The present knowledge regarding the cyclic excess pore pressure accumulation behaviour was fundamentally gained from many (cyclic) laboratory tests and an in-depth literature
1.4 Scope
analysis. The thesis will present insights into how soil behaves for undrained conditions under cyclic loading and on how to transfer the knowledge to global foundation response.
Additionally, it focuses on different modelling techniques in the numerical framework and how these affect the post-cyclic bearing capacity. Some questions were asked in advance and act as waypoints through this thesis.
• How can the partially drained cyclic loading of a design storm and the related excess pore pressure accumulation be considered within the overall design?
• What is the difference in global response between the most accurate and the most practical way to consider these partially drained accumulation effects?
• How can partially drained conditions be considered in the best possible way in terms of superposition in a numerical framework?
• What are the differences in global structural responses between implicit and explicit methods for the very same soil?
• What laboratory tests are most appropriate under which conditions (load- or dis-placement-controlled; direct simple shear or triaxial tests)? What is the influence on the element result as well as on the structural response?
• What is the best way to derive contour plots? What is the easiest way to implement these into a finite element model?
• Which modelling techniques affect the outcome to what extent? What effects must strictly be considered?
To answer these questions, the cyclic response in undrained cyclic displacement- as well as load-controlled tests is assessed and used within the numerical procedure in order to estimate the difference in global response. The main objective of the research presented is to provide a simple, easy handable concept for estimating excess pore pressure accumula-tion around cyclically loaded offshore foundaaccumula-tions that gives sufficiently accurate results and is applicable in practical design. To achieve this goal, mainly the bearing behaviour of monopile foundations is investigated, but the developed generic concept applies to all types of structures.
This thesis is divided into two parts. The first part deals with the development and understanding of the soil response under cyclic loading with a special focus on excess pore pressure accumulation (Chapter 2 and Chapter 3). In this part, general definitions are explained as a basis for the following chapters. Subsequently, the existing explicit and implicit numerical approaches are presented with their advantages and disadvantages (Chapter 4). Chapter 5 shows the results of a detailed classification of one sandy soil as well as its cyclic characteristics under mainly undrained conditions. These cyclic soil responses are to be used within the numerical procedure and a comparison of the global structural response is made. A concept for the degradation of bearing capacity due to excess pore pressure accumulation under partially drained conditions was developed in order to facilitate the inherent complex cyclic loading into a simple estimation procedure.
The developed procedure incorporates the most essential aspects. A more detailed ex-planation of the development and application of the proposed explicit design approach is given in Chapter 6.
The second part deals with the practical application of the theoretical work in the design of an offshore foundation, and the general capabilities of the approach are explored. The method is used with a simple constitutive model in conjunction with ABAQUS (Dassault Systèmes, 2016), a general purpose finite element program (Chapter 7). This work not only addresses general cyclic soil behaviour by deriving contour plots, but also compares implicit and explicit modelling for one particular sandy material. The implicit model gives insights into the element response around an offshore foundation, which can be used to gather a deeper understanding for the optimization of the explicit approach. Differ-ent explicit modelling approaches for excess pore pressure accumulation are compared against each other so that a sound recommendation for the practical application can be made. A discussion is followed in which the significance of this work regarding ex-cess pore pressure estimation and potential future applications are presented. The thesis closes with recommendations for a practical design considering all presented sub-methods and a summary. Additional applications, such as the derivation of an equivalent number of cycles as well as multistage cyclic laboratory tests to validate these concepts and also results of displacement-controlled tests, can be found in Appendix B, C and D. Additional information on specific chapters are presented in Appendix A.
2 Cyclically loaded offshore foundations
Multipod Suction bucket
monopod
Gravity Monopile Tripod Floating
Figure 2.1: Different foundation concepts with main load characteristics following Thieken (2015).
The offshore wind turbine (OWT) generates electrical energy from the kinetic energy of the wind, which reduces the wind velocity and creates a substantial force on the structure and the foundation. In general, there are four main ground-based foundation designs:
monopiles, jackets, suction buckets and gravity foundations (Figure 2.1). The concepts are briefly described below.
