Sea Technology

OCT 2012

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Deploying Sonar Systems to Create 3D Maps for Underwater Excavation Multibeam Echosounder, ROV-Mounted Sonar Used To Survey Blast Site for Construction of Reservoir Intake By Roberto Folchi Managing Director and Hans Wallin Technical Director Nitrex srl Lonato del Garda, Italy T he construction of the third water intake of the Southern Nevada Water Authority at the Lake Mead reservoir re- quired an underwater excavation at 100 meters depth, and a large basalt surface had to be deepened by 20 meters. No new technology was available to fit the project, so the old impre- cise yet effective method of using shaped demolition charges was chosen. However, no shaped charge was found in the market that fit the underwater specifications of the project, so a new one had to be conceived, along with a deployment sys- tem and technique, monitoring and survey procedures. Basalt is strong and resilient, with a high resistance to metallic jet penetration, close to that of ceramic, which is why it is used in the defense industry to protect tanks against grenade impact. The new shaped charge needed to create a high-energy jet slug to break through the basalt, which required adequate water-free standoff distance between the explosive charge and the rock to be fractured. To keep the charge standoff water-free at 100 meters depth, a specially fitted shell had to be coupled to the charge. To monitor excavation and deploy charges, Nitrex used a combination of an ROV, a single-beam sonar and a multi- beam echosounder. Excavation and Underwater Survey The rock mass to be excavated was a basalt mainly com- posed of plagioclase. There was an upper layer of stones, sand and silt. Thickness was quantified by sub-bottom pro- filing and mechanical coring. The upper layer had been weathered by atmospheric exposure before being covered by water when the Hoover Dam was completed. The progressive density increase from the top to the bot- tom of the loose sand-silt formation and from the upper layer of basalt affected accuracy, leading to overestimation of the thickness of the loose materials overlaying the solid rock. (Left) A diagram of Lake Mead's third water intake. (Bottom) Charges deployed from the barge. An open-air survey of an outcrop in the nearby village of Callville offered a general view of the consistency of the rock to be blasted. Rock mass appeared to be fractured within three main families of thermal shrinkage joints, and some minor faults were noted. Some joints showed earth filling but were generally well-closed into the rock mass. Non- interconnected cracks were seen in the rock matrix. The upper standing layer of stones, sand and silt was re- moved by air lifting and clam shell. Material was mucked aside by swinging the drag line boom, with the bucket/clam- shell kept below the water table. The hole in the rock, which was needed to host an intake shaft/riser, was excavated by means of a custom-made shaped demolition charge. Because of the water pressure at that depth, sealing for water tightness was an issue. No divers were needed. In- stead, charges were attached to a steel frame and deployed. They were monitored with Saab Seaeye Ltd.'s (Fareham, Eng- OCTOBER 2012 / st 51

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