Dive planning
Dive planning is the process of planning an underwater diving operation. The purpose of dive planning is to increase the probability that a dive will be completed safely and the goals achieved. Some form of planning is done for most underwater dives, but the complexity and detail considered may vary enormously. Professional diving operations are usually formally planned and the plan documented as a legal record that due diligence has been done for health and safety purposes. Recreational dive planning may be less formal, but for complex technical dives, can be as formal, detailed and extensive as most  professional dive plans. A professional diving contractor will be constrained by the code of practice, standing orders or regulatory legislation covering a project or specific operations within a project, and is responsible for ensuring that the scope of work to be done is within the scope of the rules relevant to that work. A recreational (including technical) diver or dive group is generally less constrained, but nevertheless is almost always restricted by some legislation, and often also the rules of the organisations to which the divers are affiliated. The planning of a diving operation may be simple or complex. In  some cases the processes may have to be repeated several times before a satisfactory plan is achieved, and even then the plan may have to be modified on site to suit changed circumstances. The final product of the planning process may be formally documented or, in the case of recreational divers, an agreement on how the dive will be conducted. A diving project may consist of a number of related diving operations. A documented dive plan may contain elements from the following list: Objective. Commercial diving contractors will develop specifications for the operation in cooperation with the client, who will  normally provide a specific objective. The client will generally specify what work is to be done, and the diving contractor will deal with the logistics of how to do it. Other professional divers will usually plan their diving operations around an objective related to their primary occupation. Recreational divers will generally choose an objective for entertainment value, or for training purposes. It will generally be necessary to specify the following: Selection of techniques and mode of diving. Detailed planning depends on the mode and techniques selected for the dive, and the choice of these depends to a large extent on  the physical constraints of the dive, but also to the legal, financial and procedural constraints of the divers. The mode and techniques chosen must also allow the dive to be done at an acceptable level of risk. There is usually more than one mode which is physically feasible, and often a choice between modes which are otherwise acceptable. In some cases detailed planning may show that the initial choice was not appropriate, and the process has to be repeated for an alternative choice. Diving without breathing apparatus. Free diving does not involve the use of external breathing devices, but relies  on a diver's ability to hold his or her breath until resurfacing. Free diving is limited in depth and time, but for some purposes it may be suitable. Scuba diving. Diving with a self-contained underwater breathing apparatus, which is completely independent of surface supply, provides the diver with the advantages of mobility and horizontal range far beyond what is possible when supplied from the surface by the umbilical hoses of surface-supplied diving equipment. Scuba has limitations of breathing gas supply, communications between diver and surface are problematic, the location of the diver may be difficult to monitor , and it is considered a higher-risk mode of diving in most circumstances. Scuba is specifically forbidden for some professional applications. Decompression is often avoided, and if necessary, is generally in-water, but may use a variety of gases. Open circuit. Open-circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure connected to a primary diving regulator, and may include additional cylinders for decompression gas or emergency breathing gas. Rebreathers. Closed-circuit or semi-closed circuit rebreather systems allow recycling of  exhaled gases. This reduces the volume of gas used, so that a smaller cylinder, or cylinders, than open-circuit scuba may be used for the equivalent dive duration, and giving the ability to spend far more time underwater compared to open circuit for the same gas consumption. Rebreathers also produce far less bubble volume and less noise than scuba, which makes them attractive to military, scientific and media divers. They also have a larger number of critical failure modes, are more expensive and require more training to use at a reasonable level of safety. Surface supplied diving. Breathing gases may  be supplied from the surface through a diver's umbilical, or airline hose, which provides breathing gas, communications and a safety line, with options for a hot water hose for heating, a video cable and gas reclaim line. The diver's breathing gas supply is significantly more secure than for scuba; communications are simplified and the divers position is either known or can be traced reliably by following the umbilical. Several major risks are thereby mitigated, but the system also has serious disadvantages in some applications, as diver mobility is constrained by the length of the umbilical, and it may  snag on obstructions. Surface-oriented, or bounce diving, is how commercial divers refer to diving operations where the diver starts and finishes the diving operation at atmospheric pressure. The alternative, while retaining surface supply, is saturation diving. For bounce dives, the diver may be deployed directly, often from a diving support vessel or indirectly via a diving bell. Decompression procedures include in-water decompression or surface decompression in a deck chamber. Small closed bell systems which include a two-man bell, a launch and recovery frame and a chamber for decompression after transfer under pressure (TUP) are reasonably