KNOWLEDGE HUB
Subsea Infrastructure Removal and Site Remediation
Subsea infrastructure removal and site remediation represent one of the final chapters of decommissioning, a complex series of subsea operations undertaken to return the seabed to its original state. While much of the industry’s focus remains on topside removal, subsea infrastructure removal and site remediation typically account for 12% to 15% of total project expenditure and involve managing thousands of tonnes of equipment that may have remained in an unclear condition for decades.
The Framework: WBS 9 and 10
The OEUK Work Breakdown Structure (WBS) is a common industry framework used to classify decommissioning-related activities. It categorises subsea infrastructure removal and site remediation within the following phases:
Subsea Infrastructure Removal - WBS 9
Covers all subsea infrastructure, excluding the jacket, including manifolds, flowlines, umbilicals, wellheads, and anchors. This phase forms the operational backbone of subsea infrastructure removal.
Site remediation - WBS 10
Focuses on Site Remediation, the final cleanup of non-oilfield debris once all major subsea infrastructure has been removed. This can include scaffold poles or items dropped overboard at any point during construction, operations, or decommissioning activities. This phase concludes the final offshore decommissioning scope.
The ultimate goal, and ideal, of subsea infrastructure removal and site remediation is to achieve a clear seabed. However, operators may apply for derogations to leave certain items, such as buried pipelines, in situ under certain conditions.
Key Components and Removal Methods
Successful subsea infrastructure removal can include the recovery of the following items:
Concrete Mattresses
Used for protection and stabilisation, concrete mattresses can be recovered via ROV-connected lifting hoops or more efficient Mattress Recovery Tools (MRT). Within subsea infrastructure removal and site remediation campaigns, traditional methods may take hours per mattress, whereas an MRT can recover five to six per hour. Stacking mattresses offshore into a single lift significantly improves safety and efficiency during onshore offloading.
Christmas Trees
Subsea trees, commonly known as Christmas trees, typically weigh around 30 tonnes, with some reaching 60 tonnes. These structures control well flow and are recovered using a dedicated lifting tool following safe unlocking and disconnection procedures.
Flexibles and Umbilicals
Flexibles and umbilicals are typically decommissioned using a reverse installation method with a lay spread vessel or by using a hydraulic shear to cut them into sections for storage on an anchor handler’s drums as part of subsea infrastructure removal and site remediation scopes.
Pipelines
Pipelines are often left in situ under certain conditions, for example if they are buried and properly cleaned. However, exposed or surface-laid sections can be cut and recovered in manageable lengths to the back deck of vessels for transport to disposal yards.
Manifolds
Manifolds are subsea structures that house valves and instrumentation to control and collect the flow of production fluids from multiple wellheads before directing them to a topside facility.
Bundles and Towheads: Integrated Flowline Systems
A bundle is a complex, integrated system in which multiple flowlines, such as production, gas lift, and water injection lines, along with umbilicals, are housed within a single large carrier pipe.
Specialised Tooling and Recovery Technology
Efficiency in subsea decommissioning is driven by selecting high-specification technology tailored to the unique size and complexity of each structure. In addition to diamond wire saws, several other specialised tools are critical for the severance and recovery of infrastructure from the seabed.
Shears, Grabs, and Recovery Tooling
These tools are the workhorses of subsea infrastructure removal, designed to handle high volumes of material recovery:
- Shear Cutters: These robust tools are used for severing pipelines, umbilicals, and flexibles. To be efficient, they must be capable of performing thousands of cuts across a campaign without requiring frequent maintenance.
- Pipe Grabs: Once pipelines have been severed into manageable sections, hydraulically driven pipe grabs are used to physically recover the sections from the seabed to the vessel deck.
- Orange Peel Grabs: For site remediation under WBS 10, these grabs are used to pick up smaller debris and non-oilfield items, such as scaffolding poles or items dropped overboard, and place them into subsea baskets for recovery.
- Mattress Recovery Tools (MRT): Using a series of hydraulic rams, an MRT grabs underneath concrete mattresses to recover them in a single lift. This is significantly more efficient than traditional ROV hoops.
High-Pressure Water Abrasive Jetting and Mechanical Cutting
For structural severance, particularly below the mudline, high-pressure water jetting, mechanical cutting tools, and emerging laser cutting technologies are used depending on the structural configuration and material composition.
- Pile and Leg Severance: These tools enter the top of the pile and sever it at the required depth, typically three metres below the mudline, without the need for extensive external dredging. This reduces cost and minimises disturbance to adjacent structures. Alternatively, externally cutting methods can also be used, with tools assessed on a case-by-case basis.
- Wellhead Recovery: The same technologies can be used in a single-trip operation to sever concentric well casings.
Specialised Interfaces
- Christmas Tree Lifting Tools: These tools hydraulically clamp onto the interface on top of a Christmas tree to allow controlled recovery to the vessel.
Ultimately, selecting the right tooling is paramount for optimising project delivery and improving health and safety outcomes. Using specialised providers to design bespoke shears, internal cutters, and recovery systems ensures that subsea infrastructure removal and site remediation remain as safe and cost-effective as possible.
Real World Professional Learning Platform
To gain real-life insight into the realities and strategic drivers of subsea infrastructure removal, you will develop a detailed understanding in a lesson delivered by Mike McHardy of DOF in the Lean Decom training course. This session moves beyond technical theory to see behind the scenes of real world execution.