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 clay, with hydraulic fracture occurring very easily along existing fracture joints. However, it would be difficult to break the chalk into smaller blocks than already exist in situ, especially in medium to high density chalks. A dredging system used in combination with the jetting system would have difficulties in removing large rock fragments to create a trench. In most weak rocks including structured medium to high density chalk, other trenching systems will be more suitable, such as a plough with rock penetrating tooth or a rock wheel cutter. In structured weak rocks, the material would be mainly fractured/cut into sizeable blocks before much of the material could be eroded or fluidised. In this case, any trench would have to be formed by a dredging system, which would lift the blocks and deposit them adjacent to the trench. In structured soft, finegrained rocks, such as chalk, there will also be some disintegration of the rock into its constituent components, and this material could become suspended in the surrounding water and transported by the current some distance from the trench. However, as mentioned earlier, jetting is highly unlikely to be used for trenching in structured rocks. In structureless rocks, the material would be broken down into a wide range of particle sizes from clay or silt-size up to boulder-size. The clay to fine sandsize particles will become easily suspended in the water and transported long distances by the current. The larger sizes, that could only be removed using a dredging system, would be deposited close to the trench. As such, the impact will be a combination of the impacts for cohesionless and cohesive soils. 4.3.3 DREDGING SYSTEMS Dredging systems used on tracked cable burial machines (Section 3.8.3), ROVs (Section 3.8.4) and burial sleds (Section 3.8.5) can be used for a range of soil types including sand, silt and certain clays. However, the larger the particle size, the harder it is to move material in a slurry suspension. Dredging systems remove soil to create a trench by a process of suction. The suction tubes are sometimes referred to as eductors. The educted soil is normally “blown” into the surrounding sea to the sides of the trench, where it is also transported as suspended sediment by the current until it settles out on the seabed. Dredging systems work best when the soil is in a slurry state. For this reason they are normally used in combination with a jetting system that is used to fluidise the soil prior to dredging. Dredging systems can remove the soil material and deposit it on a barge or, more conveniently, disperse the sediment into the sea away from the trench. The dispersal distance and thickness of deposition will depend on the particle size and current speed, in the same way as for eroded materials. The impact that the dispersed sediment makes on the local aquatic environment will depend on how well adapted the marine life is to this type of sediment deposition. 86
4.3.4 ROCK WHEEL CUTTERS AND CHAIN EXCAVATORS Physical change In hard clays and rock, mechanical chain excavators or rock wheel cutters (Section 3.8.3) are often used to create a cable trench. In these materials, a narrow slot is formed into which the cable is lowered. The action of cutting the rock or hard clay causes the material to be broken down into its constituent components, such as sand for sandstone, and silt for siltstone, limestone or chalk. In order to form an open slot the loose material has to be removed. This material naturally mixes with and becomes suspended in the surrounding seawater. Due to the protection provided by the hard ground, the slot does not need to be very deep, typically only 0.5-1.0m. Chain excavators are also sometimes used in sands and gravels. The movement of the chain fluidises the granular soil in the vicinity of the cutter, forming a low resistance “slot” for the cable to be pushed through. In this case, because the soil is being fluidised without an open slot being formed, the disturbed material can and does largely remain contained within the ground. Thus, the amount of sediment becoming dispersed is minimal. The effect of using chain excavators or rock wheel cutters in hard clay or rock is to disperse clay to sand sized particles into the surrounding seawater. However, the mass of the suspended sediment will be limited by the size of the cut slot. This suspended sediment will be transported away from the trench to some distance dependent on the particle size and current velocity. 4.3.5 QUANTIFICATION OF DISTURBANCE Ranking disturbance An attempt has been made to rank the level of seabed disturbance resulting from cable burial operations discussed above. The results of this exercise are presented in Tables 4.2 and 4.3 where a ranking from 1 to 10 has been adopted with 1 indicating a low level of disturbance and 10 a high level of disturbance. Inevitably the tables present a simplification of a complex process but, nevertheless, provide a broad indication of the level of disturbance that is likely to occur when cabling burial operations take place in a range of seabed types. It is also worth noting that other seabed interventions such as offshore dredging or certain forms of aggressive fishing which filter the upper layer of seabed would have significantly higher rankings (see Table 4.4). 87
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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3 Cable types and 18 installation t
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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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