mobile, and  suited to deep bounce dives. Saturation diving. Saturation diving lets divers live and work at depth for days or weeks at a time. After working in the water, divers are transferred in a closed diving bell to rest and live in a dry pressurized underwater habitat on the bottom or a saturation life support system of pressure chambers at the surface. Decompression at the end of the dive may take many days, but since it is done only once for a long period of exposure, rather than after each of many shorter exposures, the overall risk of decompression injury to  the diver and the total time spent decompressing are reduced. This type of diving allows greater economy of work and enhanced safety, but the capital and running costs are high and the systems are expensive to transport. Mobility of the diver is restricted because of the umbilical. Atmospheric diving suits. Atmospheric diving suits can be used for very deep dives of up to for many hours, and eliminate several physiological dangers associated with deep diving: the occupant need not decompress; there is no need for special gas mixtures; and there is no danger of decompression sickness or nitrogen narcosis. Disadvantages  include high cost, limited availability, bulk and limited diver dexterity. Diving team selection. The diving team personnel selection will depend largely on the diving mode selected and organisational requirements. Professional dive team members will generally be selected on documented evidence of proven competence or qualification for the tasks allocated. Professional diving teams will usually include (the precise terminology may vary between organisations): Technical teams will also generally base appointments on proven competence, certification or personal trust. Technical diving groups vary in complexity, but will generally comprise: Recreational groupings may be based on personal experience and trust, but are frequently relatively  arbitrary allocations by the service provider, based on certification. Recreational diving groups commonly comprise a buddy pair of divers, but may also be a solo diver or a group of divers who will be led by a divemaster. Selection may be by mutual agreement to dive together, or may simply be the result of booking on the same dive. Depth and time. Depth is often one of the more straightforward parameters, as it is often fixed by the topography of the site. Time is influenced by limitations of equipment and decompression constraints, as well as the actual time required to  perform the intended task, which in turn is influenced by the underwater environment in general, and specific to the site. Environmental factors. The specific diving environment must be taken into account during dive planning. The environment at the dive site will determine several factors which may require specific planning, such as the depth, water salinity and altitude which affect decompression planning, an overhead environment affects navigation and gas planning, water temperature and contaminants affect the choice of exposure and environmental protection, site topography affects choice of entry and exit points. Hazards of the proposed diving operation. Divers face specific physical  and health risks when they go underwater with diving equipment, or use high pressure breathing gas. A hazard is any biological, chemical, physical, mechanical or environmental agent or situation that poses a level of threat to life, health, property, or environment. 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. Diving hazards may be classified under several groups: Risk assessment. 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. Professional diving organisations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation. Dive profile. The planned dive profile is an important input parameter for gas planning and decompression planning, and is generally based on the time required to perform the task of each specific dive, and the depth at which the task will be performed, in combination with environmental  considerations and the breathing gas mixtures chosen. Limits are often due to exposure to cold, work load, decompression time, safety constraints and logistics of breathing gas supply. Route. For some dives the route to be followed and navigation procedures to follow the planned route may be important, either for achieving the objective, for safety, or for both. There may be known hazards that can be avoided by following a specific route or constraining the possible extent of diver excursion. In all penetration dives the route may be critical for safety. The diver must be assured of getting out from the  overhead zone before running out of gas. The standard method is to follow a guideline into and out of the overhead environment, and laying the line or laying and recovering the line may be part of the dive plan. In explorations and surveys the route may be unknown or uncertain, and contingency plans must be known to the divers so that the dive plan can be altered to suit the situation as it unfolds. Professional divers may follow a planned route to the worksite which prevents the diver from close approach to known hazards. This may involve limiting umbilical length  and manned or unmanned underwater tending points, downlines and jackstays Choice of equipment. Equipment will be chosen based on several constraints, including: Equipment and supplies selection would normally include: A recreational diver may expect many of these items to be arranged by the service provider (the dive boat operator, shop, or school providing thansport to the dive site and organising the dive). Decompression planning. Decompression is planned based on the intended dive profile, the chosen gas mixtures, and the chosen decompression tables or algorithms. There are two basic approaches to decompression for surface oriented dives, and one for saturation diving . The procedures chosen will to a large extent depend on the mode of diving and equipment available. Gas planning. Gas planning for diving operations where divers use open circuit equipment with breathing gas mixtures is