OMAE WET TREE SEMI-SUBMERSIBLE WITH SCRS FOR 4,000 FT WATER DEPTH IN THE GULF OF MEXICO

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1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE WET TREE SEMI-SUBMERSIBLE WITH SCRS FOR 4,000 FT WATER DEPTH IN THE GULF OF MEXICO Jingyun Cheng SBM Atlantia Inc. Houston, TX, USA Peimin Cao SBM Atlantia Inc. Houston, TX, USA Sherry Xiang SBM Atlantia Inc. Houston, TX, USA ABSTRACT This paper presents a design of a deep draft wet tree semisubmersible with steel catenary risers (SCRs) for 4,000 ft water depth in the Gulf of Mexico (GoM). The integrated system of hull, mooring, and SCRs is discussed. The design challenges of SCRs are highlighted and results of SCR strength and fatigue performance are presented. A comparison study on strength performance of various types of risers under the GoM environment criteria is performed. The assessment of extreme strength responses from various riser and hull configurations provide guidelines for the best hull selection. Sour service requirement creates challenges in the fatigue design of the production riser system at such water depth. Integrated mooring and riser design provides an optimum solution. It s found that the majority of riser fatigue damage at touch down zone is generated by wave loading & resultant vessel motion and vortex induced vessel motion (VIM). Several fatigue mitigation methods are suggested to improve the riser fatigue performance, such as planned vessel repositioning. The conclusion of this study is that deep draft wet tree semi-submersible with SCRs can be a cost effective solution for field development at 4,000 ft water depth in the Gulf of Mexico. Keywords: SCR, Semi-Submersible INTRODUCTION Deep draft production semi-submersible combined with steel catenary riser (SCR) provided a cost-effective solution for the deepwater and harsh environment in the Gulf of Mexico (GoM) and other area of the world favored by high hull steel and topside payload ratio, good constructability, and facilitating quayside hull-topside integration/commission, thereby eliminating the project risk caused by dependence on deep water heavy lift construction vessels. The concept has been demonstrated through several recent industry projects, i.e. Independence Hub (8,000 ft) [Ref.1], Thunder Hawk (6,060 ft) [Ref.2], and Blind Faith (6,500 ft). One of the major challenges for these production semi-submersibles is the extreme strength and fatigue performance of SCRs. It is generally accepted that the water depth limit for the production semi-submersible with standard SCR technology is between 5,000 and 6,000 ft. With many oil fields discovered in the GoM at shallower water depth (i.e., 4,000 ft to 5,000 ft), it is important to investigate the feasibility of the deep draft production semi-submersible concept for field development at such water depth, while using existing field proven SCR technology and maintaining commercial competiveness. This paper presents a design of a deep draft semisubmersible with steel catenary risers (SCRs) for 4,000 ft water depth in the GoM. The integrated system of hull, mooring, and SCRs is discussed. The design challenges of SCRs are highlighted and results of SCRs strength and fatigue performance are presented. A comparison study on strength performance of various types of risers under the GoM environment criteria is performed. The assessment of extreme strength responses from various riser and hull configurations provide guidelines for selecting the best hull solution. Sour service requirement creates challenges in the fatigue design of the production riser system at such water depth. Integrated mooring and riser design provides an optimum solution for riser fatigue performance. It s found that the majority of riser fatigue damage at touch down zone is generated by wave loading & resultant vessel motion and vortex induced vessel motion (VIM). Several fatigue mitigation methods are suggested to 1 Copyright 2011 by ASME

