Abstract

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.

Introduction

During the last two decades there has been a general trend towards higher oil recovery factors, and platform based or dry completed wells have a distinct advantage over subsea developments in this respect. There are a number of reasons for this, including amongst others, pressure drop in flowlines and limits on completion size, but even more importantly the cost and risk of performing subsea interventions. The latter reduces in practice the frequency of the whole range of recovery-enhancing measures such as logging, testing, reallocation, stimulation, recompletion etc.

The CAPEX costs for current platform designs with dry well completions increase however dramatically in deep water. The cost balance often tips the scale towards an FPSO and subsea development rather than the current dry completion unit alternatives, which are mainly TLP or mini-TLP in medium to deep water and Spar type platform in deep to ultra deep waters.

The most prolific deepwater exploration areas are currently West Africa, Brazil and Gulf of Mexico, ranging from benign to moderately harsh environments. The motivation for the DTS (Dry Tree Semi) development during the last 3 years has been to significantly reduce the cost of dry tree solutions for these main markets, starting with West Africa, which was subject for a paper presented at the OTC 2000 (ref. /1/). Subsequently, work has continued to extend the DTS application to Brazil and Gulf of Mexico (GoM).

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