Experimental investigation of the vertical pullout cyclic response of bucket foundations in sand
Introduction
Offshore wind turbines are constructed as renewable energy sources worldwide (e.g., Europe and North America). An offshore wind turbine includes a tall tower with a turbine that produces high overturning moment at the tower base due to wind and wave forces. The tower is supported by several types of foundations, such as gravity base, gravity base with a skirt, jacket pile, and monopile. Suction bucket foundations are competitive foundation types which supports offshore wind turbines. A bucket foundation consists of a circular surface foundation with a thin skirt around its circumference. The foundation is installed in the seabed by using its self-weight and suction force to confine the soil within the foundation.
Offshore wind turbines with low power rates are installed at shallow water depths. However, offshore wind turbines with large power rates should be installed at deep water depth, in which they are subjected to strong horizontal loads and overturning moments. Tripod bucket foundations could support offshore wind turbines at deep water depth. Tripod bucket foundations combine three single bucket foundations in a triangular shape. The basic definition of the tripod bucket foundation is shown in Fig. 1.
Previous investigations indicated that offshore wind turbines exhibit design issues, such as the accumulated displacement and unloading stiffness of the foundations under cyclic loading [1], [2], [3]. The accumulated displacement or rotation of bucket foundations increases with the increase in the number of load cycles.
Byrne [4] performed static and cyclic vertical pullout load tests on a suction bucket embedded in sand. Sand was saturated with silicon oil. The cyclic load test was limited to approximately 100 load cycles under vertical cyclic load. Similar to the test on a bucket foundation under cyclic horizontal load, the accumulated vertical displacement increases with the increase in the number of load cycles. The static vertical pullout tests were performed with displacement control at different vertical pullout velocities (from about 0.001 mm/s to 0.5 mm/s). The results showed that failure vertical pullout load is relatively independent of vertical pullout velocity. Although Byrne [4] performed the test extensively, the limitation of 100 load cycles performed might not fully represent the general behavior of bucket foundations under cyclic vertical pullout loads.
Kelly et al. [5] and Kakasoltani et al. [6] discovered that the static vertical pullout load-displacement of suction bucket foundations in sand exhibits softening behavior, and the failure tension load is reached at a small displacement. Houlsby et al. [7], [8] presented a simple analytical approach to determine the tensile bearing capacity of suction caissons under monotonic loading in sand. They considered the bucket foundations under various conditions, including drained and partially drained, as well as the effects of displacement rate.
Thieken et al. [20] performed numerical analyses to investigate the behavior of suction buckets under static vertical pullout loads. Vertical pullout capacity was found to be influenced by different factors, such as loading rate, sand permeability, and bucket dimension. A large heave of the bucket is necessary to mobilize suction pressure in the foundation.
Other studies have reported that the stability of tripod bucket foundations is mainly governed by the pullout displacement of the upward bucket, with the rotation center of the tripod located in the downward bucket, as shown in Fig. 1(c) [9], [10], [11].
These discussions reveal the importance of understanding of the accumulated displacement and unloading stiffness of the upward bucket of a tripod bucket foundation under cyclic loading. The behavior of a tripod bucket foundation can be investigated using a single bucket foundation under cyclic vertical loads because the upward single bucket governs the stability of a tripod bucket foundation. However, no study has experimentally investigated the accumulated vertical pullout displacement of a bucket foundation in sand under a large number of load cycles. Therefore, this study investigated the accumulated pullout displacements and unloading stiffness of single bucket foundations in dry and saturated sands under fully drained and partially drained conditions. A series of 1 g experimental tests was conducted on two model bucket foundations with embedment ratios of L/D = 0.5 and 1, where D is the foundation diameter and L is the skirt length. Up to 104 cyclic vertical load cycles (N) were applied at varying load amplitudes. The obtained results are applicable to the preliminary design of tripod bucket foundations for offshore wind turbines.
Section snippets
Testing equipment
Fig. 2 shows a schematic of the test equipment used in this study. A loading equipment, which is similar to that presented by Leblanc et al. [12] was designed and manufactured. This equipment comprises four pulleys, two weight blocks (m1, m2), two steel loading wires with a diameter of 1 mm, and a load lever attached to a load driving motor that was connected to a steel frame. The steel frame can be rotated around a rotational pivot during loading. The driving motor could stably produce loading
Vertical pullout failure load
Previous studies have shown that the loading rate significantly affects the ultimate pullout bearing capacity of the bucket foundation. Thieken et al. [20] investigated extensively the vertical pullout capacity of full scale bucket foundations in moderately and extremely dense sands which had an effective grain size of d50 = 0.52 mm. Permeability values of 1.11 × 10−3 m/s and 7.73 × 10−4 m/s, respectively. They found that fully drained and fully undrained conditions may be considered at loading rates of
Conclusions
This study investigated the accumulated vertical displacement and unloading stiffness of bucket foundations in dry and saturated sands under vertical cyclic loads. A series of 1 g model tests was conducted for single bucket foundations at embedment ratios of L/D = 0.5 and 1. The following conclusions were drawn from the test results.
- (1)
The vertical pullout capacity of the bucket foundation was evaluated as the sum of the skin resistances of the bucket skirt and the suction force that developed inside
Acknowledgement
This research was supported by the Basic Science Research Program funded by the Ministry of Education, South Korea (NRF-2016R1A6A1A03012812) and the Development of Life-cycle Engineering Technique and Construction method for Global competitiveness upgrade of Cable bridges (16SCIP-B119960) from the Smart Civil Infrastructure Research Program funded by the Ministry of Land, Infrastructure and Transport(MOLIT) of the Korean government and the Korea Agency for Infrastructure Technology
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