A large number of dry tree concept's have been proposed to the offshore industry over the last few years for application in deepwater fields in the Gulf of Mexico, West Africa and Brazil. Looking into the details of these concepts, they all to a varying degree apply new and unproven technology that will increase project risk when brought to first use.PGS Offshore Technology has over the last 3 years developed the Dry Tree Semi or DTS, which limits the cost and risk aspects of unproven technology by using a standard semi-submersible hull with top-tensioned risers suspended by large volume buoyancy cans. A Norwegian patent has been awarded, and US and international patents are pending for the riser support system developed for the DTS, which comprises a light weight truss tower placed in the centre of the hull. Conventional sliding or roller bearings provide lateral support to the buoyancy cans inside the riser support tower. The DTS concept avoids the disadvantage of dry tree concepts with riser tensioners, which rapidly loose payload capacity in deep water because of the increasing weights of the riser system. Compared to SPAR alternatives, the DTS has a substantially better steel weight to payload ratio. Also, installation costs are considerably reduced since offshore lifting and mating operations are not required.At the OTC 2000 (ref. /1/), a DTS platform concept for West Africa was presented. This paper presents a DTS platform concept, primarily for use in the Gulf of Mexico, but also suitable for deepwater applications offshore Brazil.

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.

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