2 improve riser fatigue performance, such as planned vessel repositioning. DESIGN BASIS The field development case investigated in this paper is located in the GoM at a water depth of 4,000 ft. The SCRs are designed for a wet tree deep draft semi-submersible capable of producing 45,000 to 60,000 BOPD. The system design life is 20 years. Table 1 lists the different risers considered in this study. The maximum production well pressure is 15,000 psi for the 8- inch production riser and 13,000 psi for the 6-inch production riser. A 12-inch riser is selected for hydrocarbon export. The topsides payload is approximately 12,000 short tons. Table 1. Riser System Description SCR OD (inch) 6-inch Production inch Production inch Oil Export inch Gas Export The generic central GoM metocean data specified in API Bulletin 2INT-MET [Ref.3] are used in the riser strength design and are shown in Table 2. The typical GoM wave scatter diagram and loop/eddy current diagram are used in the riser fatigue design. Table 2. Riser Design Metocean Criteria Extreme 100 year hurricane Survival 1000 year hurricane Significant wave height Hs (ft) Peak Spectral Period Tp (s) Peak enhancement factor γ hour mean wind speed Vw (ft/s) Surface current speed Vc (ft/s) implemented. A parametric study on different hull geometries is performed to optimize the hull configuration for riser response. For a deep draft semi-submersible, a given topside payload and associated deck requirements defines deck structural requirement. Then the column side length should be minimum necessary to provide stability of semi-submersible during quayside integration, wet-tow, installation and in-place operation. The column height consists of a combination of freeboard (airgap) requirement and operating draft. The operating draft is an important parameter for the SCR extreme performance. During the parametric study of hull configuration, for a given column size, the pontoon geometry is modified until the heave cancellation period is close to peak period of the 100- year hurricane or 1000-year hurricane so that the overall heave response is minimized. Another important factor is the polyester mooring design. The polyester mooring system is optimized for SCR wave loading fatigue and for vortex induced motion fatigue affected by surge/sway natural period and offset. The integrated hull, mooring, and riser system design flowchart is shown in Figure 2. DEEP DRAFT SEMI-SUBMERSIBLE HULL AND MOORING CONFIGURATION The semi-submersible floating production unit is based on SBM Atlantia s (SBMA s) DeepDraft Semi design. The design is similar to that employed for the Independence Hub and Thunder Hawk projects, but modified to accommodate the specified riser system and topsides payload requirements. Figure 1 shows an artist s rendering of a typical DeepDraft Semi design. This configuration enables the platform to support large payloads while maintaining stability during the various stages of integration, transportation and installation, and allows self-supported hull and mooring installation without the need for a heavy lift vessel or supplemental buoyancy. One of the main challenges in configuring the semisubmersible is the vessel motion requirements imposed by SCR strength and fatigue performance criteria, especially for fatigue consideration due to relatively shallow water depth. The integrated hull-mooring-riser system design methodology is Figure 1. DeepDraft Semi design for GOM The principal hull dimensions of the 4,000 ft deep draft semi-submersible FPU are presented in Table 3. The platform draft is constant for both operating and extreme environmental conditions. The mooring system is a 12-leg, chain-polyesterchain system anchored to the seabed with suction or driven piles. It consists of 4 bundles of 3 lines each, the angle of separation of the individual lines within a bundle is set at 5 degrees. The orientation of the mooring lines on the hull is shown in Figure 3. Each mooring line is equipped with a chain jack located on the top of column. These chain jacks can be used to reposition or re-tension vessel to improve riser touch 2 Copyright 2011 by ASME

3 down fatigue performance when needed. The chain stoppers are located immediately below the chain jacks. 1.6 Figure 3. Hull & Mooring System Plan View Deep drafted SEMI - 0 deg Heading Figure 2. Integrated Hull, Mooring, and SCR Design Table 3. Hull Principal Dimensions PRINCIPAL DIMENSIONS Pontoon Width 42.0 ft Pontoon Length ft Pontoon Height 28.0 ft Column Width 50.0 ft Column Length 50.0 ft Column Height ft Operating Draft ft Table 4. Surge, Heave and Pitch Natural Periods DOF Natural Periods (s) Surge 123 Heave 20.1 Pitch 27.5 Figure 4 shows the surge, heave and pitch motion RAOs for the hull configuration. Table 4 lists the surge heave, and pitch natural periods. Surge/Heave RAO (ft/ft) Surge Heave Pitch Period (s) Figure 4. Surge, Heave and Pitch RAOs DESCRIPTION OF SCR SYSTEM The key parameters for various SCRs considered in this study are presented in Table 5. The riser wall thickness is calculated based on the pressure requirement per CFR and API RP-2RD. A heavy wall thickness of inch is required for 8- inch production riser to meet the 15 ksi shut-in pressure. One important factor for SCR system design is the platform orientation and riser layout. In general, the SCR layout should first accommodate project specific subsea layout and interference considerations. Then the rises can be hung off from particular hull pontoon location and particular azimuth angle to avoid high wave direction for strength consideration and/or to avoid strong loop/eddy current for VIM fatigue consideration. In this study, the SCRs have been checked with different azimuth angles. All riser departure angles are assumed to be 15 degrees, although these can be further optimized per strength and fatigue requirement. All risers will be covered by strake approximately 60 percent of suspended length to suppress VIV. The production risers are also covered with thermal insulation layer for typical flow assurance considerations. Both sour service and sweet service requirements for production risers are investigated in this study Pitch RAO (Deg/ft) 3 Copyright 2011 by ASME

