Overtrawling capability of subsea protective structures

Many subsea components such as well heads and manifolds need to be fitted with a protective structure to minimise interaction with fishing gear such as trawl doors and nets. In many cases the protective structure is designed to allow the fishing gear to pass over the top without snagging.

The objective of the study was to establish the overtrawling capability of a particular design of a subsea protective structure.  This was done by performing scale model trials in a flume tank test facility.

The initial design was of a traditional subsea protection structure.  It consisted of a rectangular structure made from I beams and tubulars, with open sides, removable flat grating panels in the roof, and 60° rakers on each corner.  This is a type of design commonly found in the North Sea.

Tests showed that this design was not overtrawlable as the trawl gear tended to snag on the structure.  It was therefore modified as a result of the findings of the model tests and the new structure was tested in a similar manner.  The redesigned protection structure did not snag the trawl gear and was therefore shown to be overtrawlable.  The finished structure is now in position on the seabed.

The initial results were completely unexpected and show the power of using scale physical models in complex situations such as the interaction of trawl gear with structures and pipelines.  The flume tank tests provided readings of warp force, trawl door accelerations, and a video recording – which allowed the impact energies and pullover loads for the structural design to be assessed. The results have enabled the client to gain a better understanding of the requirements needed to make a structure overtrawlable.