Surface decompression (Sur-D) is a decompression technique used primarily in military and commercial surface-supplied diving. On a Sur-D decompression schedule, the first part of a diver's decompression is completed in the water and the remainder is completed in a deck decompression chamber (DDC) after the diver surfaces.
Surface decompression requires the use of a DDC and so is beyond the scope of most recreational and public safety diving. It is mainly used during time-sensitive diving operations where divers must be cycled in and out of the water quickly, and when sea conditions are unfavorable in order to minimize the diver's time spent decompressing in the water. Surface decompression is used in both air and mixed gas (HeO2) diving operations.
Surface decompression is almost always associated with surface-supplied diving, a technique in which the compressed gas supply is transferred from the surface to the diver's life-support equipment via a flexible hose. It requires the use of a deck decompression chamber (DDC), pictured below.
A brief explanation of the process of decompression is helpful in understanding surface decompression.
Seawater weighs approximately 64 pounds per cubic foot/1024 kilograms per cubic meter. Fresh water weighs slightly less, but the two are considered equivalent when determining diving depth and calculating decompression schedules. When a diver descends in the water, the pressure around him or her increases as a function of the weight of the surrounding water. For example:
*(1 ATA = 1.01325 bar = 760 mmHg)
In order for the diver to inflate his or her lungs, breathing gas must be supplied at a pressure equivalent to the ambient water pressure. This is the function of diving equipment, whether it is self-contained underwater breathing apparatus (SCUBA) or surface-supplied.
Commercial and military dives shallower than about 200 fsw/60 msw are generally made using compressed air, which is roughly 78% nitrogen. However, nitrogen produces measurable decrements in cognitive performance beginning at a depth of about 66 fsw/20 MSW. This effect, known as nitrogen narcosis, becomes debilitating beyond approximately 200 fsw/60 MSW. Military and commercial dives deeper than that are typically made using a mixture of helium and oxygen since helium has almost no narcotic properties. Many technical recreational divers use a combination of helium, nitrogen, and oxygen (trimix) at shallower depths to help decrease the effects of nitrogen narcosis (and the considerable cost of using only helium as a diluent). Of note, though nitrogen is not chemically inert, it is often referred to by divers as an "inert" gas when comparing it to oxygen, which is an active metabolic gas.
At atmospheric pressure, the dissolved inert gas in the body is in equilibrium with that of the atmosphere. As the pressure of the diver's breathing gas increases with increasing depth, the partial pressure of inert gas in the breathing mix rises as well. This creates a positive pressure gradient between the inert gas in the lungs and the gas dissolved in the blood and body tissues. Inert gas molecules in the lungs then pass through the alveolar-capillary interface and become dissolved in the body as a function of partial pressure and time. In other words, the farther a diver descends and the longer he or she stays at depth, the more inert gas becomes dissolved in the blood and body tissues.
As a diver ascends toward the surface, the inert gas pressure in the lungs decreases, and the pressure gradient between the lungs and the body equilibrates and then reverses. When the partial pressure of the dissolved inert gas in the body is higher than the partial pressure of inert gas in the lungs, the tissues become supersaturated. The gas molecules in the body then pass through the alveolar-capillary membrane into the lungs and are exhaled. This is a simplified description of the process known as decompression. There are detailed decompression algorithms and techniques designed to control this process and allow the diver to return safely to the surface; surface decompression is one of these techniques.
Body tissues, including blood, will tolerate some supersaturation. However, excess supersaturation will result in bubble formation within the tissues. Some bubble formation after long, deep dives is normal and tolerated; however, if bubbles form in sufficient numbers to impede circulation or displace tissue, the diver may feel symptoms of decompression sickness.
A diver on a surface decompression (sur-D) schedule accomplishes part of his or her decompression in the water. This usually includes timed stops at successively shallower depths. Surface decompression schedules plan for enough in-water decompression to prevent the development of decompression sickness (DCS) symptoms during the surface interval (the time spent on the surface before recompression in the DDC). Since some of the diver's tissues are intolerably supersaturated after the in-water phase of decompression, the diver must be transferred to the DDC quickly once he or she reaches the surface to avoid decompression sickness. For example, the United States (US) Navy's surface decompression procedures stipulate that it should take no more than 5 minutes for a diver to ascend from the last water decompression stop at 40 feet, board the diving vessel, undress, and be compressed to 50 feet in the DDC. The remainder of the diver's decompression takes place at the pressure in the DDC, typically breathing oxygen to facilitate inert gas washout.
In practice, this requires a well-trained and experienced crew. Typically, one or two diver tenders will assist the diver in removing the diving apparatus and exposure suit. In commercial diving operations, the inner lock of the DDC is often pressurized beforehand to a predetermined depth deeper than the planned chamber decompression depth. The diver enters the outer lock after undressing, the outer lock door is held closed, and the diver opens the crossover valve (see photo below) to equalize the pressure between the inner and outer locks. If this is done correctly, the two locks will equilibrate at the planned decompression depth. The diver then enters the inner lock (see photo below), begins breathing oxygen through the built-in breathing system (BIBS) mask, and closes the inner lock door. The DDC operator then surfaces the outer lock via the external exhaust valve. This procedure allows the diver to be pressurized to decompression depth in the DDC even if the air supply to the DDC is inadvertently lost during the surface interval.
Alternately, the diver will enter the inner lock after undressing, close the inner lock door, and the DDC operator will pressurize the inner lock to the decompression depth. This is the technique used by the US Navy.
If the surface interval is exceeded, sur-D procedures usually call for the decompression time in the chamber to be extended, which may slow diving operations. Diving in contaminated water can increase the surface interval as the diver must be decontaminated before entering the chamber. If the diver develops decompression sickness during the surface interval or in the chamber, he or she must be treated in the DDC on an established DCS treatment protocol, which may slow or stop diving operations depending on company policy and availability of another DDC on the job site.