Selecting the appropriate diving method is essential to any diving operations planning.
The method will dictate many aspects of an operation including personnel
and equipment.
Mixed-Gas Diving Methods. Mixed-gas diving methods are defined by the type of
mixed-gas diving equipment that will be used. The three types of mixed-gas
diving equipment are:
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Surface-supplied gear (MK 21 MOD 1)
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Semiclosed-circuit and closed-circuit UBAs
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Saturation deep dive systems
For deep dives (190-300 fsw) of short duration, or for shallower dives where
nitrogen narcosis reduces mental acuity and physical dexterity, helium-oxygen
diving methods should be employed.
Because of the unusual hazards incurred by long exposures to extreme environmental
conditions, extended excursions away from topside support, and great
decompression obligations, semiclosed-circuit and closed-circuit diving should
only be undertaken by specially trained divers. Semiclosed-circuit and closedcircuit
diving operations are covered in depth in Volume 4.
Saturation diving is the preferred method for dives deeper than 300 fsw or for
shallow dives where extensive in-water times are required. Disadvantages of saturation
diving include the requirement for extensive logistic support and the
inability of the support ship to easily shift position once the mooring is set. For
this reason, it is very important that the ship be moored as closely over the work
site as possible. Using side-scan sonar, remotely operated vehicles (ROVs) or
precision navigation systems will greatly aid in the successful completion of the
operation. Saturation diving is discussed in Chapter 15.
Method Considerations. In mixed-gas diving, the principle factors influencing
the choice of a particular method are:
-
Depth and planned duration of the dive?
-
Equipment availability
-
Quantities of gas mixtures available
-
Qualifications and number of personnel available
-
Type of work and degree of mobility required
-
Environmental considerations such as temperature,
visibility, type of bottom,
current, and pollution levels.
-
Communication requirements
Depth. Equipment depth limitations are contained in Table 13-2. The limitations
are based on a number of interrelated factors such as decompression obligations,duration of gas supply and carbon dioxide absorbent material, oxygen tolerance,
and the possibility of nitrogen narcosis when using emergency gas (air). Divers
must be prepared to work at low temperatures and for long periods of time.
Operations deeper than 300 fsw usually require Deep Diving Systems (DDSs).
The decompression obligation upon the diver is of such length that in-water
decompression is impractical. Using a personnel transfer capsule (PTC) to transport
divers to a deck decompression chamber (DDC) increases the margin of diver
safety and support-ship flexibility.
Bottom Time Requirements. The nature of the operation may influence the
bottom time requirements of the diver. An underwater search may be best undertaken
by using multiple divers with short bottom times or by conducting a single
bounce dive simply to identify a submerged object. Other tasks, such as underwater
construction work, may require numerous dives with long bottom times
requiring surface-supplied or saturation diving techniques. Although primarily
intended to support deep diving operations, saturation diving systems may be ideal
to support missions as shallow as 150 fsw where the nature of the work is best
accomplished using several dives with extended bottom times. Under these conditions,
time is saved by eliminating in-water decompression obligations for each
diver and by reducing the number of dive team changes, thus compensating for the
increased logistical complexity such operations entail..
Environment. Environmental conditions play an important role in planning
mixed-gas diving operations. Environmental factors, such as those addressed in
Chapter 6, should be considered when planning such operations. Mixed-gas diving
operations often involve prolonged dives requiring lengthy decompression and
travels that carry divers great distances from a safe haven. Special attention should
therefore be given to preventing diver hypothermia. Mixed-gas diving apparatus
are designed to minimize thermal stress, but the deepest, longest helium-oxygen
dives place the greatest stress on the diver. Exposure to extreme surface conditions
prior to the dive may leave the diver in a thermally compromised state. A diver who has been exposed to adverse environmental conditions should not be considered
for mixed-gas diving until complete rewarming of the diver has taken place,
as shown by sweating, normal pulse, and return of normal core temperature.
Subjective thermal comfort does not accurately indicate adequate rewarming.
Mobility. Some diving operations may dictate the use of a diving method that is
selected as a result of special mobility requirements in addition to depth, bottom
time and logistical requirements. The MK 21 MOD 1 is the preferred method
when operations require mobility in the water column (see Figure 13-1).
For missions where mobility is an essential operating element and depth and
bottom time requirements are great, closed-circuit diving may be the only available
option. Such diving is frequently required by special warfare and/or explosive
ordnance disposal (EOD) personnel.
Equipment Selection. Equipment and supplies available for mixed-gas diving
operations by U.S. Navy personnel have been tested under stringent conditions to
ensure that they will perform according to design specifications under the most
difficult conditions that may be encountered. Several types of equipment are available
for mixed-gas operations. Equipment selection is based upon the chosen
diving method, depth of the dive and the operation to be performed. Table 13-3
outlines the differences between equipment configurations.
