Proceedings of the ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands Proceedings of the ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE2011-50303 OMAE2011-50303 NEW DESIGN CONFIGURATION OF STEEL CATENARY RISERS WITH PULL-TUBE FOR SPARS Shan Shi Houston Offshore Engineering Houston, TX, USA Jenny Yang Houston Offshore Engineering Houston, TX, USA Charlie Mao Williams Companies, Inc. Houston, TX, USA Nishu Kurup Houston Offshore Engineering Houston, TX, USA ABSTRACT In previous Spar designs where pull tubes were used to board the risers (either export or flowline risers), the pull-tube extended a considerable distance beyond the keel and used a tapered design to form a bend restrictor that supported the riser throughout the riser/hull interface. In a current Spar design, the pull-tube is terminated at the hull keel and the bending loads are carried by a double sided stress-joint in the riser that pivots on a centralizer located near the bottom of the pull-tube. Essentially, this is an adaptation of the double-sided stress joint used for top tensioned risers exiting the bottom of their buoyancy can stems to the similar condition of an SCR exiting a pull tube terminating at the Spar s keel. This new pull-tube and SCR configuration can be applied for both Truss and Classic Spars. SCRs boarding Spars through pull tubes have several advantages over stress joints or flex-joints anchored in porches, notably, eliminating both the need for divers to make large piping connections at 500 to 600 water depths and the possibility of those connections leaking over time. Moving the bend restrictor function from the pull tube to the riser provides the additional advantage of adding flexibility for the Spar to accommodate future risers whose size and weight are not known at the time the pull tubes are designed and the platform is installed. With the stress joint as part of the riser, the bend restrictor can be custom designed for each riser since the pull tube works the same for all risers. The SCR and stress joint, pull-in and in-place analyses have been performed by using the finite element program ABAQUS. The nonlinear capabilities of ABAQUS including the hybrid, gap and contact element formulations are utilized in the analysis of the pull-in process. The nonlinear contact elements with finite sliding capability are modeled with an exponential over-closure relationship. INTRODUCTION SCR with pull-tube has been used for truss Spars by utilizing its open truss bays. All existing truss Spar pull-tubes have a long extension beyond or below its support point on the Spar hull. In contrast the Classic Spar does not have open truss bays and its center well could be either wet or dry. It thus requires the pull-tubes to exit the hull from the Spar keel with minimum extension. Therefore, the new configuration of pulltube with zero extension beyond the Classic Spar keel with a corresponding double tapered stress joint on the SCRs are designed and analyzed for installation, extreme strength, and wave motion fatigue. SCR CONFIGURATION DESCRIPTION Riser System Description The Spar risers selected for this feasibility study are summarized in the table below: Table 1 Steel Catenary Risers and Pull-tubes Sizing Type Diameter (in.) Pull-tube Diameter (in.) Export Gas 14 24 Export Oil 14 24 Import Oil Flowlines 8.625 18 1 Copyright 2011 by ASME
All Import Flowline and Export Pipeline SCRs are designed for pull-in through pull tubes installed in the Spar hull, terminating at the (+) 62-0 Elevation at the top of Spar where the vertical load of risers will be transferred to the pull tubes. Riser Properties Table 2 summarizes the Steel Catenary Riser properties used in this study. The hang-off angle is assumed to be 12 degree for all SCRs. The SCR content density for the oil export, gas export, and flowline SCRs is 55 lb/ft 3, 13 lb/ft 3, and 51 lb/ft 3 respectively. Table 2 Steel Catenary Riser Properties Parameter Gas Export Oil Export Flowlines The environment conditions are selected based on Gulf of Mexico deepwater applications. Loading Category Operating Extreme Survival Cas e ID Table 3 Flowline Riser Load Matrix Internal Fluid Internal Pressure Wave Condition Current Vessel