Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report Figure 3.19: LBT1 being deployed at the North Hoyle Offshore Wind Farm Site Previous Table 3.11 lists a selection of Tracked Cable Burial Machines, noting the various burial systems available for each machine together with corresponding details of burial capability. 3.8.4 FREE SWIMMING CABLE BURIAL MACHINES A Free Swimming Burial ROV uses thrusters for propulsion and manoeuvrability and is equipped with a work package or work skid for intervention tasks or cable burial operations. All of the ROVs in use today are unmanned; however some of the early free swimming vehicles were manned. These vehicles are now consigned to museum exhibits. The early free swimming burial machines were manned autonomous vehicles with very limited power budgets for burial operations. The early Pieces vehicles, as they were called, were generally deployed on repair and maintenance duties and would spend extended periods trying to shallow bury exposed sections of subsea telecoms cable into the seabed. However, after a near fatal incident on an early CANTAT cable which ultimately resulted in a successful full scale rescue of a stranded Pieces vehicle, the industry looked to remotely controlled vehicles for the next generation of cable intervention. The first generation Free Swimming Burial ROVs were dedicated for repair and maintenance activities to assist in the repair of damaged sections of cable and 50
Cable types and installation techniques to then attempt re-burial of the final repair splice of cable. The Free Swimming Burial ROVs used jetting systems to attempt the cable burial, but with low power budgets they only had limited success in sandy seabeds. SCARAB 1 and SCARAB 2 were the first vehicles of this type. Both of the vehicles were equipped with subsea power packs, control and telemetry functions via a control umbilical and had depth capabilities of 1000m. As subsea ploughs were introduced for the burial of subsea telecoms cables, the Free Swimming Burial ROVs developed into more sophisticated ROVs capable of a range of subsea intervention tasks with larger power budgets for cable burial. This next generation of Free Swimming Burial ROVs had cable burial tools with low and high pressure jetting tools and dredge units to allow the vehicles to work in sandy, clay and gravel seabeds. The SCARAB 4 vehicle which came into service in 1990, was fitted with a state of art rotating cylinder jetting tool with a series of high pressure jets capable of working into 400KPa clays. Free Swimming Burial ROVs have generally stayed in the niche market area of cable repair and maintenance. However, these vehicles have seen significant technological development with jetting tools now capable of deeper burial using “jetting swords” which sit either side of the cable to be buried, and with depth ratings down to 2500 and 3000m. Table 3.12 provides a listing of a number of free swimming cable burial vehicles which are used worldwide. Some of the current Free Swimming Burial ROVs can interface to a tracked work package. This provides the Free Swimming Burial ROV the opportunity to have a stable work platform for burial operations and to revert to free swimming mode when inspection and intervention tasks are required as well as more manoeuvrability, especially in the deep water operations. Figures 3.20 and 3.21 show the CM ROV3 cable burial vehicle. This vehicle can operate either in free swimming mode or can have a modular tracked unit fitted (as shown in the Figures). Figure 3.21 shows the jetting lances in the fully deployed condition. Some Free Swimming Burial ROVs now have power budgets of over 300kW and are equipped with manipulators for handling tasks, together with cable cutters, cable grippers and burial tools fitted to both the forward and rear sections of the ROV. In addition jetting lances, which are fitted to the end of a manipulator arm, allow for localised burial. Dredging units fitted to Free Swimming Burial ROVs have a particular use when working far offshore in deeper waters, where seabeds can be encountered which consist of shell beds held together by fine sands. These seabeds are particularly difficult to trench using jetting systems, as the energy from the water jets deflects off the hard shell surfaces. This can be countered by using a combination of dredging and jetting as the dredging sucks in the sand and shells and breaks up the layer composition. 51
Page 1: REVIEW OF CABLING TECHNIQUES AND EN
Page 4 and 5: Review of Cabling Techniques and En
Page 9 and 10: Abbreviations AIS Automatic Identif
Page 11: TTS Temporary Threshold Shift TWA T
Page 20 and 21: 3 Cable types and 18 installation t
Page 82 and 83: 4 Physical change 4.1 Introduction
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Review of Cabling Techniques and En
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6 Good practice measures This secti
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7 Gaps in understanding Research fo
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References Introduction Sinden, G.
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Appendix A: Standards and codes of
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Printed in UK on recycled paper. De
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