Diving Support Vessels (DSV’s), also known as Subsea Vessels, are vessels that carry out a wide range of subsea activities typically through the deployment of divers. DSV’s are most commonly used to carryout inspection repair and maintenance (IRM) on subsea pipelines and other infrastructure and to support the installation of subsea infrastructure and pipelines. DSV’s may also be modified to conduct cable-lay and/or well intervention activities.
Diving Support Vessels are built with many of the features seen on Multi-Purpose Support Vessels such as Dynamic Positioning System (II or III), large accommodation, helideck, Remote Operated Vehicles (ROV), and subsea active heave compensated crane (pedestal or knuckle boom) however also integrates a saturation diving system.
Cranes: Most DSV’s come with large capacity subsea cranes capable of handling loads in deep water (over 3000m in some instances) and at weights ranging from 100t to over 400t. Typically cranes are active heave compensated (AHC) meaning that the control systems of the crane are capable of keeping the load motionless irrespective of movements of the vessel.
Air Diving Systems: Most DSV’s will have permanent air or mixed gas diving system. Air or mixed gas diving is used at depths typically no greater than 50m depending of the mix of gas used. At any greater depth than 50m saturation diving is preferred to avoid the risks of decompression sickness and allow divers to work at depth for longer. Air or mixed gas diving relies on the supply of breathing gas from the surface. Gases used may be oxygen, or a mixture of oxygen and nitrogen (Nitrox) or oxygen and helium (Heliox). Oxygen and nitrox are used at shallower depths (50m) with heliox and provide a bottom time of no more than 50 minutes at such depth. Likewise, Heliox may be used down to 75m however with only 30 minutes bottom time. Depending on the working depth divers may be lowered to depth in a basket or a wet bell.
Typically Air or Nitrox diving requires a team of 5 (Diving Supervisor, working diver, stand-by diver, tender for working diver, tender for stand-by diver), whereas a team of 7 is required in Heliox dives (Diving Supervisor, working diver, stand-by diver, tender for working diver, tender for standby diver, short notice surface standby and tender)
Saturation Diving Systems: All DSV’s will have an integrated SAT diving system capable of accommodating as many as 24 people. The main component of a SAT diving system are:
- Personnel Transfer Capsule - The PTC is a spherical, submersible pressure vessel that can transfer divers in full diving dress, along with work tools and associated operating equipment, from the deck of the surface platform to their designated working depth.
- Deck Decompression Chamber (DDC) - The DDC furnishes a dry environment for accomplishing decompression and, if necessary, recompression. The DDC is a multi-compartment, horizontal pressure vessel mounted on the surface-support platform. Each DDC is equipped with living, sanitary, and resting facilities for the dive team. A service lock provides for the passage of food, medical supplies, and other articles between the diving crew inside the chamber and topside support personnel.
- PTC Handling Systems. Of all the elements of DDS, none are more varied than PTC handling systems. Launch and retrieval of the PTC present significant hazards to the divers during heavy weather and are major factors in configuring and operating the handling system.
SAT diving enables divers to live and work at depths greater than 50m for days or even weeks at a time. This allows for a greater economy of work and enhanced safety for the divers, since, after working in the water, the divers can rest and live in a dry pressurized habitat onboard the DSV at the same pressure as the work depth. A mixture of helium and oxygen (Heliox) is typically used for breathing as such removes the risk of illness and allows divers to remain in a saturated state for as long as 28 days and operate at depth to 600m
Team size consists of a minimum of two Diving Supervisors, one Life Support Supervisor, 1 Life Support Technician, two divers, two stand-by divers on the surface that are saturation qualified, and a dive technician.
Modular saturation diving systems may be used onboard the main deck of a vessel, such as a Multi-Purpose Support Vessel. This mode of the diving operation is more weather-sensitive as the diving bell is deployed over the side of the vessel versus an integrated diving spread which deploys the diving bell from the centre line of the vessel hull.
Remote Operate Vehicle (ROV): All DSV with come complete with at least one work class ROV and may even have two. Work class ROV’s large enough to carry sensors and/or manipulators. Larger work class ROV’s also often have multiplexing capability that allows additional sensors and tools to operate without being “hardwired” through the umbilical system. Depending on the type the size of ROV some may be rated to work in up to 4000m of water. Other common equipment used in conjunction with the ROV are:
- Launch and Recovery System (LARS) – a piece of deck machinery (A-Frame or other) that is used to safely place the ROV into the water.
- Tether Management System (TMS) – a piece of subsea equipment(top hat or side entry cage) that is lowered with the ROV from which the ROV can “fly” away from to work. Instead of being connected directly to the DSV, operating via a TMS allows the ROV to move without impact from forces placed on the vessel or the long length of umbilical to/from the DSV.