Jetting Trencher

A jetting trencher is generally active in nature with the jetting tool being typically mounted on either a tracked or free-flying ROV. A jetting trencher is fitted with a series of water jets that are housed in “swords” or arms which are positioned either side of the cable. The arms effectively fluidize the immediate seabed in non-cohesive sands and silts or “cut” the seabed to form a trench in soft clay in which the cable can be positioned. The trench is normally between 20cm and 40cm in width. In non-cohesive sands and silts the cable sinks through the fluidized sediment under its’ own weight or if needed a depressor arm can be used; the backfill of the trench where the cable is positioned is therefore almost immediate. In the case of clays the trench is usually allowed to backfill naturally.

Jetting trenchers can be deployed via a vessel/barge direct, an ROV or even on self-propelled tracked vehicles. Control of the jetting trencher is always provided by an umbilical back to the control vessel/barge.

Jetting trenchers can be used in a range of sediments, but in soils with either significant shear strength and cohesiveness (i.e. firm to stiff clays) or very coarse sediments (e.g. gravels) burial may not be possible.

Ploughs are essentially passive in nature generally being towed behind a vessel. The ploughs remotely controlled steering and motion is provided by vessel/barge via an umbilical/towing cable. Plough performance is directly related to the residual towing tension experience during operations and this in turn dictates the speed of the vessel/barge pulling the plough. Ploughs are capable of burying cables in a much wider variety of soils than jetting trenchers e.g. firm to very stiff clays and coarse gravelly sediments.

There are two distinct types of plough; displacement and non-displacement.

Displacement Ploughs

The plough cuts an open V-shaped trench which allows the cable to be laid into it. During this process a large amount of seabed material is displaced and the plough can be used in most types of sediment due to the sheer force of the forward movement. The trench can be up to 5m wide, with the plough’s footprint being up to 10m. Due to the generally large size of the plough, a very high bollard pull is required from the vessel/barge; in excess of 150 tonnes. This type of plough is normally used for pipelines and the open trench will either back fill naturally or backfilled by using a series of blades either mounted to the rear of the plough or on a separate plough. Due to the size of vessel required to pull this type of plough it is generally not suitable for shallow work i.e. <10m.

Non-Displacement Ploughs

The plough is designed to slices through the seabed, which is achieved by a shear. The shear is often thin in design, almost blade-like, which does not create a permanent trench meaning minimal disturbance is created on the seabed. The bollard pull required is generally much less than displacement ploughs, which also makes the overall operation less expensive from a vessel resource allocation standpoint.

The trench created is very narrow, normally between 30cm and 1m depending on plough size. While the plough will still have a significant footprint, up to 8m, another distinct advantage is the trench created backfills immediately due to the design of the plough.

A vibro non-displacement plough can also be used for areas of stiff clay, but the operating water depth is generally limited to less than 10m.

Carousels are used for the deployment of non-coilable flexible pipelines, umbilicals and power cables.

Tensioners are used for installation of power cables and umbilical's

A hydraulic hammer is a modern type of piling hammer used in place of diesel and air hammers for driving steel pipe, precast concrete, and timber piles. Hydraulic hammers are more environmentally acceptable than the older. They generate less noise and pollutants.