lower or deeper than the storage depth. The actual depth of the work site or PTC may be significantly different from the storage depth.
When using Table 15-8, enter the table at the deepest depth attained at any time within the last 48 hours. While the DDC may be at 400 fsw, if one diver had reached a depth of 460 fsw during an in-water excursion, the maximum upward excursion depth for the divers is 360 fsw instead of 307 fsw. After completing work at one depth and then compressing DDC to a deeper storage depth, unlimited downward or upward excursions are permitted immediately upon reaching the new storage depth. When decompressing the DDC from a deeper depth using standard saturation decompression procedures, unlimited downward excursions, as defined in Table 15-7, may begin immediately upon reaching the new chamber storage depth. A minimum of 48 hours shall elapse at the new storage depth before any upward excursions may be made.
Example. After decompression from 1,000 fsw to 400 fsw, the maximum downward excursion is 105 fsw. After 48 hours have elapsed at 400 fsw, a full upward excursion of 93 fsw to 307 fsw is permitted.
If less than 48 hours is spent at the new storage depth, the maximum upward excursion is based on the deepest depth attained in the preceding 48 hours.
Example. Decompression from a 1,000 fsw dive has been conducted to the 400 fsw depth. Twenty-four hours have been spent at 400 fsw. The dive log shows that the deepest depth attained in the preceding 48 hours is 496 fsw. The maximum upward excursion from Table 15-8, based on a 496 fsw depth, is to 396 fsw (500 -104) allowing a maximum of a 4 fsw upward excursion. After 36 hours have elapsed at 400 fsw, the dive log shows that the deepest depth attained in the preceding 48 hours was 448 fsw. From Table 15-8, the shallowest excursion depth is now 351 fsw.
The ascent rate should not exceed 60 fsw/min during an excursion. When it is detected that a diver is ascending faster than 60 fsw/min, the diver shall immediately stop and wait until enough time has elapsed to return to the 60 fsw/min schedule. The diver may then resume ascent at a rate not to exceed 60 fsw/min from that depth.
If storage depth falls between the depths listed in Table 15-7, use the next shallower depth (e.g., if the storage depth is 295 fsw, enter Table 15-7 at 290 fsw). If storage depth falls between the depths listed in Table 15-8, use the next deeper depth (e.g., if the storage depth is 295 fsw, enter Table 15-8 at 300 fsw).
Example 1. The chamber was compressed to 400 fsw from the surface. The initial depth in Table 15-7 is 400 fsw. The maximum downward excursion for an unlimited period not requiring decompression is 105 fsw, allowing a maximum diver depth of 505 fsw. If the diver descends to 450 fsw, the maximum depth achieved from the 400 fsw storage depth will be 450 fsw. Table 15-8 at 450 fsw allows a 99 fsw upward excursion to a depth of 351 fsw. Thus, these divers may move freely between the depths of 351 and 450 fsw while at a storage depth of 400 fsw.
Example 2. At a storage depth of 600 fsw, during which dives were made to 650 fsw, the maximum upward excursion that may by made to begin saturation decompression is:
■ If less than 48 hours have elapsed since the 650 fsw excursion, Table 15-8 allows a maximum upward excursion of 119 fsw from a deepest depth of 650 fsw to a depth of 531 fsw.
If more than 48 hours have elapsed since the excursion, the maximum upward excursion allowed is 115 fsw from 600 fsw to 485 fsw.
Example 3. At the new shallower storage depth of 350 fsw, divers conduct an excursion to 400 fsw. Using the deepest depth of 400 fsw achieved during storage at 350 fsw, a maximum upward ascent from Table 15-8 of 93 fsw to a depth of 307 fsw is allowed, provided the chamber and the divers have been at the storage depth of 350 fsw for at least 48 hours. Otherwise, no upward excursion is permitted.
15-21.2 PTC Diving Procedures. Actual PTC diving operations are dictated by the Unit's operating instructions. In conducting these operations, experience indicates that a maximum in-water time of 4 hours is optimal for diver efficiency. Longer dive times result in a loss of diver effectiveness because of fatigue and exposure, while shorter dives will significantly increase the time at depth for the completion of operations. Standard practice is to rotate in-water divers with the PTC operators, allowing two 4-hour dives to be conducted during a single PTC excursion to the work site. Proper positioning of the PTC near the objective is important in ensuring that the diver does not exceed the maximum permitted excursion limits (Figure 15-8).
15-21.2.1 PTC Deployment Procedures. A brief overview of PTC deployment procedures follows:
1. For initial pressurization, the PTC, with internal hatch open, is usually mated to the DDC. Divers enter the DDC and secure the hatches.
2. The DDC and PTC are pressurized to bottom depth. The divers transfer to the PTC and secure the DDC and PTC hatches after them.
3. The trunk space is vented to the atmosphere and then the PTC is deployed and lowered to working depth. The hatch is opened when seawater and internal PTC pressures are equal. The divers don diving equipment and deploy from the PTC.
4. Divers return to the PTC and secure the hatch. The PTC is raised and mated to the DDC, and the divers transfer to the DDC. Until they are decompressed in the DDC, the divers rotate between periods of living in the DDC and working
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