DEM simulation of cyclic tests on an offshore screw pile for floating wind

Screw piles need to be upscaled for offshore use e.g. being an alternative foundation and anchor form for offshore floating wind turbines, although the high demand of vertical installation forces could prevent its application if conventional pitch-matched installation is used. Recent studies, using numerical and centrifuge physical tests, indicated that the vertical installation force can be reduced by adopting over-flighting which also improved axial uplift capacity of the screw pile. The current study extends the scope to axial cyclic performance with respect to the installation approach. Using quasi-static discrete element method (DEM) simulation it was found that the over-flighted screw pile showed a lower displacement accumulation rate, compared to a pitch-matched installed pile, in terms of load-controlled cyclic tests. Sensitivity analysis of the setup of the cyclic loading servo shows the maximum velocity during the tests should be limited to avoid significant exaggeration of the pile displacement accumulation but this may lead to very high run durations.

Оценка прочности стальной винтовой однолопастной сваи при действии осевых нагрузок

The purpose of this work is resolving the problem of evaluating the structural strength of the steel screw single-bladed pile under the pressure of axial loads in various soil conditions. Calculations in this area constitute the necessary condition for assessing the possibility and feasibility of using pile-screw technology in specific construction conditions, inter alia, at main pipeline facilities. The dependence of the pressure curve on the blade on the soil stiffness modulus was revealed in the course of numerical simulation performed by the finite element method in the ANSYS software. The maximum pressures are observed at the junction of the blade and the pile shaft with a large soil stiffness modulus, if the soil stiffness modulus is small, then the maximum pressures are observed at the edge of the blade. An analytical method for assessing the strength of a single-blade screw pile is proposed based on the theory of bending of circular and annular plates, as well as the results of numerical modeling. Comparison of the analytical method for calculating the strength of a screw pile with the results of numerical modeling to assess the accuracy. The comparison results showed an accuracy sufficient for engineering calculation methods. Цель настоящей работы – решение задачи оценки прочности конструкции стальной винтовой однолопастной сваи при воздействии осевых нагрузок в различных грунтовых условиях. Расчеты в этой области – необходимое условие для оценки возможности и целесообразности применения свайно-винтовой технологии в конкретных условиях строительства, в том числе на объектах магистральных трубопроводов. По результатам численного моделирования, выполненного методом конечных элементов в ПК ANSYS, выявлена зависимость изменения эпюры давления на лопасть винтовой сваи от модуля деформации грунта: с увеличением значения модуля деформации грунта максимальные давления наблюдаются в узле сопряжения лопасти и ствола сваи, при малых значениях модуля деформации максимальные давления отмечаются у края лопасти. На основании теории изгиба круглых и кольцевых пластин, а также результатов численного моделирования предложен расчетный метод оценки несущей способности однолопастной винтовой сваи по материалу. Для оценки точности разработанного подхода к расчету прочности винтовой сваи проведено сравнение предложенного аналитического метода с результатами численного моделирования, которое показало достаточную для инженерных методов расчета точность. Общий принцип, заложенный в рассмотренном методе расчета, может быть использован для создания различных эпюр давления на лопасть в зависимости от модуля деформации грунта и характеристик винтовой сваи.

Using discrete-element method hindcasting of screw pile performance for practical design

Screw piles have been used to support a variety of structures due to their ease of installation and high axial capacity. Recently, screw piles have been proposed as an alternative foundation solution for offshore renewable structures due to their quiet or silent installation. Due to their variable geometry, design and prediction of installation requirements and its effect on in-service capacity may be challenging. In this research study, the discrete-element method (DEM) is used to numerically recreate a series of onshore field tests. The aim of the study is to investigate the ability of DEM to be used as a practical design tool for the design and deployment of screw piles. In this case study, the effect of the geometric helix pitch on the installation torque and tensile capacity of screw piles installed into sand is investigated. DEM results show that the geometric pitch of a screw pile appears to have little effect on the installation torque. The results show that DEM has the potential to be used as a practical design procedure for complex foundation installation where the simulation needs to capture installation effects.

Influence of Spacing and Cross-Sectional Shaft of Screw Piles Group on Lateral Capacity

Although there are several high-capacity screw piles in use currently, there are few studies on their lateral performance. The aim of this study is to investigate the lateral behaviour of several models of screw pile group (1×2), (2×1), and (2×2) embedded in soft clay and extended to stiff clay under lateral static load. Three spacing between pile (1.5, 3, and 4.5) Dh (helix diameter) with a shaft diameter of 10 mm, single and double helix, and embedded length ratio L/d 40 were used. The results showed that increasing the number of the piles in the group had a larger effect, the lateral resistance of group (2×2) increase to about (2.5 and 3.2) times more than groups (2×1) and (1×2) respectively. While the increase of pile spacing in the groups from (1.5 Dh) to (3 and 4.5 Dh) increases the lateral resistance about 6-23% and 16-52% respectively. Also, the result showed that the screw pile with double helix gives an increase in lateral resistance about 3-8% from the single helix.