Sonardyne International has been awarded a contract by Subsea 7, one of the worlds leading subsea engineering and construction companies, to provide its latest Sixth Generation® (6G®) acoustic technology for monitoring the position and status of towed offshore pipeline bundle systems. The bundles are manufactured at Subsea 7’s facility in Wick, Scotland and towed to their final location in the North Sea suspended between two tugs; a technique known as Controlled Depth Tow Methodology (CDTM). Under the terms of the new contract, the first tow will take place in the first quarter of 2011 when a 7km bundle will be towed out to the Bacchus field for Apache North Sea Limited. Later in 2011, four 7km bundles will be towed to BP’s Andrew field in the North Sea.
The Sonardyne 6G® acoustic equipment installed on each bundle will provide the towmaster with real time information on the pipeline’s position, depth and shape along its entire length. This data will help ensure that the bundle is not allowed to sag or snake excessively during the tow due to the effects of tow speed, tide and sea state and that it is ‘flying’ at the required depth to avoid submerged objects. Should corrective action be required, the towmaster is able to alter the depth profile of the pipeline bundle by various methods including varying the tow speed.
Sonardyne has considerable experience of this application for its acoustic technology having first developed bundle monitoring equipment for Subsea 7 in 1993. At that time, depth monitoring transponders deployed at regular intervals along the bundle were connected, via data highway cable, to master units located on the tow head. These sent data acoustically to the towmaster and received commands back, such as to open the valves that controlled flooding of the bundles. The technology proved very effective as any damage sustained to a bundle during tow-out is extremely costly to rectify.
For the latest tows, Sonardyne’s new Compatt 6 transponders will be deployed at intervals of around 700 to 1000 metres along each bundle. Sonardyne Lodestar attitude and heading reference sensors will also be deployed at each end of the bundles and at specified intervals along them. Lodestars contain highly accurate ring laser gyros that will enable the shape and attitude of the bundle to be monitored. The pressure of the carrier gas (Nitrogen) within the pipes will be monitored by sensors at each end of the bundle and this data, along with the angle of the towing wires will also be transmitted.
“This new Sonardyne 6 G technology will allow Subsea 7 to provide continuous, stable and repeatable data during the transit of the bundles to the offshore location. This is a major advance in monitoring the bundle attitude during tow operations and will provide valid and accurate data for the towmaster to safely control the depth of the bundle”.
Compatt 6 uses Sonardyne Wideband® 2 ultra-wide bandwidth signals that make it more versatile than previous versions. They can operate in more demanding acoustic conditions and are suitable for a wider range of applications than a conventional seabed positioning transponder. Acting as an acoustic network, each Compatt 6 will be able to transmit its data along the bundle to other transponders and onwards to a transceiver in a towfish that will be flown from an escorting ROV Support Vessel (RSV). The RSV will patrol the bundle going no further than 500 metres away from it. Because the towfish will be flown below the disturbed water layers near the surface it will be able to obtain clear acoustic signals from the transponders, thereby maximising the communications robustness of the system. Depth updates from all monitoring points will be received at the RSV every 30 seconds while heading, carrier pressure and trailing wire angle updates will be received at five minute intervals. The 6G® acoustic technology will also make it possible to monitor the tow wire tension, activate its release and also to operate carrier flood and vent valves remotely if required.
Sonardyne is one of the world’s most knowledgeable companies regarding the behaviour of sound through water and has already performed a detailed analysis of the acoustic properties of the sea areas in which the bundles will be towed. This will be used to ensure the optimum performance of the acoustic monitoring system is maintained throughout each tow.
“Bundles are typically towed at a relatively shallow depth of around 40-60 metres where acoustic signal propagation can be unpredictable due to the effects of shallow and aerated water, vessel noise and mixing of different water layers. Our latest generation Wideband 2 signal architecture has been designed specifically for difficult acoustic circumstances and we are confident that it will demonstrate high reliability in this challenging environment.”
Sonardyne International is based in Yateley, England. The company has offices in Aberdeen, Houston, Singapore, Brazil and Norway. It is recognised as a world leader in the design and manufacture of advanced acoustic instruments and systems for accurate survey, positioning and tracking in shallow water and full ocean depth operations within the offshore oil and gas industries.