The Simple Subsea Well Concept
Authors: Morrill David L. 
Published: December 2007

This paper describes the concept and design of six subsea wells installed in the Gulf of Mexico in 1978. Wellheads, Christmas trees, control systems, and drilling operations are described in detail. Choice of equipment and methods was based on experience, simplicity, and reliability. Diver assistance was planned and incorporated. Costs compared favorably with those of other methods of production.

Introduction

Since 1975, when the first underwater wells were completed in 100 ft (30 m) water in Lake Erie using conventional land equipment, subsea well completions have increased in number and complexity. Today, more than 300 sublake and 100 subsea completions have been installed worldwide. Major technological developments include Seal's intermediate system (SIS), Lockheed's 1-atm (1.02-kPa) wellhead cellar, Exxon Corp's submerged production system (SPS), Transworld Drilling's production system (SPS), Transworld Drilling's satellite well and central manifold concept, and Deep Oil Technology's template concept with tension leg platform-all created for deepwater production. platform-all created for deepwater production. Because of associated technological advances, new methods, procedures, and equipment now allow us to make shallow-water, single-well completions with "off-the-shelf" equipment at costs competitive with platform development. These shallow-water wells platform development. These shallow-water wells have become attractive alternatives for producing shallow reserves (especially gas) that cannot be reached from existing platforms. Fig. 1 illustrates the system concept.This paper describes the concept and design of six subsea wells that Conoco Inc. (as operator with Atlantic Richfield, Getty Oil, and Cities Service companies -the CAGC group) installed in the Gulf of Mexico in 1978. The wells were drilled to develop shallow [4,000 ft (1200 m) total depth] gas reserves in 100 ft (30 m) water. This paper describes the wellheads, Christmas trees, control systems, and drilling operations (both jackup and semisubmersible rigs.)

This specific subsea system design was chosen with the intention of paying out the investments with proven equipment; we did not intend to test new proven equipment; we did not intend to test new technology. The only new developments were improvements based on practical, field-oriented experience. Essentially, this paper updates the state of the art of wet Christmas-tree technology by presenting a simple, reliable system that can be used presenting a simple, reliable system that can be used quickly and easily. Fig. 2 illustrates the subsea system components.The wells were isolated individually at six different locations in the Grand Isle area, offshore Louisiana. In each case, the directional angles and distances were too great to allow drilling from existing platforms. Reserves would not justify installing even the platforms. Reserves would not justify installing even the least expensive type of fixed structure. However, the subsea method was a feasible alternative economically (Fig. 3). From an operational standpoint, our belief in the dependability of the equipment was justified by recent experiences.

JPT

P. 1083