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An abandoned well site is an area where a wellhead has been totally removed by cutting below seabed level. The seabed should be returned to its natural state; however, fishermen have periodically experienced snagging issues in these areas caused by debris left behind out-with of the surveyed 50 metre radius. To reduce this risk, trawl sweeps are now recommended before the site is certified as clear.

Trawl Swept Pipeline Gates are areas of seabed where a fishing vessel has conducted trawl sweep operations, to reduce the impact of heavy spoil that has been excavated when a new pipeline is installed. A fishing vessel will tow a chain mat, followed by fishing gear across the pipeline to lessen the impact of spoil and create areas of seabed that are assigned as Trawl Swept Pipeline Gates.

An FPSO – Floating Production, Storage and Offloading – is a floating vessel designed to receive, produce and store hydrocarbons from nearby platforms or subsea wells until they can be offloaded onto a tanker or through a pipeline. An FPSO is normally a vessel up to 250m long, 40m wide and weighing approximately 90,000 Gt. Most FPSOs are fixed to the seabed with pattern of mooring lines and anchors, which may extend several kilometres from the vessel.

A subsea protection structure is any structure added to, or directly integrated into a pipeline or other subsea asset with is designed to offer protection to that asset and to make that asset either fishing friendly or over-trawlable. Protection structures are most commonly constructed from steel and range from less than 1 metre to greater than 10 metres in height. A protection structure may be found outside a 500m safety zone.

A ‘Pipeline Bundle’ is a collection of smaller diameter pipelines and control umbilicals contained within a larger outer casing. They may be many kilometres long and several bundles may be connected together. Pipeline bundles are towed from shore to their location between two tow vessels at a controlled depth below the surface of the water. On arrival at the field, the bundle is lowered to the seabed.

A dynamically positioned (DP) semi-submersible rig is a mobile offshore drilling unit. The purpose of the rig is to drill new wells, alter existing wells, or suspend or abandon older wells. A DP semi-submersible rig has a propulsion system so can move itself, although may be accompanied by tugs. Normally, multiple thrusters keep the rig in position, which means they can be used in greater water depths – not requiring foundations or anchors to the seabed.

A riser base is a support structure on the seabed for the tie-in of a pipeline at one end and a flexible riser at the other. It accommodates any connection system, commonly including isolation valves and controls and provides an anchor point on the seabed for the riser load. A Riser Base may be up to 10 metres long and 3 metres wide, weighing up to 250 tonnes.

Subsea Buoys are used where it is required to lift or support an engineering feature. Examples may be support of a flexible riser from the seabed to a surface installation, or support of an anchor wire providing clearance of the wire over subsea facilities. The larger subsea buoys may provide several 1000’s kg uplift force and be 10 cubic meters in volume.

Grout Bags are large bags of material that are used to support or protect a pipeline, or structure. Each individual grout bag may contain 1 – 2 cubic meters of material that will set into a solid mass. Several grout bags may be used in succession to develop a larger structure than each individual component. Therefore the mass of the structure can vary considerably.

Concrete mattresses are laid over subsea features, such as pipelines or umbilicals, to provide additional protection. Mattresses are constructed from individual cast concrete blocks moulded into polypropylene rope to form a flexible mattresses that are approximately 5 metres long and 3 metres wide. Mattresses may be installed in succession along a pipeline, or layered to provide added protection.

A pipeline span is a section of pipeline that is not touching the seabed, leaving a gap between the pipeline and the seabed. Pipeline spans are created when seabed surrounding the pipeline shifts, meaning a section of the pipeline is no longer in contact with the seabed. Significant pipeline spans of at least 10 metres long and 0.8 metres high are included on FishSAFE.

When offshore platforms are decommissioned, an operator may be granted permission to leave the structure, or part of the structure, in place. One possibility is that the ‘platform footings’ may be left in place. This is the structure closest to the seabed, holding the platform in place. Post Decommissioning Platform Footings may project 50 metres above the seabed.