This paper presents the Multi-Column Floater (MCF), a dry-tree semisubmersible (DTS) platform concept appropriate for deepwater applications worldwide. The hull geometry is based on cell spar technology where each column consists of several individual vertical sub-columns (or cells). The use of multiple cells to provide column buoyancy results in a design flexible enough to be built anywhere in the world, and can be used to satisfy local content requirements. Hydrocarbons are produced from dry completion wells connected individually to well slots arranged in a rectangular pattern in a central wellbay. The key differentiator between the MCF and other proposed DTS solutions is that in the MCF design the drilling and production risers are supported from a vertically restrained well deck in the center of the MCF, termed the Multi-Riser Buoyancy Can (MRBC). The efficiency of the MCF design is demonstrated by configuring and analyzing the system for a hybrid development consisting of a wellhead DTS and an FPSO in 1,525 m water depth in the Campos Basin offshore Brazil.
Selection of the optimal floating production system (FPS) for a given application depends on several factors, including reservoir characteristics, well-access requirements, payload, water depth and metocean conditions. In water depths greater than 1,500m (beyond the range of conventional TLP technology) a dry-tree semisubmersible (DTS) offers an attractive FPS solution. A semisubmersible hull can be configured for quayside/inshore hull-topsides integration, hook-up and commissioning, thereby eliminating the project risk associated with an offshore lift. Furthermore, dry-trees allow easier well management, reduced OPEX and superior recovery rates than a wet-tree solution. Also, compared to Spars, semisubmersibles provide safer, more efficient wellbay and topsides layouts, while offering similar mooring characteristics.
The development of the dry-tree semisubmersible concept has attracted significant attention from all the major engineering contractors, primarily those based in Houston [1 - 6]. The proposed concepts all support the drilling and production risers using long-stroke riser tensioners, and both hanging and ram style tensioners have been proposed. The design of a semisubmersible hull to support dry trees requires careful consideration of the allowable hull motions so that existing riser tensioner technology may be utilized, and requires integrating the design of the riser system into that of the hull and topsides. Acceptable motions are obtained by a combination of heave natural period control and a large operating draft. A review of current industry efforts and the design challenges associated with the development of a DTS concept may be found in [7].
