A Subsea System for Deep Water Completion and Production
Authors: Hopkins J.W. , Dixon B.R. , Weiss F.H. 
Published: October 2008

An atmospheric system that places man, at the wellhead on the ocean floor in his normal environment is described. This atmospheric approach offers a flexible system with high reliability and significant economic advantages for the industry in pursuance of petroleum deposits at ocean depths of 400 ft or more. The system was developed to allow for early production) economic spacing of wells and an orderly subsea field development program.

INTRODUCTION

Background

With petroleum exploration and development moving into offshore areas of increasing depth) conventional drilling and production costs are rising sharply. At depths o:t 400 ft or more) fixed surface structures are subject to serious economic disadvantages as well as weather) marine traffic and civic-political problems.

Extensive economic studies of alternative offshore petroleum production systems were conducted over the past 2 years. The results indicate significant economic advantages of an atmospheric subsea system as compared to other approaches.

Design Objectives

In this new system) individual wellheads are encapsulated on the ocean floor within pressure chambers called wellhead cellars. The products of the wellheads flow into a manifold center installed on the sea floor which is also maintained at atmospheric pressure. Here the fluids from each well are controlled) commingled) and then transported by gathering lines to a separation facility. The manifold center also has necessary controls to divert individual wells for-testing. An anchored floating platform or fixed structure may serve for support of the separation facility. Locations that possess deep water reservoirs near shore may allow for onshore production equipment installation.

Manned attention at the subsea wellhead or manifold center is provided by submersible atmospheric capsules that operate from support ships. The capsules contain their own propulsion system for maneuvering into place over the wellhead cellar or manifold center. They attach themselves to the subsea structures by engagement of a gasket-type seal. Electrical power for the propulsion system) for the onboard equipment and an air supply are provided by umbilical cables from the surface support ship. The capsule work chamber is maintained at a one atmosphere shirtsleeve environment. It contains the conventional oilfield equipment with which trained oilfield personnel are familiar and use in operations that duplicate those performed on land or on platforms. In effect this total system concept is based upon the adaptation of standard oilfield methods and equipment for subsea use. Fig. 1 illustrates the design of such a subsea system.

Wellhead Cellar

The wellhead cellar [Fig. 2] is a man rated pressure shell with skin thickness dependent upon the installation depth. The cellar can be welded and run with the surface pipe, or it can be placed on the wellhead by means of an automatic connector after the well has been drilled. If the cellar is welded to the surface pipe, the BOP stack is positioned above the cellar on a short riser, and drilling is accomplished in the conventional manner. Fig. 2 shows a wellhead cellar design with provision for pump down tools. A smaller configuration becomes possible if pump down tool compatibility is not required.