Commercial offshore diving, sometimes shortened to just offshore diving, generally refers to the branch of commercial diving, with divers working in support of the exploration and production sector of the oil and gas industry in places such as the Gulf of Mexico in the United States, the North Sea in the United Kingdom and Norway, and along the coast of Brazil. The work in this area of the industry includes maintenance of oil platforms and the building of underwater structures. In this context "" implies that the diving work is done outside of national boundaries. Technically it also refers to any diving done in the international offshore waters outside of the territorial waters of a state, where national legislation does not apply. Most commercial offshore diving is in the Exclusive Economic Zone of a state, and much of it is outside the territorial waters. Offshore diving beyond the EEZ does also occur, and is often for scientific purposes.
Equipment used for commercial offshore diving tends to be surface supplied equipment but this varies according to the work and location. For instance, divers in the Gulf of Mexico may use wetsuits whilst North Sea divers need dry suits or even hot water suits because of the low temperature of the water.
Diving work in support of the offshore oil and gas industries is usually contract based.
Saturation diving is standard practice for bottom work at many of the deeper offshore sites, and allows more effective use of the diver's time while reducing the risk of decompression sickness. Surface oriented air diving is more usual in shallower water.
Scope of work The offshore diver may do a wide range of tasks in support of offshore drilling or production.
Drilling rig support Much of the offshore seabed diving work is inspection, maintenance and repair of the blow-out preventers (BOPs) and their permanent guide bases. The primary functions of a blow-out preventer system are to confine well fluid to the wellbore, provide a way to add fluid to the wellbore and to allow controlled volumes of fluid to be withdrawn from the wellbore. Dive work includes assistance with guiding the blowout preventer stack (BOP stack) onto the guide base, inspection of the BOP stack, checking connections, troubleshooting malfunctions of the hydraulic, mechanical and electrical systems, and inspection of the rig's anchors.
Production platform support There are a wide range of production platform configurations which are chosen to suit the water depth, expected sea conditions and other constraints. Diving work will depend on the details, but will generally involve inspection, maintenance and repair of the structure, wellheads, manifolds, risers, associated pipelines and mooring systems.
Divers may work on marine risers (conduits that extend a subsea oil well to a surface facility) and blowout prevention stacks on all production rig types if the depth is within diving range. Work may be surface oriented or in saturation depending on depth and duration.
Depths are relatively shallow on jack-up rigs, which stand on the seabed, and are limited to depths less than about 90m. Much of the diving may be on air. Diving work may include inspection of the seabed before jack-up, measurement of penetration depth of legs, monitoring and management of scour around the legs where they rest on the seabed, inspection and repair of underwater structural components of the rig and support of anchor deployment and recovery.
Semi-submersible platforms operate in a wider range of depths, mostly relatively deep, and bottom work is more likely to be in saturation and using heliox. There is also shallow work on the hull which is likely to be on air: inspection of thrusters, pontoons and the rest of the underwater structure of the rig and when necessary repair.
Wellheads Wellheads control the production of oil or gas from the well, and can be used for water injection. They are mounted on production guide bases, which are the upper endpoint of the well casing at the seabed. A well head can be directly connected to a subsea manifold or indirectly via other wellheads, which may be arranged in groups called a subsea template. Most diving work on wellheads and manifolds is installation and maintenance.
Manifolds Subsea manifolds are structures mounted on the seabed where pipelines and connections to wellheads are connected to control the flow of product from the wells to their next destination. They will include valves and control mechanisms for the valves, and diving work mostly involves inspection and maintenance work, but can also include installation and repair, and connecting in new wellheads.
Pipeline work A large amount of offshore diving work is associated with submarine pipeline work, particularly with pipeline connections, inspection and repair. Depths may range from deep to shallow, and procedures and diving mode will be chosen to suit. Work includes aspects of pipe laying and trenching, and work on existing pipelines and ancillary equipment.
Pipelay barges
A pipelay barge, or lay barge, is used to lay submarine pipelines by welding pipe sections to the pipeline on board, inspecting and coating the joint, and lowering the assembled part to the seabed over a projecting supporting structure called a stinger which guides the pipe and controls the curve at the end of the barge (overbend) in a more or less continuous process. As the pipe approaches the seabed, the pipe takes a convex-downward curve (the sagbend) before making contact with the bottom. the sagbend is controlled by tension applied from the vessel and tension wires from the pipeline on the bottom to anchors which prevent it from being dragged out of position by the barge.
