Dry Tree Semi - Reduced Costs for Dry Well Completions in Deepwater West Africa by Application of Proven Semisubmersible and Riser Technology
Authors: Often Ola 
Published: October 2008

The DTS (Dry Tree Semi) wellhead platform is a novel platform design based on proven components, with the aim to drastically reduce the cost of deepwater dry wellhead platforms in benign ocean environments.

The DTS concept includes the following features:A conventional 4-column ring-pontoon semi-submersible platform. The platform is optimised for benign weather conditions with low airgap and square structural members.A lattice structure riser support tower, which can be jacked to an elevated position in transit, and lowered to reach below main wave and current region during operation."Dry completion" rigid risers with surface X-mas trees at deck level, supported by buoyancy cans mainly within the lower part of the riser support tower."Taut leg" mooring with combined wire and synthetic rope, which allows the platform to be relocated over individual wells for drilling or workover.Topsides accommodating wellheads and manifolds and drilling/workover/processing facilities depending on chosen configuration.

The platform design is aimed at cutting costs for deepwater dry completions solutions in benign waters, but the technology has the potential to be extended into harsher environments.

The concept has been verified through model testing, riser, mooring and structural analysis up to class approval. In typical West African conditions the motions, accelerations and offsets of the DTS are confirmed by model tests to meet requirements for a dry completion system.

Comparisons have been made with alternative dry completions units. The concept enables dry well completions on a conventional platform design, at significantly reduced costs.

Introduction

The DTS (Dry Tree Semi) reflects the application of well proven technology to the current industry challenge of providing cost effective dry tree completions for fields in ultra deep waters. The aim of this paper is to present the simple design principles of the DTS and the characteristic features of the concept.

Main concept principles

The DTS concept combines the well-established principles of a production semisubmersible with buoyancy supported rigid risers and dry wellheads (Figure 1).

The subsea wells are tied back directly to the platform deck through a tieback casing ending in a dry wellhead and X-mas tree. The risers are held in tension by buoyancy cans (air tanks) attached to each individual riser below the upper wave and current zone with the highest hydrodynamic loading. Figure 2 shows the distribution of weight and buoyancy in the upper part of the riser for a 1500 meter water depth case and Figure 3 shows the resulting hydrodynamic drag loading from waves and current in Angolan deepwater conditions.

To maintain the separation between the buoyancy cans and their position relative to platform, the risers are supported horizontally by an open truss tower structure extending from deck level down to the lower buoyancy cans. Horizontal support is typically provided every 14 meters. This riser support tower is kept in an elevated position for tow and transport, and only lowered to permanent position prior to final installation.