Tri-Pod Suction Buckets Suction buckets were selected as the foundation structure mainly because of their advantages compared to conventional underwater foundation systems: simplicity, efficiency and environmental friendliness. More technically, the tri-pod suction buckets were used as the foundation for the wind turbine to convert the external overturning moments into mostly axial tension or compression loads applied at the top of suction buckets. To resolve all of the applied loading (i.e., vertical and horizontal loads, and the overturning and twisting moments) into equivalent vertical and horizontal loads applied to each suction bucket, all practical load combinations and directions of load application were considered in the analyses from which the worst loading condition was used in the final design of the suction buckets. Per the final design, each of the tri-pod suction buckets comprising the foundation system is about 6 m (20 ft) in diameter and 12 m (39 ft) in length. For each stage of the installation, a given suction pressure is applied to the bucket, and the penetration of the bucket continues until the point when the pushing force-soil resistance equilibrium is reached. The suction pressure is then in- creased (i.e., pushing force is greater than the soil resistance), and the bucket will continue to penetrate into the soil until the next stage of equilibrium is reached. This procedure is repeated until the desired depth of penetration is achieved. Suction bucket mechanism As discussed by Bang and Cho (2000), geotechnical engineers Fabrication of lower wind turbine system and suction buckets Mechanism and Installation A suction pump attached at the top of the bucket provides the necessary reduction in water pressure inside the pile (i.e., outside ambient water pressure minus water pressure inside the bucket) to facilitate the entire installation operation. During installation, pump- ing water out reduces the water pressure inside the bucket, which creates a driving force that pushes the bucket down into the seafloor. Therefore, the capacity of the suction pump must be greater than the amount of seepage flow (i.e., the flow of water from outside of the bucket to inside). If the pushing force is large enough to overcome the soil resistance, the bucket will penetrate into the seafloor, and the penetration of the bucket will cease at the point of equilibrium when the pushing force is equal to the soil resistance. Positioning and installation of the suction bucket foundation must carefully control the suction pressure so that the complete installation of a suction bucket is possible. The designer must determine the correct bucket length-to-diameter ratio to ensure the bucket is installed to its intended penetration depth. The soil resistance corresponding to the bucket penetration dictates the lower limit of the necessary suction pressure. That is, if the applied suction pressure is less than the estimated value of soil resistance, the vertical pushing force will be less than the soil resistance, which will impede or prevent the penetration of the bucket. Conversely, instability of the soil inside of the bucket dictates the upper limit of the suction pressure. That is, if the suction pressure is too great, the soil inside the bucket becomes unstable; consequently, this instability will allow the soil to fill the inside of the bucket, which will prevent the bucket from reaching the desired penetration depth and the bucket installation will be incomplete. Loading Capacity The loading capacity of a suction bucket must consider three separate loading conditions (i.e., vertical, horizontal and inclined loading) and combinations thereof. The designer can estimate the vertical compressional loading capacity based on conventional theories for large diameter open-ended piles. The vertical tensile loading capacity, however, requires consideration of three different failure mechanisms: 1. Bucket slip, which occurs when the bucket itself slips out of the soil 2. Bucket pull-out, which occurs when the bucket and the soil inside are pulled out simultaneously, as a unit 3. Reversed bucket bearing capacity, which occurs when the soil outside the bucket experiences failure similar to shallow foundations but in a reverse fashion Bucket slip is a dominant failure mechanism for large-diameter suction buckets and/or for low-strength soils, whereas the bucket pull-out mechanism is the dominant failure mechanism for small- diameter suction buckets and/or high-strength soils. The least resistance of the three mechanisms is selected as the final vertical tensile loading capacity. DEEP FOUNDATIONS • NOV/DEC 2017 • 15