4 Description Table 5. Riser Design Data 8-inch Production 6-inch Production 12-inch Oil Export 12-inch Gas Export Pipe OD (inch) Wall thickness (inch) Material Grade X70 X70 X65 X65 Maximum Pressure (psi) 15,000 13,000 3,650 3,650 Thermal Insulation Thickness (inch) n/a n/a Hang-off Angle (deg) Top Termination TSJ or TSJ or TSJ TSJ Unit FJ FJ Strake Coverage 60% 60% 60% 60% Service sour or sour or sweet sweet sweet sweet SCR STRENGTH DESIGN The strength design of SCRs is implemented in accordance with API RP-2RD [Ref.4]. The analyses for each riser were performed in the near, cross and far riser directions with multiple three (3) hours random dynamic simulations using nonlinear finite element software FLEXCOM [Ref.5]. The average of the extreme observed values from each realization was used to estimate the extreme expected value. For semisubmersible, large vessel heave motion may cause compression at the riser touch down zone. The design requirements are no compression for the 100-year hurricane condition, and limited compression for the 1000-year hurricane condition, while satisfying the API RP-2RD stress criteria. The results of the critical 100-year hurricane and 1000-year hurricane intact conditions are presented in Table 6. The typical API RP 2RD von Mises stress distribution and effective tension distribution envelopes are shown in Figure 5 and Figure 6. It s found that all risers meet the API RP 2RD stress criteria for the 100-year and 1000-year hurricane conditions. No compression is expected at the touch down zone for all risers. Some compression has been expected at the touch down zone. However, the compression level is low and will not cause overstress and buckle of the riser pipe. These results demonstrate that the SCRs meet the strength design criteria. The SCR compression at touch down is primarily associated with the SCR cross section properties and semisubmersible porch heave motion, both of which are water depth independent. Therefore, the SCR strength feasibility is less sensitive to the water depth change. SCR FATIGUE DESIGN The long term fatigue design of SCRs is performed in accordance with the following criteria: Factored Life = (1-β)/ (SF wave D wave + SF VIV D VIV + + SF VIM D VIM ) > Design Life Design Life is 20 years. D wave is the annual fatigue damage due to wave loading and the resultant vessel motions. D VIV is the annual fatigue damage due to current induced VIV. D VIM is the annual fatigue damage due to current induced VIM. β is the percentage of total allowable fatigue damage during installation, which is typically taken as 10%. Safety factor for SCR wave loading fatigue damage, SF wave = 10 Safety factor for SCR current VIV fatigue damage, SF VIV = 20 Safety factor for SCR vessel VIM fatigue damage, SF VIM = 10 The short term fatigue design of SCRs should also be performed, but considered as standalone criteria. API X curve with a stress concentration factor of 1.2 is used for all riser fatigue calculations. The production risers may subject to mildly sour due to reservoir souring as a consequence of water injection. A knockdown factor is applied at the welding root to investigate the fatigue feasibility. The sour service criteria are typically developed based on projected specific testing. Knockdown factors of 10 and 45 are used for illustration in this study. The riser Vortex Induced Vibration (VIV) fatigue was analyzed using SHEAR 7 version 4.5 [Ref.6], with modal information calculated by FLEXCOM. This version introduces time sharing effect, thus removes some of the conservatisms in the earlier versions. The VIV fatigue lives for each riser are summarized in Table 7. It s found that VIV fatigue damage for the straked risers are insignificant. It should be mentioned here that fatigue damage due to heave induced VIV was also evaluated, and found to be negligible. The riser wave loading fatigue was analyzed in the time domain. Vessel motion timetraces generated by the coupled analysis software AQWA [Ref.7] was used as the riser analysis input. A total of 108 consolidated fatigue seastates were used in the wave loading fatigue analysis. Fatigue damage was calculated using rainflow cycle counting approach, and calculated at eight (8) circumferential points around the riser pipe. The calculated wave fatigue lives for each riser are summarized in Table 7. A typical fatigue distribution is shown in Figure 7. It s found that the majority of the wave induced fatigue damages at riser touch down zone are caused by the median seastates in the scatter diagram, induced mainly by the vessel surge motion in the riser near/far plane. The deep draft semisubmersible is also subject to Vortex Induced Motion (VIM) as it responding to surface current. 4 Copyright 2011 by ASME

