The driven monopile is currently the preferred foundation type for most offshore wind farms. While the static capacity of the monopile is important, a safe design must also address issues of accumulated rotation and changes in stiffness after long-term cyclic loading. Design guidance on this issue is limited. To address this, a series of laboratory tests were conducted where a stiff pile in drained sand was subjected to between 8000 and 60 000 cycles of combined moment and horizontal loading. A typical design for an offshore wind turbine monopile was used as a basis for the study, to ensure that pile dimensions and loading ranges were realistic. A complete non-dimensional framework for stiff piles in sand is presented, and applied to interpret the test results. The accumulated rotation was found to be dependent on relative density, and was strongly affected by the characteristics of the applied cyclic load. Particular loading characteristics were found to cause a significant increase in the accumulated ro...

https://www.icevirtuallibrary.com/doi/pdf/10.1680/geot.7.00196

Response of stiff piles in sand to long-term cyclic lateral loading

Offshore wind is a source of clean, renewable energy of great potential value to the power industry in the context of a low carbon society. Rapid development of offshore wind energy depends on a good understanding of technical issues related to offshore wind turbines, which is spurring ongoing research and development programmes. Foundations of offshore wind turbines present one of the main challenges in offshore wind turbine design. This paper reviews the present state of knowledge concerning geotechnical and structural issues affecting foundation types under consideration for the support structures of offshore wind turbines, and provides recommendations for future research and development.

/pdf/foundations-of-offshore-wind-turbines-a-review-3yvgufr6nj.pdf

Foundations of offshore wind turbines: A review

A programme of testing on suction caisson foundations in an artificially prepared sand test bed near Luce Bay, in Scotland, is described The tests are relevant to the design of either monopod or quadruped foundations for offshore wind turbines Records are presented for suction installation of the caissons, cyclic moment loading under both quasi-static and dynamic conditions to simulate the behaviour of a monopod foundation, and cyclic vertical loading and pullout of caissons to simulate one footing in a quadruped foundation Variations of stiffness with loading level of the foundation are observed, with high initial stiffness followed by hysteretic behaviour at moderate loads and degradation of response at high loads Some implications for the design of wind turbine foundations are briefly discussed

/pdf/field-trials-of-suction-caissons-in-clay-for-offshore-wind-k3rsur2f94.pdf

Field trials of suction caissons in clay for offshore wind turbine foundations

A new concept with respect to offshore wind turbines, the bucket foundation, is known in the oil and gas industry, although the load conditions for wind turbines are significantly different. An extensive number of loading tests with small-scale bucket foundations subjected to combined loading were carried out in the geotechnical laboratory at Aalborg University in Aalborg, Denmark. Tests were performed on buckets of various sizes, embedment ratios, and load paths on saturated dense Aalborg University Sand No. 1. The experimental results were used to evaluate the behavior of the bucket foundations in accordance with the macromodel approach. An expression to describe the combined capacity of bucket foundations was developed by calibrating the failure criteria for bucket foundations.

Calibration of Failure Criteria for Bucket Foundations on Drained Sand under General Loading

This paper reviews foundations for offshore wind energy convertors considering the significant growth of offshore wind energy since the early 2000s. The characteristics of various foundation types (i.e., gravity, pile, suction caisson, and float type) and the current status of field application are discussed. Moreover, the mechanical characteristics of soil are described in the sense that these characteristics including modulus, strength, damping, and modulus degradation of soil play critical roles for the design of offshore foundations. By using these mechanical properties of soil, theoretical studies to consider structure-soil interaction are classified (into equivalent spring models, distributed spring models, and continuous element models) and explained. Field and laboratory experiments on the response of structure embedded in soil to static and dynamic loads are discussed. Based on the review of previous studies, directions for future research and study on offshore wind turbine are suggested.

A review of foundations of offshore wind energy convertors: Current status and future perspectives

Laboratory tests applying vertical and moment loads to suction caissons founded in sand and clay have been conducted to simulate an equivalent series of field tests. The caissons used in the laboratory were 0·15 m, 0·2 m and 0·3 m in diameter, whereas those for the field tests were 1·5 m and 3·0 m in diameter. The loads applied to the caissons in the laboratory tests were scaled from those in the field tests, and the models were loaded in a near-identical manner to the field trials. The test results are presented in non-dimensional form for comparison. The non-dimensional laboratory data from moment loading tests were similar to the field data in most cases. However, the non-dimensional data from vertically loaded caisson tests in the laboratory and in the field show more significant differences, and possible reasons for these are discussed.

/pdf/a-comparison-of-field-and-laboratory-tests-of-caisson-3zgj8n792v.pdf

A comparison of field and laboratory tests of caisson foundations in sand and clay

Vertical monotonic and cyclic loading tests on model suction caissons have been carried out to explore conditions relevant to the design of multiple-footing foundations for offshore wind turbines. The tests were conducted in a pressurised chamber, which simulates up to 20 m of water depth, to investigate the effect of cavitation on the ultimate tensile response of the caisson. Cyclic loading tests were also conducted at elevated pressures. Data from cyclic tests are presented to compare the response of caissons subjected to varying ambient pressures and rates of loading. The ambient pressure had little effect on the cyclic response. Faster rates of loading generated larger pressures beneath the lid of the caisson and increased the load-displacement stiffness. The ultimate tensile capacity was significantly affected by the ambient pressure and rate of loading. Implications for the design of offshore caisson foundations are discussed.

/pdf/transient-vertical-loading-of-model-suction-caissons-in-a-2u21p5jw13.pdf

Transient vertical loading of model suction caissons in a pressure chamber

The viability of multiple footing structures that use suction caisson foundations would be improved if the up-wind leg(s) could resist significant tensile loads. In the particular case of offshore wind turbines, large moments are applied at foundation level by the action of wind and waves on the structure. For a multiple footing structure the applied moment is resisted primarily by vertical reactions on opposing foundations. If a significant tension can be allowed at the upwind side, then the spacing of the foundations, and therefore the overall size of the structure, can be greatly reduced. Bye et al (1995) suggest that the tensile capacity of caissons is dependent on the rate of loading, compared to the rate of drainage of excess pore pressure beneath the caisson. To investigate this, model caisson tests were carried out in test beds made of two different gradings of sand. Whilst the ultimate capacity is large in both cases (and controlled by the cavitation of the pore fluid) the displacements required to mobilise the loads are large compared to the diameter of the footing. These displacements are of a magnitude that would cause serviceability problems. The experimental results suggest that the upwind footing should be designed for tensile loading no greater than (at most) the drained friction on the skirt.

Tensile Loading of Model Caisson Foundations For Structures On Sand

Within the next few years a number of wind farms will be constructed around the coast of the United Kingdom. In the first instance many of the wind turbine structures will be founded on piles. These foundations, although simple to design as they are a well-established technology, are a significant proportion of the overall installed cost for these structures; of the order of 30%. Various options are being investigated that may reduce the installed cost, and therefore increase the economic viability of these wind-farm developments. One possibility is to use skirted shallow foundations installed by suction either as a single foundation (

R. B. Kelly

Papers

Field trials of suction caissons in clay for offshore wind turbine foundations

A comparison of field and laboratory tests of caisson foundations in sand and clay

Transient vertical loading of model suction caissons in a pressure chamber

Tensile Loading of Model Caisson Foundations For Structures On Sand

Pressure Chamber Testing of Model Caisson Foundations in Sand