Condition Stress / Fatigue Criteria OP1 Product 3,000 psi 10-yr winter storm Associated Intact 0.67Fy OP2 Product 3,000 psi Associated 10-yr loop current Intact 0.67Fy OP3 Product 1,5000 psi 1-yr winter storm Associated Intact 0.67Fy FW Product 3,000 psi Fatigue waves Associated Intact 10 EX1 Product 1,500 psi 100-yr hurricane Associated Intact 0.8Fy EX2 Product 3,000 psi Associated 100-yr loop current Intact 0.8Fy EX3 Product 1,5000 psi 10-yr winter storm Associated Intact 0.8Fy EX4 Product 1,5000 psi Associated 10-yr loop current Intact 0.8Fy SV1 Product 1,500 psi 1,000-yr hurricane Associated Intact 1.0Fy SV2 Product 1,500 psi 100-yr hurricane Associated 1-line broken 1.0Fy Pipe OD (in.) 14 14 8.625 Wall Thickness (in.) 0.875 0.875 1.333 Material X-65 X-65 X-70 Min. yield strength (ksi) 65 65 70 Min. ultimate tensile strength (ksi) Modulus of Elasticity (ksi) Poisson 抯 Ratio Density (lb/ft 3 ) 77 77 82 29,500-30,000 Pipe Fabrication method Seamless Seamless Seamless 0.3 490 Loading Category Operating Extreme Survival Case ID Table 4 Export Riser Load Matrix Internal Fluid Internal Pressure Wave Condition Current Vessel Condition Stress / Fatigue Criteria OP1 Product 3,705 psi 10-yr winter storm Associated Intact 0.67Fy OP2 Product 3,705 psi Associated 10-yr loop current Intact 0.67Fy FW Product 3,705 psi Fatigue waves Associated Intact 10 EX1 Product 3,705 psi 100-yr hurricane Associated Intact 0.8Fy EX2 Product 3,705 psi Associated 100-yr loop current Intact 0.8Fy SV1 Product Ambient 1,000-yr hurricane Associated Intact 1.0Fy SV2 Product Ambient 100-yr hurricane Associated 1-line broken 1.0Fy Wall thickness manufacturing tolerance 15% 15% 15% -5% -5% -5% Maximum ovality* 0.50% 0.50% 0.50% Corrosion allowance (in.) 0.118 0.118 0.157 External FBE coating thickness (in.) 18 mil 18 mil 3 to 4 in. FBE coating density (lbs/ft 3 ) 90 90 46.8 for 3 in. 43.7 for 4 in. Design temperature ( 癋 ) 120 120 190 Operating density (lbs/ft 3 ) 13 55 51 Riser Pull-tubes The pull tubes are steel tubes which are supported by the platform at several elevations and exit from the platform keel. The pull-tube extension could be 0 ft to 20 ft long depending on the performance requirements under the design environmental conditions. In this study, all the riser pull-tubes are modeled conservatively as 0 ft extension for analysis. The extension could be adjusted up to 20 ft based on the analysis results if required. Design and Analysis Requirements The riser analysis load matrix for the flowline SCRs and the export SCRs are presented in Tables 3 and 4, respectively. The riser systems will be designed for an operating life of 25 years. The fatigue parameters in Table 5 are used for preliminary import flowline riser fatigue assessment. Table 5 Flowline Riser Fatigue Analysis Parameters Parameter S-N Curve Thickness Correction Safety Factor Value SCR Weld OD: DNV-E (in seawater), SCF=1.21 (see note) SCR Weld ID (all locations): Sour Service Curve, SCR=1.21 Stress Joint: DNV-B1 (in seawater), SCF=1.05 Stress Joint Weld: DNV-C (in seawater), SCF=1.05 Exponent = 0.15 Tref = 1.000 in. 10 for wave fatigue damage 20 for VIV and VIM fatigue damage 25 for short term extreme event fatigue 2 Copyright 2011 by ASME
The key fatigue analysis parameters for export risers are summarized in Table 6. Parameter S-N Curve Thickness Correction Safety Factor Table 6 Export Riser Fatigue Analysis Parameters Value SCR Welds: DNV-E (in seawater), SCR=1.21 Stress Joint: DNV-B1 (in seawater), SCF=1.05 Stress Joint Weld: DNV-C (in seawater), SCF=1.05 Exponent = 0.2 Tref = 1.260 in. 10 for wave fatigue damage 20 for VIV and VIM fatigue damage 10 for short term extreme event fatigue RISER DESIGN AND ANALYSIS Pull-tube Configuration The SCR pull-tubes are designed using 0.75 inch wall thickness steel pipe of 24 and 18 OD for export and flowline SCR, respectively. The export SCR pull-tube shown in Figure 1 has a 20 ft extension and a 40 ft straight section inside of the platform keel, the flowline import SCR pull-tube shown in Figure 2 also has a 20 ft extension and a 30 ft straight section inside of the platform keel. Doubled curve sections are used between the top of the straight section and the bottom of the hard tank for