Aliyu Abdullahi
My research project
Towards digital twin technologies for structural integrity assessment of offshore wind turbines under long-term vibrations
Offshore wind turbines (OWT) have become hugely important to modern societies as they strive to meet their future targets of sustainable and environmentally friendly energy utilization. With the advantages OWT turbines offer with respect to the foregoing, comes the challenges of the ends of their initial design lives, fast approaching as well as an insufficient track record. This inadvertently will lead to unanticipated breakdowns in need of unplanned maintenance, which are carried out at huge costs as well as unnecessary downtime. Also, the nature of forces (i.e. cyclic and dynamic) that an OWT experiences as well as the environment (both sea and supporting soil) where it operates, make it difficult to accurately capture, ab-initio, the conditions it may be exposed to, while in operation.
These thus warrant a monitoring system capable of keeping track of the conditions of OWT systems at regular intervals or better still, in real time. Other researchers have developed monitoring methods targeting specific damage types and limited to the first two damage detection levels. In this sense, this research embarks on the development of calibrated FE models to be used for the assessment of the long term effect(s) of vibration on the structural integrity of OWT systems and sets the stage towards the automation of the monitoring procedures (i.e. Digital Twins) for this purpose.
This strategy involves building Finite Element Models of OWTs; Experimental modal testing of OWTs; Updating of the Finite Element Models and finally, linking the updated FE models with their physical counterparts. With the developed FE model, the structural states of the physical OWTs can be interrogated after long term load applications for the identification of damage in the OWT structure.
My publications
Publications
Offshore wind turbines (OWTs) have emerged as a reliable source of renewable energy, witnessing massive deployment across the world. While there is a wide range of support foundations for these structures, the monopile and jacket are most utilised so far; their deployment is largely informed by water depths and turbine ratings. However, the recommended water depth ranges are often violated, leading to cross-deployment of the two foundation types. This study firstly investigates the dynamic implication of this practice to incorporate the findings into future analysis and design of these structures. Detailed finite element (FE) models of Monopile and Jacket supported offshore wind turbines are developed in the commercial software, ANSYS. Nonlinear Soil springs are used to simulate the soil-structure interactions (SSI) and the group effects of the jacket piles are considered by using the relevant modification factors. Modal analyses of the fixed and flexible-base cases are carried out, and natural frequencies are chosen as the comparison parameters throughout the study. Secondly, this study constructs a few-parameters SSI model for the two FE models developed above, which aims to use fewer variables in the FE model updating process without compromising its simulation quality. Maximum lateral soil resistance and soil depths are related using polynomial equations, this replaces the standard nonlinear soil spring model. The numerical results show that for the same turbine rating and total height, Jacket supported OWTs generally have higher first-order natural frequencies than Monopile supported OWTs, while the reverse is true for the second-order vibration modes, for both fixed and flexible foundations. This contributes to future design considerations of OWTs. On the other hand, with only two parameters, the proposed SSI model has achieved the same accuracy as that using the standard model with seven parameters. It has the potential to become a new SSI model, especially for the identification of soil properties through the model updating process.
Offshore Wind Turbines are a complex, dynamically sensitive structure owing to their irregular mass and stiffness distribution and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoon, hurricane, earthquake, sea-bed current, tsunami etc. As offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable, and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occurs during the design lifetime. Traditionally, engineers use conventional types of foundation system such shallow Gravity-Based Foundations (GBF), suction caissons or slender pile or monopile owing to prior experience with designing such foundations for the oil and gas industry. For offshore wind turbine, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse met-ocean and geological conditions. The paper, therefore, has the following aims: (a) Provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) Discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project. (c) Provide examples on applications of scaled model tests.