Saturation diving:

 

Saturation diving is one way to reduce the overall risk to the diver while improving productivity. It is a very expensive, fairly hazardous and high-tech way of controlling risk economically. This apparent contradiction can be explained by the amount of time the diver can spend working productively for the time spent decompressing.

The time required for decompression depends on the exposure pressure and duration, but it reaches a maximum for a given depth when the diver is saturated with inert gas at that depth.

During decompression there is a risk of decompression sickness, which is, as a general rule, reduced by decompressing more slowly. In-water decompression can only be tolerated for relatively short periods, as it exposes the diver to other hazards, some of them proportional to the duration, so decompression in a dry chamber is preferred. Preferably the chamber can be removed from the water during decompression, for further reduction of exposure to hazards, so the chamber must be pressurised. This chamber should be reasonably small to keep down the cost of deployment, so it is an advantage to transfer the divers into a more spacious and comfortable chamber on the surface platform, which also allows the bell to be used for the next shift while the first divers are decompressing.

The procedure described so far is known as bell bounce diving, and it is used for work where the amount of time spent at depth is relatively short. When the time spent decompressing would exceed the time between shifts, the diver would be more profitably employed underwater, and the time in the chamber would be less risky if the diver was not being decompressed, so a larger set of chambers can be used, in which the divers spend off-shift time under the same pressure they will experience at the underwater worksite. At the end of the job they are all decompressed together slowly, but the total time in decompression is reduced. This is cost-effective and puts the divers at less risk of decompression sickness than bounce diving for the same amount of time at the worksite.

The personal diving equipment used by saturation divers is similar to that used by surface supplied divers, with the addition of the closed bell and saturation system. The longer deeper dives and helium based breathing gases expose the saturation diver to greater heat loss, so hot-water suits are more likely to be used, and the cost of the helium makes it more likely that breathing gas reclaim systems will be used. These are systems where the exhaled gas is piped back to the surface for recycling.

Diving support vessels

Offshore diving may be from a stationary platform or a diving support vessel. Most diving is from moored or anchored vessels as live-boat diving exposes the diver to additional hazards of thrusters and propellers. Special precautions are needed when diving from dynamically positioned vessels. The size of the vessel may range from small boats capable of supporting a dive team using scuba replacement to large support vessels with full saturation systems, launch and recovery systems and heavy lifting gear.

Moonpools

Some diving support vessels have an opening through the bottom of the hull called a moonpool to facilitate diver deployment. This is usually the part of the vessel with the least vertical motion in a seaway, which makes launch and recovery of the bell or stage easier, safer and more comfortable for the divers.

Diving from dynamically positioned vessels

Dynamically positioned vessels (DPVs) are vessels which can hold a position and heading by computer-controlled operation of thrusters and propellers. In many cases this can be done sufficiently precisely to use as a platform for diving operations, but there are specific hazards associated with this mode of diving. The vessel control system monitors its position by reference systems which may include taut wire, radar surface stations (Artemis), Seabed acoustic transponders (Hydracoustic Position Reference) or Differential Global Positioning System, using satellites and a terrestrial base station. International Marine Contractors Association (IMCA) guidance requires at least three independent referencing systems of at least two different types for DPV diving operations, to minimise the risk of loss of position. The DP footprint is the scope of movement of the vessel, and it is limited for safety of the divers. Three alert levels are provided to the diving team to indicate the current capacity of the vessel to maintain position. Green indicates normal status, where diving work can be done, yellow indicates partly degraded status, where the divers would be recalled to the bell, and red indicates emergency status, where the dive would be aborted. The particular hazards of DPV diving include loss of position and thruster hazards. Special precautions are taken to prevent divers from getting into the danger zones of thrusters and propellers. These include limiting umbilical length, and other physical restraints.

Management and control of diving operations

Offshore diving practices are basically similar in principle to inshore diving practices, but are extended to include practices specific to the equipment and environment.

Personnel

The usual commercial diving management system of having a diving supervisor of appropriate competence in direct and immediate control of a diving operation is also standard for offshore work. IMCA has a system for certification of offshore air and saturation diving supervisors, which is recognised and used by all signatory contractors. This system is fairly representative of most offshore diving operations, but details may differ.

