Pipe-in-Pipe Riser Solution for FPSO’s Offshore Brazil

Abstract

As part of an early ultradeep water field development study for offshore Brazil, the extreme dynamic responses of various compliant steel riser configurations; Steel Catenary Riser (SCR), Shaped-SCR (SSCR) and Steel Lazy Wave Riser (SLWR), are investigated in combination with Floating Production Storage and Offloading units (FPSOs) and Semisubmerible Floating Production Units (SEMI FPUs). As part of this investigation, the feasiblity of Pipe-in-Pipe (PIP) SCR design when hosted on FPSO offshore Brazil is explored which, if successful, enables a simpler and more cost effective configuration with improved flow assurance capabilities. Several FPSOs and Semis are designed and vessel motion screening results used to identify potentially feasible riser porch hang-off locations along the side of the different hull types and sizes under consideration. A relationship between pipe terminal velocity and riser’s Maximum Allowable vertical Velocity (MAV) at the riser porch is developed to predict an acceptable limit against overstressing at the Touch Down Point (TDP) for a range of pipe sizes and configurations, under extreme wave loading conditions. Both near and far cases are analysed. Preliminary wave fatigue loading analyses are then performed for the most promising configurations. A Rainflow cycle counting method is used to compute the fatigue damage. Riser behaviour is highly influenced by the insulation applied. For FPSOs, it is found that uninsulated heavy walled SCRs are feasible with respect to strength for some porch locations, whereas insulated SCRs are not feasible. PIP SCRs are found to exhibit superior strength behaviour, mitigating the normally limiting compression buckling phenomena in the TDP, enabling this concept in deepwater. Fatigue life assessment on the other hand, shows the PIP SCR will not work without further development of specific fatigue life enhancement solutions. As an option, a considerably reduced amount of buoyancy can potentially make the PIP riser work, in a so called Shaped SCR (SSCR) configuration. Other technical qualification gaps to meet PIP SCR strength and fatigue requirements are identified and discussed.