Underwater Cutting By Abrasive-Waterjet

The abrasive-waterjet (AWJ) cutting technology was adapted for underwater use and its performance was evaluated itnd optimized. A series of parametric tests was carried out on a variety of AWJ configurations using secondary gas injection of abrasives, direct pressurization of abrasive slurry, and secondary injection of abrasive slurry. The tests were conducted in hyperbaric chambers at simulated water depths of up to 9000 feet. At a simulated depth of 8000 feet, a 1.4-inch-thick steel plate was cut at a rate of 1 inch/minute using secondary injection of abrasive slurry. At a simulated depth of 140 feet, 8-inch-thick steel was cut in a single pass using secondary gas injection of abrasives, and basalt was cut to a depth of 1.6 inches at a traverse rate of 24 iinches/minute. These results demonstrate that AWJs hold great promise as safe and effective underwater cutting tools. INTRODUCTION Underwater cutting applications include repair, rescue, and salvage operations, offshore platform removal, nuclear plant service, and deep ocean rock sampling. These operations are expensive, often dangerous, and are complicated by the physical limitations of available cutting equipment. The development of manned submersibles and remotely operated vehicles (ROVs) has iextended the maximum working depth for underwater operations, thereby rnagnifying the shortcomings of existing underwater cutting techniques. Traditional methods for cutting metals underwater include divermanipulated torches, pyrotechnic devices, electric arc processes, and mechanical cutters. Most of these methods require large quantities of compressed gases, which complicates operation in deep water and is impractical at open-ocean depths. For cutting thick metals, mechanical cutters that enter the kerf, such :IS saws, require precise and heavy fixturing and create the risk of binding. t is observed by Beebe et al.,' the abrasive-waterjet (AWJ) cutting technique has the potential to address many of the problems associated with existing underwater cutting techniques, and can significantly reduce the time required to perform underwater cutting. The AWJ cutting technique is currently used in a wide variety of in-air industrial cutting applications. These applications have ranged from cutting 3-foot-thick granite, employing jets of over 500 hp, to drilling small-diameter holes in advanced ceramic materials with jets of less than 5 hp. An AWll can cul: virtually any material and, since the process is a cold cutting proccss, it does not create a heat-affected zone. This paper presents the preliminary results of a program to evaluate and optimize AWJ cutting for underwater and deep ocean operation. BACKGROUND Details of In-Air AWJ Cutting AWJ cutting is a micromachining process. PL high-velocity waterjet (Mach 2 3) is created by pressurizing water to 35,000 55,000 psi and forcing it through a small-diameter nozzle (0.005 0.060 inch). Normally, abrasives are added to the high-velocity .iet in a secondary mixing chamber. The abrasives are entrained in the jet and accelerated in the mixing tube to create a Ihigh-velocity composite jet. A drawing of an AWJ nozzle is shown in Figure 1. As shown in Figure 2, a typical system for AWJ cutting in air consists of three components: an ultrahigh-pressure (LIHP) pump, an abrasives system (including a storage hopper, a motering system, ULTRAHIGH-PRESSURE I WATER INLET