• A gravity-based foundation supports itself with its own dead weight under moment load, so design considerations such as sliding, tilting and gapping must be taken into account. The foundation is hollow and can be rafted in place. No installation by pile driving or suction is required. However, the seabed may need to be prepared.
They can be designed with a skirt, which is used to prevent erosion from water seeping under the foundation.
• Suction buckets are installed primarily with suction and are therefore more environ-mentally friendly. They can be used in a monopod or multipod arrangement. They also have the advantage of easy decomposition.
• Monopile foundations are open-ended steel piles which are subjected to mainly lat-eral loads. The vertical loads play only a minor role, because of the large diame-ter and, hence, high axial capacities. Monopiles currently have a diamediame-ter of 8 m
with rotor diameters of 167 m with approximately 12 MW (Zachert and Wichtmann, 2020), with larger foundations already in planning. Research projects investigate the feasibility and implementation of 20 MW plants (Schuster et al., 2021). Mo-nopiles are driven into the subsoil in water depths of up to 40 m. Their diameter has increased in recent years due to higher water levels. They are connected to the tower by a transition piece, which is usually grouted, bolted or flanged. The general concept is well known and easy to handle, both in terms of transportation and in-stallation. Almost no seabed preparation is required unless heavy scour is expected.
The main problems are decomposition and noise immissions during installation.
The overall system has lower stiffness than a multipod arrangement. The length-to-diameter ratio has steadily decreased over the years. Larger drainage paths are created, which pose an additional risk in partially drained soils and may create sub-stantial amounts of excess pore pressure. Low permeability and rapid loading can lead to excess pore pressure accumulation. Moreover, monopiles account for 81% of all foundations in Europe and are the most widely used ones (Wind Europe, 2021).
• Jackets are a lattice-type structure with a square or triangular footprint. The mo-ment load is converted into vertical loads in the corner piles.
• Floating OWT are deployed in deep waters and towed to their location where they are anchored, moored, or partially submerged (e.g., Tension Leg-Platform). Floating or moored systems have advantages e.g. when other structures would become too large and expensive to transport.
The foundation concepts can be divided into monopod and multipod foundation. For multipods, the exact soil-structure interaction and overall response depends on the foot-print (i.e. location of the footings), number of legs and loading direction. Overall, the choice of foundation depends on an interaction between structure (turbine), soil profile, water depth and resulting loading condition. The foundations also differ in their bearing mechanism. Monopods are single supporting structures and bear with higher moment loads (H-M). Multipod structures are loaded in tension and compression since the global lateral effect generates axial, moment and lateral load (H-V-M) at each pile or bucket (Figure 2.1).
2.1 Geotechnical design of OWT
Although various foundation concepts can be used, this thesis will focus on monopiles since it is the most common foundation type.
2.1.1 Analytical design of monopile foundations
Analytical methods are important for a fast estimation of the foundation response, which is essential for any subsequent cyclic design. For monopiles the Winkler model is a well-known tool in the design process (Winkler, 1867). It uses p-y springs (for lateral be-haviour) and t-z springs (for axial bebe-haviour). The springs are independent of each other
2.1 Geotechnical design of OWT
(Cox et al., 1974; O’Neill and Murchison, 1983; Reese et al., 1974). The method of calcu-lating the lateral response was introduced by Reese and Matlock (1956) and McClelland and Focht Jr. (1958). The p-y curves define the relationship between the soil resistance p and the lateral displacement of the spring y. However, even if this approach is used in daily practice, cyclic degradation can only partially be approximated (Dash and Bhat-tacharya, 2015; Byrne et al., 2017; Zhang et al., 2019). Liquefaction due to seismic events is not part of this thesis. However, the same problems apply here. Similarly, degraded p-y curves can be used for a simplified analysis of monopiles under seismic loading or a finite element model is needed (Bhattacharya et al., 2021).
