RIASSUNTO
ABSTRACT
In the Dutch sector of the North Sea, on-bottom stability of pipelines can be achieved in most cases without burial of the pipeline, but by the addition of a concrete coating, and the integrity of large diameter pipelines is generally unaffected by fishing activities.
To analyze the effect of unburied pipelines on fishing operations, the interaction between a beam trawl and a pipeline has been investigated. A numerical simulator of this interaction has been developed and this simulates the perpendicular crossing of beam-trawl gear over a concrete-coated rigid, i.e., large diameter, pipeline. In addition, model tests have been performed.
Through comparison of the predictions of the numerical simulator with the results of the model tests, it has been established that the numerical simulator describes the behavior of the beam trawl in the interaction correctly and predicts both the magnitude and duration of the interaction forces within 10%.
The results of the numerical simulations and model tests not only confirm, but also explain, the significant reduction of the interaction forces resulting from fitting hoop-bars to the beam-trawl vshoes or the use of shoes with a curved front. In addition, it has been established that the forces exerted on the pipeline are lower in magnitude and of shorter duration than the warp-line forces.
The general conclusion of this investigation is that the warp-line force during a pipeline crossing is of the same order as the maximum warp-line force during normal fishing operations, when irregular seabed conditions, clay boulders or stones are encountered.
INTRODUCTION
The on-bottom stability of offshore pipelines can usually be achieved without burial, but by the addition of a concrete coating. However, a pipeline resting on the seabed, even temporarily, can be subjected to frequent crossings of bottom trawl gears. During these crossings, interaction forces, are experienced by both the pipeline and the fleshing gear.
In the Dutch sector of the North Sea, beam trawling, see Fig. 1, is the predominant fishing method. Maximum weights of the steel beam and beam trawl-shoes (Fig. 2) of between 5 and 6 tons have been recorded, and trawling velocities of 7 knots are quite common. Therefore, the effect of beam trawl - pipeline interaction on both the pipeline integrity and fishing operations were investigated.
Pipeline integrity has been the subject of several investigations [1, 2] dealing with impact tests and pull-over calculations. Results indicate that the integrity of large diameter pipelines is normally not affected by fishing-gear crossings.
The effect of beam trawl - pipeline interaction on fishing operations is the subject of the present investigation. The interaction between fishing gear and pipeline has been investigated both numerically and experimentally, with special attention being given to the beneficial effect of fitting hoop-bars to the beam-trawl shoes (Fig.3). The investigation concentrated on perpendicular crossings of beam trawl gears over large diameter pipelines, since in this situation the highest interaction forces are found.
In this paper the beam-trawling characteristics, the numerical simulator and the model tests are described, the numerical simulator is verified and conclusions are given.