Pipeline Replacement Using Owner Company Inhouse Equipment
Authors: Brown Robert J. 
Published: December 2007

Introduction

It is conventional practice to design offshore pipelines for an operating life of around 20–25 years. During this period they are subject to routine inspection, maintenance and repair to ensure their continuing safety and stability on the seabed. Experience is showing however, that a surprisingly high proportion of lines, including even those installed in recent years, are already in urgent need of partial or full replacement. This situation is the result either of external damage incurred during installation of the lines, or of internal defects such as corrosion of the steel pipe. In such circumstances, the cost of installing new pipelines can be greatly reduced if owner companies make use of their existing equipment performing normal field support operations. Typically, this equipment includes supply vessels, work boats, survey vessels and possibly also submarines.

This paper describes an innovatory method which was employed in the UK Sector of the North Sea during 1977, to redeploy an existing marine pipeline (shown in Figure 1) from one area of Hamilton Brothers' Argyll Field complex to another. The line had not previously been in use, and was repositioned as a flowline connecting a new wellhead to a wellhead already linked to the production facility. The overall objective was to make maximum use of existing field support equipment in cutting the line to length, connecting the necessary hardware at either end, deflooding the section, towing it to its new location and tying it into the two wellheads.

The successful outcome of this project has important implications for the redeployment, maintenance and repair of existing pipelines, and for the installation of new lines.

Method

The technique adopted utilizes supply vessels and work boats, and comprises the following elements of work:Inspection of the pipeline to be redeployedSurvey to establish a suitable tow routeEngineering preparation of the pipeline for the towing operationCutting-to-length of the pipeline already existing on the seabedInstallation of connection hardware to both ends of the pipelineTowing of the pipeline to its new positionConnection of the pipeline

This procedure can be employed over a wide range of water depths, providing that the correct engineering approach is taken. A key requirement is the ability to cut the existing pipeline and attach the appropriate hardware. The cutting can be done either with the pipe on the sea bottom, or after raising the line to the surface. Where a vessel of sufficient horsepower is available the pipe, in some instances while still flooded, can be dynamically raised by attaching a cable, applying tension and spooling-in the cable until the pipeline reaches the surface. The thrust of the vessel is then maintained until the pipe end is brought onboard at a suitable angle of departure. The line is cut and pullhead/connection hardware is attached.

If there is sufficient compressor capacity onboard, the line can be dewatered while still at the surface. This enables towing of the line to proceed prior to spooling-out on the winch and before lowering the line again. On completion of towing to the new location, the pipe end can be placed on the bottom and connection to the subsea facilities performed.