm (m) Angle of Heel (degrees) Figure 18: Curve of Static Stability at Draft Figure 18 shows that at operating draft the maximum righting arm of 2.21 m occurs at 25 degrees of heel. Table 13 is a summary of the intact stability parameters. This shows that StabCAD calculated a displacement of metric tons. From the weight calculations, it can be seen that the StabCAD output longitudinal center of gravity corresponds with the calculated LCG of meters. This is largely due to the added water ballast tanks added in the bow. Table 13: Intact Stability Parameters Intact Stability Parameters Draft at no Heel M Displacement M.Tons Center of Gravity (X,Y,Z) = LCG M TCG 0 M VCG M Wind Speed Wind Loading 51.6 M/Sec Wind Direction is Normal to Tilt Axis Range of Stability Range of Stability Deg Downflooding Angle AFT STARBOARD POINT Weathertight Angle AFT STARBOARD POINT Damage Stability The criterion to be used for damage stability as per ABS 2005 is: Design wind speed for all topside modules is 50 knots The angle of inclination should be no greater than 25 degrees 24

41 The minimum extent of weather tight integrity line must be greater than the first intersection of the righting moment and heeling moment (static angle) Horizontal penetration should be at least 1.5m Longitudinal damage extent should be 1/3L 2/3 or 14.5 m, whichever is less Damaged compartments are completely filled The designed KG must not exceed the maximum allowable KG For damage conditions the minimum extent for water tight integrity angle known as the downflooding angle has to be greater than the first intersection of the righting moment and heeling moment. Figure 19 shows the ABS representation of what is required. Figure 19: ABS MODU 2005 Damage Stability Curve The damage stability curve represents the worst case scenario that the vessel must forego until proper repairs can be made. The vessel was constructed of 3 meter deep ballast tanks to fulfill the requirement of the double hull design. The double hull design ensures that the extent of the damage will not penetrate the crude oil tanks. ABS codes also specify that the damage encountered must be at least 14.5 meters long. In this case, the worst case scenario will be damaging 2 ballast tanks as if the damage occurred at the intersection of the tanks. This ensures that the worst case scenario is achieved. Through several iterations of damaging a combination of ballast the worst case can be seen by damaging tanks 7 and 9 either on the port or starboard sides, which fulfill the ABS MODU criteria for damage. These tanks are shown in Figure

42 Figure 20: Damaged Ballast Tanks - 2 total From the damaged analysis the stability curve in Figure 21 could be analyzed. Figure 21: Damage Stability Curve From the damage stability curve, the downflooding angle must not cross the first intercept at 9.50 degrees. As shown, the worst case scenario passes the established criteria. 26

43 Table 14 shows the conditions that must be satisfied. ABS MODU rules stated that the angle of inclination when flooded must not exceed 25. The optimum angle is 12.72, which passes this criterion. Also, the input KG of m must not be greater than the least allowable KG calculated from StabCAD. The least allowable KG is m which is greater than m. Condition To Satisfy Table 14: Damage Conditions to Satisfy Allowable Optimum Range Of Static KG Tilt Angle Stability Angle 1st Intercept 2nd Intercept (M) (Deg) (Deg) (Deg) (Deg) (Deg) For Input KG = Heeling Arm = Righting Arm Static Angle = Area Ratio = RM/HM Ratio = Overall, both the intact and damaged stability of a 100% loaded vessel passed the ABS rules and regulations. For an intact hull, the range of stability was found to be 15.1 degrees before the first downflooding point was reached by the water line. The design and calculated vertical center of gravity was less than the maximum allowable KG as dictated by ABS. The maximum angle of inclination was found to be less than 25 degrees, satisfying all conditions of a stable vessel. For the damaged stability, the minimum requirements were met as well. The stability curve calculated a downflooding angle at degrees which is greater than the first intercept between the righting moment and heeling moment at 9.5 degrees. The range of stability for the damaged condition was found to be 3.51 degrees. For a draft of meters, the input vertical center of gravity of meters was less than the maximum allowable KG of 19.2 meters. Throughout the analysis, a maximum stability analysis was run, and the vessel proved stable if all of the ballast tanks were damaged proving a optimum vessel design. 27

