Hydraulic Fluid Viscosity (Centistokes)
Shaded area shows viscosity range recommended for hydraulic tools.
WARNING Dispose, handle, and transport hydraulic oil in accordance with hazardous materials procedures.
Rock drilling is necessary for many UCT operations. Cable and pipeline stabilization may require that rockbolts or U-bolts be used to anchor an armored cable or split-pipe protection system to the seabed. A hole is drilled in the seabed rock or coral into which an anchoring device is inserted. In some cases, grouting of anchors is required (refer to Section 2.11 for a discussion of grouting techniques). Rock drilling may also be necessary in explosive excavation operations. If a trench must be excavated or an obstruction removed so that a cable or pipeline can be laid, holes are drilled into the rock or coral and an explosive charge is placed in the hole and detonated (See Section 2.7.7 for a discussion of explosive excavation).
Drilling holes in seabed rock is best done with hydraulic rock drills. Pneumatic rock drills were used in the past, but have proved undesirable because of high maintenance requirements and the percussion waves produced by the exhaust gas. Also, in cold weather the pneumatic equipment tends to freeze up.
Commercially available underwater hydraulic rock drills are extensively used by the UCTs. These tools come in three models:
• A light-duty hammer drill
• A heavy-duty hammer drill
• A sinker drill as illustrated in Figures 2-2, 2-3, and 2-4, respectively.
These tools are capable of drilling holes up to 3 inches in diameter and up to 20 feet deep. Hydraulic rock drills should be powered by sources capable of 10 gpm and pressures up to 2,000 psi.
If rock drills must be used while diving with scuba, it is advisable to wear a wet suit hood to minimize the effects of the noise generated by the equipment (scuba should only be used as a last resort).
CAUTION The U.S. Navy Diving Manual requires the use of voice communications with the Dive Supervisor when using scuba gear with powered tools. Surface-supplied diving gear is assumed to have working communications.
Drilling rates vary with the type of drill, diameter of the hole, hardness of the rock, skill of the diver, and the working conditions. Average drilling rates for sound rock
UiVUttiMMICti WUKK / tUH/y IQ Ut S
Figure 2-3 Commercial heavy-duty hand drill (Stanley HD-45).
Figure 2-4 Commercial sinker drill (Stanley SK-58).
for the various model drills and drill diameters, assuming good working conditions and an experienced diver/operator, are given in Table 2-2.
Table 2-3 provides a summary of characteristics for the three rock drills. Of all the oil hydraulic tools used underwater by the UCTs, the rock drill performance is most heavily dependent on providing adequate flow and pressure at the tool. Drilling rate can be seriously reduced by both flow restriction in the return hose and high fluid viscosity (thickness). For this reason, a 3/4-inch minimum inside diameter is recommended for the return hose. Pressure in the return hose should be maintained below 150 psi.
When the tool is in use, the supply line pressure is a function of the load on the tool and frictional losses in the hoses. Poor performance may be related to hose restrictions, a worn tool, or improper viscosity of the hydraulic fluid.
The performance of oil hydraulic rock drills is sensitive to the configuration and physical arrangement of the hydraulic circuit. A common misconception with hydraulic tools is that tool performance can be corrected by increasing system pressure and flow rate. The rock drills, however, are sensitive to fluid viscosity and back pressure in the return hose. Care must be taken to be sure the return hose diameter is large enough to minimize return flow restriction. The return hose acts like an accumulator that can damp out movement of internal mechanisms.
It is also important to ensure that the hydraulic oil used is compatible with the tool and power source (see Section 2.2.2). If the fluid viscosity is too high (thick), the tool may become sluggish or inoperable. It may be necessary to change to a lower viscosity fluid in cold climates. Follow proper procedures when changing or dis
posing of hydraulic fluids as most are considered hazardous materials.
In extremely cold climates the hydraulic oil operational temperatures tend to run cooler than normal, which can result in higher hydraulic viscosity and sluggish performance. Tool manufacturers recommend maintaining operational viscosity between 20 and 85 centistokes. Figure 2-5 shows how hydraulic oil viscosity varies for oil operational temperature. At very low temperatures, fluids may need to be preheated to avoid damage to the tools.
REMEMBER that hydraulic rock drills are equipped with rotary bits. Use extreme care when carrying the tool to avoid the drill bit becoming entangled in the divers dress.
Recommend that the power supply flow be off before moving the drill. This can be done either at the topside power source or by using the bypass valve installed in the hose 10 feet from the tool.
Was this article helpful?