How do you safely remove a stack of concrete higher than Mount Everest from the sea floor?
The subsea engineering challenge
The scale of the challenge for removing subsea mattresses has been calculated for the North Sea alone to be in excess of forty thousand mattresses. Piled on top of each other these mattresses would exceed the height Mount Everest. The global challenge is even greater.
The removal of these mattresses is laborious, challenging and potentially hazardous/dangerous due to the proximity of these mattresses to critical subsea infrastructure. The condition can vary wildly from mattress to mattress and so there is no one-size-fits-all solution but Fathom focused on how we could deliver a system to solve as many challenges as possible. Recovery operations are expensive which is why in 2019 Fathom Group took on the challenge to find a better way to tackle this industry challenge. We were honoured to receive a Decommissioning Challenge Fund award for this project and are thankful to the team at the Scottish Government for their support on the project, as well as the team of engineers, operational specialists and suppliers who supported it.
Our solution
To take on the challenge, FATHOM Managing Director Richard McGowan conducted detailed research, working closely with operational specialists, to fully understand the challenge and constraints of the mattress recovery operations, laying out many possible concepts which could solve the multitude of challenges faced across the whole operation. Meanwhile, Technical Director Krzysztof Mackojc developed a through BoDA using his knowledge of subsea engineering and marine operational analysis to ensure robust design and analysis of the short-list of concepts. The final FATHOM Modular Recovery System (MRS) developed allows for diverless removal, whilst also incorporating several efficiency improving features with the potential to reduce recovery costs by 50% compared to more traditional diver methods.
Initially targeting the industry challenge of safe and efficient mattress recovery, the system has been developed with our holistic approach to ensure efficiency during every step of the operational life-cycle including:
1) Low fabrication cost
2) Ease of transport & mobilisation
3) Operational effectiveness & adaptability
4) Safety and efficiency of subsea & deck operations
5) Ease of maintenance, repair & adjustment
6) Minimised environmental impact, allowing re-use and re-purpose of mats instead of costly land-fill
7) Modular design allowing for adaptation to client specific operational procedures and a further range of subsea handling activities, including:
- Pipeline Recovery
- Boulder Clearance
- Mattress & Scour Protection Deployment
- Shipping Container Recovery
- Chain and Crane Wire Recovery
The impact
Taking conventional methodologies as our base case the cost saving per mat recovered are significant. Taken across a campaign the savings are as much as 50% over diver employed methods. The large cost savings come from the removal of the DSV and the increased efficiency from the improved handling, minimised deck operations and logistics.
What we learned
Not all of our initial assumptions were correct. For example we originally envisaged unloading the mattresses after each run, cutting down on equipment costs. However, during the technology validation we listened to the end-users and revised the operating methodology to reduce double handling during mattress removal, and the risks and restrictions related to SIMOPS. We also noted that the approach and system was readily transposable to other sectors such as offshore renewables and salvage.
We’d be keen to hear from any groups looking to decrease the time and cost of mattress recovery and would be happy to share more information about this exciting design.