44 5 Local and Global Loading In order to determine the strength of main girder required to fulfill ABS standards of strength, loading conditions must be determined. Figure 22 below shows the local load conditions starting at the stern and taking measurements every 15 meters forward from that point for the empty, ballasted, and full storage tank cases: Local Loading Load (mt) Distance from Stern (m) Figure 22: Local Loading empty ballasted This figure shows that the largest amount of the load comes from filling the storage tanks. Also the elevated loading in the forward section of the ship represents the effect of the permanent trim tanks that were added to the vessel. While this data can help to determine which loading condition is most likely to cause the largest deflections, shears and moments, it is not complete enough to get accurate data. In order to model the loads more completely, a global loading profile, which can be seen in Figure 23, was created. This profile will provide more accurate values for the displacement, shear, and bending moments that the hull experiences. full 28

45 Global Loading Load (mt) full ballasted empty Distance From Stern (m) Figure 23: Global Loading 29

46 6 General Strength and Structural Design Using the 2005 ABS Rules for Building and Classing Steel Vessels (section 3.2.1) the minimum section modulus and moment of inertia for the beam were calculated and shown in Table 15. Table 15: ABS 2005 Regulations Parameter Limiting Value Hog/Sag Section Modulus ± 0.5 m Maximum m^3 Minimum Moment of Inertia m^4 Minimum After determining these values, visual analysis was used to analyze the ship hull. This was accomplished by modeling ship hull as a beam with the specifications above. Next, the global loading profile was entered as a set of distributed loads oriented in the negative y direction. Then the buoyant force was applied in several different configurations in order to determine the maximum levels of hog, sag, shear and bending moments. The section modulus of the beam had to be increased after the initial runs due to excessive sagging and hogging of almost 4 meters. The section modulus of the beam had to be increased to 2400 m 4 it meet the deflection requirements. The first of the configurations analyzed was the evenly distributed buoyant load show in Figure 24 below: Figure 24: Evenly Distributed Buoyancy Case This case will represent conditions where wave action is small and the ship is floating level. The results from the analysis of this case can be seen in Figure 25. The linear springs in the model are used to represent the force the water below the ship as it resists the downward bending of the vessel. The springs stiffness was found my calculating the amount of force required to deepen the draft of the vessel by one meter. This force was then distributed evenly among the springs. Finally, the rotational spring located at the location of the internal turret represents the force exerted by the water in response to the ship pitching around the turret. Figure 25: Evenly Distributed Buoyancy Case Deflection 30

47 Next, two cases were designed to simulate the worst case waves that our ship will experience. This wave had a height of 13.5 meters and a wavelength of 169 meters. The maximum hog case was found to occur when the wave troughs were at the ends of the vessel. Figure 26 and Figure 27 show the input and beam deflection for this load case. The positive deflection present at the bow of each of these configurations is due to the fact that the ship was fixed at the location of the turret instead of at the beam. However in each of the conditions this end beam displacement was minimal. Figure 26: Maximum Hog Buoyancy Case Figure 27: Maximum Hog Buoyancy Case Deflection Finally, a case representing the design wave from above with a crest at each end of the ship was implemented. This case was chosen because the lacks of buoyant force in the center of the ship lead to an extreme case of sagging. The input and deflection from this load case can be seen in the following Figure 28 and Figure 29. While this loading case has more than twice the displacement then the evenly distributed buoyant force, the deflection is still within the limit of half a meter. Figure 28: Maximum Sag Buoyancy Case 31

48 Figure 29: Maximum Sag Buoyancy Case Deflection The next step in the analysis was to find the wave induced shear and moments generated by the hag and sag loading conditions. This was done by taking the total shear and moments for those conditions and subtracting the Stillwater shear and moment from them. This value was then compared to the shear and moment envelopes determined using the ABS 2005 Rules for Building and Classing Steel Vessels (sections and 3.5.3). Figure 30 and Figure 31 below show the wave induced shear and bending moment under the max hag and sag conditions compared to the calculated envelope Wave Induced shear Moment (kn/m) Positive Envelope Negative Envelope Sag Worst Case Hog Worst Case Distance from Stern (m) Figure 30: Wave Induced Shear 32

49 Wave Induced Moment Moment (kn/m) Hog Envelope Sag Envelope Sag Worst Case Hog Worst Case Distance from Stern (m) Figure 31: Wave Induced Moment The figures above show that the vessel meets the ABS requirements for both wave induced sheer and moment. It can be noted that if the weight near the stern of the ship were distributed differently, the shear in that area of the ship could be improved. It would be recommended that more weight be added to the stern section of the vessel in order to offset the much larger buoyancy force found in the sag and stillwater conditions. 33