both flowline and export SCR pull-tubes to maximize bend radius. The doubled curved sections provide a minimum radius of 800 ft for export risers and 600 ft for flowline risers. Three middle supports are placed for the doubled curve section with a maximum spacing of 80 ft. The pull-tubes can be made of 60 ksi material. If the 20 ft long pulltube extension is required, the pull-tube bottom section can use 80 ksi for extra strength. In the classic Spar application, the SCR pull-tube is shielded inside the Spar hull; it is not exposed to environmental loads, such as wave and current. SCR Stress Joint Design Figure 1 Export SCR Pull-tube Layout The SCR stress joints provide first contact interface between the hull and the SCRs. The doubled tapered section design is configured to minimize the SCR stress concentration at the hull interface. The stress joint is the key structural member for SCR strength and fatigue. The stress joint assemblies presented in Figure 3 and 4 consist of a middle 40 ft long doubled tapered section and two 10 ft long end sections using 80 ksi material. The design is used as a preliminary configuration, and can be optimized. Figure 5 shows the pulltube and SCR stress joint detail at the platform keel. 3 Copyright 2011 by ASME
Figure 2 Flowline SCR Pull-tube Layout Figure 5 SCR Pull-tube and SCR Stress Joint at Platform Keel Detail Figure 3 Import Flowline SCR Stress Joint Configuration, SCR Analysis Model The SCR is modeled using beam elements while the pull tube and stress joint are modeled using the pipe-in-pipe feature in Flexcom3D. In order to effectively capture bending and tension response, the critical regions like the touchdown zone, hang-off region and stress joint are modeled using finer elements. The touchdown zone is modeled using 5 ft elements and the hang off area including the stress joint is modeled using 1 ft elements. 3,000 ft of the SCR from the hang-off is assumed to be covered by strakes. Analysis performed in this study, assumed that there is no pull-tube extension for conservative consideration. The 0 ft extension pull-tube will not provide flex support at the SCR stress joint region. Therefore, the SCR stress joint will experience higher stresses and the pull-tube strength and fatigue becomes less critical. For this reason, only the SCR strength and fatigue are reported here. Figure 4 Export SCR Stress Joint Configuration 4 Copyright 2011 by ASME
SCR Strength Analysis The riser system strength of the flowline, gas export and oil export SCRs are analyzed for critical load cases defined in the riser design load matrix. The results are satisfactory. The maximum strength utilization ratio for all the load cases defined in the load matrix is less than 0.9. Figure 6 and 7 are the von Mises stress distribution at the SCR touchdown zone. Figure 6 shows the import flowline under 1-yr winter storm condition and Figure 7 shows the export SCR under 10-yr loop condition. Figure 8 and 9 are the von Mises stress distribution at the stress joint section. Figure 8 shows the import flowline under 1,000-yr hurricane condition and Figure 9 shows the export SCR under 10-yr loop condition. It is noticed that the second centralizer of the export SCRs have relative larger stress, it can be reduced in the future detailed analysis by rearrange the 2 nd centralizer and the pull-tube support locations. Riser corrosion allowance provided in the design basis is applied in the strength calculation. The analysis results confirm that the SCRs satisfy the strength design requirements for all the controlling load cases. Figure 8 Flowline SCR Maximum von Mises Stress 1000- yr Hurricane Far (Load Case SV1) Figure 9 Export SCR Maximum von Mises Stress 10-yr Loop Far (Load Case OP2) Figure 6 Flowline SCR Maximum von Mises Stress 1-yr Winter Storm Near (Load Case OP3) Figure 7 Export SCR Maximum von Mises Stress 10-yr Loop Near (Load Case OP2) Short Term Fatigue Damage The SCR short-term fatigue analyses are performed using the 100-yr hurricane seastate based on middle period, maximum offset. The riser damages are counted for a continuous, 30-hour storm simulation without ramp-up or ramp-down. Different fatigue SN curve and stress concentration factors for