Fishing interaction with subsea structures

The main issues
The oil and gas industry shares the waters of the world with fishermen and problems can arise when fishing gear comes into contact with subsea pipelines, cables, bundles and other subsea structures. 

Issues resulting from these interactions include:

• Safety of fishermen
• Damage to subsea structures
• Damage to fishing equipment
• Impact on fishing activity and the environment
• Communication between the two industries

Where direct contact between the fishing gear and subsea pipelines / structures may occur it is important to consider the design and protection philosophy of these in order to reduce the number and consequence of these interactions.  Currently, there is no comprehensive recommended practice for the design of overtrawl protection to wellheads and manifolds.   This makes it difficult for the designer, who needs to take account of a number of factors depending on the type of structure being designed. These include:

Pipelines: the main issue with pipelines is determining whether or not trenching or burial is required.  Different parts of the world have different philosophies for this.  For example, in the UK sector of the North Sea the general practice is that pipelines less than or equal to 16in diameter require trenching.  In the US, the general practice is that larger pipelines require trenching whereas smaller lines self bury.  This difference is due to soil conditions and size of fishing vessel operating in the areas.  From research into fishing practices, it is found that many trawlers intentionally follow pipeline routes because they are Fish-Attractant Devices (FAD).

Cables: cables are generally trenched and buried for protection.  However, it is sometimes possible for a cable to have sufficient protection by shielding from a neighbouring pipeline.  This can significantly reduce installation costs.

Protective structures:  subsea structures, including manifolds and wellheads, are covered by protective structures in order to protect against trawl gear damage.  These structures are conventionally designed  in two ways:

• Overtrawlable: designed to allow trawl gear to pass over them without snagging or stopping the vessel
• Fisher friendly: designed to either deflect fishing gear or to deliberately snag the gear but allow easy retrieval

Bundles:  Bundles often have intermediate and end termination structures which can be hazards to trawl gear.  These can also require protective structures.

It is important to understand the different types of fishing activity and how they interact with subsea structures.  Fishing methods that have caused problems in the past include otter and beam trawling, seine netting and longline fishing.  The specific practices of fishermen in a particular area should also be considered.  Vessel size and activity varies considerably.

Jee technology and experience
Jee has many years of experience of fishing interaction with subsea structures both in the waters around the UK and elsewhere.  We have developed an understanding of fishing activities and have many contacts with government agencies and fishing organisations world wide.  Jee has specific experience in the following areas:

• Trenching guidelines Joint Industry Project (JIP): determined the effects of trawl gear interaction with pipelines and set out a methodology for assessing whether the pipeline needs to be trenched for protection.  The JIP included the development of:
• Interaction models
• Acceptability/limit state criteria for pipeline damage
• Requirements for protection
• Risk/cost models – optioneering studies
• Overtrawling of large diameter pipelines (JIP):  the purpose of the JIP was to help identify impact and pull-over loads on fishing vessels when overtrawling large diameter pipelines.  It involved full-scale and flume tank trials and drawing conclusions about the acceptability of performing model-scale trials in place of full-scale trials. 
• Performed interaction analyses for a number of projects:  these include analyses on pipelines and other subsea structures and involved:
• Assessment of fishing activity
• Assessment of effects and consequences on pipelines
• Specification of protection and trenching requirements
• Flume tank testing
• Assessment of fishing practices:  These studied have included:
• Review of oilfield operator experience
• Identifying high risk pipelines
• Quayside interviews and surveys of trawlermen
• Dissemination of information:
• Improving the flow of information from oil industry to the fishing industry
• Educate the oil industry in fishing activity
• New pipeline:
• Assessment of the effect on the fishing industry of a new pipeline through an established fishing area
• Environmental impact of pipeline installation

• Feasibility: development of protection philosophy (trenching or burial of a pipeline can often be very costly and can effect the economic viability of a development, especially in marginal fields).  Assess the impact of new pipeline on fishing industry.
• FEED: perform design of protection structure. Perform risk assessment and consequence analysis. Determine requirement for trenching and trenching depth.  Determine method of burial.  Analyse pipeline self-burial potential and timescale.  Assessment of impact, pullover, snagging and gear retrieval loads on structures and pipelines.
• Technical support: Provide flume tank or full scale trials, test specification, management of tests.  Communication with fishing industry.  Dissemination of information to fishing industry.  Management of JIPs.  Surveys and liaison.
• R&D: Development of protective structures