Abstract
Offshore wind-turbine support structures are largely made of steel since steel monopiles have accounted for the majority of installations in the last decade. As turbines become bigger, steel structures have led to an exponential increase in material and installation costs. From this point of view, the use of concrete for future support structures has been initiated. In this study, concrete–steel composite piles have been investigated. A pre-tensioned high strength concrete pile was placed in the lower part, mainly to support the axial load, and a steel pile in the upper part to resist the lateral load. A mechanical joint was adapted to connect the two different types of piles. Static axial, dynamic axial, and lateral load tests were performed to evaluate the load-displacement response of the composite pile, verify the integrity of the mechanical joint, and investigate its potential application to offshore wind foundations. This paper focused on the load-displacement response and the connection integrity; in particular, it investigated the lateral load-displacement response by comparing it to the results of beam-spring analysis. Based on the results from the field tests, a site-specific lateral load-displacement curve was suggested, and the connection integrity was verified.
Highlights
The overall objective of this study was to investigate the behavior of concrete–steel composite pile foundations by field pile load tests: three dynamic axial load tests, one static axial load test, and five lateral load tests for three small-sized piles with a diameter of mm and four large-sized piles with a diameter of 1000 mm to verify the possible application to offshore wind foundations
The small-sized composite piles having a diameter of 500 mm were investigated by field pile load tests
The applicability of large-sized composite piles and PHC piles with a diameter of mm was comprehensively evaluated to determine its potential use in offshore wind foundations
Summary
The monopile is the dominant foundation system for current and planned offshore wind farm developments in shallow coastal waters, in Europe [1]. The main reason for its predominance is a straightforward design and relatively simple installation and transportation processes. They have been used on wind farms for wind turbines larger than 8 MW located in water more than 30 m deep. Such large monopiles have led to an exponential increase in material and installation costs [2]
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