Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Giulio Nicolai (1) F P Lars Bo Ibsen (1)
(1) Aalborg University, Aalborg , Denmark
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
PhD student in the department of offshore foundations of the Aalborg Univerisity. My working field is the experimental testing of small-scale pile models, focusing on the investigation of cyclic lateral loading effects on the piles response.
Response of monopiles under cyclic lateral loading in sand
Offshore wind energy has a large potential for development although represents yet a small percentage of the total wind energy capacity installed. Large availabilities and high mean wind speeds make the offshore wind energy technology to be an attractive solution. Nevertheless, the costs for such technology are higher compared to the onshore converters and thus a cost reduction is a goal that has to be reached. The foundation represents approximately one third of the total cost of an offshore wind turbine and therefore researches aiming at a costs reduction are necessary.
The present paper deals with small-scale testing of a monopile model in dense saturated sand. The testing rig used to perform the experimental investigation is able to apply thousands of load cycles and static loading to the monopile model. Such foundation model is a small-scale prototype pile with diameter equal to 10 cm and length 50 cm, in order to have the stiff behavior of real monopiles. The purpose of the laboratory tests is to investigate how the static ultimate lateral resistance of monopiles is affected by the cyclic loading.
Main body of abstract
There are several foundation concepts for offshore wind turbines but the most common is the monopile foundation. Such foundation is a hollow steel pile that is driven into the seabed. The current design methodology of offshore piles, regarding lateral loading, is not well developed in the main design standards, i.e. API and DNV. The design approach for offshore piles was formulated testing slender piles used in the oil and gas sector. Such typologies of piles have diameters in a range of 0.5 m to 2 m approximately and have a flexible behavior, whilst piles for offshore wind turbines have diameters in a range of 4 m to 6 m and have a stiffer behavior. Further, the current design formulation does not account for the number of cycles and was made by analyzing experimental results of tests with less than 1000 cycles. In real offshore conditions, a wind turbine structure is subjected to millions of cyclic loads during its lifetime that induce to an accumulated rotation that might be critical for the turbine. Therefore, further investigations are necessary to fully understand the response of monopiles under cyclic lateral loading in order to improve the design standards. A more appropriate design standard methodology might lead to a reduction of the foundation size and hence to a decrease of the quantity of material, achieving a reduction of the fabrication costs.
A comparison between the static resistance of the pile with the static resistance of pre-cycled piles is presented. The cyclic loading is supposed to reduce the static ultimate lateral resistance of a pile, according to the design guidelines. In the present paper it is shown that such response is not achieved in the experimental test results, but in contrast it is found that the static resistance of the pile is increased after applying cyclic loading.
The design methodologies proposed by the main standards provide formulations that are in contrast with the main findings of the present test results, regarding the static ultimate lateral resistance of piles. Therefore further investigations are required in order to be capable of predicting the real response of cyclic laterally loaded monopiles.