The influence of combined loading on the response of monopiles used to support offshore wind turbines (OWTs) is investigated in this paper. In current practice, resistance of monopiles to vertical and lateral loading is considered separately. As OWT size has increased, the slenderness ratio (pile length, L, normalised by diameter, D) has decreased and foundations are tending towards intermediate footings with geometries between those of piles and shallow foundations. Whilst load interaction effects are not significant for slender piles, they are critical for shallow footings. Previous research on pile load interaction has resulted in conflicting findings, potentially arising from variations in boundary conditions and pile slenderness. In this study, monotonic lateral load tests were conducted in a geotechnical centrifuge on vertically loaded monopiles in dense sand. Results indicate that for piles with L/D = 5, increasing vertical loading improved pile initial stiffness and lateral capacity. A similar trend was observed for piles with L/D = 3, when vertical loading was below 45% of the pile’s ultimate vertical capacity. For higher vertical loads considered, results tended towards the behaviour observed for shallow footings. Numerical analyses conducted show that changes in mean effective stress are potentially responsible for the observed behaviour.

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The majority of offshore wind structures are supported on large-diameter, rigid monopile foundations. These piles may be subjected to scour due to the waves and currents that causes a loss of soil support and consequently decreases the pile capacity and system stiffness. The results of numerical models suggest that the shape of the scour hole affects the magnitude of pile capacity loss; however, there is a dearth of experimental test data that quantify this effect. This paper presents a series of centrifuge model tests on an instrumented model pile that investigates the effects of scour-hole geometry on the response of a laterally loaded pile embedded in sand. The pile instrumentation allowed load–displacement and p–y (soil reaction – displacement) curves to be derived. Three scour geometries (global, local wide, and local narrow) and three scour depths (1D, 1.5D, and 2D; where D is pile diameter) were modelled. For all three scour types, pile moment capacity decreased almost linearly with increase of scour depth. Simple empirical relations were proposed to evaluate the detrimental influence of scour on the pile moment capacity. A new method has been developed to allow designers to quantify the effect of scour-hole shape and severity of scour on the pile response.

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In this study, a total number of 20 centrifuge tests were carried out to investigate monopile behaviour under lateral cyclic loading. The instrumented model pile simulates an offshore wind turbine foundation with an embedment ratio of 5 installed in sand layers with two relative densities of 80% and 50%. The influence of the directional characteristic and amplitude of cyclic load on pile lateral behaviour was studied. The data analysis focused on the influence of cyclic load on the accumulation of lateral displacement and evolution of secant stiffness of the foundation system. The most damaging cyclic load type (which can cause the most accumulated pile displacement) is identified as two-way loading, and it was observed that cyclic load always increases the pile secant stiffness. A new model for the prediction of evolution of accumulated displacement and change in secant stiffness has been formulated. An example of the procedure developed is presented for a typical field monopile subjected to cyclic loading. Lastly, the performance of the new model is demonstrated and predicted results are compared with field test data.

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The majority of installed offshore wind turbines are supported on large-diameter, open-ended steel pile foundations, known as monopiles. These piles are subjected to vertical and lateral loads while in service. In current design practice, interaction of vertical and lateral loads are not considered, rather piles are designed to resist vertical and lateral loads independently. Whilst interaction effects are widely studied for shallow foundations, the limited research on this topic for pile foundations often produces conflicting results. This paper reviews the research of the influence of vertical loading on the lateral response of pile foundations under combined loads, from the perspective of analytical research, numerical research, and experimental research from tests performed on 1-g (gravitational acceleration) model, centrifuge, and full-scale piles. The potential reasons for the differences among the results of previous research are discussed. Some guidance for future research on the effect of vertical loads on the lateral response of piles is provided.

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The large diameter monopile is a commonly used foundation concept for offshore wind turbines. The advantages of geometrical simplicity and reliable performance make it often the most attractive solution. Despite the concept’s high popularity, optimisation of the current design models can still be made. To address fundamental understanding of modelling effects in centrifuge testing of laterally loaded monopiles in sand, a large coordinated centrifuge-testing program across 11 different centrifuge centres worldwide is ongoing. This extended abstract presents the initial results of global benchmark testing.@en

The influence of scour on the lateral response of monopile foundations for offshore wind turbines is investigated in this paper. Application of lateral load-displacement (p-y) curves to predict the lateral pile behaviour is subject to uncertainty as many of the presently used design approaches have been derived for long, slender piles. These piles, with typical length/diameter ratios (L/D) of greater than 10 behave differently compared to rigid monopiles, with L/D typically less than 6. In this paper, centrifuge tests are conducted on a monopile model under various scour scenarios and p-y curves are derived from strain gauges embedded along the model pile wall. Global scour and two different shapes for local scour holes are studied. Using the piecewise polynomial method for extraction of p-y curves from sparsely distributed strain measurements, it is recommended to use a 4th order polynomial for the moment profile to extract soil reaction and a 7th order polynomial for the moment profile to calculate pile deflection. Results indicate that the pile behaviour is significantly influenced by the nature (size, shape) of the scour holes affecting the pile–soil system and suggest that the p-y curves should be appropriately modified to account for this behaviour.

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Although wind energy capacity has increased significantly in the last few decades, the installed capacity of offshore wind turbine still lags far behind that of onshore wind turbines due to the installation and foundation cost. The aim of this research project has been to clarify the influence of combined vertical and lateral loads, lateral cyclic load, and scour erosion on monopile foundations, in order to achieve more realistic and cost beneficial solutions for offshore wind turbine foundations and thereby increase its competitiveness when compared with other energy sources. Monopiles are the most popular foundation system today for offshore wind turbines installed in shallow to medium water depths. These relatively light structures (low vertical load), need to resist substantial lateral and moment loads. There have been a dearth of studies conducted to investigate the influence of vertical load on the lateral response of these rigid monopiles and the few available have drawn contradictory conclusions. In addition the lateral and moment loading exerted on monopiles due to wind, wave, and water currents is cyclic in nature. This type of loading can lead to the accumulation of lateral displacement/rotation and possible degradation of soil resistance over time. This evolution of pile head displacement and the change in soil stiffness with increasing cycles of load is poorly understood. Cylindrical structures, like monopiles, founded in offshore regions are commonly subjected to scour erosion caused by flowing water and currents, which induces loss of soil support around the pile, reducing the lateral load capacity and causing increased pile displacement. As a result, the system dynamics of the structure might be adversely affected. The results of numerical models suggest that the shape of the scour hole affects the loss of pile lateral capacity, however, there is a shortage of experimental test data that measure this effect. More than 60 centrifuge tests which are categorized into three groups are presented in this thesis, which consider the interaction of combined vertical and lateral loads, lateral cyclic load and scour erosion on the behaviour of rigid monopiles. The tests have been performed in homogeneous dry Geba sand in order to mimic simplified drained offshore soil conditions. @en

Pile foundations used for offshore wind structures are subjected to large lateral loading from wind and waves while in service as well as significant vertical loading from the top structure. Erosion of soil from around these structures, termed scour, poses a significant problem for the structural stability. In order to better understand the performance of piles facing scour problems, the effect of local scour on the behavior of mono-piles installed in sand under combined lateral and vertical loading has been investigated using the Finite Element Method (FEM) using PLAXIS in this paper. The simulation results showed that vertical loading can decrease pile lateral displacement and improve the lateral capacity of piles in the absence of scour and under scour. The increase of scour depth will largely reduce lateral capacity of piles.@en