only in the 1970s , that cable burial was introduced as the optimal protection technique . The standard burial depth was approximately 0.6m for many years mainly to provide protection against fishing nets and trawls .
A statistical analysis done by the US Department of Energy on 1061 submarine cable faults and accidents in various sea areas showed that 82 % of cables were damaged by external activities such as ship anchors , icebergs , fishing activities , seabed relief or even shipping containers that fell off vessels while the remaining 18 % was due to internal faults [ 5 ].
Today trenching machines have widespread use and are classified based on their operational mode . There are four major categories , including : ( i ) burial sleds , ( ii ) burial remotely operated vehicles ( ROVs ), ( iii ) tracked burial machines , and ( iv ) burial ploughs [ 7 ] ( see Table 1 ).
In normal circumstances , subsea cable burial projects are seldom carried out using a single trenching method due to variable conditions of the seabed and depending on the depth and location . Because of these factors , several burial technologies may be selected according to their different range of application . Figure 1 shows various trenching methods with different capabilities .
2.0 PRINCIPLE
The ROV-based jet trenchers are often preferred to the other trenching methods in deep-sea soft soil regions , due to their excellent versatility and operability . In reality , except for some trenching tools such as cable ploughs and pipeline ploughs , conventional trenching devices are heavy and complex machines ( with several moving parts ), which makes them difficult to control in deepwater conditions [ 6 ].
The jet trenching method modifies the initial seabed matrix with high-speed water jets so that the heavier pre-laid ‘ products ’, cables or pipelines , sink to a pre-determined depth , dragged down by their own weight . As shown in Figure 2.1 , this modification is usually achieved through erosion / fluidisation or cutting and relies on specific methodologies [ 6 ].
The basic configuration of a trenching ROV is similar to the common ROV with the additional features of a jetting unit . In most circumstances , the power rating of a trenching ROV is higher than the normal work-class or heavy work-class ROV which can vary from 200HP to 1000HP . Significantly higher power rated machines are very rare as there are only a few manufacturers involved in manufacturing such units .
Figure 2.1 : General principle of the jet trenching method : Conceptual illustration ( adapted from Warringa et al . [ 9 ])
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