The discoveries made in the pre-salt basin offshore Brazil are among the world's most important in the past decade. The pre-salt province comprises large accumulations of excellent quality, high commercial value light oil, however, the new discoveries are located in ultra-deepwater in areas lacking infra-structure, which makes the Floating Production Storage and Offloading (FPSO) vessel the preferred solution.
Steel Catenary Risers (SCRs) are the preferred solution in wet-tree applications due to their simplicity, robustness and low CAPEX and OPEX compared to other riser options. However, due to its relatively high dynamic motions, FPSO is not a feasible host for SCRs in most environments. Also, for efficient production from such rich reservoirs, facilities supporting high production rate and large diameter risers are necessary. This makes it more challenging to find a robust and commercially attractive riser solution.
In this paper a novel design for an FPSO with the ability to host large diameter SCRs is presented and evaluated for applications in deepwater field developments. The new design, namely, Low Motion FPSO (LM-FPSO), has a hull form with a generally rectangular cross-section. The platform is moored in-place using a conventional mooring system. The LM-FPSO performance is enhanced with the robust low-tech feature, namely, free-hanging Solid Ballast Tank (SBT). The SBT is located at a certain distance below hull keel and connected to the hull through four groups of short tendons. All tendon components are the same as those used in conventional TLPs. Through the mass and added mass of the SBT, the LM-FPSO provides heave, roll and pitch responses better than or comparable to any floating production structure in use today.
The paper presents detailed description of this novel design and its in-service performance. A case study is presented where the LM-FPSO is used in persistent swell in ultra deepwater offshore Brazil to support large diameter SCRs. The SCR's feasibility is demonstrated and reported. The identified risks and associated mitigations for the new design compared to the conventional FPSO are investigated and reported. The paper concludes with discussions on the project execution plan, cost and schedule benefits when developing fields using this novel design.