Session description

This session considers the broad topic of fixed offshore foundation systems and includes papers addressing the primary elements of global or whole-structural-system analysis and assessment. A range of speakers will represent academia and industry with contributions covering different aspects of bottom-fixed support structures and foundations, their design, analysis and optimisation. Topics addressed will include hydrodynamic loads, soil-structure interaction and geotechnical issues, support structure dynamics and simulation technology, field testing and laboratory experiments as well as pile design.

Learning objectives

• Better understand soil-structure interaction mechanisms and analyse methods
• Appreciate how to analyse and assess structural dynamic behaviour
• Examine fatigue damage models applied to offshore wind foundations
• Recognise performance indicators for the whole-structure
• Identify methods to objectively assess optimum foundation configuration

Paul Doherty University College Dublin, Ireland
Co-authors:
Paul Doherty (1) ^{F P} Carl Brangan (1) Luke Prendergast (1) Kenneth Gavin (1)
(1) University College Dublin, Dublin 4, Ireland

Design tools available for monopile engineering

Introduction

This paper addresses the limitations of current design codes and highlights the potential impact for existing foundations that are currently installed offshore. A suite of design tools are assessed, ranging from conventional analytical procedures to 3-D non-linear coupled finite element analysis. The impact of using more advanced design procedures is discussed.

Approach

Monopiles have been by far the most commonly used support structure for offshore turbines, with approximately 75% of existing wind farms founded on these large diameter steel tubes. Monopile geometries have mirrored the growth in the offshore wind sector, with monopile sizes approaching diameters of 7 and 8 meters, with expected penetrations of up to 40 m depth. The elegant and simple steel structure, the quick installation process and the broad experience base makes monopiles an increasingly attractive solution. As water depths increase at future wind farms it was anticipated that monopiles would become less feasible due to their flexibility, however technological advancements in hammer equipment and supply chain improvements in heavy steel, now means that supersized monopiles are becoming a reality in ever increasing water depths. However, despite the widespread prevalence of monopiles across the wind sector and the major advances in many areas of foundation technology, the design tools commonly used in practice have not evolved. As a result, codes originally developed for oil and gas applications are now being implemented by the wind industry for engineering situations for which they were never intended. Conventional p-y based design may not be applicable for offshore wind farm applications and therefore more advanced tools are required in practice.
A suite of monopile design cases are considered in this study, which examine a range of water depths and soil conditions using conventional analytical approaches. The implications of simple design assumptions were assessed by investigating the impact of base shear and selection of appropriate spring stiffnesses. A range of possible failure scenarios were considered under a holistic framework that examined the ultimate, serviceability, fatigue and dynamic limit states. The governing design cases under specific conditions were identified and the limitations of existing analysis procedures are discussed. A suite of 3D Finite Element Analysis were then completed using non-linear constitutive modelling to assess the relative accuracy of the traditional analytical approaches.


Main body of abstract

Offshore wind farms are typically supported on large-diameter monopiles. The dimensions of these foundation elements are outside the scope of current experience and research findings. As a result, the current design procedures for lateral loading (API, 2007) in cohesionless soil are being extrapolated well outside the original dataset. The most popular method of analysis for laterally loaded piles, and the method adopted in the offshore design codes of the American Petroleum Institute (API, 2007) and Det Norske Veritas (DNV, 2007), is based on the Winkler model and is commonly referred to as the p–y approach. This method of analysis assumes that the pile acts as a beam supported by a series of uncoupled springs, which represent the soil reaction. These springs can be characterized by a linear or non-linear curve, which describes the soil reaction p at a given depth as a function of the lateral movement y. The spring stiffness is defined as the secant modulus of the p-y curve.

The empiricism of the p-y approach results in a number of discrepancies, which require urgent research attention. The main limitations and differences between the 2007 API design code and industry practice include (a) The mode of failure is considerably different ; (b) Components of resistance are neglected (side shear/base shear) ; (c) Diameter effects are uncertain ; (d) The linear increase in stiffness with depth is questionable ; (e) The underlying earth pressure coefficient is unverified ; (f) Pile properties are ignored in the existing approach (g) Cyclic loading and accumulated rotations are poorly considered.

