CABLE PROTECTION FEATURE
ADDITIONAL PROTECTION Where additional protection is required, mattresses of interlinking concrete blocks are often used to stabilise and shield the cable. These can be installed from a vessel-mounted crane using a special frame and manoeuvred into position with the assistance of ROVs. They can also be fitted with fronds to encourage the settlement of mobile sediments to provide extra protection to the cable and prevent secondary scouring.
Rock dumping is also used widely to protect cables (and structures) in the offshore wind industry. Rocks are dropped from a specialised vessel either via a fall- pipe or by side dumping. Split hopper vessels are also used which can jettison their load by a release mechanism which effectively parts the hull of the vessel. Rocks must be sized and berms designed to ensure that they can withstand the hydrodynamic loading as well as being of a suitable size that scour around the berm is not initiated by their presence.
Another possibility for cable protection comes in the form of cable-mounted articulated shells made from polyurethane or cast iron. The extra armouring provides impact protection and additional ballast to the cable. The shells are typically formed as pairs of interlocking half-pipes in 0.5-2m lengths which are strapped or bolted together around the cable whilst still on deck. They can also be retro-fitted underwater, although this can be difficult and time-consuming particularly where the cable is already in contact with the seabed.
OTHER VULNERABLE AREAS The cable is also vulnerable as it approaches the wind turbine structure, between the seabed and the termination point on the work platform. The cable commonly enters the structure via a J-tube or an I-tube (so-called because they form the shape of the corresponding letter) which protects it through the splashzone. Where the cable enters the tube, a bellmouth is fitted to prevent overbending and facilitate installation and a Cable Protection System (CPS) is used to manage the shift between the dynamic subsea environment and the rigid tube. It is commonly made from moulded polyurethane elements which provide a gradual transition in stiffness, dampening the movement of the cable and preventing overbending.
J-TUBES The use of J-tubes in particular is widespread in the offshore wind industry, and installation contractors tend to be familiar with the issues associated with cable pull-in into such systems. When used in conjunction with monopile foundations, however, they have to be fitted after the pile has been driven to avoid being displaced by the impact. To avoid the need for this operation, a number of products have been designed which avoid the need for a J-tube by protecting the cable through a reinforced aperture in the base of the pile. Once inside the monopile, the cable spans freely, in the absence of hydrodynamic loading.
CABLE-MOUNTED ENTRY SYSTEMS Cable-mounted entry systems are functionally similar to the CPS described previously, but they also effectively act as a permanent guide to the cable, managing the transition between horizontal and vertical alignment. The system will feature a latching mechanism to secure and seal the cable to the wall of the monopile at the entry point, which may be actively or passively operated. A second, smaller polyurethane assembly is located on the front end of the device to protect the cable against overbending on the inside of the pile. Such systems are still in relatively early stages of development.
Wood Group Kenny
www.woodgroupkenny.com
www.windenergynetwork.co.uk
79
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100