Development of the Subsea Completion System for the Highlander Field
Authors: Davis P.J.R. , Pond R.J. 
Published: December 2007

Summary
The Highlander field is being developed with a remote subsea manifold system connected to production facilities on the Tartan platform some 8 miles away. To accommodate the need for gas lift, water injection, equipment interchangeability, and service flexibility, a completely new, innovative subsea system was devised. Only 19 months elapsed between start of work on the comprehensive bid package and delivery of the first complete set of completion equipment ready to run subsea. This was achieved by thorough project organization, preparation of some 1,800 new drawings, intense manufacturing effort, and stringent quality control. Close coordination between operator and supplier, with considerable operator involvement, proved essential to the successful execution of the project.

Introduction
While subsea completion technology has been developing rapidly since its introduction more than 20 years ago, refinement of equipment and optimization of approach are still far from their ultimate. The recent increase in the use of subsea completions as a means of oilfield development has resulted in an upsurge in the general level of subsea-system-design expertise.
As operational experience has widened and the demands of certification and quality control have become greater, subsea systems have continuously been improved, but the range of up-to-date products is still relatively small. fThus when a present-day operator attempts to meet a new, specific subsea completion application, it is unlikely that he will find an existing system within these product ranges that completely fulfills his requirements. In general, the nearest the operator will get to an "off-the-shelf" system is a selection of standard building-block components e.g., valves, connectors, tees, and guide frames - that can be pieced together to provide the basis for his required system.
This paper describes how the Highlander subsea development project was organized to take advantage of this building-block approach. It emphasizes all aspects of project organization, including comprehensiveness of bid project organization, including comprehensiveness of bid package, timing, project control, manufacturing, and package, timing, project control, manufacturing, and quality control. To furnish a full insight into the scope of such a project, a technical outline of the entire system, including field-related features, is provided. Because the paper is written jointly by the operator and the supplier concerned, the two viewpoints should prove useful to other operators and suppliers.

The Highlander Development
The Highlander field is a small oil accumulation located in 420 ft [128 m] of water in U.K. Block 14/20. It contains an estimated 35 million barrels [5.6 × 10 m3] of recoverable oil. This marginal field will not economically support a dedicated production facility, so reserves are instead produced through a remote subsea manifold system tied back to the existing production facilities on the Tartan platform some 8 miles to the southeast. Flow from a number of subsea wells is commingled through a common 12-in. [0.3-m] subsea pipeline (see Fig. 1). Field development is complicated by a low GOR and low reservoir energy, resulting in the need to include both gas-lift and water-injection systems to maximize oil recovery. Separate pipelines are able to deliver the gas and water from the Tartan platform to the template manifold. The flow of gas and water from the common headers into the subsea wells is controlled by variable subsea chokes. This is the first use of remote-controlled variable chokes in a subsea application.
As in many such marginal developments, an early cash flow was critical in reaching commercial viability. This was achieved in the Highlander field by early production; flow from three of the four existing delineation wells was directed along separate pipelines to the Tartan platform. These wells were later tied back as satellite wells to the Highlander template manifold when it was installed in the summer of 1985, and the service to which each pipeline was put during the early production phase was changed.

Development of the Completion Specification
The design of the Highlander completion equipment had to encompass a number of essential production-related functions and, at the same time, follow a general philosophy of incorporating simple components to make a philosophy of incorporating simple components to make a fully integrated, yet versatile, system. These aspects of the specification are elaborated in the next section.
JPT
P. 453