50 7 Wind and Current Loading In order to obtain the environmental loads, the areas of the vessel and components for both above and below the draft must be found. The draft was determined to be 22.57m from the keel. This draft line was modeled in AutoCAD and the resulting surface areas were calculated as shown below. Figure 32 shows how the area calculations and coefficients were distributed about the ship shape. Table 16 displays the calculated areas of each designated A. Figure 32: AutoCAD Area Distribution Table 16: Area Calculations of Ship Shape A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² A m² Once these areas were established, the actual environmental loads could be calculated. Wind, current, and the mean wave drift force will all have an effect on the vessel. Both wind and current speeds, along with the significant wave height employed to determine this was verified to be the following values as seen in Table 17. Table 17: Wind, Current, & Significant Wave Height Implementation Wind Speed (m/s) 35.7 Current Speed (m/s) 1.71 Significant Wave Height (m) 8 These values were used to figure the total environmental force for bow, beam, and quartering seas on the vessel as seen in. Table 18: Total Environmental Forces on Vessel Force (kn) Bow Seas Beam Seas Quartering Seas Wind Current Mean Wave Drift Force Total Force (kn)

51 The quartering seas force depicted above was calculated with a set angle of 22.5 degrees. When this angle increases the total force will also increase. This being known, it is unlikely that this vessel will be subject to environmental conditions at this angle since a single point mooring design has been chosen. This single point mooring design will allow the vessel to weathervane in order to avoid extreme environmental loads. These hand calculated results were compared with the environmental external forces obtained through Mimosa, and are discussed in the Mooring/Station Keeping section. With the aid of the mooring software, the wind, current, and wave data was projected for the bow of the vessel to be kn, kn, and kn. These values differ somewhat drastically to the bow seas calculations shown above in. Therefore, there must be a discrepancy in the hand calculated forces, as the majority of the confidence lies in the accuracy of the mooring results. In going through the process of determining the wind, current, and wave through hand calculations, one assumption was strictly implemented. Due to limited resources, the coefficients used to determine these loads were characteristics of an average drill ship. Seeing as drill ships have an overall length that would be considerably less than an FPSO and a more hydrodynamic bow which is able to cut through the environment, this assumption can be agreed upon to be the cause of the inconsistency. 35

52 8 Mooring/Station Keeping An integrated motion-mooring analysis was performed for the South China Sea FPSO facility. The environmental conditions considered are shown below in Table 19. Table 19: Environmental Conditions Considered Wave Data Hs (m) Hmax (m) Ts (s) Tz (s) 1yr Typhoon yr Typhoon yr Typhoon yr non-typhoon yr non-typhoon yr non-typhoon Wind Data Current Data Speed (m/s) Direction Speed (m/s) Direction 1yr Typhoon yr Typhoon yr Typhoon yr non-typhoon yr non-typhoon yr non-typhoon Environmental Loads were calculated both by hand and using MIMOSA. Table 20 shows a comparison of the environmental forces. The loads found through hand calculations are much smaller, because the wave coefficients used in those calculations were based on a drill ship. The wave force in MIMOSA is thought to be high due to the shallow operating depth. Table 20: Environmental Load Comparison ENVIRONMENTAL LOADS Collinear Environmental Forces From MIMOSA Wind (kn) Wave (kn) Current (kn) Total (kn) Hand Calculated Environmnetal Forces Wind (kn) Wave (kn) Current (kn) Total (kn) Mimosa, a program used for the design and analysis of mooring systems for offshore vessels, is used to conduct all mooring analyses/design. The referenced API guidelines for measuring the factors of safety in the analysis are shown below: API RP-2SK Mooring Code: Dynamic Intact FOS = 1.67 Dynamic Damaged FOS = 1.25 The maximum vessel offset, as recommended by API for intact condition, is 8% of the water depth for rigid risers and 10% of the water depth for flexible risers. For damaged condition, API allows for an offset of 12% to 15% of the water depth. Vessel offsets are controlled by adjusting the pretension and total line length associated with each leg. For single point mooring systems ABS (section of the Guide for Building and Classing Floating Production Installations) dictates that there are a minimum of three wind, wave, and current configurations must be analyzed. The first of these is when all three of the conditions are collinear and acting on the bow of the ship. The second consist of wind and current collinear and both at 30 degrees to 36

