As oil and gas fields deplete and offshore wind farms reach end-of-life, the offshore structures must be dismantled, removed and recycled. DeepOcean is one of many supplier companies that have specialist capabilities and extensive experience in such services.

Prior to any offshore work, early-phase feasibility studies and detailed environmental impact assessments are conducted to ensure that forthcoming offshore operations can be managed safely and without any unnecessary harm to the environment.

In some instances, following the environmental assessment, ‘leave in place’ is the better choice, as infrastructure that has been in place for many years may create new ecosystems which have greater biodiversity than surrounding areas. If the removal of the subsea infrastructure is considered to be beneficial, then restoration of the good environmental status of marine ecosystems in the execution of the project is paramount.

Dunlin Alpha 
One project where removal of subsea infrastructure was deemed the most environmentally responsible choice was at the Dunlin Alpha field in the UK sector of the North Sea.

The Dunlin Alpha platform was installed in the 1970s when North Sea development was in an earlier phase. The platform was located approximately 100 nautical miles north-east of the Shetland Isles.

The Dunlin Alpha topsides was removed in 2022, and all seabed decommissioning and clean-up activities were already concluded when DeepOcean was in March 2023 awarded the final decommissioning work-scope for the field.

The scope of work entailed the removal of the upper and central conductor guide frames, estimated at around 400 and 250 tonnes each (in air), respectively, and measuring approximately 50 by 10 metres in size. Both guide frames spanned two platform legs horizontally at 10 and 70 metres seawater depth. The scope also included the removal of six problematic 32-inch conductors that were unable to be removed by the topside decommissioning contractor, due to various stuck drill strings and/or previous external repairs preventing them being retracted through the guide frame. The physical offshore work was scheduled for July 2023.

At the initial risk assessment/engineering evaluation stage, it was decided that the following constraints needed to be applied to deliver the safest, most cost-effective and practical solution for removal of the subsea equipment.

If the removal of the subsea infrastructure is considered to be beneficial, then restoration of the good environmental status of marine ecosystems in the execution of the project is paramount.

Minimising crane connection times  
The first constraint was to minimise crane connection times during the guide frame cutting process. This also provided an option to abort should problems arise during the final cutting/release process, or any other factors arising that would either prolong the activity past an acceptable crane connection time or affect the ability to remove structures.

DeepOcean’s solution was to design, manufacture and install special clamps at each corner of the guide frames. Structural assessment showed that the structures were able to support their own weight via the internal horizontal diagonal supports, while each corner would be individually cut using a diamond wire saw, and a ‘strongback’ clamp fitted to bridge the cut, thus supporting the weight of the guide frame temporarily until all four were installed. Once all four were installed and the frames resting on their clamps on all four main structural members, the remaining internal members were cut, lift rigging installed and the guide frames lifted free of the platform structure within a relatively short timeframe.

Minimising operations in shallow areas 
The second constraint was to simplify and minimise operations in the shallow areas to maximise remotely operated underwater vehicle (ROV) operations and limit environmental downtime.

Simplification came in the form of lightweight sectional HMPE (high modulus polyethylene) rigging installed on all four corners, choked around the main structural members and utilising ROV shackles, with suitable rigging length to allow connection deeper than the upper guide frame. This limited exposure to environmental conditions for this task.

Safe cut and recovery 
The third constraint was to provide a workable solution to safely cut and recover the vertical conductors directly to the vessel deck with no risk of dropped objects. This had to be done while navigating around the relatively unknown integrity of the outer conductors and connectors and historic repairs which prevented direct vertical lifting through the existing guide frame conductor slots.

Given the restricted access by vessel crane in many locations, all tooling was developed and selected to allow the ROVs to manoeuvre into location and operate independently from the vessel and crane. Rigging attachment points were added by drilling into each conductor and through the internal drill string, followed by the installation of securing/lifting pins.

The pins not only provided a suitable rigging attachment point but also structural integrity to the weaker outer conductors. Approximately seven pins per conductor were installed to allow shorter sections to be cut and recovered safely to deck. All cutting was conducted using diamond wire saws, selected for their size and manoeuvrability and outfitted to allow the ROVs to independently manoeuvre into place.

Full structural assessment 
The fourth constraint, which is common in long established oil and gas fields, was limited original design information and historical data. This meant that a full structural assessment and analysis was needed to quantify general integrity. A weight assessment was conducted of the original structure, including consideration for marine growth, grout filling and/or flooded members. Further evaluations were made of historic repair clamps, in addition to identifying suitable lift points along with the ability of the structures to remain fully intact during the removal process.

Reverse engineering and modelling conducted in-house gave conservative assessments of structural integrity and weights, and also included dynamic analysis to determine rigging requirements and suitable safety factors to widen the environmental parameters as far as practical to allow safe and successful lifting operations at all stages. This engineering study also determined that the most practical method of recovering the guide frames to the vessel deck was to place the removed frames on the seabed, and further downsize. This significantly de-risked and simplified the deck lifting aspect, where smaller sections were lifted to deck. This also improved environmental parameters for the activity significantly.

Extensive engineering evaluation and modelling from DeepOcean’s in-house engineering team, and close collaboration with the offshore team onboard the construction support vessel Edda Freya helped to minimise risks and simplify challenges of such a heavy and complex scope.

The project also required significant collaboration with specialist subcontractors, together with whom subsea diamond wire saws, drill and pin tools were identified, developed and tested specifically for the work-scope. Overall, 31 holes were drilled and pins inserted into the vertical conductors, 67 diamond wire cuts were made with an additional 28 cuts utilising subsea decommissioning shears.

96% recycling

Some 96% of Dunlin Alpha structures were recycled
Photo: DeepOcean Group

Utilising the Edda Freya and two ROVs, removal and recycling of all components – with a combined weight of 848 tonnes of waste material – was successfully achieved as a diverless operation. All material were brought ashore to the Dales Voe facility in Lerwick for recycling, of which 96% of waste material was reused or recycled.

The preparation timescale (12 weeks) was a particular challenge. It was also clear that some significant engineering challenges affected the cutting and removal of two significant structures spanning two concrete platform legs mid-water; the upper guide frame and upper conductor sections were subject to environmental conditions near, but below the surface which presented additional challenges to ROV operations.

Decomissioning of Dunlin Alpha was concluded successfully within time and budget expectations, with zero harm to people or the environment.  

Part of the energy transition 
Accomplishing decommissioning projects while maximising the recycle and re-use potential requires support from suppliers that specialise in waste handling. DeepOcean continues to seek efficiencies in recycling by engaging with vendors in relation to the re-use of components where it is cost- and environmentally-efficient, and in alignment with circular economy principles.

Working closely with recycling vendors to minimise the CO₂ footprint throughout the entire onshore recycling process is also key to responsible recycling.

Utilising hybrid vessels such as Edda Freya is another way of bringing equipment onshore with the smallest environmental footprint. All vessel emissions are tracked through an online platform, which helps to optimise energy efficiency plans. The development of remote operation centres, which allow ROVs to be operated remotely, also contributes to reducing CO₂ emissions and reduces risk to offshore personnel.

• Further reading: ‘UKCS decommissioning cost estimate drops 25% to £44.5bn’. The UK has made great strides in reducing oil and gas infrastructure decommissioning costs over the past five years, according to the North Sea Transition Authority (NSTA).
• Find out more about the scale of decommissioning prospects in a number of regions across the globe, as the transition towards a greener energy path leaves numerous stranded fossil fuel assets worldwide.