Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

M. Hesham El Naggar

2nd Supervisor

Timothy Newson

Co-Supervisor

3rd Supervisor

Jonathan A. Black

Affiliation

University of Sheffield

Co-Supervisor

Abstract

Offshore foundation systems are constantly evolving to meet the needs of new developments in energy sector. Moving into ever deeper water for hydrocarbon recovery or creating foundation systems for renewable energy sources, such as offshore wind Turbines (OWT) farms, creates specific challenges. Large fixed vertical tower structures are typically used to support OWT inducing complex loading on foundations as a result of combined wind, wave and self-weight loading effects, all of which must be accommodated within very small rotation envelopes and natural frequency band to allow the turbines to operate effectively. In this thesis, a hybrid foundation (H.F) comprising a plate with a diameter (W) fitted with a pile with a length (Lp) at the center is proposed as an alternative to monopiles (MP) with aim of reducing needed pile diameter and penetration. This type of foundation system can benefit OWT since the turbine are subjected to high overturning moments. Since there is no guidance available regarding the performance of this system to support OWTs, the objective of this research is to evaluate the behavior of this system and the typically used monopile in various clay profiles and examine the effects of various parameters on their performance.

The research methodology includes three aspects: (i) developing a calibrated and verified 3D finite element model (FEM) (ii) conducting a detailed FE modelling on the behavior of a 5MW wind turbine supported by hybrid foundation and monopiles considering various pile/monopile length to plate width/diameter to characterise their lateral ultimate capacity, compare their structural response under serviceability loading, establish their stiffness properties and study their dynamic response (iii) performing a series of geotechnical centrifuge tests on scaled models representing a monopile and a hybrid foundation to investigate the benefits of the proposed system and compare it to monopiles.

Results indicated adding a plate to monopile improves the relative lateral ultimate capacity, whilst enabling a reduction of monopile penetration depth and diameter. Specifically, when the hybrid foundation was used in stiff clay sites, similar lateral capacity was reached for the hybrid foundations as that of a 30m depth monopile well as comparable stiffness response. Reduction of bending moment and tower tip displacement of 30-45%, 4-7% were recorded. Soil compliance reduced the 1st natural frequency compared to the equivalent fixed base by 52-10% and 40-0% for the monopile and the hybrid foundation, respectively. Formulae considering foundation geometry and soil profiles are proposed to (i) estimate the lateral ultimate capacity of both systems under loading eccentricity representative of medium depth water (ii) evaluate the three spring stiffness values (iii) estimate the 1st natural frequency. Centrifuge testing results indicated the performance of the proposed hybrid foundation system was similar to that of monopile in stiff clay profiles indicating potential use.

Summary for Lay Audience

Offshore foundation systems are constantly evolving to meet the needs of new developments in energy sector. Moving into ever deeper water for hydrocarbon recovery or creating foundation systems for renewable energy sources, such as offshore wind Turbines (OWT) farms, creates specific challenges. Large fixed vertical tower structures are typically used to support OWT inducing complex loading on foundations as a result of combined wind, wave and self-weight loading effects, all of which must be accommodated within very small rotation envelopes and natural frequency band to allow the turbines to operate effectively. In this thesis, a hybrid foundation (H.F) comprising a plate with a diameter (W) fitted with a pile with a length (Lp) at the center is proposed as an alternative to monopiles (MP) with aim of reducing needed pile diameter and penetration. This type of foundation system can benefit OWT since the turbine are subjected to high overturning moments. Since there is no guidance available regarding the performance of this system to support OWTs, the objective of this research is to evaluate the behavior of this system and the typically used monopile in various clay profiles and examine the effects of various parameters on their performance.

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