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An Insight Into Trenching, Subsea Ploughing & Offshore Operations

Author: Marky Tyrell
by Marky Tyrell
Posted: May 15, 2015

Offshore operations of gas and oil industries have been pioneered by innovative thinkers and risk takers. Trenching and other subsea procedures have been pushed into deeper waters that are far from the shore. As technology advances, companies experience more challenges and greater risks. As a result, the industry has responded by prioritising safety and system integrity.

Innovations have to undergo industry-standard evaluations to ensure that they are ready for the risks. There are some companies that continue to push the limit and introduce innovations and, as a result, the subsea process market continues to grow.

Offshore pipe laying on the seafloor is a challenging process. The deeper the water, the harder it is to accomplish trenching and offshore cable burial. There are three ways in which subsea pipe is installed. These are J-lay, tow-in or S-lay. With the tow-in system, the pipeline is assembled onshore and then towed towards the location. In the S-lay system, the pipes are assembled at the installation site. The vessel has all the equipment needed for joining the pipe segments. The J-lay system is used in very deep water, where S-lay system is not applicable because the end of the pipeline might go straight down. Then there’s the Reel-lay system, where the pipeline is assembled onshore and then placed on a vessel that will then go out to the offshore location.

One of the factors that can affect the pipeline installation is the buoyancy, which affects the process both in negative and positive ways. When submerged in water, the pipe weighs less when filled with air. This puts less stress on the barge. But once the pipe is resting on the seabed, the pipe must be applied with a downward force to keep it in place. The force can be provided by the substance that will pass through the pipeline. The problem is when the substance is gas, which doesn’t weigh enough to keep the pipe down. In shallow water, the solution is to pour concrete over the pipe to keep it in place. But in deep water scenarios, the right thickness and the insulation of the pipe must be determined to ward off hydrostatic pressure and keep the pipeline in place.

Trenching and Offshore Pipeline Burial

Pipeline can be laid inside a trench that will protect the cable from anchors, fishing gear, and trawling activities. Trenching is also recommended in shore approaches to protect the pipes from wave action and currents. The process can be done before the laying of the pipeline or after the pipeline has been laid. In the post-trenching method, the trenching device is placed on top on the pipeline and removes the seabed underneath it. There are several trenching systems used in offshore operations. These are jetting, mechanical cutting, subsea ploughing, and dredging or excavation.

Jetting is a post-trenching process that removes the soil underneath the pipeline with the use of powerful pumps. Water is blown on each side of the pipeline to remove the soil. Mechanical cutting utilises cutter disks or chains to dig through harder soils and boulders below the pipeline. Subsea ploughing was first used for pre-trenching but it has evolved into complex systems that are lighter in size. Modern ploughing devices are safer and faster than their predecessors. Dredging or excavation is often used in shallower water. A dredger is used to remove the soil before the pipes are placed. Dredging can be done in several ways that includes with the use of a backhoe or buckets.

It is common knowledge that a buried pipeline is better protected than a pipe that is placed in an open trench. Pipes are often covered with rocks from a nearby shoreline. Soil that has been excavated from the seabed during the trenching can also be used as a backfill. The disadvantage of offshore pipeline burial is that it is harder to locate a leak when it arises. The ensuing repair operations will also be difficult.

Subsea Lifting

As offshore operations go deeper, the risk connected to subsea lifting operations has been increasing. The development of lifting devices has been driven by the demand of higher lifting capacity, offshore operations in deeper waters and compensating subsea movements. Subsea lifting involves moving objects to be placed on the seabed or remove objects from it.

Through the use of devices specifically designed for subsea lifting, the team can lift, hover, or lower any object underwater. Technology has allowed people to control buoyancy in order to handle all the lifting needs of the offshore operations.

Ecosse Subsea Systems is a subsea technology, equipment hire and offshore engineering consultancy specialising in offshore cable lay, subsea lifting, subsea plouging.

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Author: Marky Tyrell

Marky Tyrell

Member since: Dec 05, 2014
Published articles: 68

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