Dry Completion Units for West Africa Field Development
Authors: Luo Y. , Huang E. , Lai N. , Baudic S. , Chen H.T. 
Published: October 2008

This paper deals with the selection and evaluation of Dry Completion Units (DCUs) for West Africa field developments. The main considerations in selecting a DCU are highlighted and discussed. The DCU hull concepts are then evaluated in both the technical performance and economic terms. A number of technical challenges in implementing the DCU concept are highlighted and analyzed using the advanced engineering analysis tools.

The paper concludes that the dry tree completion development via DCU is an attractive option and should be carefully considered for West Africa field development. The DCU development in deep waters offers interesting technical challenges. Potential cost savings can be achieved by adopting innovative approaches.

Introduction

As an alternative to the full subsea completion, the deployment of the Dry Completion Unit (DCU) is a viable option to the deepwater West Africa field development. The DCU hosts dry completion trees and also serves as a permanent drilling, production and workover unit. It is usually deployed with an FPSO that hosts the main production facilities and provides storage of the produced crude. Compared with the full subsea wet tree development option, the dry tree development via DCU can often offer potential life-cycle cost savings.

One of the important criteria in implementing the dry tree solution is that the relative movement between the platform and the risers due to platform's vertical and horizontal movements (riser stroke) can be suitably accommodated. This requirement sets a limit on DCU's motion. Up to now, the most commonly selected hull concepts for DCUs are the SPAR and the Tension Leg Platform (TLP). The TLP has excellent vertical motion characteristics, while the SPAR decouples the relative platform and riser movement by using riser buoyancy cans. Due to West Africa's benign environment, other platform concepts with less satisfactory motion characteristics such as the semi-submersibles and barges have been proposed and may be able to meet the design requirements.

The main objectives of this paper are as follows:To review and highlight the selection criteria for implementing DCU;To evaluate the pros and cons of various DCU hull concepts;To illustrate the state-of-the-art work that ABB has conducted in recent field development studies;To identify future areas of studies.

In the following sections, the general benefits of deploying the DCU have been outlined and the main considerations in selecting a DCU are discussed. The discussion covers a wide range of issues related to reservoir, drilling and workover operations, flow assurance, technical feasibility and development economics. The DCU hull concepts are then reviewed from both technical and economical points of view. The evaluation focuses on the proven technologies of the TLP and SPAR concepts, while the semi-submersible and barge concepts are also discussed.

Finally, a number of areas that offer potential cost savings and technical challenges are studied. The areas studied include the FPSO/DCU close proximity arrangement, tender assist drilling, DCU/tender linked operation mode, well pattern and riser spacing, and linked FPSO/SPAR operation.