Evaluation of Dry Tree Platform Concepts
Authors: Huang Edward , Bhat Shankar , Luo Yong , Zou Jun 
Published: October 2008

Dry completion by means of a surface wellhead platform is a viable alternative to subsea wet completion in all water depths. Dry completion offers the benefits of better reservoir testing and monitoring, drilling and workover capabilities, lower operating costs due to ease of well intervention, better flow assurance, and increased recovery of oil and gas.

This paper evaluates a variety of dry tree platform concepts, including both the established Spar and TLP and several newly developed platform concepts, for application in the Gulf of Mexico, offshore West Africa and Brazil. A single column medium draft floater, three and four column extended base TLPs are the new concepts that are included in this paper. A wide range of payloads is considered. Main particulars, global performance characteristics, advantages, and limitations and cost saving features of the new concepts are presented. Using the results from a screening study, trends in the global response, weights and costs are presented and discussed. A map of application domains for the dry tree platform concept is presented on the basis of cost comparisons.

Introduction

When oil companies develop a reservoir in deep water, a floating platform with the dry wellheads is often determined to be more economical than subsea wells. The optimum platform, however, will depend on many factors specific to the field development, such as geographic location, local wind, waves and current, water depth, number of wells, facility payload, environmental issues, etc. To implement the surface tree option, the relative motion between the production riser and the platform needs to be within certain allowable limits. This requirement limits the allowable platform vertical motion.

Only two deepwater dry tree platform concepts are field proven: The conventional Tension Leg Platform (TLP) and the Conventional Spar. A comparison of TLP and Spar weight and cost efficiencies with regard to variations in payload and water depth is provided in Reference 1. However, engineering innovation never stops. In recent years, a variety of new, innovative, dry tree platform concepts have been proposed. The new concepts are aimed at reducing the cost of dry completions in deepwater. Three of the innovative new concepts are included in the comparative evaluation presented in this paper. These are:Single-Column Floater (SCF) 1Extended-Base 4-Column TLP (ETLP-4) 1Extended-Base 3-Column TLP (ETLP-3) 1

In addition, we include some comparisons with the proven conventional TLP and Conventional Spar concepts and the emerging Truss-Spar concept.

Main objectives of this paper are to review and compare these dry tree platform concepts and to illustrate the trends of payload and cost efficiencies. The findings generate insightful knowledge for future concept selection in GOM, West Africa, and Offshore Brazil.

Review of Design Environments

The severity of design environment depends on the location of application. A summary of some of the main environmental design criteria for deepwater GOM, West Africa, and Brazil is provided in Table 1.