Arany et al. (2017) present a simplified analytical design of a monopile in 10 steps. Cyclic design is done with empirical correlations, which however, can only be used for a very approximate result. The number of cycles is derived by using an assumed peak storm duration divided by an assumed wave period. Nevertheless, such a simplified calculation may give a first good estimation in preliminary analysis. For an accurate cyclic design proof, numerical calculations are unavoidable due to the spatially distributed and soil-specific accumulation of deformations and excess pore pressures. For this reason, the main design under cyclic loading is nowadays performed with finite element models at almost all locations within a wind farm. In these models, the whole stress-strain relationship of the different elements around the foundations is modelled. Finite element calculations can be applied to complex soil and system geometries by using sophisticated soil models and accounting for spatial variations in soil properties. Their use is permissible according to DIN 1054:2021-04 and DNV-RP-C212.
2.1.2 Design load cases
Offshore structures are designed to withstand harsh environmental conditions. Several de-sign standards state requirements to ensure sufficient safety such as ANSI/API RP 2GEO, DNV-RP-C212, DNV-ST-0126, DIN EN ISO 19901-4:2017-01, DIN EN ISO 19902:2021-03, DIN EN 1997-1:2014-19902:2021-03, DIN 1054:2021-04 and DIN 18088-4:2019-01. The design is aimed at different aspects. DNV-ST-0126 distinguishes four different design limit states, these are the ultimate limit state (ULS), serviceability limit state (SLS), fatigue limit state (FLS) and accidental limit state (ALS). These are intended to include all possible (geotechnical) failure mechanisms.
In this context, the ULS aims at analysing the bearing capacity using a 50-year design storm and is intended to ensure sufficient lateral capacity. Here, the soil condition at failure is of interest. The SLS estimates settlement and tilting. The permanent accumu-lated head rotation needs to be smaller than a project-specific limit value; often the total structure’s inclination at seabed level should not exceed 0.5◦. The FLS targets cyclic loading in terms of foundation stiffness affecting natural frequency. The initial stiffness of the structure is used to investigate the overall natural eigenfrequencies. Herein, the initial stiffness and damping are important (Thieken et al., 2018; Saathoff et al., 2019).
The ALS targets accidental impacts. The ULS and ALS calculations are performed with safety factors applied to characteristic values of loads and resistances in order to establish a predefined level of safety.
Normalized load [1]
-2.5 2.5 7.5 12.5 17.5 -7.5
-12.5 -17.5
0.0 0.8 1.0
0.6 0.4
10h 6h 3h
0.2
Time t [h]
Wave Wind
Figure 2.2: 35-hour design storm (BSH No. 7005).
2.1.3 Loading conditions
Design proofs for all four design limit states are carried out. Herein, not all loads that act on an offshore foundation originate from the same source. DNV-ST-0126 defines the main load components as:
• permanent loads such as weight, ballast or equipment. These loads do not vary in direction, magnitude or time period
• variable loads varying in direction and amplitude
• accidental loads from technical failure, fire, collision, breaking wave or impact
• deformation loads from temperature or settlement
• environmental loads from ice, marine growth, earthquake, tidal current, snow, wind and hydrodynamic (cyclic nature)
All load cases need to be considered in different combinations (DNV-ST-0126; DIN 18088-4:2019-01). The loads are categorized as quasi-static, cyclic or transient. For the cyclic SLS design, a distinction must be made between a short-term (partially drained) storm load event and long-term (drained) soil behaviour during the lifetime of the OWT when dealing with sandy material. The storm load, under which partially drained conditions are present, consists of a spectrum of different wave heights, periods and directions. Wave conditions during the storm are required for the design. Therefore, wave heights and wave periods including their probability of occurrence are needed. To simplify this, a 35 h-design storm is used (Figure 2.2).
The pre-defined design storm consists of two storm build-up phases, in which the loads are increasing in their magnitude and a peak phase, which considers the largest acting loads. The duration of this peak phase is assumed with 3 h according to BSH No. 7005.
Afterwards, the storm calms down again. Here, especially for cyclic loading, not only the maximum force, but also the load history plays an important role in evaluating the