A major development program for large ploughs to trench submarine pipelines began in 1975, and has led to the construction of 13 ploughs, which have been used in many parts of the world. Deep trenches are best cut in a sequence of separate passes. The paper describes the development of the first multi-pass plough, used to trench the Gullfaks pipeline in 1984, and the application of the concept to Arctic pipelines, which may require trenches more than 4 m deep.
Many submarine pipelines have to be trenched. A trench protects the pipe against damage from fishing gear, much reduces hydrodynamic forces from waves and currents, gives a degree of protection from small anchors and from construction vessels' mooring lines, and may be desirable for security, aesthetic or environmental reasons. The relative importance of these factors naturally varies from project to project. If the trench is backfilled, protection against fishing gear and waves is complete, security against other kinds of damage is increased, and heat transfer between the pipe and the sea is reduced.
In some areas, regulatory authorities insiston trenching, at least for small-diameter pipelines. In the early years of North Sea development, for instance, authorities asked for pipelines to be lowered below the natural sea bed, sometimes to a depth of 2 or 3 m. It was widely argued that these requirements were excessive, and the generally excellent experience of more than 15 years operation of an extensive and growing network of North Sea pipelines, in the British, Norwegian, Dutch and Danish sectors has led to a moderation of requirements. Authorities are now sometimes ready to accept that pipelines need not be trenched, particularly if they are large or if it can be demonstrated at they will bury themselves naturally1,2
Until about five years ago, jetting was the technique almost always used to trench submarine pipelines. Since jetting techniques were first developed in the 1950s, they have been substantially modified, but their efficiency is much affected by geotechnical 'conditions on the sea bed. In medium clay, for instance, jetting cuts a neat rectangular trench, but in loose sand it leaves a wide and shallow trench, with side slopes less than 100, which does little to protect the pipeline. In the wrong conditions, jetting is slow an expensive. In one notorious instance in the Netherlands sector, 13 passes over the same spot were required before the pipeline was lowered to the required depth.
A creative dissatisfaction with the high cost and limited efficiency of jetting led to a search for better trenching methods. There have been two principal lines of development, ploughs and mechanical cutting systems. Each is appropriate in the right conditions. At first, mechanical cutting systems were plagued by mechanical and electrical faults, and by sensitivity to bottom soils and topography.
