Cable burial tools

Most commonly, this is expressed as a single burial depth for a project. Over recent years typical specification burial depths have increased from around 0. How these burial depths are selected is often not clearly defined.

What may not be appreciated is the significant cost associated with such depths and how the cost of burial can increase with depth. Clearly a seabed comprising stiff clay, or dense sand, provides a higher level of protection to the cable than a soft clay or loose sand. Having identified the appropriate burial depths, it is necessary to select suitable equipment which will achieve the required burial depths efficiently. The primary burial technique currently in use is a cable plough.

As this is, essentially, a passive tool, there are minimal moving parts and high reliability is possible together with rapid trenching speeds. In certain circumstances, such as in deep water it is necessary to use alternative burial techniques. The use of such techniques is discussed together with the soil conditions in which they are capable of operating.

The primary human threats are fishing and anchoring, while sediment mobility and submarine slides are the primary natural threats. These threats have been discussed in some detail by other workers eg Shapiro et al, , Evans, and it is not proposed to discuss them fully here, however some notes are provided which expand on the published work. The different types of fishing operations have particular characteristics which affect the threat they pose to a cable. With the notable exception of shell fish dredging, fishing techniques are generally intended not to penetrate the seabed to any great depth.

Trawling is one of the most common forms of fishing and the one most often associated with damage to cables. The net is held open by trawl doors otter boards that are typically of 2 to 4 tonnes in weight, but greater than 10 tonnes is possible.

However, trawl doors are designed to skim over the surface, and inspection of wear marks suggests that in most seabed, penetration would be limited to or mm. An alternative method is by a beam. Shell fish dredging using toothed dredges which scrape the seabed can penetrate the seabed to a depth of mm in a single pass. However, multiple passes are probable and this potentially extends the depth of penetration.

Note may be made of the fact that shell fish generally prefer sandy seabeds which have a reasonable degree of stability. As fishermen generally have a very good knowledge of the seabed conditions within their particular area, it would be unusual to find a high degree of such fishing in, for example, an area of sandwaves. Bottom set fishing, placing nets or pots on the seabed to catch fish over a period of time, generally uses relatively small equipment and only poses a low degree of threat to a correctly buried cable.

However, the technique does pose some particular problems, for example lobster fishing is performed in rocky areas where cable burial is particularly difficult. While anchors of many types are available for different applications, the most significant in the context of submarine cables are ship anchors. Ship anchors are designed to penetrate into the seabed to generate the maximum holding power for the weight of anchor.

However deep penetration is not desirable in practice as this increases the difficulty in recovering an anchor. Therefore anchors are generally designed to skim over the surface of the seabed, supported by the shank, with only the flukes penetrating in firm seabeds.

It is therefore possible to accurately predict the maximum penetration of a ships anchor in most seabed soils. The sizing of a ship anchor is dependant on a number of factors, with guidance on appropriate sizes being provided by Lloyds classification rules.

These rules assign a number to a ship based on weight, physical dimensions and proposed use. The recommended anchor size is given purely on weight, with no consideration given to use of high holding power anchors which are specifically designed for deeper penetration. However, most ships use the standard Admiralty pattern stockless anchors and therefore, using the Lloyds rules, and dimensions for standard anchors, it is possible to make a rapid assessment of the likely size and penetration of an anchor in most seabed soils.

Of these, anchoring accounts for approximately one third. The databases indicate that most faults occur in water depths of less than m, however as this accounts for most of the continental shelf, this is not surprising. This implies that anchoring threats are limited to relatively shallow waters and therefore smaller ships and anchors.

Therefore, it should be possible to identify the level and extent of risk from anchoring and to select an appropriate burial depth. Although fishing and anchoring may account for two thirds of all cable faults, natural hazards must also be assessed.

These hazards take many potential forms and the normal preference is to route round a particular threat such as a sand wave field or an area with an unstable slope. However, as the seabeds become increasingly congested this is not always possible and cables have to be routed through such areas. The variability of natural hazards makes a review on an individual basis an essential requirement.

However it is possible to assess the conditions under which, for example, sands will become mobile and the rate of mobility. Once the threats have been identified, some assessment should be made of the likely depth of interaction. This will be a function of the geological and metocean conditions along the route.

The cable route survey should be designed to provide sufficient information to carry out this assessment. Problem areas should be investigated by techniques such as cone penetration testing CPT and sampling. Lateral correlation by other techniques such as resistivity or seismic refraction is possible, however these techniques do not, in themselves provide the required engineering parameters. Additional work that may be required, particularly as part of a sediment mobility study, are some simple laboratory tests on soil samples.

There is very little documented data on the depth to which normal fishing gear penetrates in different soil types. However it is known that burial to between 0. Unfortunately no detailed soil strength data is available to assess how this factor affects cable protection. However, there is some evidence to suggest that reasonable protection would be provided at between 0. The deepest point of penetration of most fishing gear will be when it is dropped to the seabed. As such it may be likened to a dropped object which will penetrate the seabed to a depth dependant on its weight and the bearing capacity of the soil, effectively acting as a retarding force.

This was intended to quantify the protection provided by seabed soils. A tentative correlation was proposed for a range of soil types. Based on a burial depth of 1m in 40kPa clay, with extrapolation to a range of strengths, a recommended depth to achieve a BPI of 1 is presented. This estimation allows for increasing depth of burial with reducing strength of a clay seabed.

Applying the same methodology to an increasing clay strength is not fully valid. A certain depth of burial is required as the lay tension will tend to reduce the apparent buried depth in any undulating seabed. A nominal minimum depth of 0. The resulting optimum burial depth is presented as Figure 2. This is due to the different geotechnical strength parameters associated with a sand.

However it is possible to apply the analogy of the depth of penetration of an object using a correlation with bearing capacity. This is most reliably measured by in situ tests, such as cone penetration tests which are routinely performed as part of most cable route surveys.

The results of this exercise are presented as Figure 3. It is stressed that at this stage, this assessment is tentative only and makes no allowance for the mobility of sands under the action of waves and currents. The frictional nature of sands means that strength is derived from the weight of overlying soil. However, a progressive increase in burial depth with reducing relative density is recommended to allow for the lower level of protection provided by loose sands.

Equipped to bury up to 2. Webwork by Ape Red Media of Romsey. Power Cable Subsea Jetting Sleds. Power Cable Laying Jetting Sleds. Outline Spec. Burial Tools: Trench depth 2.

Control system housed in a rugged transit case and includes: HMI for graphical display of vehicle Deployment ram controls RS survey data link Analogue inputs for optional deck equipment e. The following sensors are fitted: Burial depth Pitch Roll Subsea jetting pressure Surface jetting pressure Surface tow tension Analogue inputs are available subsea for optional additional sensors e. Hydraulic System: Deployment ram, subsea specification with spherical bearings both ends, fitted with ROTA linear transducer Directional control valve with speed control, relief and check valves, fitted onto hydraulic power pack 7.

Burial Tools: Trench depth 3. Control system housed in 2 rugged transit cases and includes: 2 x monitors for video 4 channel DVR PC based control system with touch screen control PDU and SPS for supply, monitoring and safe operation of pod, subsea HPU and subsea water pump Deployment ram controls RS survey data link m umbilical fitted on an umbilical winch.