A major diving project or offshore installation may also have a diving superintendent on staff. The diving superintendent is usually a senior diving supervisor appointed by the diving contractor and is responsible for the overall planning and conduct of diving work, and will be responsible for allocating a diving supervisor for each diving operation.

A saturation system will be managed by a Life Support Supervisor and operated by Life Support Technicians (LSTs), and there will usually be one or more Diving Medical Technicians(DMTs)on site, and an off-site standby contract with a suitably rated Diving Medical Practitioner, who is trained in diving medicine and able to advise on treatment under hyperbaric conditions.

Diving team

The diving team will include at least one working diver and at least one standby diver, a diving supervisor and a tender for each diver. Other personnel may be needed to operate special equipment like winches and a bell launch and recovery system, and to operate cranes and other equipment related to the work to be done. If the divers are deployed using a diving bell, the standby diver stays in the bell and is called the bellman. The bellman acts as tender for the working diver's umbilical, but must tend his own umbilical during a rescue. The working diver and bellman may swap functions during a shift to give the diver a break. A standard practice is for the standby diver's umbilical to be about 2m longer than the working diver's umbilical, to ensure that the standby diver can reach the diver in an emergency.

Saturation dives

Saturation divers will live under pressure in the saturation system between dives. They are pressurised at the beginning of a tour of duty and remain under storage pressure at as close as reasonably practicable to the working depth until they are decompressed at the end of the tour, which may take up to two weeks, depending on the storage pressure. Excursions to deeper and shallower working depths are carefully planned and controlled to minimise the risk of decompression sickness. Limited excursions may be possible without special decompression, but larger excursions may require part of the saturation system to be isolated for additional decompression, or if short, it can be done in the bell. The bell can be locked onto the saturation system, and the divers transfer from the saturation system living quarters to the bell under pressure. When the bell reaches working depth the bottom lock is opened and the divers get out and back in through it. Before surfacing the bell the lock is closed and sealed to maintain internal pressure, and the divers transfer back to the saturation system living quarters under pressure.

Bounce dives

Surface oriented dives are those in which the diver is not under saturation. These are also referred to as bounce dives, and the divers may be deployed using a diving stage, wet bell or closed bell, or for shallow dives directly from the vessel or platform, depending on what water access is available. Launch and recovery systems (LARS) are used to lower the stage or bell and to lift it out after the dive.

Risk management

Once risks have been identified and assessed, all techniques to manage the risk fall into one or more of these four major categories:

Avoidance (eliminate, withdraw from or not become involved)

Reduction (optimize – mitigate)

Sharing (transfer – outsource or insure)

Retention (accept and budget)

Ideal use of these strategies may not be possible. Some of them may involve trade-offs that are not acceptable to the organization or person making the risk management decisions.

Health and safety

Offshore diving generally takes place at remote sites, and emergency medical facilities may be far away, so it is common to include relatively complex and expensive emergency facilities and personnel on site. The actual diving work is usually done by one or two divers, backed up by a team of support personnel, both to facilitate getting the work done, and to provide an acceptably low level of risk for the diver and other affected personnel. Offshore diving operations are expensive and inherently hazardous, so extensive planning and effective management are necessary to control risk and ensure that the necessary work is done effectively.

Legislation and codes of practice

National regulations

The Approved Code of Practice and guidance for Commercial diving projects offshore published by the HSE provides guidance on compliance with the UK Diving at Work Regulations 1997

IMCA guidance[

IMCA members are obliged to comply with IMCA guidance in their diving operations. This is guidance is provided in a group of documents detailing industry recognised good practice for various aspects of offshore diving, including:

IMCA D 006 Diving operations in the vicinity of pipelines

IMCA D 010 Diving operations from vessels operating in dynamically positioned mode

IMCA D 014 IMCA international code of practice for offshore diving

IMCA D 018 Code of practice for the initial and periodic examination, testing and certification of diving plant and equipment

IMCA D 019 Diving operations in support of intervention on wellheads and subsea facilities