5 Based on the model test observations from previous SBMA s projects, i.e., Independence Hub and Thunder Hawk, the lock-in VIM motion is highly sinusoidal and current heading dependent. The key dimensionless parameter of VIM is reduced velocity Vr defined as following: VT V r = D Where V is the current velocity, T is the period of natural oscillation in the + A, A directions (Figure 8), and D represents the column width projected onto a plane perpendicular to the current direction. The typical semisubmersible VIM A/D curves are shown in Figure 9. The details of the VIM calculation methodology is discussed in Ref.8. As shown in the definition of reduced velocity, the vessel surge/sway periods will affect the lock-in surface current speed. Therefore, it s important to design the riser and mooring system using an integrated approach to achieve an optimum fatigue performance. The key design parameter is mooring system pretension. Typical GoM long term loop/eddy current was used in the riser VIM fatigue evaluation. The estimated VIM fatigue lives for each riser are also summarized in Table 7. From the table, VIM is the largest fatigue damage contributor at the touch down, accounting for about two thirds of the overall damage. It s important to note here that Table 7 conservatively assumes the fatigue damages from all sources occur at the same location along the riser and around the pipe circumference. The combined fatigue life is 691 years for the 8-inch production riser (sweet service), 1,299 years for the 6-inch production riser (sweet service), and 442 years for the export risers. All risers exceed the un-factored design life target of 222 years considering a 10% installation damage allowance. For the sour service production risers with a knockdown factor of 10, the combined fatigue life is 345 years for the 8-inch production riser, and 478 years for the 6-inch production riser, which exceed the un-factored design life target. For a more stringent knockdown factor of 45, the combined fatigue life is 77 years for the 8-inch production riser, and 106 years for the 6-inch production riser, which are below the fatigue life target. Figure 7 presents a closer look at the typical fatigue life distribution at the 8-inch production riser touch down zone. It shows that the worst fatigue damages from wave loading and VIM occur at the different locations along the riser. The reason is that in the GoM, the worst fatigue seastates and loop/eddy currents typically come from the different directions, which causes a natural spreading of the fatigue damages due to vessel offset. As a result, for a knockdown factor of 45, the combined fatigue life for the 8-inch production riser will be 109 years instead of 77 years, which is still below the design target. One measure to improve touch down fatigue performance is to spread the damage by repositioning the vessel during the service life. Figure 10 illustrates the fatigue damage rate for the 8-inch production riser by repositioning the vessel every 50-ft in the plane of the riser. It shows that the worst fatigue damage location moves with minimal overlap from each vessel position. As expected, the damage rate increases as the vessel offsetting in the near direction of the riser because of the reduced hang off angle. The operational strategies to achieve this may range from planned vessel reposition only in the non-hurricane season, planned vessel reposition in all seasons but return to nominal position in preparation of inoming hurricane events, to planned vessel reposition in all seasons without returning. These strategies offer increasing operational flexibility, while imposing more stringent design of the mooring and riser systems. Depending on the operational procedure, the fatigue life can be improved 2 to 5 times, which meet the fatigue life target. Another measure is to increase the mooring pretension when the vessel observes VIM responses. This measure may reduce the sway period of the vessel enough to be in the lock out region. It requires an EMS system to monitor the platform and sufficient jack capacity to adjust pretension during a loop/eddy event. Finally, the riser touch down fatigue life can also be improved by riser local enhancements such as better welding, using upset end, or mechanical connector that typically provide better fatigue performance than field welds [Ref.9]. CONCLUSIONS This paper has presented a design of a deep draft wet tree semi-submersible with SCRs for GoM field development in the 4,000 ft water depth. The integrated design of hull, mooring, and SCRs is discussed. The hull is configured for the SCR extreme and fatigue performance based on the SBMA s DeepDraft Semi design. Extreme strength design, wave induced fatigue design, and current induced VIM fatigue design are performed on the sweet/sour service production risers and export risers. It s demonstrated that the deep draft wet tree semisubmersible with SCRs is a feasible solution for 4,000 ft water depth in the GoM. For more stringent sour service criteria, operational measures and/or local riser enhancements may be required to achieve a robust fatigue design. ACKNOWLEDGMENTS The authors would like to acknowledge permission from SBM Atlantia Inc. to prepare and publish this work. The authors would like to thank S. Schuurmans for his review and valuable comments. REFERENCES [1]. Kindel C, Rijken O, Khodr R, Cao P, Galvin C, Hofslot T, Barnett P (2007), Independence Hub Turnkey Delivery of Ultra Deepwater Hull and Mooring System. OTC [2]. Cao P, Xiang S, Chabot L, Fourchy P, Boubenider R. (2010) Thunder Hawk Riser System Design, An Integrated Experience, OMAE [3]. API Bulletin 2INT-MET (2007) Interim Guidance on Hurricane Conditions in the Gulf of Mexico. 5 Copyright 2011 by ASME

