Offshore Wind Turbine Foundation

I. The Pile Soil Analysis (PISA) project
The Pile Soil Analysis project is a joint industry initiative which is run by the Carbon Trust's Offshore Wind Accelerator program. The aim of PISA is to investigate and develop 'improved design methods for laterally loaded piles, specifically tailored to the offshore wind sector' (University of Oxford, n.d.).
Currently monopiles used in offshore winds applications are designed following the guidance found in (API, 2010) and (DNV, 2014), which were written many decades ago for use in the oil and gas industry,
where piles are long and slender and do not experience large lateral loads. The issue is that piles designed for offshore wind applications have much larger diameters and are significantly shorter as they
need to resist larger lateral loads, resulting in an overly conservative pile being built with the soil response being altered from what was expected.
The PISA project has carried out 'large scale field tests' onshore to gather data (Byrne, et al., 2015). The locations were chosen such that the soil was similar to that found in the North Sea. In order to validate
the results of the field tests, computational analyses were carried out at Imperial College London (Zdravkovic, et al., 2015). The design methodology report is to be handed over to industry at the start of
2016.
In order to gain a better understanding of soil response with respect to laterally loaded piles further computational modelling is required. Other areas which may be looked at include foundation response:
- depending on the wider parameter space
- in layered soils, made up of both clay and sand
-under varied loading.
The modelling will be carried out either using Imperial College Finite Element Program (ICFEP) or another commercial software e.g. AbaqusFEA. The soil will be modelled as a 'constitutive soil' similar to
the method carried out by Zdravkovic, et al. (2015).
II. Continuous real time heath structural monitoring
The majority of the turbines that have been built have a basic data collection system in the rotor assembly called the Supervisory Control And Data Acquisition (SCADA). This data includes a range of different variables ranging from wind speed to oil temperature (Antoniadou, et al., 2015). The data also includes acceleration of the turbine from which it is possible to calculate the fundamental frequency of the structure.
A recent study carried out in the Duddon Sand Offshore Wind Farm has found that there is a significant difference between the designed and the actual fundamental frequency for the wind turbines. The consequence of underestimating the fundamental frequency is that this allows for the possibility of increasing the operational lifetime of the structure. Another study looking at a single wind turbine in the
Walney Offshore Wind Farm showed the potential increase in the operational lifetime of the structure to be 88% (Kallehave, et al., 2015)
In order to further improve the optimization of wind turbines and allow monopoles to be used in locations with greater confidence the following areas will require further research:
- Improve design with the use of measurements - where the design process is refined using data that is gathered. In addition to looking at the data that is gathered with the SCADA, other data collection techniques should be utilised.
- More accurate modelling of the soil-structure interaction - this is related to the PISA project see above.
- Improved understanding of overall structural damping in the turbine - overall structural damping is the total damping minus the aerodynamic damping. Currently there is difficulty in accurately determining the individual damping contributions to the overall damping, especially
during operation (Kallehave, et al., 2015).
The research falls within the EPSRC Energy theme and is sponsored by Mott MacDonald.