Catenary Offset Buoyant Riser Assembly for Malaysian Deepwater

Transcription

1 Catenary Offset Buoyant Riser Assembly for Malaysian Deepwater F.E. Jamaludin, a and J. Koto, a,b,* a) Faculty of Mechanical Engineering,Universiti Teknologi Malaysia, Skudai, Johor Bahru, Malaysia b) Ocean and Aerospace Research Institute, Indonesia *Corresponding author: jaswar.koto@gmail.com & jaswar@utm.my Paper History Received: 10-November-2017 Received in revised form: 20-December-2017 Accepted: 30-December-2107 ABSTRACT Steel Catenary Riser is the preferred solution for riser system in offshore oil & gas production. They are more structurally reliable, technically simple and cost effective compared to other types of riser. As the offshore oil & gas production move to deep and ultra-deepwater regions, the optimum solution have to be determined to overcome the challenges of steel catenary riser such as its weight and size. Catenary Offset Buoyant Riser Assembly (COBRA) is a concept conceived to address most of the problem of steel catenary riser in deepwater. It combines the advantages of steel catenary and hybrid risers and is made up of two parts. The bottom part consists of a rigid steel pipeline in catenary configuration and connected to a sub-surface buoy. A flexible jumper connects it to the floating structure. The flexible jumper and the buoy effectively absorbed the forces acting on the riser and floating structure essentially making the steel catenary undisturbed by the dynamic motions, thus improving its fatigue performance. Numerical simulation of the COBRA concept in a Malaysia deepwater project is carried out by considering the environmental conditions from the Kikeh Field to analyse its static and dynamic performance. The static and dynamic analyses showed that. at the touch-down point, the tension for COBRA is about 50% smaller that for steel catenary riser maing it suitable for deepwater projects in Malaysia. KEY WORDS: Riser, Deepwater, COBRA, Oil & Gas. NOMENCLATURE Catenary Offset Buoyant Riser Assembly International Energy Agency Empirical Orthogonal Functions First Order Reliability Method Floating Production Storage Offloading Steel Catenary Riser 1.0 INTRODUCTION In offshore oil & gas engineering, risers are essentially pipes that connect an offshore floating structure and subsea wells to each other. They are the conduit through which fluids are transferred between the floating structure and the wells. A riser is a unique common element to many floating offshore facilities and is critical to safe field operations. Therefore, it must be designed to be able to maintain its integrity under external and internal loadings throughout its service life. Risers can be categorized into two based on their type of operation. Drilling risers, as shown are used to contain fluids for well control. Production risers are used to convey hydrocarbons from the seabed to the floating structure [1]. The selection of riser solution for deepwater is governed by a set of much more intricate factors compared to shallow water, such as water depth, weight and size. As deepwater fields are becoming more important as the source of hydrocarbon, different concepts of risers have been conceived and studied to consider the limiting effects of these factors. Many shallow water oil & gas fields around the world have matured and depleted making deepwater as the new frontier for exploration and production activities. The International Energy Agency (IEA) estimated that there could be around 270 billion barrels of recoverable oil alone in deepwater worldwide. The impact of deepwater exploration and production trend is also felt in Malaysia. The estimated hydrocarbon deposit in deepwater 9 JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]

2 fields in Malaysia is approximately 1 billion barrel of oil equivalent and 6 trillion cubic feet of gas. Deepwater resources are expected to contribute to about one third of national oil production by 2020 [2]. As the industry moves further to deep and ultra-deepwater, the engineering challenges for riser become tougher as well. The COBRA concept was conceived to address some of the challenges. This concept consists of a steel catenary riser section from the wellhead connected to a long, slender sub-surface buoy which in turn is connected to the floating structure via a flexible jumper [3]. It combines the conventional steel catenary riser with hybrid riser and inherits the desired advantages of both. This project will simulate the performance of COBRA in a Malaysia deepwater project. 2.0 COBRA CONCEPT COBRA concept was developed by Karunakaran and Baarholm [4]. It is a modification of the hybrid riser concept. The aim is to combine the flexibility features of the hybrid concept with the simplicity and economic features of the steel catenary riser. The concept is an assembly of a rigid steel pipe laid in catenary configuration connected to a subsurface installation, a sub-surface buoyancy can which is tethered down to the seabed and a flexible jumper connected from the buoy to the floating structure as shown in Figure ENVIRONMENTAL DATA The main particulars of the Kikeh field used in the COBRA simulation are shown in Table 1. Table 1: Kikeh field particulars Water Depth 1350 m Water density 1025 kg/m 3 Wave height 6 m Wave period 11.7 s Wind speed 19 m Current speed Varies with depth Soil Stiffness Horizontal 200 kn/m Vertical 50 kn/m The selection of the most suitable of wave spectrum for a particular seaway is essential for hydrodynamic time domain analysis. Maimun et al. [5] had carried out the measurements and analysis of wave heights for the Malaysian waters at two locations. One was carried out in the Straits of Malacca near Kukup using the Shipboard wave radar and another was carried out near Pulau Sibu in South China Sea using wave pressure sensor. Although the results were not conclusive, it has been successfully shown that the standard spectra of Pierson- Moskowitz type could be used for Malaysian waters. The theoretical formulations for the Pierson-Moskowitz spectra are based on Gran [6]. The formulation used the significant wave height and peak period of the measured spectra and since wind speed is not included, uncertainties in wind speed measurement are eliminated. The Pierson-Moskowitz spectra can be represented as: (1) Where:! " is a significant wave height in meter, Ω is a peak frequency Figure 1: COBRA The numerical simulation of the COBRA concept is carried out by positioning the sub-surface buoy, at water depths250 m. The rigid steel pipe is 1550 m long while the flexible jumper is 485 m long. A simulation using Steel Catenary Riser is also carried out to directly compare the performance of the two concepts. The rigid steel pipe is 1945 m long. The touch-down points are similar in both configurations. For both simulations, three positions of the vessel are considered, at minimum offset of -65 m, at origin and at maximum offset of +65 m. The study of extreme oceanographic currents at deepwater locations offshore Borneo in the South China Sea was carried out by Sheikh and Brown [7]. Current speeds over the height of the water column were approximated using Empirical Orthogonal Functions (EOFs) and their extreme profiles derived using the inverse First Order Reliability Method (FORM). These profiles are then compared with consideration of different types of current events and used as a basis to formulate and idealised deepwater design vertical current speed profiles for locations offshore Borneo that are application specific. Figure 2 shows the vertical current profile for the area approximate to the Kikeh field. 10 JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]

