Developing a decommissioning strategy for offshore wind farms
Photo Credit: Unsplash/Nicholas Doherty
As part of global climate change targets, wind has become a popular mechanism to help companies achieve their decarbonisation strategies. The worldwide installed offshore wind capacity reached over 35 GW in 2020, with this being forecasted to triple by 2026. This rapid rate of installation, paired with the global demand for sustainable energy, makes the life cycle of a wind turbine incredibly important.
Offshore wind turbines are typically designed with an operating lifespan of 20-25 years. This means that turbines erected in the early 2000s are nearing the decommissioning phase of their life cycle. Alternatively, wind turbines can have their lives extended through repowering or refurbishment; however, this does not negate the need for decommissioning, but merely postpones it.
At some point in time, offshore wind turbines will need decommissioning, making it crucial for industry to look to the future to create cost-effective, sustainable and effective strategies for decommissioning.
Decommissioning methods
Decommissioning methods for offshore wind farms are still in their infancy, but equipment from the oil and gas industry and decommissioning strategies for onshore wind can be applied and adapted.
For offshore wind, there are two main options when decommissioning monopiles: partial removal and complete removal. Currently, partial removal is the most common method for decommissioning. This process means monopiles are removed by cutting them off several metres below the seabed, leaving behind tonnes of steel buried in the seabed. To cut the monopile, decommissioning engineers typically use either diamond wire cutting or water jetting.
Partial removal is popular as it not only keeps costs and complexities down, but also complies with both the IMO Guidelines and OSPAR Guidance. IMO Guidance allows for partial removal if entire removal involves an unacceptable risk to the marine environment. Likewise, the OSPAR offshore windfarm guidelines say that national authorities can decide whether components of a wind farm should remain at site as long as they have no adverse impact on the environment, the safety of navigation or any other uses of the sea.
However, as decommissioning matures, advanced engineering methods and technologies open the door for complete removal techniques that are sustainable and environmentally conscious. This development will be crucial, particularly as monopiles are constantly getting larger. Moreover, complete removal makes future uses of wind farm sites and ocean space easier as the ocean bed is available for installations without the costly removal of deteriorated debris.
Hydraulic pile extraction
Developing new methods for monopile removal will allow the offshore decommissioning business to improve its efficiency, knowledge and cost-effectiveness. One emerging method is the use of pressure, which has been studied by research groups in the Netherlands and Germany.
In 2018-2020, the Dutch HyPe-ST consortium investigated monopile (MP) decommissioning by pressurising the pile internally to push it out of the soil. They investigated extractions from multiple soil types, with the soil configuration playing an important role in the required extraction pressure. Scales used for the experiment were 1:20 and 1:30 steel piles. While in 2018-2021, the German DeCoMP project investigated a combination of technologies for decommissioning monopiles, including a similar concept to pile pressurisation on a scale 1:10 test stand.
Compared to other removal techniques, hydraulic extraction is considered more environmentally sustainable than partial pile removal as it disturbs the soil less and allows for all the steel to be recovered and recycled. This method would contribute significantly to a circular economic model and help provide a sustainable life cycle model for wind turbines.
Proven by research
A Wodcon paper found that this methodology works best for a decommissioning strategy if excavation and pile pressurisation are combined. For hydraulic extraction to be successful excavation of any impermeable top soil layer is required.
The paper, which explored an experiment conducted in the Royal IHC dredging laboratory, tested multiple soil variants to create several suggestions for possible decommissioning methods. Homogenous soil (sand), homogenous soil (clay), homogenous soil (sand) with scour protection, homogenous soil (sand) with a clay top layer, and stratigraphy with interbedded sand, clay and (weak) rock layers were tested. In all cases, the disposal of excavated sediments would result in a sediment plume in the water, meaning that legislation regarding turbidity limits needs to be taken into account when creating a decommissioning strategy.
Decommissioning strategies shown in the paper follow more or less the standards of offshore lifting operations with the addition of complementary arrangements for dredging operations. Operations should be organised according to the soil conditions at the monopile site, as this determines the equipment needed e.g. jets, hydraulic extraction units, lifting tools, and suction systems. This highlights how essential it is for a multidisciplinary approach to be taken when decommissioning offshore wind farms.
Future of decommissioning
In February 2023, classification society DNV launched a joint industry project to develop a decision tool and standard that can be used by all stakeholders during the specification, design, manufacture, procurement, and approval of any equipment planned for the installation and decommissioning of bottom-fixed wind turbines.
If offshore wind continues to show a 14% average annual growth, bottom-fixed wind turbines could constitute 11% of the global grid-connected electricity supply by 2050. This makes it imperative that discussions on decommissioning equipment and methods start happening now. Collaboration with leading experts can help the industry formulate a standardised approach to decommissioning, which would help make practices consistent, improve safety, streamline operations and reduce costs.
The company is currently in advanced discussions with Caley Ocean Systems (a Pryme Group company), Huisman Equipment, IMECA (a brand of Reel), Jan de Nul Group, Osbit, Remazel Engineering, Saipem, SSE Renewables, and TWD regarding the initial scoping proposal. The joint industry project is planned to start by May 2023.
All of this research will contribute towards making monopile decommissioning more cost-effective and environmentally friendly. The growing demands for renewable energy generation will also create growing demand for more efficient wind-power technologies, which will naturally shorten the lifespan of older wind farms. Alternative options such as hydraulic extraction show a promising step towards the expansion of offshore wind decommissioning techniques.
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