Long-term performance assessment of offshore wind turbine structures based on monitoring data
Creators: name: ABDULLAHI, ALIYU
Publisher: name: University of Surrey
Year: 2022

description: As the world strives for cleaner and more sustainable energy utilisation, interests in the offshore wind turbine (OWT) industry have massively grown over the last decade. These have attracted sustained investment efforts from top global economies (even as emerging ones gradually embrace the technology), based mainly on the offshore wind farm’s (OWF) levelised cost of energy (LCOE). Foundation has been shown to have the most influence on both the LCOE and structural stability of OWTs, accounting for up to a third of the entire OWF project’s LCOE, with an equally high failure criticality rate. Therefore, to derive optimum benefits from OWTs, their foundations must be carefully selected based on the two factors of water depths and turbine ratings. While there is a wide range of OWT support foundations, the monopile and jacket are the most deployed. However, the foundation recommendations in the literature are often violated due to a lack of universal guidance for their selection, leading to cross-deployments of the two foundation types with implications on the integrity of OWT systems. By their nature, OWTs are almost always in states of continuous vibrations, especially under operational loads, leading to modal property and stability-condition changes. Where tolerable limits are breached, foundation-soil interaction-induced resonance and excessive foundation rotations (tilts) are two of the most likely consequences. These make the OWTs susceptible to unpredictable and sometimes difficult-to-locate structural or system damage(s), affecting their effective long-term functioning. In older OWTs (found today across many wind farms) that are at/close to their initial design lives, these damages are even more likely, increasing susceptibility to unexpected failures requiring unplanned and expensive maintenance. Two prominent solutions that can guarantee the safe and effective operations of OWTs, as identified in the literature, are their long-term comprehensive characterisation and the implementation of reliable structural health monitoring (SHM) regimes that monitor the conditions of the OWTs. The former solution contends with a gross shortage of sufficient track records due to the relatively young age of the OWT technology. In contrast, the latter solution is faced with measurement difficulties (especially bathymetric) based on access restrictions. Furthermore, with model updating-based SHMs, there is a limit on the maximum number of updating parameters an algorithm can effectively accommodate, thereby often forcing the use of oversimplified FEMs that may misrepresent the prototypes, especially real OWTs.
description: This study investigates the dynamic implications of foundation cross-deployments in OWTs (focusing on monopile and jacket) to incorporate the findings into their future analyses and designs. Extensive FE modal analyses of the fixed and flexible-base cases of the monopile and jacket supported OWTs (MSOWT and JSOWT) are carried out with natural frequency chosen as the comparison parameter. Furthermore, this study develops a preliminary OWT foundation selection chart based on water depths and turbine ratings to avoid such cross-deployments. The comprehensive characterisation of OWTs is also achieved by conducting a range of model tests (medium and long-term) on two different-sized laboratory OWT (LOWT) models under varying amplitude high cycle application (HCA) operating loads. These loads are modelled after environmental field data. Acceleration and displacement sensor readings are respectively used to monitor the LOWTs’ structural responses leading to the natural frequency and foundation rotation (tilt) changes throughout the tests. To address the limitation on the allowable number of updating parameters for model updating, few-parameters soil-structure interaction (SSI) models are developed for the MSOWT and JSOWT with only two parameters (instead of seven). The relationship between the maximum lateral soil resistance and soil depth