3 Factors such as high external pressure can results in pipeline failure such as collapse and buckling propagation. Consideration or these issues were taken towards pipeline wall thickness design process. The thickness determination was made in accordance with DNV class rule. Table 4 summarised the main properties of the rigid steel pipe used in the simulation. Table 4: Rigid Steel Pipe Properties Material Carbon Steel API 5L (PSL2) Grade X65 Density 7850kg/m3 Young s Modulus 2120MPa OD 0.254m Thickness 0.027m Operating Pressure 35MPa Maximum Temperature 60 Figure 2: Offshore Borneo vertical current profile 4.0 VESSEL DATA The Kikeh field is served by FPSO Kikeh. It has a storage capacity of 2,179,000 bbls and supports 5 production, 1 export and 3 water injection risers. Table 2 shows the main particulars of the FPSO used in the simulation. Table 2: FPSO Kikeh Particulars Length 337 m Breadth 55 m Depth 27 m Mass 273,000 tonnes VCG 8.47m from keel LCG 8.68m fwd of amidship Maximum offset 65 m (5% water depth) Length 337 m 5.0 COBRA DATA The COBRA concept is made up of a steel catenary riser section connected to a subsurface buoy tethered to the seabed by two mooring lines. The floating structure is connected to the steel catenary section at the buoy via a flexible jumper held in place by a tensioner located at the structure. 5.1 Flexible Jumper The important criterion of the jumper is the minimum bending radius. It is a definitive radius to which the riser can bend without damage. Table 3 shows the properties of the flexible jumper. Table 3: Flexible Jumper Properties Outer Diameter m Inner Diameter m Minimum Bending Radius 5 m 5.2 Rigid Steel Pipe Selection of wall thickness based on the paper by Junaidi.et.al [8]. 6.0 ANALYSIS ACCEPTANCE CRITERIA The design analysis result shall meet particular limiting criteria and requirements. The following points describe the criteria that need to be fulfilled by the COBRA concept in this study: 1. Maximum/minimum top tension of the flexible jumper The catenary load is supported by the tensioner located at top section of product on the vessel. The wave and current motions create variations of tension load on the riser. The limiting capacity on the tensioner shall be reviewed with maximum tension load experienced by the flexible jumper. In addition, large range of variations on tension load should give more attention in the analysis. 2. Compression No compression load is permitted along the flexible jumpers. 3. Minimum Bend Radius (MBR) of flexible jumper Bending radius is the minimum radius of the riser can be bended without damaging it or making it buckle. The smaller the bend radius, the greater is the flexibility. The product shall not exceed the permissible bending radius that given by the manufacturer. 6.1 COBRA The location of the buoy is at 250 m beneath the water. The current speed at this depth is m/s. The length of the flexible jumper is 485 m and the length of the rigid steel pipe is 1550 m. The buoy is tethered to the seabed by a couple of mooring lines. Static Analysis In static analysis, the static equilibrium configuration is achieved by considering only static loading Flexible Jumper Table 5 shows the outcome of the simulation. Table 5: Flexible Jumper Static Analysis Angle at hang-off point ( ) Angle at buoy ( ) Effective tension at hang JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]

