Boskalis tests new operation on bridge simulator

Boskalis Offshore is working on a project for the installation of two submarine power cables between Java and Bali. This is quite a complex operation, particularly because currents in the area are up to 7 kts. Dimitri van Heel,

To cope with these currents a new concept was developed and tried out on MARIN’s bridge simulators. This followed on from earlier simulations and calm water model tests. Boskalis’ new Cable Lay Vessel Ndurance, which is fitted with four azimuthing and one tunnel thruster for dynamic positioning, will be deployed on the project.

As cable laying is a precision job, the chute - the point where the cable leaves the vessel - needs a continuously shifting offset from the cable route. Based on cable and seabed parameters, current loads and lay tension the ideal offset to keep the cable on the route, can be calculated for different currents. The DP system needs continuous updates of these offsets, whilst at the same

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time it must control the gradual and fluent motion along the route. A sudden loss of position may cause the cable to buckle and hence, lead to a mission failure.

A new concept was developed that involved using an offshore tug to assist Ndurance in maintaining position when the current increases above a certain value. The tug provides redundancy and is only necessary in case of a major DP failure aboard the Cable Lay Vessel. Two bridge simulators were used during training. Ndurance’s own DP system was used and the external forces of the tow wire and cable were fed into the DP system.

In two sessions, the cable laying operation was practised by the crew and the assist-

ing tug. In the first session a powerful, con ventional AHSV was used and although the concept looked very promising, manoeuvring the vessel was not entirely satis factory. Therefore, a smaller and more manoeuvrable ASD-type tug was used in the second session, which gave very good results. Training resulted in numerous lessons learnt and identified possible tech- nical and procedural improvements. But most of all, it proved that the concept is viable. After an extensive sea trial period to further prepare ship and crew for the task at hand, both cables have been suc- cessfully installed. MARIN is proud to have been of assistance during this very interest- ing project right from the concept through to the actual operation.

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