Review of Cabling Techniques and Environmental Effects Applicable

More documents

Recommendations

Info

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report for the planning of the offshore cable installation works rather than to assess the fate of disturbed sediment. 4.3 Level of Sediment Disturbance The level to which the seabed is disturbed is primarily related to the nature of the ground and the type of tool selected to bury the cable. The type of device used to support the tool is likely to have a secondary influence. This section discusses the application of the cable burial tools described in Section 3 in the range of ground conditions described in Section 4.2. The following cable burial tools have been considered: ● ● ● ● ● 82 Ploughs Simple Advanced Jetting Deep Burial Rock Ripping Vibrating Jetting Fluidisation Erosion Dredging Rock Wheel Cutter Mechanical Chain Excavator The section covers the likely effect of these systems in terms of the level of disturbance to the seabed. Section 4.3.5 attempts to quantify this disturbance by ranking the effects of the cable burial systems and provides limited advice on the calculation of the volume of sediment disturbed and brought into suspension. 4.3.1 PLOUGHING Cable ploughs (Section 3.8.2) are generally used in sand, silt, all types of clay and weak rock such as structureless chalk, which generally consists of sand to cobble-sized fragments in a matrix of silt. They can also be used in harder rock if the plough is fitted with a rock penetrating tooth, or in chalk and gravel beds if a vibrating plough share is used. The controlled operation by which cable ploughs work, “displacing” the sediment into which the cable is lowered, followed by the natural backfilling of the trench ensures that soil disturbance is kept to a minimum, and also minimises mixing between soil particles and the surrounding water. The type of soil most susceptible to mixing with water during ploughing is silt, because silt possesses no internal cohesion and the particles are small enough to be eroded by gentle water turbulence. Silt may remain in suspension for days giving the current chance to transport the sediment some distance away from the trench.
Physical change Structureless chalk, as for silty soils, is also susceptible to mixing and dispersion in the surrounding sea water during ploughing. 4.3.2 JETTING SYSTEMS Jetting systems are one of the most commonly employed cable burial tools used by tracked vehicles (Section 3.8.3), ROVs (Section 3.8.4), and burial sleds (Section 3.8.5). The mechanisms employed by jetting systems for developing a trench will largely depend on the soil type. Cohesionless soils In cohesionless soils, the soil can be liquefied or fluidised with jets at relatively low pressures (i.e. low jet exit velocities). Liquefaction or fluidisation occurs when the pore water pressures in the soil become equal to the total overburden stresses, reducing the effective stresses to zero. In both cases, the soil particles and water behave very much like a dense fluid. However, there is a distinct difference in the two conditions. In liquefaction, the volume and bulk density is more or less constant (Ishihara, 1993). In fluidisation, the water content is increased and the soil structure is completely broken down to give a material of lower density. An increase in the jet flow rate at low jet pressures simply causes an increase in the volume of the liquefied/fluidised soil (N.B. an increase in flow rate at constant pressure can only be achieved by increasing the total nozzle area). If the jet pressure/velocity is increased, the velocity of the fluid flowing over the surface of the soil will increase until eventually the soil particles are lifted off and transported away from the soil mass as suspended sediment. This is the process of erosion or scour. A trench in cohesionless soil is thus created by a process of erosion/scour. Jetting systems are sometimes used in combination with a dredging system to increase the rate of removal of soil (see Section 4.3.3). The problem with liquefaction in cohesionless soil is that the trench walls collapse and flow back into the trench. This means that a lot of soil has to be removed before there is a significant increase in trench depth. The final trench shape in cohesionless soils tends to have very gentle sloping sides (Lincoln, 1985). It may require several passes before a trench is created with sufficient depth. Due to the difficulty in forming a trench in cohesionless soils, some jetting systems do not even attempt to create one. The “fluidisation train” used for burying offshore pipelines in cohesionless soils is one such example (Boom, 1976). The train works by fluidising the soil beneath a pipe so that the pipe can sink under the combined weight of itself and the train. Water is injected into the soil around the pipe at relatively low pressure, which is just enough to cause fluidisation but not necessarily erosion. The likely effects will depend on which mechanisms take place. If a trench is formed by erosion, a substantial amount of material may be transported away 83
Page 1:
REVIEW OF CABLING TECHNIQUES AND EN
Page 4 and 5:
Review of Cabling Techniques and En
Page 6 and 7:
Page 9 and 10:
Abbreviations AIS Automatic Identif
Page 11:
TTS Temporary Threshold Shift TWA T
Page 14 and 15:
Page 16 and 17:
Page 18 and 19:
Page 20 and 21:
3 Cable types and 18 installation t
Page 22 and 23:
Page 24 and 25:
Page 26 and 27:
Page 28 and 29:
Page 30 and 31:
Page 32 and 33:
Page 34 and 35: Review of Cabling Techniques and En
Page 82 and 83: 4 Physical change 4.1 Introduction
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
6 Good practice measures This secti
Page 142 and 143:
7 Gaps in understanding Research fo
Page 144 and 145:
References Introduction Sinden, G.
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157:
Appendix A: Standards and codes of
Page 158 and 159:
Page 160 and 161:
Page 164:
Printed in UK on recycled paper. De
show all

Review of Cabling Techniques and Environmental Effects Applicable

Create successful ePaper yourself

Delete template?

Save as template?