This paper investigates the feasibility of mooring and riser systems for a Gulf of Mexico FPSO in a 10,000 ft water depth. System concepts are reviewed and the technical challenges facing the deepwater mooring and riser designs are highlighted. Engineering analyses are then conducted to evaluate various mooring and riser concepts and to establish their suitability for ultra-deepwater applications. The paper concludes that even though the design of moorings and risers for the GOM FPSO in 10,000 ft waters is technically challenging, innovative solutions can be developed. The deep water depth is not a showstopper to the deployment of the FPSO in the GOM.
The ship shaped Floating Production Storage and Offloading system (FPSO) offers a cost-effective field development solution especially in deepwater areas that lack an adequate pipeline network. While a GOM FPSO in 10,000 ft water depth differs very little in terms of topsides and vessel structure (except that the the vessel structure should be adequately reinforced to withstand the high mooring/riser vertical load), its mooring and riser designs face many new challenges [Ref.1].
The mooring horizontal stiffness reduces significantly in ultra-deep waters unless a high pretension is adoped, which is detrimental to the mooring line tension. Synthetic mooring materials (polyester) has advantage of being more elastic and light weight and therefore are shown to be more capable in coping with the deep water depth.
The deepwater moorings and risers impose high loads to the turret bearing. The bearing capacity design and its related impact to the vessel structure should also be carefully considered.
There is no lack of challenges in the riser system design. For flexible risers, the diameter is limited to small sizes in order to resist the hydrostatic collapse near the seabed and the large hang-off tension at the vessel end. The SCRs offer a more robust solution but face the problems of compression-induced overbending and excessive fatigue at the hang-off and touchdown regions.
This paper aims at providing an overview of the issues facing the mooring and riser designs in the harsh deep water environment in the GOM region. Various concepts are evaluated and innovative solutions are proposed to demonstrate that the FPSO can be successfully deployed in 10,000 ft waters.
Unless in a very benign environment (West of Africa) or an environment with high directionality (Offshore Brazil), an FPSO has to be turret moored to allow weather vaning. By aligning its heading with the predominant environment, it attracts the least environmental load. Because of the harsh hurricane condition, the GOM FPSO has to be turret moored.
The mooring line configuration can be catenary or taut leg. The catenary configuration is more applicable to shallow to intermediate waters in which the vessel wave frequency motions can be accommodated by the change of line geometric shape.