The UBA MK 21 MOD 0 is an open circuit, demand-regulated diving helmet
designed for saturation, mixed-gas diving at depths in excess of 300 fsw and as
deep as 950 fsw. With the exception of the demand regulator, it is functionally
identical to the UBA MK 21 MOD 1, which is used for air and mixed-gas diving.
The regulator for the MK 21 MOD 0 helmet is the Ultraflow 500, which provides
improved breathing resistance and gas flow over the MK 21 MOD 1.
The UBA MK 22 MOD 0 is an open circuit, demand-regulated, band-mask
version of the UBA MK 21 MOD 0. It is used for the standby diver for saturation,
mixed-gas diving at depths in excess of 300 fsw and as deep as 950 fsw. It is
provided with a hood and head harness instead of the helmet shell to present a
smaller profile for storage.
Operational Characteristics. Equipment operational characteristics are reviewed
in Table 13-2 and specific equipment information can be found in paragraph 13-8.
All diving equipment must be certified or authorized for Navy use. Authorized
equipment is listed in the NAVSEA/00C Authorized for Navy Use (ANU) list. For
proper operation and maintenance of U.S. Navy approved diving equipment, refer
to the appropriate equipment operation and maintenance manual.
Support Equipment and ROVs.. In addition to the UBA,
support equipment must not be overlooked. Items commonly used include tools,
underwater lighting, power sources, and communications systems. The
Coordinated Shipboard Allow ance List (COSAL) for the diving platform is a reliable source of support
equipment. Commercial resources may also be available.
Occasionally, a mission is best undertaken with the aid of a remotely operated
vehicle (ROV). ROVs offer greater depth capabilities with less risk to personnel
but at the expense of the mobility, maneuverability, and versatility that only
manned operations can incorporate.
Types of ROV. There are two types of ROVs, tethered and untethered. Tethered
ROVs receive power, control signals, and data through an umbilical. Untethered
ROVs can travel three to five times faster than tethered ROVs, but because their
energy source must be contained in the vehicle their endurance is limited. ROVs
used in support of diving operations must have ground fault interrupter (GFI)
systems installed to protect the divers.
ROV Capabilities. Currently, much of the Fleet’s requirements for observation
diving are being met by using ROVs. They have been used for search and salvage
since 1966. State-of-the-art ROVs combine short-range search, inspection, and
recovery capabilities in a single system. A typical ROV system includes a control
and display console, a power source, a launch and retrieval system, and the vehicle
itself. Tethered systems are connected to surface support by an umbilical that
supplies power, control signals and data. Untethered search systems that will
greatly increase current search rates with extended endurance rates of 24 hours or
more are currently under development. Figure 13-2 shows a typical NAVSEA
ROV.
Diver’s Breathing Gas Requirements. In air diving, the breathing mixture is
readily available, although pump and compressor capacities and the availability of
back-up systems may impose operational limitations. The primary requirement for
mixed-gas diving is that there be adequate quantities of the appropriate gases on
hand, as well as a substantial reserve, for all phases of the operation. The initial
determinations become critical if the nearest point of resupply is far removed from
the operation site.
Gas Consumption Rates. The gas consumption rates and carbon dioxide absorbent
durations for various types of underwater breathing apparatus are shown in
Table 13-1. Refer to Chapter 4 for required purity standards.
Surface-Supplied Diving Requirements. For surface-supplied diving, the diver
gas supply system is designed so that helium-oxygen, oxygen, or air can be
supplied to the divers as required. All surface-supplied mixed-gas diving systems
require a primary and secondary source of breathing medium consisting of
helium-oxygen and oxygen in cylinder banks and an emergency supply of air from
compressors or high-pressure flasks. Each system must be able to support the gas
flow and pressure requirements of the specified equipment. The gas capacity of
the primary system must meet the consumption rate of the designated number of
divers for the duration of the dive. The secondary system must be able to support
recovery operations of all divers and equipment if the primary system fails. This
may occur immediately prior to completing the planned bottom time at maximum
depth when decompression obligations are the greatest. Emergency air supply is
provided in the event all mixed-gas supplies are lost.
Deep Diving System Requirements. A deep diving system must be able to store
and supply enough gas to support saturation diving to the maximum certified
depth. Deep diving systems can handle and store pure gases, and mix the required
percentages of helium-oxygen as needed. When DDS-type equipment is
employed, additional quantities of gas must be included for DDC and PTC
charging and for replacing losses due to leakage, transfer trunk and service lock
usage and scrubber cycling. A DDS must also have an air system capable of
supporting surface-supplied air diving operations and initial pressurization of the
DDS for saturation operations.
