Underwater work techniques Ways to Improve Capacity and Troubleshooting Anchor Problems. An anchor can foil to hold because it is improperly set, is inappropriate for the seaf-loor or load conditions, or the scope is too small for proper anchor performance. Some typical problems and solutions are listed in Table 2-20.

The minimum scope required depends on the expected load and the characteristics of die anchor rode. Scope can vary from 5 (ratio of line length to water depth) for chain to 10 or more for wire or synthetic line. The scope must be adequate to ensure that the line angle at the anchor is less than about 6 degrees at the mooring design load. Scope can be reduced by the addition of line clumps which depress the line and reduce the line angle at the seafloor. Line clumps can be preinstalled on permanent moorings or they can be added during emergencies when small boat anchors fail to hold.

Table 2-20 Common Anchor Problem Tests



Sand/Hard Soils

Anchor not embedding

Reduce fluke angle Sharpen anchor flukes Add piggyback

Use larger or heavier anchor or more chain Use diver to help set flukes with jet or airlift Add scope or add kellet to the chain Change to a hooking anchor


Anchor not embedding

Fix the flukes in the open position and place anchor on seafloor Change points down to plow-type anchor or anchor with large tripping palms

Sand/Hard Soils/Mud

Anchor unstable Anchor not holding

Add or extend stabilizers Change to more stable anchor

Use larger anchor Add piggyback Increase scope/add kellet Check anchor for fouling

The first option when a small boat anchor drags is to increase scope. Continued dragging may require use of engine power to augment the anchor. Another option, illustrated by Figure 2-41, is to slide a line clump down the anchor rode to depress the line. The weight is called a "kellet" and it can be constructed from materials on hand. The best position of the kellet is about midway along the rode, but it may have to be adjusted if there is too much bounce in the line.

Another method to increase holding capacity is to use multiple anchors, in tandem (piggyback) or side-by-side (Figure 242). Tandem anchors can hold 20 to 30 percent more than two anchors pulled individually provided that the correct anchors are used in tandem. The piggyback is also useful in helping die leading anchor penetrate harder soils. The simplest connection for the piggyback anchor is direcdy from the piggyback's shank to the crown end of the leading anchor. For this to be effective, the leading anchor must be of a stable design, like the Bruce, plow, or Navmoor. Light weight anchors become unstable when loaded by the piggyback method and roll out of the seafloor. The preferred connection for lightweight anchors is to the leading anchors' shackle; although, this complicates installation. Side-by-side and tandem anchors should be separated longitudinally by a distance equal to at least two anchor lengths to avoid interference.

2.8.2 Deadweight Anchors

Deadweight anchors can vary from sophisticated (concrete/steel anchors with cutting skirts) to engine blocks, railroad wheels, concrete clumps, etc. Some options are shown in Figure 2-43. Although inexpensive to fabricate, they are cumbersome to handle; thus, they are seldom used. Deadweights can be constructed from concrete or steel. The material has an important effect on capacity because concrete loses 43 percent of its weight in water compared to 15 percent for steel, and holding capacity is related to the submerged anchor weight.

Scuba Encasement
Kellet (balled chain, weight)

Figure 2-41. Use of a kellet to improve anchoring.

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