Abstract

Cables and pipelines are buried in deepwater to provide thermal insulation, on-bottom stability and physical protection. In the soft soils associated with deepwater regions cable ploughs and ROV based jet trenchers are the tools of choice.

The Offshore Engineering Division of the Coflexip Stena Offshore (CSO) Group and Perry Slingsby Systems have contributed significantly to the development of deepwater cable and pipeline burial systems. Significant resources continue to be allocated to geotechnical research programmes to support product developments.

This paper discusses the key technical issues with respect to deepwater cable and pipeline burial. The state-of-the-art of geotechnical engineering and research, and it's application to new burial product developments, is also presented.

Introduction

It is now common for telecommunications cables to be buried across the world's continental shelves in water depths up to 1,500m. Such burial is primarily to provide physical protection, against fishing and shipping activity, and onbottom stability against environmental forces. Traditionally, burial of subsea cables has been achieved by simultaneous lay and trench using a cable plough pulled by the cable installation vessel. As such the installation speed of the cable is governed by the rate of burial, not the rate of cable lay. As a result there is a growing desire to de-couple cablelay from burial, i.e. use a separate vessel and/or tool to bury the product post-lay. This has led to technical issues for both subsea ploughing and jetting, which may be able to provide an alternative to ploughing in some circumstances. Proposed oil and gas developments, particularly in the Gulf of Mexico, offshore West Africa and Brazil, are now commonplace between 1,000 and 2,000 metres of water, with plans for 3,000 metres plus being discussed in the industry.

It is well known that the requirements for pipeline burial change as water depth increases (3,5). Whilst physical protection and on-bottom stability issues still exist in deep water, as water depth increases their importance tends to diminish. As water depth increases flow assurance requirements becoming increasingly important and the potential benefits of flowline burial become clear (7). Thus the issues of pipeline burial in deepwater are focussed on achievable performance, e.g. burial depth, backfill parameters (particularly thermal conductivity and density) and mitigating issues associated with pipeline installation in deepwater soils, particularly floatation of light pipelines.

This paper discusses the issues associated with burial of cables and pipelines in deepwater. The current state-of-the-art is reviewed, the knowledge gaps and technical risks are highlighted, and research programmes, which have been designed to address these issues, presented

Deepwater Cable Burial

Ever since the first two transatlantic telecommunication cables were installed by AT&T Submarine Systems in the late 1950's, the need for protection of subsea cables has been understood. It was quickly recognised that the sections of cable on the continental shelves were most at risk from fishing and shipping activity. It has been estimated that up to 95% of all subsea cables failures can be directly attributable to fishing activities (11).

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