Søren Dam Nielsen

Advanced Laboratory Setup for Testing Offshore Foundations

This paper describes a test setup for testing small-scale offshore foundations under realistic conditions of high pore-water pressure and high impact loads. The actuator, used for loading has enough capacity to apply sufficient force and displacement to achieve both drained and undrained failure modes for small-scale offshore foundations. Results from trial tests on two small-scale bucket foundations, subjected to transient or cyclic loading, are presented. Tests showed that cavitation limits the undrained bearing capacity. Hence, a high pore-water pressure is important for simulating offshore conditions.

Transient Monotonic and Cyclic Load Effects on Mono Bucket Foundations

The mono bucket foundation is a cost-effective foundation concept for offshore wind turbines and is a competitor the monopile, which currently supports almost 80 % of all installed European offshore wind turbines. Since the 1990s the bucket foundation concept has been widely used in the oil and gas industry. Arround the turn of the millennium, the industry started considering the mono bucket foundation for offshore wind turbines. The transition from supporting oil and gas structures to supporting offshore wind turbines, also meant a change in the loading conditions, and the use of existing design methods was not a possibility. The dominating load for an offshore wind turbine is the overturning moment, mainly coming from wind and waves. Especially wave loads affects the foundation with repeated loads where a wave load is succeeded by the load from the next coming wave. This is denoted as cyclic loading, which is transferred via the foundation to the soil. The soil response to cyclic loading is complex, and there is still no generalised design methods that account for cyclic loading of the soil. Beside cyclic loading, large waves may also lead to impact loads with a short duration. This can lead to the generation of excess pore pressure in the soil, of which the effect is ambiguous. Small-scale tests have shown that a short load duration creates suction in the pore water inside the bucket. This suction acts as a stabilising force and enhances the bearing capacity of the foundation. The shorter load duration the more enhancement, and thereby a higher bearing capacity. The behaviour of the mono bucket foundation exposed to cyclic loading is also investigated by small-scale testing. Compared to previous similar investigations, the used load frequency is 1.0 Hz, which is 10 times faster compared to previous tests. In contradiction to previous findings, the experiments showed that two-way loading leads to the highest accumulation of permanent rotation. The tests have been used to calibrate a model, which can predict the accumulated rotation of a mono bucket foundation exposed to cyclic loading.

Response of cyclic-loaded bucket foundations in saturated dense sand

AbstractOne of the major concerns of designing offshore wind turbine foundations is to address the effects of cyclic loading coming from, especially, wave loading. This paper describes the results ...

Transiently Loaded Bucket Foundations in Saturated Dense Sand - Demonstration of the Boot Effect

The mono bucket foundation is a cost-effective foundation for offshore wind turbines. During a storm, these foundations are exposed to large wave loads of short duration. This paper investigates the effect of increased loading rate on the bearing capacity of two mono bucket foundations installed in dense sand inside a pressure tank. The foundations had aspect ratios (skirt lengths [L] relative to the diameters [D]), L/D, of 0.5 and 1.0, respectively. Foundations were brought to failure with varying loading rates, resulting in drained, partly drained, or undrained behavior. Increases in bearing capacity were observed as the loading rate increased. This behavior was caused by a combination of dilative soil behavior and suction created by upward movement, known as the boot effect. For mono bucket foundations, the boot effect resulted in an increased bearing capacity that was 18 to 25 times higher than the drained capacity. Furthermore, the boot effect also led to an increase in stiffness. The stiffness of the partly drained response is measured up to four times higher compared with drained behavior.