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Abstract

The continuous operating protection system called COPS is a diverless solution to achieve the stabilisation and protection of subsea pipelines and cables : the system is based on the use of a continuous fabric form-work mattress which is spread on the sea bed over the pipeline or cable to be protected by a remotely controlled underwater crawler and simultaneously rilled with cement grout.

The method has been successfully used in the GULLFAKS field where about 3.6 km of grout mattresses having a cross section of 2 meters by 0.2 meter have been laid.

The performances of the system are presented as well as a trade off com-parison with the other stabilisation and protection methods currentl used: burying, rock dumping or placement of covers.

Introduction

In many cases, sections of pipelines or cables which cannot be buried for technical or economical reasons require by national and international regulations to be protected by an alternative system to avoid mechanical damage caused by environmental conditions or human made hazards (fishing activities).

The two main systems currently in use are rock dumping and placement of (short length) covers : both of these systems have technical limitations and involve the use of specialized vessels or diver intervention. On this basis, COFLEXIP and VSL international have jointly developed in 1985 and 1986 an innovative protection system based on an original VSL patent which allows the covering of pipelines or cables by continuous grout mattresses placed by a diverless machine operated from a non specialized vessel: the system has been offshore tested in 1986 in the GULLFAKS field for the laying of 3.6 Km of grout mattresses over an 8" OD pipeline and two control umbilicals. The level of performances achieved in this project has clearly demonstrated the effectiveness of the method.

The system has been qualified for use in the Norwegian sector and is presently under review by the United Kingdom authorities.

PRINCIPLE OF THE SYSTEM

The system mainly consists of the following devices (figure 1) :

The continuous fabric formwork mattress is composed of a double polyethylene layer interconnected with spacer straps giving the specified cross section of the mattress. The top layer is closed by a longitudinal sealing strip (VELCRO band).

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