IMCA D 021 Diving in contaminated waters

IMCA D 022 Guidance for diving supervisors

IMCA D 025 Evacuation of divers from installations

IMCA D 030 Surface supplied mixed gas diving operations

IMCA D 034 Norway/UK Regulatory Guidance on Offshore Diving (NURGOD)

IMCA D 042 Diver and ROV based concrete mattress handling, deployment, installation, repositioning and decommissioning

IMCA D 052 Guidance on hyperbaric evacuation systems

IMCA D 054 Remotely operated vehicle intervention during diving operations

Hazards

A hazard is any agent or situation that poses a level of threat to life, health, property, or environment. Most hazards remain dormant or potential, with only a theoretical risk of harm, and when a hazard becomes active, and produces undesirable consequences, it is called an incident and may culminate in an emergency or accident.

Divers face specific physical and health risks when they go underwater or use high pressure breathing gas. When a diver enters the water there is inherently a risk of drowning, and breathing while exposed to pressure imposes a risk of barotrauma and decompression sickness.

There are some hazards which are more common in the offshore environment and in offshore diving operations. There is more diving at extreme depths than in other applications, and the solutions to this bring their own hazards. In order to reduce the risks of compression arthralgia and decompression sickness, saturation divers decompress only once at the end of a tour of duty, but this introduces hazards associated with living under pressure and requiring a long decompression schedule. Helium gas is used in breathing mixtures to reduce work of breathing and nitrogen narcosis, which would make deep diving work difficult or impossible, but the consequences include accelerated heat loss and higher risk of hypothermia, so hot-water suits are used for active warming, but they introduce a risk of heat injuries if something goes wrong with the temperature control system.

Work on oilfields may result in exposure to crude oil and natural gas components, some of which (such as hydrogen sulphide) can be highly toxic.

Much of the diving work involves moving and handling large and heavy objects, and inherently hazardous tools and equipment. These hazards are usually aggravated by the underwater environment.

The inherent problems with offshore evacuation in emergencies like fire or sinking, which are problematic for ordinary crew, are much more difficult to deal with for divers in saturation. The methods of controlling the risks due to these hazards are usually engineering solutions, and are expensive, and often introduce secondary hazards which must also be managed.

Risk[edit]

Hazard and vulnerability interact with likelihood of occurrence to create risk, which can be the probability of a specific undesirable consequence of a specific hazard, or the combined probability of undesirable consequences of all the hazards of a specific activity.

The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident.

The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations are constrained by occupational health and safety legislation, but also by the physical realities of the operating environment, and expensive engineering solutions are often necessary to control risk.

Assessed risk

Risk assessment is the determination of an estimate of risk related to a well-defined situation and a recognized set of hazards. Quantitative risk assessment requires calculations of two components of risk : the magnitude of the potential loss, and the probability that the loss will occur. An acceptable risk is a risk that is understood and tolerated, usually because the cost or difficulty of implementing an effective countermeasure for the associated vulnerability exceeds the expectation of loss.

A formal hazard identification and risk assessment is a standard and required part of the planning for a commercial diving operation, and this is also the case for offshore diving operations. The occupation is inherently hazardous, and great effort and expense are routinely incurred to keep the risk within an acceptable range. The standard methods of reducing risk are followed where possible.

Statistical risk

Statistics on injuries related to commercial diving are normally collected by national regulators. In the UK the Health and Safety Executive (HSE) is responsible for the overview of about 5,000 commercial divers, and in Norway the corresponding authority is the Petroleum Safety Authority Norway (PSA), which has maintained the DSYS databse since 1985, gathering statistics on over 50,000 diver-hours of commercial activity per year.

In 2013 the UK HSE reported a fatal accident rate for commercial offshore and inland/inshore diving of typically 20–40 per 100,000 workers per year. That is much more than the rate found in construction or agricultural activities and results in diving being classified as "high hazard" by the HSE.

According to a 2011 report to PSA, the last recorded saturation diving fatality in Norway occurred in 1987, and few serious incidents happened over the preceding 25 years. In 2010 there were two reported incidents leading to injuries.