more complex than operations where atmospheric air is supplied via low pressure compressor from the surface, or the breathing gas is reclaimed, processed and re-used. Scuba gas planning. Scuba gas planning is the aspect of dive planning which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive profile, and can be critical to  the safety of the dive. The scuba diver by definition is independent of surface supply and, in general, must carry all gas needed for the dive, though in limited circumstances depots of drop cylinders may be placed along the route of the dive for use on the return. This requires the route to be marked and the divers to return along the marked route, and is particularly suited to penetration dives, such as wreck and cave dives. Deep dives with open water ascents can also occasionally make use of surface standby divers who can provide contingency gas to ascending divers  whose position is marked by a shotline or decompression buoys. The calculations assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Scuba gas planning includes the following aspects: Surface supplied gas planning. Open circuit surface supplied diving mostly uses air as the breathing gas, though mixed gases may also be used. Surface supplied air is generally supplied by low pressure compressor, and the continuous supply is limited only by the compressor continuing to  run effectively, and to provide air of suitable quality. There is also a reserve air supply, either from a second compressor, or from fairly large high pressure cylinders. Each diver also carries a scuba bailout cylinder, which should carry sufficient gas to safely surface from any point in the planned dive. Running out of air is a relatively low risk with these facilities, and gas planning centres on ensuring that the primary and, if present, backup compressors are correctly sized to provide the necessary pressure and flow rates. These are specified by the breathing equipment manufacturer based on depth and  workload, and by the compressor manufacturer for the standard running speed of the machine. Reserve surface supply cylinder contents are based on the gas requirement for safe ascent from any part of the dive, allowing for reasonably foreseeable delays, and for a rescue by the standby diver. The diver's bailout cylinder should contain adequate gas in case of an emergency at the planned depth. Critical pressure should be calculated based on the planned profile and must allow change-over, ascent and all planned decompression. In some jurisdictions the stand-by diver must be supplied from an air source which  is independent of that supplying the working divers, as the cause of an emergency may be failure or contamination of the main air supply to the working diver. Low pressure compressor delivery. Compressors are rated according to the volume of air taken in each minute. This is also the free gas volume that will be supplied to the divers. The volume of air used by the divers will depend on work rate and depth. Short term variations are compensated by the air receiver on the compressor. The delivery volume at maximum ambient pressure for the planned dive must be sufficient  for all the divers to be supplied from the compressor. The supply pressure must be in excess of minimum functional pressure for the regulator to be enough to get air to the diver. In practice a delivery pressure of about 20 bar is commonly used. The manufacturer of the helmet or full-face mask will specify a pressure range which will deliver sufficient air for a given dive depth, which is usually from 6 to 10 bar more than the ambient pressure due to depth. Free flow helmets. Free flow helmets generally require a considerably higher compressor delivery than demand  helmets, as the flow is continuous, and should never drop below peak inhalation rate of the diver. Flow rates up to 1500 litres per minute surface equivalent are quoted for the Divex AH-5 helmet at 50 metres sea water for heavy work. Delivery pressure at the AH-5 helmet is recommended at 3.5 bar above ambient. Saturation gas planning. Saturation systems frequently use gas reclaim equipment to minimize the loss of expensive helium, and this makes the gas usage relatively independent of dive duration and depth, however reserves must be available in case of loss or leakage. Scrubber systems are used  to remove carbon dioxide from the breathing gas, and other filters to remove odours and other contaminants. Booster pump systems are used to return gas to high pressure storage. Contingency plans. Contingency planning covers what to do if something happenes that is not according to the planned operation. The hazard identification and risk assessment will suggest the range of foreseeable contingencies, and the specifics of how much to organise to deal with them will depend on the consequences. In general, contingencies that have serious health and safety consequences should have plans in place to deal with them, while those which  are merely an inconvenience may be accepted if they occur. Some contingency classes are listed here: Plans for technical contingencies may include arrangements for alternative equipment, spares, alternative boat etc. The level of contingency planning will depend on the project, and the importance of the task. Plans for adverse conditions may include arrangements for alternative dates, or in some cases alternative venues. Emergency plans. In general, there should be plans to deal with reasonably foreseeable emergencies that pose a risk to health and safety wherever there is a duty of care, these may include where relevant: Some of the action  generally taken to prepare for possible medical emergencies will include: Permits and permission. It may be necessary to arrange for clearance to dive. Permits or permission for access or to dive at the site may be required, and making the arrangements can be considered part of dive planning. This may include, but is not limited to: Budget. Estimating the cost of a diving operation is an important component of the planning process, and is dependent on almost all the factors described above.