Experience from land-based and offshore installations of cables, umbilicals and pipelines has shown that the majority of faults are caused by external damage.
Protection of installations against external impact can be done by the following methods, alone or in combination:
external armour/protection applied to the cable/umbilical/pipeline;
trenching;
additional protection on seabed.
This contribution will cover more closely the trenching part of the protection aspect.
In most cases the most economical method of protection is achieved by trenching the cable/umbilical/pipeline just below the surface of the seabed. Total protection might be very expensive as this might require a substantial burial depth. The burial depth is often a compromise between the technical/economical aspects and the risk factor. If there is a very low probability of a ship's anchor hitting the subsea line, some companies disregard this possibility when burial depth is decided An effective burial depth will then normally be 0.7-1.5 m. Fishing gear is expected not to penetrate more than 0.3-0.6 m into the seabed.
Burial or trenching equipment can be classified roughly into five main groups according to their working principles:
ploughing equipment;
cutter/suction equipment;
water jetting equipment;
dredging;
drilling and blasting.
Equipment exists that combines the principles from two or three groups. It is designed to cover a wide range of seabed soil conditions.
There is a variety of equipment available. Most of the equipment was developed for trenching work for cables and pipelines in shallow waters As the need for burial at deeper waters increased, many contractors developed new equipment for subsea trenching. This new generation of equipment has later been modified to take care of both cable and pipeline burial. Most of the available equipment has therefore reached a considerable size that necessitates both high lifting capacity onboard the support vessel and high power consumption.
Owing to its large size and weight, there is always a possibility that the burial equipment itself may damage a subsea cable, pipeline or fixed installation. Once the subsea line is buried, possible damage may be concealed from visual inspection.
As remarked above, the market has for many years been dominated by heavy machines, often developed for trenching rigid, large pipelines. The CAPJET concept was based on the philosophy of directing the attention to the trenching operation itself and less to the support equipment.
The weight and the propulsion force of the machine should be so low that hazard to the subsea line during burial is negligible.
The weight (in air) of the machine should be so low that the equipment can be handled by a wide range of surface vessels.
Instead of a multipurpose machine, the capacity should cover at least 80% of actual seabed conditions.
There should be no or few wearing parts, so as to give low downtime during operations.
Environmental impact should be kept to a minimum.
