Deep Water Moored Semisubmersible with Dry Wellheads and Top Tensioned Well Risers
Authors: Wanvik Leiv , Johnsen John Magne 
Published: October 2008

Depletion of deep-water reservoirs by means of dry completion units (for short: DCUs) have been confined to TLPs, Spars, and Deep Draft Floaters (DDF). The preference for DCUs is well motivated as they give maximum well access with ample well and riser condition monitoring and flow assurance control.

This paper proposes another step forward and that is, to exploit the potential of semisubmersible hulls as carriers of dry tree solutions.

The paper describes the findings from a development study by Kvaerner Oil & Gas Field Development (KOG FD). The proposed solution may be seen as an extension of the classic semisubmersible/top tensioned riser combination technology, with which the industry has several thousand rig-years of experience.

This paper demonstrates that a DCU semisubmersible hull can be arranged and operated more or less similarly to present art TLPs, in terms of overall, structural, and drilling layout, favourable construction as well as operational and safety procedures.

The enabling technology is connected to the recently developed large stroke multi-riser tensioning system, which takes away the need for tension legs and floater vertical motion suppression. Additionally the freely ventilated wellbay and riser area of the DCU semisubmersible has several safety advantages in mitigating the risks from riser leaks/explosions/fire within the platform or its vicinity. The reduced consequential damages lead to less onerous riser specifications relaxing barrier/weight requirements and cost.

The DCU semisubmersible will be a competitive alternative to Spar/ DDF solutions, and will fill the shortcomings of the TLPs beyond present waterdepth limitations.

Introduction

Significant new discoveries in offshore West Africa, offshore Brazil, and the Gulf of Mexico are currently attracting a lot of attention. The larger finds, however, are in ultra-deep locations with water depths of 3000 ft to 6000 ft.

Ultra-deep discoveries around the world have much of the same challenges i.e. frontier drilling/wells technology, unconsolidated sediments, and flow assurance problems. Economic exploitation of the large finds may be troubled by extreme water depths, lack of pipeline or other off-take infrastructures, and distance to terminals and refineries from consumer market.

While WA has favorable weather conditions its location imposes some special boundary conditions, which includes lack of local contractors of a sufficient size to handle major projects, limited local yard capacities and skilled work force and few harbors to provide operational service.

Lack of offshore infrastructure implies that an FPSO concept may be more attractive, as some form of storage vessel will be required. A storage vessel will offer a large deck surface thus emerging as a kind of "free issue property" for a process plant for which one would otherwise have to build a separate platform.

The "first wave" of WA projects of this type has homed in on large (or rather gigantic) stand alone FPSO's

The "next wave" will show some preference for field developments that includes "on-board" drilling and workover functions. In addition, some reshuffling in the target fields queue has taken place.