The number of submarine pipelines in the North Sea continues to grow and with it comes the need to monitor and service such systems. In most areas of the North Sea, pipeline inspections are required annually by government inspectorates. These twelve-monthly inspections must be capable of determining, as far as reasonably practical, the following:

  • movement of the pipe;

  • unsupported spans;

  • loss of cover in buried lengths;

  • damage to the pipe;

  • debris adjacent to the pipe.

For the most part these conditions required visual inspection to determine the extent and magnitude of damage. However, to inspect the entire pipeline visually is slow and costly, especially when it is considered that huge sections of the pipeline show no anomalies.

A sonar and sub-bottom profiler inspection indicating possible problem areas on which a visual inspection may be planned eliminates the need to inspect visually the entire pipeline. Recent advance on sonar equipment, towing platforms and data handling combine to allow this type of reconnaissance sonar survey to be carried out swiftly and accurately, with results being available at the end of fieldwork or soon afterwards (depending on data volume). In many cases the sonar/profiler survey even eliminates the need for a following visual inspection, thus further reducing time and cost.

Obviously there are some problem areas, for example trenched sections, but it is foreseeable that continuing developments in sonar imaging and data handling will limit the need for ROV/diver inspection to points of proven damage.

This paper firstly points out the requirement for confident sonar inspection, details some systems currently in use and attempts to suggest possible future developments.

Field Planning and Procedure

In order to view conditions on both sides of the pipeline the sonar must traverse the pipeline length on both sides. With normal passive tow fish this necessitates two separate passes of the pipe.

The position of the vessel and the position of the towing frame relative to the vessel must be accurately determined with all data acquisition systems, then integrated to the on-line navigation computer. A typical example of a vessel set-up is shown in Fig 1.

The towing vehicle chosen should provide a stable platform capable of being maintained at an optimum position relative to the pipeline. This optimum position should be determined by the sonar frequency and beamwidths, and the desired vessel speed.

With the introduction of steerable towing frames it may now be possible to run two tow vehicles simultaneously, one on either side of the pipe, thus eliminating the need for two passes of the pipe and so cutting the field time in half.

The sonar must have a frequency and meanwidths capable of resolving actual damage or features indicating that damage may have occurred or could occur in the future. This question of resolution is particularly important when considering free spans. A span may be only a few centimeters high yet of sufficient length to cause stress on the pipeline.

Fig 1 Side-scan sonar survey on-line system(available in full paper)

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