‘Before we deployed the vessel to lay the mattresses we needed actual cable locations, but because of high cross current conditions in the coastal waters we chose a tracked ROV initially, said Chris Henson, survey projects manager for ot2k. ‘But once the operation began no ROVs were used for sub-surface positioning, just Easytrak.’

The mattresses were deployed from a vessel on four point mooring using a lifting frame with a transponder at each end and sub-surface gyro to monitor heading. A scanning sonar mounted on a seabed frame was used to check mattress positions relative to each other. When married to high precision GPS (either RTK or HP DGPS) and motion sensors, absolute accuracies in the order of 2 to 2.5m and relative accuracies of better than 1m in 25m of water were achieved.

Henson added, ‘Easytrak enabled us to undertake mattress deployment without the usual diver or ROV intervention in near zero visibility, resulting in a very safe and extremely cost effective operation.’

The Easytrak Portable USBL system is rapidly deployed and easy to use. Traditionally popular in the oil and gas sector, its potential is being realised for  underwater activities in shallower waters , such as locating and tracking the movements of ROVs engaged in salvage work, searching for wrecks, monitoring the impact of dredging on the seabed, contributing to the accurate positioning of harbour construction materials, and performing harbour wall inspections.

Advances in acoustic technology mean that divers need not be involved in such dangerous inspection tasks as much as previously.

Although some of this high-tech USBL equipment may be expensive to buy for smaller operators or one-off tasks, it is now available to rent from tow Aberdeen based offshore companies.

MJ Information No: 22517