Thesis
Investigation of decommissioning of offshore wind mono-pile foundations
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2023
- Thesis identifier
- T16807
- Person Identifier (Local)
- 201661723
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Decommissioning process intricacy level varies depending on the offshore wind turbine components. Decommissioning superstructure components -blades, nacelle, tower, etc.- is a straightforward process, which is the reverse of the installation. The full removal decommissioning strategy that applies to the superstructure extends to include a few offshore wind foundations. For instance, the decommissioning of suction bucket foundation is through release and extract by applying pressure. Nonetheless, for the pile foundation, which is the most common type, with a total installation of more than 7,000 [4,914 single and 2,934 group piles], the decommissioning is complicated; one reason is that excavation is required in the two strategies - full and partial removal [cutting the pile externally]. However, the current industry-proposed methods may not always be adopted, and developing novel foundation decommissioning methods such as extraction [full removal] becomes mandatory/ beneficial. The reason is that offshore wind currently initiated to alter the method for decommissioning some of its turbine components, e.g., blades, from landfilling the material to recycling. For extraction method feasibility validation, an investigation was carried out through an experimental campaign and a techno-economic assessment. For the experimental campaign, a series of 1g model pile extraction tests using displacement and force control and 400mm long open-ended steel piles with diameters of 88.9 and 101.6 mm were conducted in unsaturated and saturated soil. Experimental campaign results showed the tensile capacity of both piles had reduced by up to 90% compared to the compression capacity in both conditions. In the unsaturated case, with maintaining the soil density constant, results had evident the impact of extraction rate -by reducing- the tensile capacity of the pile regardless of diameter. Nonetheless, in the saturated soil, the tensile capacity decreased due to the pore water pressure; for the 101.6 mm pile, the decline was by 50 and 18% for displacement and force extraction applications, respectively. Compared with displacement extraction application, the tensile capacities of the 88.9 and 101.6 mm piles under force decreased [lowest] by 25 and 23% in unsaturated soil and saturated soil by 6 and 11%, respectively. Despite higher capacity under displacement extraction, the application exhibited local shear failure -partial drainage occurrence due to the application mechanism- where piles had extracted soil-free, which was not the case for the force application. The experimental results validated the theoretical method developed in aiding extraction of OWPile foundations and demonstrated the feasibility and efficiency of extraction application, displacement, and required total energy [velocity and time]. For the techno-economic assessment, a model developed to determine the most economical decommissioning strategy and method for offshore wind foundations in terms of timing and costs by comparing the industry-preferred strategy [partial removal] with the novel-proposed one. The model inputs’ parameters included foundation removal operations duration, wait-on-weather, and vessels’ strategies and types, which are key drivers that significantly influence the total decommissioning costs. Additionally, sensitivity analysis was carried out for the input parameters because are subject to a high degree of uncertainty. For the sensitivity analysis, the parameters’ baseline estimated values had increased by conservative percentages; offshore wind foundations decommissioning methods activities’ estimated duration by 100%, ranging the weather adjustment factor by 50 – 100%, and vessels’ day rate by ± 20%. The model results showed that the economical vessels’ strategy, regardless of decommissioning strategy/ method, is the transiting -utilising two vessels of the same type-compared to the sender one. Analysing results further, heavy lift vessels (HLV) carry out decommissioning activities in less time than wind turbine installation vessels (WTIV), nevertheless increasing the costs due to their low availability. The sensitivity analysis results showed that despite increasing weather factor adjustment factor and vessels’ day rate, the total cost for the novel decommissioning proposed method is more economical than the industry-proposed one. Including the learning curve in the analysis, the total cost of offshore wind foundations decommissioning, regardless of methods, could reduce by up to 35%.
- Advisor / supervisor
- Brennan, Feargal
- Resource Type
- DOI
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