ABSTRACT

The use of pipes or cables laid on seabottom calls for the adoption of means to guarantee protection against possible damage caused by dragging anchors, trawler nets and the action of currents on free spans.

The preferred protection method, most commonly used, is the burying of the cable or pipe in a trench of proper size dug along the line route: at present, for the most important cable and pipe laying projects, the national authorities prescribe a line burying for the whole route. To give a reliable solution to this construction problem, Tecnomare, on the basis of the design and construction experience gained during a previous project TM 102, has designed, built and tested a new system for the burying of underwater cables and pipes, named TM 402, which is now at an operating stage.

Such a system consists of an underwater remotely operated crawler, linked by means of an umbilical cable to surface equipment for power supply and control; it can operate under manual or fully automatic computer control in a various spread of soil conditions and in water depths up to 160 meters. The underwater vehicle supports a special chain trenching tool by which a narrow trench with vertical walls is made. The cable or flexible line is supported by a proper handling device which lays it with a controlled geometry onto the trench bottom. A complete instrumentation system allows a continuous monitoring of the system performances and of the cable/pipe conditions.

In this paper a detailed description of the system and of its main components and subsystems is given, together with its operating capabilities and procedures.

The results of the sea trials carried out in the Summer of the 1980 campaign are, moreover, reported.

INTRODUCTION

The construction of underwater hydrocarbon transportation lines and of underwater electric power or telecommunication links requires the adoption of means which may guarantee their protection against possible damage caused by human activities or environmental unforeseen loadings.

This protection is necessary, not only for the technical-economical reasons connected with the, usually, very high downtime and repair costs consequent to the disruption of the line's economic utilization, but also, in the case of oil/gas transport lines, for ecological and safety reasons due to pollution risk in case of line failure. Safety rules in many countries impose the burial of oil and gas underwater pipelines; the number of underwater electric power line projects for which authorities have prescribed partial or total burying is increasing.

The main causes of damage to underwater lines is the impact with dropping or dragging anchors, trawl net dead weight and the action of currents and waves on free-spans.

For large size pipelines an alternative to a protection technique can consists in an overdesign of the pipe and coating mechanical characteristics in order to make it capable of taking up possible accidental impact loads.

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