Diving support vessel

CSV Skandi Singapore departing Fremantle, Australia

A diving support vessel is a ship that is used as a floating base for professional diving projects.

History

Commercial diving support vessels emerged during the 1960s and 1970s, when the need arose for offshore diving operations to be performed below and around oil production platforms and associated installations in open water in the North Sea and Gulf of Mexico. Until that point, most diving operations were from mobile oil drilling platforms, pipe-lay, or crane barges. The diving system tended to be modularised and craned on and off the vessels as a package.

As permanent oil and gas production platforms emerged, the owners and operators were not keen to give over valuable deck space to diving systems because after they came on-line the expectation of continuing diving operations was low.

However, equipment fails or gets damaged, and there was a regular if not continuous need for diving operations in and around oil fields. The solution was to put diving packages on ships. Initially these tended to be oilfield supply ships or fishing vessels; however, keeping this kind of ship 'on station', particularly during uncertain weather, made the diving dangerous, problematic and seasonal. Furthermore, seabed operations usually entailed the raising and lowering of heavy equipment, and most such vessels were not equipped for this task.

This is when the dedicated commercial diving support vessel emerged. These were often built from scratch or heavily converted pipe carriers or other utility ships. The key components of the diving support vessel are:

• Dynamic Positioning – Controlled by a computer with input from position reference systems (DGPS, Transponders, Light Taut Wires or RadaScan), it will maintain the ship's position over a dive site by using multi-directional thrusters, other sensors would compensate for swell, tide and prevailing wind.
• Saturation diving system – For diving operations below 50 m, a mixture of helium and oxygen (heliox) is required to eliminate the narcotic effect of nitrogen under pressure. For extended diving operations at depth, saturation diving is the preferred approach. A saturation system would be installed within the ship. A diving bell would transport the divers between the saturation system and the work site lowered through a 'moon pool' in the bottom of the ship, usually with a support structure 'cursor' to support the diving bell through the turbulent waters near the surface. There are a number of support systems for the saturation system on a diving support vessel, usually including a remotely operated vehicle ROV and heavy lifting equipment.

Modern diving support vessels

The 2015 launched DSV Curtis Marshall

Gulmar Da Vinci in Albert Dock

The Skandi Arctic supply vessel in Leith docks

Most of the vessels currently in the North Sea have been built in the 1980s. The semi-submersible fleet, the Uncle John and similar, have proven to be too expensive to maintain and too slow to move between fields. Therefore, most existing designs are monohull vessels with either a one or a twin bell dive system. There has been little innovation since the 1980s. However, driven by high oil prices since 2004, the market for subsea developments in the North Sea has grown significantly. This has led to a scarcity of diving support vessels and has driven the price up. Thus, contractors have ordered a number of newbuild vessels which are expected to enter the market in 2008.

More recent vessels are designed and built to support both diving activities and remotely operated vehicles (ROVs) operations with dedicated hangar and LARS for ROV's, and to support seismic survey operations and cable-laying operations. They may have carry 80 to 150 project personnel on board, including divers, diving supervisors and superintendents, dive technicians, life support technicians and supervisors, ROV pilots, ROV superintendents, survey team, clients personnel, etc. For all these personnel to carry out their contracted job with an oil and gas company, a professional crew navigate and operate the vessel according to the contract requirements and instructions of project superintendents. However, ultimate responsibility lies on the master of the vessel for the safety of every person on board. In expanding the utility of the vessel, these vessels provide in addition to the usual domestic facilities. specialised diving mixed gas compressors and reclaim systems, gas storage and blending facilities, and saturation diving accommodation systems where the divers live under compression. These vessels are available to be hired by diving contractors or directly by oil and gas contractors who then will subcontract a specialist service-provider to use the vessel as a platform to carry out their activities.

Special features

Dynamic positioning

Main article: Dynamic positioning

Dynamic positioning (DP) is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyrocompasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position.

Moon pool

Main article: Moon pool

A moon pool is an opening in the base of the hull, giving access to the water below, which allows divers, diving bells, remotely operated underwater vehicles or other equipment to enter or leave the water easily and in a relatively protected environment.

Saturation accommodation

See also: Saturation diving § Architecture of a surface saturation facility

Bell launch and recovery system

See also: Diving bell § Deployment of a modern diving bell

Diving bells are deployed over the side of the vessel or platform using a gantry or A-frame from which the clump weight and the bell are suspended. On dive support vessels with in-built saturation systems the bell may be deployed through a moon pool. The bell handling system is also known as the launch and recovery system (LARS).

Is is also used to move the bell from the position where it is locked on to the chamber system into the water, lower it to the working depth and hold it at that depth without excessive movement, and recover it to the chamber system. The system used to transfer the bell on deck may be a deck trolley system, an overhead gantry or a swinging A-frame. The system must constrain movement of the supported bell sufficiently to allow accurate location on the chamber trunking even in bad weather. A bell cursor may be used to control movement through and above the splash zone, and heave compensation gear may be used to limit vertical movement when in the water and clear of the cursor, particularly at working depth when the diver may be locked out and the bell is open to ambient pressure.

Diving from a DSV

Diving from a DSV makes a wider range of operations possible, but the platform presents some inherent hazards, and equipment and procedures must be adopted to manage these hazards as well as the hazards of the environment and diving tasks.

Hazards

• Hazards of the positioning system
  • Anchor patterns
  • Thrusters

Equipment

See also: Surface supplied diving § Equipment, and Saturation diving § Auxiliary and support equipment

• On board recompression facilities
• Equipment to transport the diver through high risk zones
• Equipment to limit access to known hazards
• Hyperbaric evacuation facilities

Procedures

See also: Surface-supplied diving § Diving procedures, and Saturation diving § Operating procedures

• Use of stages and bells to transport the diver through the interface between air and water, to avoid hazards, and for decompression.
• Surface-supplied diving with limited umbilical length
• Underwater umbilical tending

See also

• Dive boat – Boat used for the support of scuba diving operations
• Commercial offshore diving – Professional diving in support of the oil and gas industry
• Saturation diving – Diving decompression technique
• Professional diving – Underwater diving where divers are paid for their work
• Dynamic positioning – Automatic ship station- and heading-holding systems
• Diving bell – Chamber for transporting divers vertically through the water