In particular, the rigid mode of failure casts considerable doubts on the validity of applying the existing p–y curves (which were developed to match the response of flexible piles) to predict the behaviour of offshore monopiles, which are typically much stiffer. API RP2A needs to be urgently calibrated for rigid pile behaviour to determine the initial stiffness and ultimate capacity. This paper presents a suite of 3D finite element analysis (FEA) undertaken within the Plaxis software environment to determine the load distribution behaviour of rigid piles in cohesionless soil. Of particular relevance to this study was the accuracy of the material models and the calibration of the soil parameters to ensure the analysis is representative of offshore conditions.

Three constitutive models were investigated within the Finite Element Analysis: (i) The Mohr Coulomb (MC) model , (ii) The Hardening Soil (HS) model and (iii) The Small-strain Hardening Soil (HSS) model. The MC model is a linear elastic perfectly plastic model with the initial linear loading curve specified by a single Young's modulus value (E). The Hardening soil (HS) model is an advanced model available in PLAXIS which can be used for both soft soil and stiff soils. The HS model uses a Mohr-Coulomb failure surface and hardening plasticity in the pre-failure stress state. Plastic strains are calculated by the multi-surface yield criteria. The hardening yield surface defines the irreversible shear straining due to deviatoric loading and the compression hardening yield surface defines the irreversible volumetric straining due to isotropic loading. The stiffness of the soil is governed by triaxial loading stiffness, the oedometer loading stiffness, and the unloading-reloading stiffness. The HSS model is similar to the HS model but provides an option to include the small strain shear stiffness (G0) of the soil. The increased level of complexity with the various models requires increased levels of calibration due to the increased number of material parameters required.

The results of the analysis are used to critically compare the different constitutive models. The FEA analysis is also bench-marked against the conventional API code, to highlight where significant cost savings could be made in industry by implementing more advanced design procedures.


Conclusion

Finite Element Analysis has been used to highlight the limitations of traditional p-y based design criteria. Existing design solutions based on p-y analysis are shown to be overly conservative in some instances and under conservative in others (depending on the specific water depths and ground conditions). Particular limitations with respect to cyclic loading and dynamic modal analysis have been highlighted.
The results demonstrate the potential advantages and cost savings that can be achieved when using more advanced FEA based design, however caution is also advised in the use of these tools as they are shown to be very sensitive to the geometric configuration, meshing details, material model and most importantly the material parameters selected. Accurate site specific calibration is critical to the successful implementation of FEA in practice. Selection of the design material parameters and accurate modelling of the soil stratification is not a trivial process and significant experience is needed in comparison to the implementation of the routine API design guidelines.
Three separate material models were investigated and the relative advantages and disadvantages discussed. The major advantage of the HSS model was demonstrated for the serviceability limit state, where the seabed monopile rotation under a typical 3.6MW and 6MW design turbine load are shown to be significantly less than those predicted using the API design guidelines. The HSS model provides the option of inputting the small strain shear stiffness (G0) of the soil, which can lead to stiffer loading responses in comparison to the other models. As the G0 value is not routinely measured in offshore site investigations, a process for estimating the G0 value from CPT correlations was also suggested. However, for accurate implementation in practice, it is recommended to tailor the site investigation to obtain the parameters required for subsequent numerical analyses. Overall, this will lead to the most accurate soil-structure interaction estimates and should lead to the most efficient and reliable designs.



Learning objectives
The key learning objectives will be:
(i) Existing analytical p-y methods for the design of offshore monopiles have significant limitations
(ii) There are a range of more advanced design tools available for use in industry
(iii) Site specific calibration of FEA software is critical to the successful implementation of advanced numerical analysis
(iv) FEA could offer significant cost savings through optimized and more efficient design of offshore monopiles