Feasibility of Steel Lazy Wave Riser System Configuration in Deepwater Using CRA Clad Steel Pipes

Abstract The application of steel lazy wave risers (SLWRs) is relatively a novel solution in subsea industry. Compared with free hanging steel catenary risers (SCRs), the targeted provision of custom designed buoyant section in SLWR results in decoupling the vertical displacement at hang-off point (HOP) driven by floater production unit (FPU) motion from the rest of the riser, hence limiting fatigue loading at the critical touchdown point (TDP) and reducing top tension and bending stresses. However, due to the highly corrosive nature of some hydrocarbon reservoirs, the wall thickness that must be added to account for corrosion along the design life can increase riser system loading, hence limits the pipe diameter that can be used for the target water depth. Therefore, and as a viable alternative solution, the application of steel pipes internally clad with a limited thickness of corrosion resistant alloy (CRA) was analyzed for deepwater SLWR application. In this work, configuration/shape analysis with basic catenary method was conducted for one of the existing twin 10-inch, grade X65 production SLWRs, as case study in Mediterranean Sea, with spread moored FPSO at 1700 meters water depth. Then after, one of SLWRs planned for project future expansion was designed according to API STD 2RD code, while adopting the analyzed shape. A wide range of commercially available steel grades for CRA Clad pipes, including X70, X65, X60 and X52 were covered in the design. Moreover, different alternatives of pipe diameters including 10, 12 and 14-inch were considered. The static and dynamic analysis for combined loading code check, as well as bending stress range for fatigue life calculations were performed using finite element analysis (FEA) software OrcaFlex version 11.2b. Although the result for design of the new SLWR concluded that all combination of steel grades and pipe sizes are safe for all extreme load cases, when it comes to fatigue, it was found that the 10-inch grade X65 is the only case that can achieve minimum service life of 20 years based on the input design parameters. This result suggests that further detailed analysis for SLWR is needed to optimize the shape parameters and obtain the best configuration that results in minimum payload on the FPSO and stresses on the riser system, sufficient length of the flowline until the anchor point or PLET, and reduced CAPEX by using minimum number of buoys for the buoyant section. All that besides achieving the targeted fatigue and service life. Throughout this study, a comprehensive background about CRA Clad pipes and SLWR is covered. Moreover, the feasibility of their combined application for the first time in the Mediterranean Sea is introduced.