Cost Effective Dry Tree Field Development in Deepwater West Africa
Authors: Shivers Robert M. , Sikes Carlton T. , Chianis John W. , Aggarwal Rajiv , Berner Dale E. 
Published: October 2008

An innovative, low cost, deepwater development concept has been devised forWest Africa. The concept utilizes a simple, cost effective deep draft floatingdrilling/production platform (DDFP) with surface wellheads to minimizedevelopment drilling cost and life-of-field well intervention cost. MultipleDDFP's built to a standardized design provide capex phasing and enabledevelopment drilling coverage for large area extent fields.

Introduction

West Africa has seen several large deepwater discoveries in recentyears1. Vanco Energy Company is the largest license holder ofdeepwater acreage in West Africa with 19 million net acres in 6 countries. Amap comparing this area to the Gulf of Mexico Deepwater for scale is shown inFig. 1. Most prospects on these licenses lie in 5,000 – 8,000 ft. WD. The prospects are characterized by shallow (4,000 – 6,000 ft. BML), normallypressured reservoirs. This paper describes recent studies aimed at devising apractical, cost effective deepwater development concept that can bestandardized and replicated for any deepwater West Africa field.

Several recent deepwater field development projects in West Africa have optedfor sub-sea wells tied back to an FPSO2,3,4,5. Concerns with thisdevelopment approach include:Cost and complexity of sub-sea facilities such as trees, control systems, flow lines, manifolds and risersFlow assurance with low reservoir pressure and temperature in deep, coldwaterHigh cost of drilling and completing development wells from a MODUHigh life-of-field well intervention cost, particularly for sandcontrol

Development schemes featuring surface wellheads generally offer reduceddrilling/completion and well intervention costs and less acute flow assuranceproblems cf. sub-sea development. However, the capital cost of a deep draftfloating platform hull, mooring, topsides and riser system can be higher thanfor comparable sub-sea development. Furthermore, it is generally not practicalto reach shallow reservoirs over a large area from a single floating platform;therefore, sub-sea satellite drilling centers are still required. This studyindicates that simplifying and standardizing the floating platform design cansignificantly reduce the cost of surface wellhead development. This approachenables several streamlined platforms, each with a modest number of wells, tobe installed under a phased field development approach. A development conceptbased on these principles that is applicable for deepwater West Africa isdescribed further below.

Concept Description

Concept Overview.

The concept is predicated on dry trees for alldevelopment wells. Wells are drilled and completed from a deep draft floatingplatform (DDFP) for drilling and production, equipped with a movable, modularplatform rig. Well slots are limited to 16 to minimize the size and weight ofthe platform, limit the directional reach on development wells and facilitatestandardization and replication of hull and topsides. Two or more DDFP's spaced 2 – 4 miles apart will cover large fields. The central DDFP exports de-gassedoil and produced water to a turret moored FPSO through a transfer line. Outlying DDFP's export de-gassed fluids to the central DDFP where flow iscommingled into the transfer line to the FPSO.