A chain-wire-buoy integral mooring system was developed for a semi-submersible based deep water floating production system (FPS). State-of-the-art computer analysis was incorporated in the design process. Large diameter subsurface buoys installed along the mooring line constitute a unique feature of this mooring system. The computer code has never, been validated against full scale or model test data for configurations with submerged buoys in the wave zone. Model tests were conducted to provide a guideline on the adequacy of the proposed mooring design, and confirm aspects that are difficult to simulate ana1ytically. Test resu1ts on the i so1ated mooring line and the mooring line connected to the FPS concluded that the design would meet stringent requirements of 1OO-year environmental conditions and complying with excursion, vertical and dynamic load limitations.
The semi-submersible producer is a proven concept in shallow to medium water depths. A production system of such design, however, needs high well productivity and low maintenance to be economic. Brazil's recent experience in medium water depths indicates the design can be used in deeper water. With no experimental data to refer to, a design that pushes technology into a new dimension must be dealt with in an innovative and yet cautious manner.
Traditionally, moorings on semi-submersible vessels were not considered as permanent. Such vessels were moored during operations by composite cable and chain lines forming a catenary array. Whenever the weather became unfavorable the operation was ceased, the moorings dropped and the mariner had to ride his vessel out of the storm using its own propulsion system. The new generation of semisubmersibles designed as floating production platforms for application in marginal fields along the continental shelf have one aspect that requires a more profound consideration. These vessels incorporate a purpose built semi-submersible hull carrying an oil or gas production facility as a payload. Whereas they have retained their mobility, the requirements for station keeping due to riser excursion limits, and the inability to relocate have led to much more stringent design requirements of permanent catenary moorings.
Figure 1 depicts an overview of the components of a typica1 mooring leg of the proposed eight point mooring system for the FPS. The mooring setup includes submerged buoys near the surface in order to minimize the angle of the mooring (relative to horizontal) leaving the vessel. On each mooring line there are two submerged buoys. Their buoyance counteracts the weight of the mooring line and enables the vessel to carry a higher deck load. Under high wind conditions, the rig offset moves the upper s ubmerged buoys close to the surface. Under high sea states these buoys could enter the wave zone. The wave loading on the buoy could result in high dynamic loads on the connectors between the buoys and the mooring line. Excessive dynamic loads on these buoys could threaten the integrity of the mooring system. Computer analysis was extensively used throughout the design process.