Controlled Buoyancy Subsea Recovery Device Concept for cost effective seabed recovery
In 2017, the North Sea Transition Authority (NSTA) outlined a goal of reducing the UKCS decommissioning costs by 35%. One solution considered to help achieve this was the development of a buoyancy controlled subsea recovery device capable of lifting between 300 and 500 tonnes, to effectively remove equipment from the seabed for return to shore without the requirement for a heavy lift vessel.
This concept was researched by a University of Aberdeen student as part of the MSc in Decommissioning. Over a three-month period, a high-level feasibility study focussed on an evaluation of a controlled buoyancy subsea recovery device designed to lift a 300-tonne payload from the seabed – specifically the removal of a subsea mattress. By using the controlled buoyancy subsea recovery device, it could remove the requirement for divers and a dive support vessel, instead using a smaller vessel such as an anchor handling vessel.
The study, supported by NZTC, compared the controlled buoyancy subsea recovery device to existing mattress recovery technology and methodologies in the market, comparing potential cost savings.
Controlled buoyancy subsea recovery device technology can be used to recover concrete mattresses and other subsea infrastructure from the seabed. With more than 40,000 concrete mattresses alone in the UKCS, this method is more cost effective than complex heavy lift operations.
The initial study acknowledged feasibility of the technology, suggesting further development and understanding of the market. The study highlighted a prototype would be beneficial in confirming fabrication and operational requirements.
NZTC supported a further in-depth technical and commercialisation study, undertaken by Sealand Projects Limited. This second study compared the controlled buoyancy subsea recovery device methodology to existing mattress removal technology. It also assessed the technical, commercial and reputational risks of the concept.
The report concluded that the submersible buoyancy device could be used to recover not only concrete mattresses, but other subsea infrastructure from the seabed. It could also be utilised for subsea debris clearance utilising a less expensive vessel such as an anchor handling vessel.
Lessons learned / Next steps:
The Sealand Projects study concluded that the controlled buoyancy subsea recovery device is a technical, feasible concept and does not represent an increased technical risk than other towing operations in the marine environment. The technical review concluded the concept is considered structurally sound and the functionality applicable to decommissioning as well as installation activities.
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