Advanced laboratory testing for offshore wind turbine foundations

Started: January 2014
Supervisor: Jardine, R.J.
Funding: PISA Project

Background

The increasing demand for renewable energy imposes an urgent demand for reliable and economic offshore wind generation. However there exist major uncertainties regarding the foundation behaviour of deep water wind turbines particularly with mono-pile structures. Offshore structures are subjected to strong wind and wave actions which impose lateral and axial cycling loading to the foundations. There exist significant uncertainties regarding the soil stiffness, response and ultimate lateral and axial capacity available during and after cycling loading. In particular, displacement accumulation could be large after storm events and so affect  wind turbine performance. Field monitoring shows significant discrepancies between expected and measured dynamic responses that affect turbine design.

Research aims

This research forms part of the Pile Soil Analysis (PISA), offshore Joint Industry Project (JIP) which aims to reduce the cost of wind turbines and find better design methods for foundations. The research is funded by Carbon Trust, DONG Energy and other offshore energy companies. The Academic Work Group (AWG) comprises Oxford University, Imperial College London and University College Dublin. The main objective is to reduce the cost of mono-piles and other piled foundations by finding better design approaches. The strategy is to conduct advanced numerical modelling studies that will allow simplified industrially applicable tools to be developed. These will be tested against full-scale pile tests and refined as required. Central to these activities is the accurate characterization of the full-scale test sites.

Research objectives

The main objective for the research by Emil Ushev is to perform high quality Triaxial, Hollow Cylinder and Ring Shear tests that will both feed into the numerical analysis and provide key insights into several issues related to the full scale foundations. This input is vital to calibrate the numerical modelling and reveal the true constitutive behaviour of the soils which model geomaterials that are frequently encountered offshore.

The PISA programme involves two field tests, one in sand, at Dunkerque, France and another in glacial till at Cowden, Humberside. The key features to be explored in the laboratory tests are:

-     The non-linearity and pressure dependence of the soils` stiffness
-     Shear failure and yielding characteristics for both soils, explained within the Critical State framework
-     Rate and ageing effects on stiffness and shear strength
-     Shear strength and stiffness anisotropy
-     Behaviour under cyclic loading
-     Differences between intact and reconstituted behaviour
-     Interface shear characteristics

EMIL USHEV

PhD Candidate - Geotechnics 
Department of Civil & Environmental Engineering 
Imperial College London SW7 2AZ 
emil.ushev11@imperial.ac.uk