Re-assessment of overtrawlability

Our client was operating several pipelines on the UKCS that were lying untrenched on the seabed. These had been assessed for fishing gear interaction when the lines were designed and installed in the mid-1990s and changes in fishing practice in the intervening years meant that the client was no longer sure if the conclusions of the original study were still valid.

Our client therefore requested that a new study be carried using current data on trawlers and fishing gear.

Jee carried out the following work:

• Data gathering on current fishing practices and pipeline operational conditions
• Use of 2D analysis to assess the pullover forces of the new fishing vessels
• Writing of a 3D ABAQUS FE model (which considers coatings, contents, buoyancy, seabed friction, pressure, temperature and hydrodynamic forces) to predict bending due to trawl gear pullover and hooking
• Assessment of the results of the FE analyses in accordance with HSE guideline OTH 561 in order to verify that it was still acceptable to leave the pipelines untrenched

Our study concluded that although trawl gear loads are now higher than those originally predicted, the predicted dents and bending due to trawl gear are within acceptable limits and the pipelines can safely be left untrenched on the seabed.