HYDROGRAPHIC SURVEY
an array length of 25 centimetres at 100 KHz will have a beam width of about one degree. At 300m, 1degree of arc covers about 5m along track – the smallest resolvable object. Narrowing this beam width requires a physical 100kHz transducer many metres long. In SHADOWS, this requirement is replaced by
a virtual transducer integrating multiple pings with towfish position and attitude. Tis comes from two sources, 1) the onboard Inertial Navigation System (INS) and 2) the position correlation between consecutive pings. Tis assumes that each ping has all the information needed to determine the change in fish position and attitude by identifying the same target on two consecutive pings and resolving the movements needed to place the target at the same point on the seabed. At 300m range and 5knots a target will be insonified by over 35 separate pings. iXSurvey undertook extensive testing, before
commercial operations began, to determine optimum settings and scenarios. Systematic variation of power output, pulse length, range and fish height were made whilst surveying point targets and seabed features, eg sands, gravels and muds. Image quality and absolute
positioning accuracy were then compared. Te system was found to excel at broad, regional seabed mapping where it gives high quality data in very efficient timescales, but is also able to acquire stunning quality images over specific targets at short range. SHADOWS represents a huge advance in technology, and for economic use it requires a similarly large change in mindset for survey planning. For example, the traditional line plan for a 1000m pipe or cable route survey may be reduced by two thirds by replacing 15 lines of SSS and sub-bottom profiler with only 2 lines of SHADOWS to cover the entire area. Te real time mosaic would then be used to determine the optimum route, by passing hazards, and this would then be surveyed with three lines at 25m spacing. Tis reduces the survey time and focuses data in the most important zones. iXSurvey recently used SHADOWS on a seagrass mapping project in Cavailaire, South of France.
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Dominic Koe is with iXSurvey, Edinburgh, Scotland.
www.ixsurvey.com
Deep ocean sampling and observation H
ydro-Lek’s deep ocean sampling and observation system used in discovery of world’s deepest undersea volcanic vents
Berkshire, UK-based remote handling specialists, Hydro-Lek Ltd, has partnered with National Oceanography Centre in Southampton to develop HyBIS, the deep ocean sampling/observation system. HyBIS was deployed last month by a British scientific expedition to discover and film for the first time the world’s deepest undersea volcanic vents located 5000m (3. 1 miles) down in the Cayman Trough in the Caribbean. HyBIS, a mnemonic for Hydraulic Benthic
Interactive Sampler, enables seabed sampling at extreme depths and conditions together with video instrumentation observation. Highly robust, flexible and inexpensive, HyBIS is designed to operate in conjunction with existing deck handling and cable systems used on extended towed sonar arrays, thereby eliminating the need for additional and costly ROV deck handling equipment. It also enables sonar surveys to be followed up with localised observation and sampling during the same voyage. HyBIS measures 1.5m x 1.4m x 1.8m high
and comprises two subsea modules : an upper module which houses hydraulic and electric power modules with thrusters, release mechanism,
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video and lighting, and fibre-optic telemetry for instruments and sensors; and a lower module which houses a detachable sampling grab, automatic closing covers and hydraulic drive cylinders. Hydro-Lek has used corrosion resistant
materials throughout the manufacture of HyBIS and it is designed to withstand the high pressures and harsh environment found at depth. Indeed, the pressure 3 miles deep at the bottom of the Trough is 500 times the normal atmospheric pressure – this is equivalent to the weight of a large family car pushing down on every square inch of the seabed. Electrically-driven thrusters enable the vehicle to be manoeuvred above the seabed under its own power. In operation, the HyBIS acts as an ROV which
is controlled at depth via the umbilical winch. All other functions are self-generated using the on- board electrical services. The system is operated from maximum 7Kw 220-240V single phase power source at the surface via the umbilical cable. All 3 phase power and control voltages for lighting and instrumentation are contained in 6000 metre rated pods. The control of HyBIS from the surface is via a hand held control box. This contains the command telemetry for remote operation of all hydraulic and
electrical functions including remote switching for power to lights, cameras, and other demands for ancillary equipment. This is transmitted th rough an RS 232 link via the optical fibre to the subsea instrumentation pod. This then controls a range of driver and relay boards to control all subsea systems. The system comprises two 3 phase power packs and two 4-way valve packs which provide power for all hydraulic functions. The HyBIS has two reversible thrusters, one
fitted each side of the vehicle which allows manoeuvrability about the umbilical. Each thruster can produce 40 kg of thrust from the 1.5 Kw motors. This allows HyBIS to manoeuvre around a radius of one hundred metres from the surface ship and survey the sea floor at a speed of 0.5kts. The sampling module on the lower section of
the HyBIS comprises a 900 x 800 mm clam-shell grab capable of picking up to 0.3 cu metres of soft sediment, shingle and loose material which can be hydraulically ejected remotely in an emergency. The HyBIS is also fitted with one camera and two lights working horizontally for steering and observation, and one camera fitted vertically for observation during descent and monitoring of grab contents during ascent.
For more information,
www.hydro-lek.com
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