Buoyant Tower: Experimental and Numerical Studies on Novel Application of Suction Caisson Foundation

Abstract

Abstract The buoyant tower, or BT, is a unique concept that stems from, and is a hybrid of, spar and compliant tower technologies. A BT is composed of one or a group of cylinders anchored to the seabed by a single suction caisson foundation (SCF). The BT rolls in a compliant manner while keeping the base from moving in any direction. The first application of BT is CX-15, a production platform in the Corvina Field offshore Peru. The 10-year-long operation since 2012 has provided a vast volume of data and experiences that support the IP-holder to re-visit this concept for extending the design to offshore wind, i.e., w.BT. This paper summarizes a recent experimental and numerical study on SCF, one of the key components of a BT or a w.BT, subjected to cyclic loads. Unlike most of SCFs used for offshore O&G industries, the SCF of a BT or a w.BT needs to maintain adequate load-bearing capacities after enduring repeated rolling motions. The vertical loads come from the difference between the weight of the platform and the buoyancy, which varies with the draft changes due to waves and tides. The horizontal loads mainly come from cyclic wave loads on the hull. These conditions are seldom studied, and little is known about the bearing capacity of SCFs under such loading conditions. Also because of the unique configuration of the BT, there is no design guidance available in the existing standards that are fully applicable. The current study aims to understand the reduced vertical and horizontal load-bearing capacities of the BT's SCF after constant cyclic rolling as well as the failure mechanisms of the soil surrounding the SCF. The relevant working conditions were studied with 1:100 geotechnical centrifuge model tests. The bearing capacities of the SCF were carefully studied with a focus on reduction of the SCF's bearing capacity subjected to cyclic rolling of the BT. The test results were also used to calibrate a numerical analysis procedure that will be used for the design of SCF for w.BT.