On the seabed in most parts of the world where oil and gas production takes place, there are many pipelines and cables, which must be crossed when installing a new pipeline. To prevent damage to the existing pipelines and cables when installing a new pipeline across them, ‘crossing preparations’ must be performed.
Mattress installation
At the location where a new pipeline needs to cross existing infrastructure, ‘mattresses’ will be installed by Allseas’ Calamity Jane. A ‘mattress’ consists of concrete blocks connected by steel or synthetic cables. The pipeline will be installed on top of the protective mattresses.
Trenching of pipelines
To protect an installed pipeline on the seabed, for example from damage by the trawler boards of fishing boats, currents or other forces, many pipelines in water depths up to 350 m are trenched. Allseas’ Digging Donald, a unique mechanical trencher, digs a V-shaped trench underneath the pipe, in which the pipe gradually sinks behind the machine.
Digging Donald is capable of trenching pipelines with an outer diameter of up to 48 inches in a wide range of soils, achieving a trench depth with a maximum of 2.1 m (7 ft) depending on the soil conditions, from the original seabed level by means of the mechanical digging arms and multi-pass jetting.
The trench can be backfilled by using Digging Donald with adjustable backfilling blades, which can be mounted on the trencher.
Trencher handling and operations are diverless. In order to ensure pipeline integrity at all times, Digging Donald makes no physical contact with the pipe during the trenching and backfilling processes.
Digging Donald can also be used for free-span rectifications and umbilical trenching.
In water depths from 0 to 25 m, Allseas’ Jet Sled, a shallow water jetting tool, executes diverless pipeline burial operations by jets and eductors.
