Hyperbaric evacuation and rescue is the emergency transportation of divers under a major decompression obligation to a place of safety where decompression can be completed at acceptable risk and in reasonable comfort.
Divers in saturation inside a diving system cannot be quickly decompressed to be evacuated in the same way as other installation personnel. The divers must be transferred to a pressurised chamber which can be detached from the installation's saturation diving system and transported to a safe location. A hyperbaric evacuation unit (HEU), also known as a hyperbaric rescue unit (HRU), with the capacity to evacuate the maximum number of divers that the diving system can accommodate, is required, with a life support system that can maintain the hyperbaric environment for at least 72 hours. After the initial evacuation, the HEU and its occupants are taken to a designated location where they can be safely decompressed to surface pressure.
The preferred way is to provide a self-propelled hyperbaric lifeboat (SPHL). Hyperbaric rescue chambers without propulsion (HRCs) are also accepted, but requirements for life support and recovery are complicated by limitations of design and configuration, and the unit must be towed clear of the evacuated installation by another vessel. Detailed guidance on hyperbaric evacuation is provided in IMCA D 052 - Guidance on hyperbaric evacuation systems.
After launching, the HEU is recovered by the standby hyperbaric rescue vessel (HRV) and transported to the standby hyperbaric reception facility (HRF), where the divers are transferred under pressure and decompressed in relative safety and comfort. In remote locations the HRF may be mounted onboard the HRV.
Another type of hyperbaric evacuation is for medical purposes, usually for a single diver, and may be done in a portable chamber for one or two occupants or a hyperbaric stretcher, The diver may be in saturation or being treated for decompression illness, so the pressure will be either the saturation pressure or treatment pressure, which is usually much lower, at about 18 msw (2.8 bar absolute), with the diver on an oxygen treatment table. The second occupant is usually a hyperbaric chamber attendant, to provide any necessary emergency medical assistance. Portable chambers may be transported by any vessel of opportunity, road transport vehicle or helicopter capable of carrying the load.
Purpose
Saturation diving is normally done from a saturation system on a diving support vessel or an offshore platform. While under saturation, the divers cannot be decompressed quickly in response to an emergency as that would be rapidly fatal, and though unusual, emergencies requiring personnel evacuation have occurred on such platforms due to extreme weather or accidents. Any attempt to evacuate saturated divers must be done at or very near their saturation storage pressure, in a hyperbaric chamber provided with life support systems which maintain the pressure and temperature, and keep the breathing gas correctly oxygenated and free of excessive carbon dioxide. A hyperbaric evacuation and rescue plan is necessary, and the equipment is normally required to be on standby during all saturation diving operations.
The International Maritime Organization's Code of Safety for Diving Systems, 1995, provides guidance and recommendations on saturation diving systems including the provision of hyperbaric evacuation facilities. The owners of the ships and installations involved in the diving operations are responsible for providing these contingency plans and equipment, which must cover irreparable damage to life support systems, explosion, fire and other damage or circumstances making it untenable for divers to remain in the saturation habitat.
The risk to personnel in a hyperbaric evacuation unit is greater than in a full saturation installation, particularly when it is floating independently in the sea, and it should be possible to delay launch until it is clear that no further delay is acceptable, and this implies that the platform or vessel should be designed and constructed to provide a high level of protection to the diving system against fire, loss of stability and critical loss of buoyancy. Hyperbaric evacuation is the last resort, but when it is no longer avoidable it must be possible to launch quickly and safely.