several critical locations defined in the design basis are used for riser fatigue damage evaluation. The requirement for short-term fatigue is that the total damage from a single event should be less than 1/25 = 0.04 = 4%. The calculated short term fatigue damage results are less than the limit of 4%. Therefore, the short term fatigue requirement for the import and export SCRs is satisfied. Long-Term Fatigue Life The SCR long-term fatigue is analyzed. One hour simulations are performed for each fatigue seastate. Fatigue SN curves and stress concentration factors for different several critical locations are used for riser fatigue damage evaluation. 5 Copyright 2011 by ASME
The riser fatigue damages are summarized in the following Tables 7 and 8. Figures 10 to 15 show the fatigue life distribution along the SCRs touchdown zone and stress joint section for the flowline and export SCRs. The calculated fatigue lives for all risers satisfy the required minimum fatigue life of 417 years, when assume the riser VIV plus installation damage is up to 30% of riser life. In the fatigue life calculation, riser weld (including transitions between the stress joint and SCR pipes) and sour service at inner diameter for production riser are considered with different S-N curve and SCF. Table 7 Import SCR Long Term Fatigue Damage Table 8 Oil and Gas Export SCRs Long Term Fatigue Damage 100000 Long Term Fatigue Life Flowline Figure 12 Oil Export SCR Long Term fatigue Life (TDZ) rs) (y life e u F a tig 10000 1000 OD ID 100 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Touch Down Zone (ft) Figure 10 Flowline SCR Long Term Fatigue Life 100000 10000 rs) (y life e u 1000 F a tig Long Term Fatigue Life Flowline OD ID Figure 13 Oil Export SCR Long Term Fatigue Life (Stress Joint) 100 5000 5005 5010 5015 5020 5025 5030 5035 5040 5045 5050 5055 5060 5065 5070 5075 5080 Stress Joint (ft) Figure 11 Flowline SCR Long Term Fatigue Life 6 Copyright 2011 by ASME
Tension (kips) Figure 14 Gas Export SCR Long Term Fatigue Life (TDZ) to the original position when the pull-in is completed. Figure 16 displays SCR pull head reaches lower end of the pull-tube. Figure 17 presents the tension at the SCR top. Figure 18 presents the reaction force at the guide during pull-in. The stress in pull-tube and SCR during the pull-in process are shown in Figure 19 and Figure 20. The maximum pull-tube and SCR stress are 9.8ksi and 36.5ksi. The pull-tube double curvature is pre-fabricated, its pre-bending stress is close to zero. Figure 15 Gas Export SCR Long Term Fatigue Life (Stress Joint) SCR INSTALLATION ANALYSIS Analysis of the import and export SCRs installation procedure focused on the 1st end and the 2nd end SCR pull-in methods. SCRs installation analysis results have shown that both the first-end and the second-end SCR pull-in procedures are feasible based on the riser installation stress and strain design criteria. First End SCR Installation From the strength point of view, the 1 st end installation procedure requires a minimum pull-chain length of 960ft which will results the acceptable maximum SCR bending stress of 35ksi. The corresponding pipe lay vessel distance to the Spar for pull-in operation is about 1000ft. Second End SCR Installation The 2 nd end installation procedure can be performed at the Spar offsets from 200ft to 400ft with no additional tilt the platform. Spar offset of 400ft will results better SCR stress during installation. It is required to move the spar back to its mean position associate the pull-in process in order to minimize the bending stress at the SCR stress joint. The 2 nd end pull-in becomes feasible at the minimum spar offset of 200ft benefits from use of the no keel extension pull-tube design with a contoured bell-mouth at its lower end. This could allow SCR rotate about 3 degree at the spar keel with acceptable stress/strain levels of the SCR. The maximum SCR stress during 2nd end pull-in is about 45ksi from the flowline SCR. The maximum SCR pull-in load is 420kips, which is also from the flowline installation case. Pull-tube stress is not an issue for both SCR in-place and installation conditions. The worst installation case in terms of riser stress is the second export SCR pull-in at Spar 200ft offset using the 2 nd end installation procedure. The analysis results are shown next. The Spar along with the pull-tube and pull-chain are offset by 200 ft where the pull-in starts, and then the spar moves back Figure 16 2 nd Export SCR Pull Head Reached the Lower End of the Pull-tube 390 380 370 360 350 340 330 320 310 Export SCR Pull In Analysis Export SCR Top Pull Tension 0 100 200 300 400 500 600 Pull in Length (s) Figure 17 2 nd Export SCR Top Pull Tension during Pull-in 7 Copyright 2011 by ASME
