Bundle towhead over-trawling – Scale model tests

Fishing vessels frequently trawl throughout the North Sea. Problems can arise when fishing gear, such as trawl doors, beams or nets, come into contact with subsea infrastructure such as pipelines, cables, wellheads, bundles or bundle towheads..

It is sometimes necessary therefore to fit these with protective structures to minimise the consequences of interactions with fishing gear.

The client was working on a project in the northern North Sea that included two bundles and four towheads.  One specific requirement for the towhead design was that they were to be overtrawlable by fishing vessels.

An overtrawlable structure may be defined as one that does not snag the fishing gear causing either the stopping of the vessel or breaking its warp line during overtrawling.  The structure must also be able to withstand any loads or impacts experienced during overtrawling.

Jee provided technical support on fishing interaction and confirmed the overtrawlability of the towhead design and derived the pull-over loads and impact energies on the structure by conducting flume tank tests on a scale model of the towhead.  The project involved:

• Defining the type and intensity of fishing activities in the area
• Defining the number of anticipated interactions with each towhead
• Designing and building a scale model of the towhead
• Performing the trials
• Deriving the pull-over loads and impact energies on the towhead
• Establishing the over-trawlability of the towhead design

This project provided the client with the information they needed to ensure the overtrawlability of their towhead design.