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

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

Numerical investigation on rigid and flexible pipelines embedded in granular and self-compacting materials

Self-compacting filling material or controlled low-strength material (CLSM) is a cementitious material which is liquid during filling, and it is used primarily as backfill, e.g., in trenches. Several products are currently used as CLSM such as flowable fill, controlled density fill, flowable mortar and low-strength plastic soil–cement. The low-strength requirement is necessary to allow for future excavation of CLSM. A two-dimensional numerical model was developed using the finite element system ABAQUS. In this model, the material behavior of granular soil and CLSM is described using an elasto-plastic constitutive model with Mohr–Coulomb failure criterion. Rigid and flexible pipes were modeled once embedded in sandy soil and once embedded in self-compacting material. The numerical model allows the modeling of the effect of hardening process on the overall behavior of the pipe–soil system. The main objective of this study is to investigate the behavior of rigid and flexible pipelines embedded in CLSM as a filling material numerically and to show advantages and disadvantages in comparison with the presently widely used filling materials like sand. © 2020, The Author(s)