Subsea activities

Subsea engineering and construction involves the building and maintenance of underwater equipment and pipelines.

Survey operations

Survey ships with specialist technology are used to image, map and examine seabed conditions in advance of drilling wells or installing pipelines. Their work influences decisions over where equipment should be installed and the route of pipelines.

Excavation and trenching

Pipelines and umbilicals are buried below the seabed by digging down before being covered. This is carried out to protect them from damage by currents or items such as fishing nets or anchors.

Flowline assurance

Crude oil varies in its temperature and thickness (viscosity). It also contains many different chemicals which can hamper its flow through a pipeline. Flow assurance examines how different oil behaves as it moves through a pipeline to help improve the flow.

Separation and processing

Equipment is used to separate oil, gas and water beneath the water surface. Each substance can then be processed individually back on the surface.


Inspections are use to determine the need for maintenance work on underwater equipment. This is normally carried out using divers, ROVs and AUVs, often with specialised electronic equipment to take detailed images or readings.

Corrosion management

Equipment can suffer corrosion as a result of being in the sea and from being exposed to chemicals. Equipment is checked by divers, ROVs and intelligent pigging to assess how it is being affected. Corrosion can weaken equipment which may result in leakages or structural failure.

Subsea control systems

Control systems involve the manipulation and monitoring of all underwater equipment such as christmas trees, manifolds and valves. A surface master computer is connected with via umbilicals to underwater control pods on subsea equipment. This allows the operator to monitor and operate equipment to maintain safe production.

Diving operations

Depending on the depth of water, commercial divers can go underwater in a diving bell to carry out inspections, perform maintenance, and install new equipment.

Subsea equipment

The subsea industry uses a variety of equipment to extract oil and gas form beneath the seabed including pipelines and offshore drilling machinery.


Remotely Operated Vehicles (ROVs) are used to access difficult to reach locations in the sea. The unmanned vehicles are connected to the surface by umbilicals and controlled by technicians using a joystick a bit like playing on a PlayStation. ROVs carry out tasks such as video monitoring, inspection, repairs and construction.


Autonomous Underwater Vehicles (AUVs) are similar to ROVs but they can be pre-programmed and piloted by an onboard computer. Once put into the water, AUVs can be left to carry out their tasks unsupervised before returning to base (the surface).

Christmas tree

This vital piece of subsea equipment is placed above the well on the seabed to control the flow of oil or gas. Once opened the flow passes through a line to the ship or platform at the surface. Trees can provide additional functions including chemical injection into the well and intervention.


Forcing a pig (a device with blades or brushes) through a pipeline for the purposes of moving or separating fluids, cleaning or inspection is called ‘pigging’. Inspection pigs are sometimes called intelligent pigs and can be used to check for corrosion and the wall thickness of the pipeline to help identify potential leaks.

Dynamic positioning, deepwater positioning

Onboard computers use satellite signals to keep a vessel in a stationary position over a subsea oil well or construction site using thrusters or propellers on the vessel.


Umbilicals are enclosed bundles of cables and pipes housing a combination of electrical, hydraulic, chemical or fibre optic connections to link the surface and underwater equipment. Any underwater system requiring to be operated remotely, such as an ROV, can be contacted by an umbilical.

Flexible riser

A tube or pipe carrying oil, gas or water to the surface. The riser is flexible to allow it to move with the motion of the sea without breaking.

Pumps and compressors

Pumps and compressors build up pressure to push oil and gas through underwater pipelines or to the surface. Pumps are used for oil with compressors aiding the movement of gas.