Force Total Total Stress Stress 40 35 30 25 20 15 10 Figure 18 2 nd Export SCR Pull-tube Support Reaction Force during Pull-in Figure 19 2 nd Export SCR Pull-tube Stress Envelope 40 35 30 25 20 15 10 5 0 12 10 2 5 0 8 6 4 2 0 Export SCR Pull In Analysis Pull Tube Support Reaction Force during Pull in 0 100 200 300 400 500 600 Pull in Length (ft) Export SCR Pull In Pull Tube Stress Envelope 0 100 200 300 400 500 600 Curve Length from Pull Tube Bottom End (ft) Export SCR Pull In Analysis Export SCR Stress Envelope 0 100 200 300 400 500 600 700 800 900 1000 Riser Length from Top (ft) Guide @ 508f Guide @ 436f Guide @ 364f Guide @ 284f Guide @ 212f Figure 20 2 nd Export SCR Stress Envelope SUMMARY AND CONCLUSIONS Preliminary strength and fatigue analysis has been performed for the flowline and the export risers. The strength results for the flowline and export risers satisfy design criteria. Short term and long term fatigue requirements for both the flowline and export risers also satisfy design criteria. The riser interface at the pull tube is at the keel support for the pull-tube rather than at the end of a pull-tube extension. With this interface location, the pull-tube does not participate significantly in the primary horizontal loading on the riser. More detailed assessment of the pull-tube need to be performed in the future, but the design is not expected to be critical. The pull-tube double curvature is pre-fabricated; its pre-stress can be very low. In the classic Spar application, the entire tube is shielded inside the classic Spar hull; the pull-tube dynamic stress due to environment is also low. SCRs installation analysis results presented in this report have shown that both the first-end and the second-end SCR pull-in procedure are feasible based on the riser installation stress and strain design criteria. Each installation method has its pros and cons in terms of risk, requirement, accuracy and schedule. It is found that the double tapered steel stress joint design and the riser pull-tube could satisfy the design requirements. SCRs installation analysis have shown that the second-end SCR pull-in procedure is feasible and may require less platform trim and offset compared to the truss Spar case. The new SCR and pull-tube design configuration was developed based on Gulf of Mexico deepwater applications. This configuration reduces the pull-tube extension nearly zero beyond hull structure and put the stress components on the riser. It thus leads to the possibility of using pull-tubes for Classic Spars. In addition, it not only has the advantages of pull-tube for support and installation of SCRs, but also shows better performance on issues such as installation requirement of platform offset, pull-tube strength/fatigue and arrangement, and is applicable for various SCR sizing. It is thus a good choice for Spars, allowing for the pre-fabricated pull-tube has the flexibility of fit for a variety of future SCRs. ACKNOWLEDGMENT The authors would like to thank management of Williams for clearance to publish this work. Special thank goes to Robin Converse for his support. REFERENCES [1] API RP 1111, Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines Limit State Design. [2] API RP 2RD, Design of Risers for Floating Production System (FPSs) and Tensions Leg Platforms (TLPs). [3] API Spec 5L, Specification for Line Pipe. [4] ASME B31.4, Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids. [5] ASME B31.8, Gas Transmission and Distribution Piping Systems. [6] AWS D1.1, American Welding Society, Structural Welding Code. [7] DNV OS F201, Dynamic Risers. [8] DNV RP C203, Fatigue Design of Offshore Steel Structures. 8 Copyright 2011 by ASME