4 off point (kn) Effective tension at buoy (kn) Minimum bending radius (m) Rigid Steel Pipe Table 6 shows the outcome of the simulation. Table 6: Rigid Steel Pipe Static Analysis Angle at buoy ( ) Effective Tension at Effective Tension at Figure 3: Flexible Jumper Static & Dynamic Effective Tension at hang-off point Dynamic Analysis Nonlinear time domain analysis with irregular wave is considered for the dynamic analysis. The Pierson-Moskowitz wave spectrum with 100-year return period is used in the simulation. Flexible Jumper Table 7: Flexible Jumper Dynamic Analysis Minimum bending radius (m) Hmin (m) Minimum tension (kn) Maximum tension at hang-off point (kn) Maximum tension at Minimum angle at hangoff point ( ) Maximum angle at hangoff point ( ) Minimum angle at buoy ( ) Maximum angle at buoy ( ) Figure 4: Flexible Jumper Static & Dynamic Effective Tension at buoy Figure 5: Flexible Jumper Dynamic minimum and maximum angle at hang-off point 12 JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]

5 Figure 6: Flexible Jumper Dynamic minimum and maximum angle at buoy Rigid Steel Pipe Table 8: Rigid Steel Pipe Dynamic Analysis Maximum tension at Minimum tension at Stress at buoy (MPa) Stress at taper stress joint (MPa) Stress at touch-down point (MPa) Figure 8: Rigid Steel Pipe Static and Dynamic tension at touchdown point 6.2 STEEL CATENARY RISER (SCR) The whole riser system is made entirely of rigid steel pipe for this configuration. The rigid steel pipe used is the same as the pipe used for the bottom part of COBRA and laid in catenary configuration. The total length of the pipe is 1945 m and it also includes a 10 m taper stress joint with 2.5 inch wall thickness. Static Analysis Table 9: Steel Catenary Riser Static Analysis Angle at buoy ( ) Effective Tension at Effective Tension at Dynamic Analysis Nonlinear time domain analysis with irregular wave is considered for the dynamic analysis. The Pierson-Moskowitz wave spectrum with 100-year return period is used in the simulation. Figure 7: Rigid Steel Pipe Static and Dynamic tension at buoy Table 10: Steel Catenary Dynamic Static Analysis Maximum tension at Minimum tension at Stress at buoy (MPa) Stress at taper stress joint (MPa) Stress at touch-down point (MPa) JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]

6 therefore reducing its weight and length even more. Reducing its length will help to reduce the riser spread area and thus reducing the probability of clashing with other seabed installation facilities. At the touch-down point, the tension for COBRA is about 50% smaller that for steel catenary riser. Therefore, the COBRA concept is suitable for application in deepwater projects in Malaysia. REFERENCES Figure 9: Steel Catenary Riser Static and Dynamic tension at hang-off point Figure 10: Steel Catenary Riser Static and Dynamic tension at touch-down point 6.3 COMPARISON BETWEEN COBRA & SCR Maximum Tension at Hang-off point (kn) Maximum Tension at Stress at touch-down point (MPa COBRA SCR Chakrabarti, Subrata K., 2005, Handbook of Offshore Engineering, Elsevier 2. Khalid, N., 2008, Kikeh - A Prized Catch, Deepwater Southeast Asia Congress 2008 Shanghai, 9-10 October Karunakaran, D., Aasen, H., Baarholm, R., 2011, New Uncoupled Deepwater RiserConcept for Harsh Environment Catenary Offset Buoyant Riser Assembly (COBRA), In: Deepwater Offshore Technology Conference. New Orleans, USA, October Karunakaran D. and Baarnholm, R., 2013, Cobra Riser Concept for Ultra-DeepwaterCondition, Offshore Technology Conference, Houston, Texas, USA, 6 May Maimun, A. et al., 2006, Seakeeping Analysis of a Fishing Vessel Operating in Malaysian Water, Jurnal Mekanikal, No. 22, Gran, S., 1991, A Course in Ocean Engineering, Elsevier Science Publishers B.V., The Netherlands 7. Sheikh, R., Brown, A., 2010, Extreme Vertical Deepwater Current Profiles In The South China Sea, Offshore Borneo, Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2010, June 6-11, 2010, Shanghai, China 8. Junaidi, M.N., Koto, J., 2016, Review on Design of Oil Subsea Pipeline in Kikeh Field, Malaysia, Journal of Ocean, Mechanical and Aerospace Science and Engineering, Vol.32(1), pp Det Norske Veritas, 2010, Dynamic Risers, Offshore Standard DNV-OS-F CONCLUSIONS The outcome of the simulations showed that the COBRA configuration has better performance than Steel Catenary Riser in terms of tension requirement. The tension requirement for COBRA at the vessel hang-off point is only 10% of the tension requirement for Steel Catenary Riser. This is because for COBRA the tensioner is used to hold flexible jumper which is lighter that rigid steel pipe for the same diameter and thickness. The tension of the rigid steel pipe for COBRA at its hang-off point is absorbed by the buoy. The higher tension of the Steel Catenary Riser at the hang-off point is due to its length and weight. The length of the steel pipe, meanwhile can be further shorten 14 JSOse Received: 10-November-2017 Accepted: 30-December-2017 [(12) 1: 9-14]