6 [4]. API RP 2RD (1998) Design of Risers for Floating Production systems (FPSs) and Tension-Leg Platforms (TLPs), first edition. [5]. FLEXCOM user s manual, Version 7.9. MCS limited, Galway, Ireland, [6]. SHEAR 7 user s Guide, Version 4.5, MIT, 2007 [7]. AQWA reference manual, Version 5.7A, ANSYS, [8]. Xiang S., Cao P., Rijken O., Ma J., Chen Y. (2010) Riser VIM Fatigue Induced by Deep Draft Semisubmersible FPU, OMAE [9]. Pollack J, Riggs D.C, (2011) Improved Concentric Thread Connectors for SCRs and Pipelines, OTC Load Case 100-year Hurricane 1000-year Hurricane Table 6. Riser Strength Design Summary Results Extreme Response 8-inch 6-inch 12-inch Oil 12-inch Gas Production Production Export Export Expected Min. Effect Tension at TDZ (kips) Expected Max. von Mises Stress at TDZ (ksi) Stress Utilization Ratio Max. Top Rotation Angle (deg) Expected Min. Effect Tension at TDZ (kips) Expected Max. von Mises Stress at TDZ (ksi) Stress Utilization Ratio Max. Top Rotation Angle (deg) Riser 8-inch Production 6-inch Production Table 7. Riser Fatigue Design Summary Results (un-factored fatigue touch down zone) Location Sour Wave Loading Knockdown (yrs) Factor VIM (yrs) VIV (yrs) Combined (yrs) Sweet Welding Root Welding Cap Sweet Welding Root Welding Cap Welding Root inch Oil Export Welding Cap inch Gas Welding Root Export Welding Cap Copyright 2011 by ASME

7 4000ft DeepDraft Semi - 12inch-Gas Export VonMises Stress (API-2RD) along the riser Survival, 1000yr Hurricance, Intact Max. Von Mises Stress (API-2RD) (ksi) Near Cross Far Curvilinear Distance along Structure (ft) Figure inch Gas Export Max. API RP 2RD von Mises Stress Distribution ft DeepDraft Semi - 12inch-Gas Export Effective Tension Envelope Survival, 1000yr Hurricance, Intact Effective Tension Envelope (kips) Near Cross Far Curvilinear Distance along Structure (ft) Figure 6.12-inch Gas Export Riser Effective Tension Envelope 7 Copyright 2011 by ASME

8 ft DeepDraft Semi - 8-inch Production SCR - Combined Fatigue Unfactored Fatigue Life (yrs) Weld Root - Wave Weld Root - VIM Weld Root - Combined Weld Cap - Wave Weld Cap - VIM Weld Cap - Combined TDP -- Arc Length (ft) -- Hangoff Figure 7. 8-inch Production Riser Long Term Fatigue Life Touch Down Zone Figure 8. Sketch of Semi VIM response 8 Copyright 2011 by ASME

9 0.50 DeepDraft Semi VIM A/D deg 15-deg 30-deg 45-deg A/D Vr Figure 9. Typical Deep draft semi-submersible VIM A/D Curves ft DeepDraft Semi - 8-inch Production SCR - Wave + VIM Fatigue - ID Fatigue Damage w/ factor Vessel Position- 100ft - Wave+VIM Vessel Position - 50ft - Wave+VIM Mean Position - Wave+VIM Vessel Position + 50ft - Wave+VIM Vessel Position + 100ft - Wave+VIM TDP -- Arc Length (ft) -- Hangoff Figure inch Production Riser Annual Fatigue Damage Distribution using Vessel Reposition (Sour knock down factor of 45 at the welding root) 9 Copyright 2011 by ASME