53 the waves. The third configuration has the wind at 30 degrees to the waves and the current at 90 degrees to the waves. Three different mooring systems were selected as potential candidates for the design of a FPSO to be located in the South China Sea. Each mooring system will be developed and drawn out using the MIMOSA program within the SEASAM package. Then, each system will be analyzed using MIMOSA and then revised until it meets current design criteria. Mooring of the FPSO will be accomplished by utilizing one of three systems listed below. 1. An external disconnectable turret with a single point mooring system 2. An internal disconnectable turret with a single point mooring system 3. A spread mooring system The mooring system is configured to meet certain design characteristics with respect to maximum allowable offsets and specified safety factors for both intact and damaged conditions. Since the FPSO will be designed to operate in a water depth of around 100 m, the maximum allowable offset for operating conditions will be 10m, which is 10% of the water depth. The minimum safety factors that must be met are 1.67 for intact conditions and 1.25 for damaged conditions. It is required that the mooring system be able to allow the FPSO to operate in a 1 year non-cyclonic storm and survive on location a 100 year noncyclonic storm or a 1 year cyclonic storm. The mooring systems to be considered for the FPSO design are an internal and external disconnectable turret mooring systems. Advantages of the turret type mooring systems are the vessels ability to weathervane, or move with the environmental forces to minimize forces on the mooring system and vessel. Another advantage of the turret type mooring system is the availability of a disconnectable turret. With the disconnect-able turret system, a vessel can shut-in the well and disconnect when there is a threat of severe weather, such as typhoons that are frequently encountered in the South China Sea. Some disadvantages of the turret type mooring system is the limited space inside the turret for risers and the additional cost of the turret system. If a turret is designed to house 10 risers, it can hold a maximum of 10. Depending on the type of environment that the FPSO will be moored in, the extra cost and limited production ability of the turret design could outweigh the risk of a spread mooring system. Figure 33 (courtesy of SBM IMODCO) shows internal turret and an external turret mooring systems on ships. It is important to notice how the vessel s ability to weathervane is affected with the internal turret. The further astern the turret is moved, the lesser the vessel s ability to weathervane. This is effective because when the vessel is disconnected from the system, the turret sinks down to an equilibrium point where it is well below the force of wind and waves. 37

54 Figure 33: Internal and External Turret Mooring Systems A single point, internal, disconnectable turret was chosen for this FPSO application. The reasoning associated with this decision can be viewed in Table 21. According to the weighted mooring system selection chart, the most beneficial type of mooring system to utilize for the environmental conditions and water depths given is the internal turret with a disconnectable feature. The internal turret will allow the vessel to weathervane to minimize the forces acting on the vessel due to the environment, reducing the force that the mooring system has to comply with. With the disconnectable mooring system, if severe weather threatens the vessel, the mooring system can easily be dropped and the vessel either move under its own power or be towed out of the area until the threat of the severe weather has passed. Once the weather has passed the production site, the vessel can be placed back over the production field, reconnect to the mooring system and resume production in a relatively short period of time. Objective Weight Parameter Need to weathervane Ability to disconnect Reconnection Efficiency Vessel motion 0.1 Table 21: Weighted Mooring System Selection Chart % time vessel is headed into current/winds Disconnect Time (hrs) Reconnection Time (hrs) Effective increase on vessel motion (%) Magnitude Score Value Magnitude Score Value Cost 0.1 Cost (million) OVERALL UTILITY VALUE External/Internal Permanent Mooring Internal Disconnectable Turret The mooring system for the FPSO was designed through an iterative process where a system was designed then analyzed then improved upon many times. The mooring system is comprised of 12 lines of chain with 4 groups. The 4 groups are spread out evenly 90 apart with a 5 spacing between each line within the 4 groups. The lightest possible chain that was selected for the system was 142 mm R4 stud link chain. Each length of chain is 500m long from the mooring point on the turret to the anchor 38

