Cyclic overlay model of p–y curves for laterally loaded monopiles in cohesionless soil

The bearing behaviour of large-diameter monopile foundations for offshore wind turbines under lateral cyclic loads in cohesionless soil is an issue of ongoing research. In practice, mostly the p–y approach is applied in the design of monopiles. Recently, modifications of the original p–y approach for monotonic loading stated in the API regulations have been proposed to account for the special bearing behaviour of large-diameter piles with small length-to-diameter ratios. However, cyclic loading for horizontally loaded piles predominates the serviceability of the offshore wind converters, and the actual number of load cycles cannot be considered by the cyclic p–y approach of the API regulations. This research therefore focuses on the effects of cyclic loading on the p–y curves along the pile shaft and aims to develop a cyclic overlay model to determine the cyclic p–y curves valid for a lateral load with a given number of load cycles. A stiffness degradation method (SDM) is applied in a three-dimensional finite element model to determine the effect of the cyclic loading by degrading the secant soil stiffness according to the magnitude of cyclic loading and number of load cycles based on the results of cyclic triaxial tests. Thereby, the numerical simulation results are used to develop a cyclic overlay model, i.e. an analytical approach to adapt the monotonic (or static) p–y curve to the number of load cycles. The new model is applied to a reference system and compared to the API approach for cyclic loads.</p

An experimental study on the parameters affecting the cyclic lateral response of monopiles for offshore wind turbines in sand

The design of monopile foundations for offshore wind turbine structures is dominated by requirements resulting from serviceability and fatigue limit state. To fulfil these criteria, the load deflection-behaviour and therefore long-term accumulations of permanent deflections and rotations of the monopile foundation due to cyclic occurring wind and wave loads have to be predicted. In this paper a brief overview on current design code practice as well as other proposed methods for the prediction of accumulated deflections or rotations is given. Further, the results of a systematic model test study dealing with the response of monopiles to lateral cyclic loading in medium dense sand at different cyclic load ratios, load eccentricities and pile embedment lengths are described and evaluated. The observations of the model test study are supplemented by results of a second test series involving the visualisation of displacement fields around laterally loaded piles by means of particle image velocimetry. Based on the findings and the results of previous experimental investigations, recommendations regarding the prediction of displacement accumulations for large diameter monopiles in sand are given. © 202

On the influence of thermally induced radial pipe extension on the axial friction resistance

Within the design process of district heating networks, the maximum friction forces between the pipeline and the surrounding soil are calculated from the radial stress state and the coefficient of contact friction. For the estimation of the radial stresses, the soil unit weight, geometric properties such as the pipe's diameter and the depth of embedment, as well as the groundwater level are taken into account. For the coefficient of contact friction, different values are proposed, dependent on the thermal loading condition of the pipeline. Although this is an assumption of practical use, physically the coefficient of friction is a material constant. To revise the interaction behavior of the soil-pipeline system with respect to thermally induced radial pipe extension, a two-dimensional finite element model has been developed. Here, the frictional contact was established using Coulomb's friction law. For the embedment, sand at different states of relative density was considered. This noncohesive, granular material was described by the constitutive model HSsmall, which is able to predict the complex non-linear soil behavior in a realistic manner by stress-dependency of stiffness as well as isotropic frictional and volumetric hardening. In addition to the basic Hardening Soil model, the HSsmall model accounts for an increased stiffness in small strain regions, which is crucial for the presented investigation. After a model validation, a parametric study was carried out wherein a radial pipe displacement was applied due to thermal changes of the transported medium. Different combinations of geometry and soil property were studied. We conclude by presenting a corrective term that enables for an incorporation of thermal expansion effects into the prediction of the maximum friction force

A model test study on the parameters affecting the cyclic lateral response of monopile foundations for offshore wind turbines embedded in non-cohesive soils

During their service life, monopiles supporting offshore wind turbines are subjected to a large number of lateral cyclic loads resulting from complex environmental conditions such as wind and waves varying in amplitude, direction, load eccentricity and frequency. The consequential accumulation of displacements and rotations of the foundation structure with cyclic loading is one key concern in the design of monopiles. Nevertheless, the relevant offshore guidelines do not provide suitable procedures for predicting such deformations. Although there are several methods for this purpose in the literature, some of them produce very different or even contradictory results, which prevents a consistent approach to dimensioning. This paper briefly summarizes the current standardization regarding design of monopiles for cyclic lateral loading and provides some examples of possible prediction models from the literature. To highlight the need for further research, the predictions according to different approaches are compared and evaluated by a calculation example and a parameter study. Further, the results of a small-scale 1 g model test campaign on the load-displacement behaviour of monopile foundations subjected to lateral cyclic loading and the influencing parameters are presented, evaluated and compared with the findings of other research groups. In this way the test results can help to support or improve model development and provide insight into key issues relevant to monopile design. The parameters that have been assessed include the cyclic load magnitude, cyclic load ratio, load eccentricity, soil relative density, the grain size distribution of the non-cohesive bedding material and the pile embedment length

Estimation of capacity decrease due to accumulated excess pore pressures around cyclically loaded offshore foundations in sand

In particular during storm events, a build-up of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. Such accumulated excess pore pressure reduces the effective stresses in the soil and hence negatively affects the structural integrity. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no generally applicable and accepted method for the calculative verification currently exists. The paper presents an approach which allows for the transfer of the soil behaviour observed in cyclic direct simple shear tests to the foundation structure system by means of a three-dimensional numerical model. The method is modular and can easily be assessed with engineering judgment in each substep. The used approach enables the consideration of site-specific cyclic laboratory test results by taking into account the mean stress, the cyclic shear stress amplitude and the number of load cycles at each integration point of the numerical model. Hence, the numerical approach may contribute to the optimisation of common foundation solutions as well as to the verification of innovative foundation structures even in complex soil conditions

A new method for the analysis of foundation behavior in sand under drained high-cycle loading

In offshore technology, especially in offshore wind energy converters, permanent deformations of the structures must be limited. For that purpose, the accumulation of permanent deformation due to cyclic loading must be predicted as accurately as possible. To account for this accumulation in non-cohesive soils, different approaches such as semi-empirical methods, p-y curve methods and numerical methods are available. Among the numerical approaches, the Stiffness Degradation Method (SDM) has the advantage of practical feasibility. However, it is only compatible with a relatively simple constitutive law and does not consider the influence of un- and reloading stress paths in the soil. With the basic concept of SDM, a new method termed Cyclic Strain Accumulation Method (CSAM) is proposed. In CSAM, the weaknesses of SDM, especially its incompatibility with advanced constitutive laws, are overcome, while retaining the practical feasibility as the main advantage of SDM. Through numerical calculations of a monopile, it is found that the CSAM is able to reproduce SDM results if the same material law is applied. The results of SDM and CSAM for the case of a vertical loaded strip footing have been presented. The comparison shows that the CSAM results are more realistic than the SDM results. Besides, CSAM is computationally more efficient and open for further optimisation. The effects of sophisticated material law and the consideration of un- and reloading are investigated. Results show that CSAM is a promising new approach to account for the deformation of foundations under cyclic loading in non-cohesive soils