ABSTRACT

A review of the advances made in the Control Depth Tow Method (CDTM) of installing pipeline bundles, including an outline of the design philosophy, the way in which the design was implemented and a summary of the experience gained during the installation.

The paper described the changes to the CDTM process which reduced the size of the fleet required to install the bundle and outlines the way in which parts of the installation process were achieved remotely, without the use of divers or ROVs.

Reduction in installation cost and improved operational flexibility are also highlighted.

INTRODUCTION

Rockwater has a long history of building and installing pipeline bundles in the North Sea using the well known CDTM. As with all technologies it is continuously developing, rising to the challenges of competition and continuing low oil prices.

Rockwater builds bundles at the Wester fabrication facility located near wick in Caithness. The site is 66 km in length and is equipped with fabrication shop facilities and two rail tracks. The bundle is manufactured from carrier pipe, flowlines, service lines and umbilicals, and is launched from the railway track into Sinclairs Bay. After a series of engineering checks the bundle is towed to the installation site using CDTM.

The Cyrus bundle is designed to carry oil and services between the Andrew and Cyrus fields. The bundle is approximately 65 km long and consists of a 10″ flowline insulated with 2″ of low density PU foam, which in turn are protected by a 14″ sleeve pipe. This combination provides very effective low cost pipe line insulation. The bundle also contained a 4″ gas lift pipe, a 2″ service line, 9 hydraulic control tubes, 2 electrical power cables and 2 electrical signal cables.

Several developments of the standard bundle launch, tow and installation methodology were used on the Cyrus project. This document highlights these developments and the reasons for their use.

Design Philosophy

In a conventional CDTM tow the depth and the shape of the bundle are monitored using transponders which communicate acoustically with a survey vessel travelling alongside the bundle. Limitations in acoustic range in the medium frequency band (MF) mean that for a bundle longer than 45 km two survey vessels are required to monitor the bundle during tow. Thus a typical towing fleet for "long" bundles consists of

  • One or more leading tugs to provide the forward towing force

  • Trailing tug to provide the back tension

  • Two survey vessels stationed alongside the bundle to monitor bundle depth

  • One patrol vessel to prevent other vessels from crossing the bundle during tow

The Cyrus project made use of an internal "data highway" to transmit data from depth sensors positioned along the bundle to any transponder on the bundle. This decision decoupled the depth and shape monitoring from the survey vessels, since either the leading or the trailing tugs could monitor the depth using acoustic telemetry. It also removed the requirement for one of the survey vessels

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