55 positioned on the seafloor. Table 22 contains a description of the mooring system as well as the chain properties of the R4 studlink chain. The factored break strength is the strength of the chain factored for corrosion over the expected lifespan of the mooring system (10 mm of corrosion over the life of the project). Table 22: Mooring Line Properties Lines 12 Line Length 500 m Modulus kn/m 2 Break Strength Factored Break Strength Diameter Weight in Water Line Properties kn kn 142 mm 442 kg/m When MIMOSA is used to plot the mooring system, the product can be seen in Figure 34 and Figure 35 which contain the horizontal view and the vertical view of the mooring system, respectively. The mooring lines have a catenary shape which aids the vessel in returning to its equilibrium position when it is displaced by forces. Figure 34: Horizontal Layout of Mooring Lines Figure 35: Vertical View of Mooring Lines Before environmental forces were taken into consideration, a few preliminary calculations were run concerning the mooring system using MIMOSA. First, the amount of chain resting on the sea floor was found to be 136 meters. This length of chain on the sea floor is critical in the ability of the mooring system to maintain a satisfactory watch circle. The anchor selection was determined after the survival conditions for the mooring system was analyzed and will be discussed later in this section. Next, the amount of 39

56 chain suspended between the seafloor and the turret was found to be 364 meters. The horizontal distance from the anchor to the point where the chain attaches to the turret was found to be 488 meters. Finally, the weight of each leg of the mooring system was calculated to be approximately 221 metric tons which reflects a total mooring system weight (less anchors) of approximately 2652 metric tons. The total weight of chain that must be supported through the water column is approximately 1930 metric tons. First, the mooring system was analyzed for the intact survival condition. The mooring system must survive and remain intact during a 1 year typhoon event, whose parameters can be found previously in Table 19. The three different environmental conditions were utilized, per API rules in analyzing the mooring system. Co-linear environmental forces Non Co-linear Case 1: Wind and current are collinear and both at 30 to waves Non Co-linear Case 2: Wind at 30 to waves, and current at 90 to waves For these conditions, the vessel could not offset more than 12m from its original location and the mooring system had to have a factor of safety greater than The results for the intact mooring system analysis can be found in Table 23. The three cases mentioned earlier in this section were analyzed. As can be seen in the table, for the intact survival conditions, the maximum displacement and factor of safety allowed in each case were met and surpassed. Table 23: Intact Survival Analysis Maximum Offset (m) Intact Analysis Factor of Safety Environmental Case Allowable Co-linear Actual Wind and current are collinear and both at 30 to waves Wind at 30 to waves, and current at 90 to waves Actual Actual After the intact condition was satisfied, the mooring system was analyzed in the damaged survival condition with co-linear environmental forces acting on the bow of the vessel. For the damaged condition, one mooring line was damaged. The damaged analysis was completed for each of the 3 lines in the group of lines with the highest tensions. The maximum offset and minimum factor of safety allowed for the damaged survival analysis was 12 m (12% of the water depth) and 1.25, respectively. For the damaged line, line #1 was selected because that particular line had the greatest maximum tension on it during the intact analysis. The results from this analysis can be found in Table 24. A graphical representation of the offsets from each analysis compared to their respective allowable offsets can be seen in Figure 36 and Figure 37 for intact and damaged cases, respectively. 40

57 Table 24: Damaged Survival Analysis Maximum Offset (m) Damaged Analysis Factor of Safety Environmental Case Allowable Co-linear Actual Wind and current are collinear and both at 30 to waves Wind at 30 to waves, and current at 90 to waves Actual Actual Figure 36: Offsets for Survival Conditions 41

58 Figure 37: Offsets for Damaged Conditions After the survival analysis was performed, the operating conditions were investigated. In order to be able to produce, the vessel cannot offset more than 12 m from its original location during the intact and damaged conditions while maintaining a line factor of safety of 1.67 and 1.25 for intact and damaged, respectively. Again, mooring line #1 was the candidate for damaging due to its tendency to hold the greatest maximum tension. The mooring system must be designed to allow the vessel to be able to produce continuously through a 1 year non-typhoon event. This mooring system keep the vessel in a tight enough watch circle (less that 12m), so that theoretically, production could occur during survival conditions if the production equipment would allow. Upon completing the analysis of the mooring system, critical components such as the type and size of anchors used and the number/types of risers to be used could be selected. Determining the anchor for the vessel was done using was the Vryhof manual. Due to the soft clay bottom composition, the Stevpris Mk. 5 (which can be seen in Figure 38) was selected based on its ability to dig deeply into soft clay, which would add security to the mooring system. Next, an anchor weight of 12 metric tons was selected based on the MIMOSA output for the largest line tension present in all of the analysis combined. 42