In the North Sea and elsewhere on the Norwegian continental shelf there is heavy fishing activity, and especially bottom trawling represents a hazard to pipelines on bottom. Theoretical studies as well as models and full scale experiments have been undertaken to clarify important factors which influence the behaviour of the trawl doors including the forces which are acting when trawl doors pass pipelines. The aim has been to obtain more accurate knowledge of the risk of damage to a pipeline lying on the bottom.
Offshore pipelines normally have to be protected on the sea floor against effects of the environmental conditions as well as human activities. A completely buried pipeline with some cover is well protected against the majority of effects which could damage the line. In the North Sea, and elsewhere on the _ Norwegian continental shelf, there is very intense fishing activity by most of the European fishing nations, using different types of gear. Some fishing gear, especailly bottom trawls, may represent a hazard to unburied pipelines. Bottom trawling takes place also in the deeper parts of the Norwegian continental shelf, even down to 300 – 400 meters. At such depths current velocities are low and wave action insignificant, and only the trawling activity could justify special protection of a pipeline.
A normal pipeline burial operation leaves the pipeline in an open trench. At depths where currents and wave actions are insignificant, a gradual backfilling by natural means usually will take place.
At greater depths, the normal pipeline trenching operation will be increasingly more complicated and costly, and may ultimately not be possible at all.
Due to low current action at greater depths, the natural backfilling of a trench may not be obtained.
It is not sufficiently known what could happen when? Pipeline on the sea floor, or in an open trench, is hit by trawl doors. Some practical experience exist concerning damage to pipeline coating by bottom trawl, and pipeline coating has also been tested in laboratory for effects of blows from trawl doors . Further, an' engineering approach has been developed to design pipeline protection to resist impact from dragging anchors and trawl doors .
In November 1973 the Norwegian Deep Water Pipeline Project Committee, a committee appointed by the Royal Ministry of Industry, initiated a research program on bottom trawl influences on submarine pipelines.
The project was divided into three phases. Phase No. I was a paper study, while phases Nos. II and III consisted of model tests, and field tests, respectively.
A typical situation with trawl and pipeline is shown in figure 1. The part of the bottom trawl gear which is considered to represent the main hazard to a pipeline is the trawl doors. They are made of steel or wood, having rectangular or oval shapes, and are towed ahead of the trawl net in order to spread the Opening of the net.