Diving work on lay barges includes shallow inspection and maintenance work on the stinger, and deeper work at the bottom associated with the pipe and pipeline connections. Shallow work is usually on air, and may include stinger inspection, Checks that the pipeline runs smoothly and is not damaged before leaving the stinger, placing and checking CCTV cameras for monitoring the pipe transit, operating manual valves on the stinger buoyancy system and work on the mooring system. Deep work will use breathing gas appropriate to the depth, and may include checking the as-laid condition of the pipe and coating, grout bagging long spans for support, connection of flanges, bolt tensioning and hyperbaric welding, attachment, operation and removal of pigging equipment, and attachment, removal and moving of constant tension wires.
Pipeline stabilisation
There are several ways that a pipeline can be stabilised on the seabed, and diver intervention is required on most of them. Concrete mattresses are used to weigh down the pipeline to hold it in place, particularly on bends, but may also be laid under the pipe to stabilise the substrate. Frond mattresses are mainly used to stabilise the sand, and are usually anchored to the bottom. Sandbags and gravel help prevent scour, and can be used to support areas where the pipe spans a local depression. Trenching protects the pipeline from current and external loads such as from iceberg gouging, trawl nets and anchors, and saddles and ground anchors can secure the pipeline against lateral movement when located by piles or grouted to the substrate. Deploying these items usually requires diver input for controlling the placement, and may also require the diver to physically move components like sandbags.
Trenching barges
Trenching barges are used to excavate a trench on the seabed for a pipeline or cable to protect and stabilise it. Infilling is usually left to the natural action of the water on the seabed. The actual excavation is done by a sled which may use water jets, airlifts or mechanical plough systems to remove material and form the trench. This is generally done by towing the sled along using the pipeline to guide it after the pipeline is in position. The trench is progressively excavated under the pipeline, which sags into it as it is formed. Motion of the trenching barge is generally controlled by heaving in on the forward anchors and slacking away the stern anchors, which will periodically be moved by a service vessel. Diving tasks may include inspection of the pipeline and trench, including measurements where applicable, setting the sled and dealing with problems which may arise during the trenching operation.
Connections Connections may include tie-in of a pipeline to a platform riser, wellhead, manifold, or another pipeline by end or tee connection, insertion of valves, and connection to flexible hoses at single point moorings. The work may include measurement and survey of the components to be connected, welded and flanged connections, including inspection, alignment, surface preparation, fitting of gaskets and bolts, tensioning of bolts and testing. Disconnecting may also be necessary, which may involve splitting, cutting and burning work. When welding is necessary a hyperbaric welding chamber may be fitted around the pipeline so that welding can be done in an inert gaseous environment rather than wet, as this improves the weld quality. Depending on circumstances, the chamber may be large enough for the welder to work inside, but as the chamber must be accessed at depth and is under ambient pressure, the work must be done by a diver.
Pipeline inspection Pipeline inspection is done by divers or remotely operated vehicles (ROVs). Inspection requirements may be set by owners, certifying authorities, insurance companies and government departments, and there are a number of tasks which may be specified. These generally include inspection for pipeline stability, damage and fouling, and the inspection is usually recorded on video with a running commentary, and by still photography for details. Damage will be measured and the position and extent recorded. Repairs will also usually be done by divers.
Pigging A pig is a tool that fits closely into the bore of a pipe and is driven along the inside of a pipeline by a pressure differential and used to perform a specific task for which it is specially designed. Pigging is done to inspect the bore, scrape off deposits, sediment or corrosion products, or separate two different product batches. Access to the pipeline for inserting or removing pigs must be done by divers when the access point is underwater. The diver may also connect pressure hoses, open and close valves, and monitor progress of the pig past indicator stations. Pigs are designed to pass through fully opened gate and ball valves, and round bends, but the negotiable bend radius depends on the pig design.
Single point mooring systems A single point or single buoy mooring system is a facility for loading and unloading petroleum products to tankers, which moor to them only by the bow. One or two flexible hoses are used to transfer the products between the pipeline and tanker. A SPM may be used in deep water to take oil from the oilfield via a production platform, and in shallow water to export or import crude oil or oil products from an oilfield or refinery, usually by way of a storage facility. A range of single point mooring configurations are in use, and installation, maintenance and inspection work on all types is commonly done by divers. The work may include placing the pipeline end manifold (PLEM) and connecting it to the submarine pipelines, installing anchor chains and checking and adjusting chain angles, installing the submarine hoses between the PLEM and the buoy, installing a ships mooring system and operating subsea valves. The work is likely to involve the use of winches and cranes, rigging, including use of tirfors, chain hoists, strops and spreaders, flanging, using wrenches, hammers and gaskets, oxy-arc burning and welding.
Diving equipment