Risk to offshore installation mooring lines outside 500m safety zone

Health and Safety Executive - Safety notice

Department name: Energy Division – maritime integrity, offshore

Bulletin number: ED3-2023

Issue date: 05/23

Target audience: Operators of floating production and non-production installations in the UK continental shelf (UKCS). Marine control operators and standby vessels. Designers of floating offshore installations.


There is a risk of fishing vessels colliding with offshore installation’s mooring systems when trawling near to, but outside of, exclusive 500m safety zones.

Outline of the problem

Mooring lines of offshore installations extend well outside the exclusive 500m safety zone protected by international agreements, duty holders of offshore installations must manage the risk of collision with mooring lines.

Risk of collisions outside safety zones

Navigation and fishing outside safety zones presents a risk of collision with mooring systems. A collision could lead to loss of containment of hydrocarbons.

Duty holders operating offshore installations must ensure that other users are made aware of the vulnerability of mooring systems when navigating or fishing within the vicinity of a mooring spread.

Collisions, and fishing gear snagging on mooring lines, have occurred despite standby vessels and control room operatives issuing warnings to trawlers of possible contact with mooring lines.

Action required

Clear warnings

If mooring lines are comprised of fibre ropes and subsurface buoys, be explicit about snagging hazards and the extent outside the 500m safety zone when issuing navigational warnings.

Duty holders must clearly warn fishing vessels and other users of the sea about the:  

  • dangers of navigating or trawling within the mooring spread
  • potential for a mooring failure when over-trawled
  • consequences of a failure when an installation is engaged in drilling or extraction of hydrocarbons from the seabed.

Emergency response

Skippers of emergency response vessels (ERRVs) should be aware of the exact location and extent of mooring spread and the composition of lines. They should also be aware of the consequences of trawling within the mooring spread.

Duty holders should review the Collision Risk Management System and Emergency Response Plan to consider the possibility of collision between fishing gear and moorings extending beyond the 500m safety zone.

Designers and operators

Designers and operators of moored floating installations should be aware of the potential for mooring failure due to over-trawling.

Duty holders of oil and gas installations deploying moored installations should issue adequate information about planned mooring spread.


Relevant legal documents

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