A Suspended Wellhead is a structure and equipment located above the seabed which relate to a well that is not currently in production and has been suspended awaiting further intervention. Suspended wellheads can be found outside of safety zones and are in effect immovable objects to fishermen. They may project 4 metres above the seabed.

Rock Placement Berms – or ‘rock dumps’ provides protection to pipelines or umbilicals. Vulnerable pipelines, such as pipelines spanning the seabed or crossing other pipelines, may require additional protection, which may be provided by rock “dumping” along the length of vulnerable section. The rock will generally be 10 – 20 centimeters in diameter and may cover the pipeline in excess of 0.5 meters.

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.

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.

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.

A Clay Berm is a mound of excavated seabed material that is produced during the trenching process, as a pipeline or umbilical is mechanically buried for protection. The material and size will depend upon the local seabed and diameter of pipeline. Large diameter trenched pipelines may exhibit large ‘berms’ up to 1 meter in height. There may often be two berms, one either side of the pipeline.

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.

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.

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.

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 ‘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 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.