Anchorage Mechanism for Ocean Compressed Air Energy Storage Including Gravity-based and Suction Caissons Approaches

Ocean renewable energy sources including the energy imparted by wind, waves, and gulfstream currents are intermittent in nature. Therefore a viable energy storage solution to provide a dependable and economically viable feed to the power grid is vital for the ocean energy utilization. Ocean Compressed Air Energy Storage (OCAES) is an innovative concept that is based on converting energy into compressed air and storing the compressed air in the storage vessels on seabed. At the seabed, the compressed air pressure is balanced by the hydrostatic water pressure, so the OCAES vessel does not have to resist high internal air pressure. Also the air-water interface acts as a piston to maintain a steady air pressure during compressed air inject /retrieval process.;The two main objectives in this study are focused on the configuration and anchorage of OCAES vessel, with the anchorage approaches including ballasting and suction caissons. The structural analysis of the reinforced concrete storage vessel, via multi-physics program COMSOL, and design based on ACI codes are performed. The operational loading on OCAES vessel has cyclic characteristics due to the internal pressure change during the compressed air inject/retrieval process. The configuration of an OCAES vessel with the storage capacity of 12410 MJ is recommended to be a reinforced concrete tank with inner dimensions of 20 m x 20 m x 3 m, wall thickness of 0.6 m, and 5 m thick ballast on its top. Suction caissons are also considered as an anchorage option for OCAES vessel. The criteria for three failure modes of suction caisson in sand under axial pullout loading are developed as functions of pullout rate, sand hydraulic conductivity, length of drainage path, and shear strength properties of the sand. The research approach included analytical developments and numerical analyses via multi-physics program COMSOL. The validity of criteria is verified by the experimental data reported in literature.;In Failure Mode I, the sand is assumed to be in non-flow state and the core sand remains in position during the pullout process. The pore water is extracted to fill the emerging chamber space beneath the caisson's lid. If the pore water inflow velocity is smaller than the pullout rate of suction caisson, the vacuum occurs in the chamber space and the pullout capacity experiences a sudden drop of magnitude at the moment of vacuum vanishing.;In Failure Mode II, the sand around the caisson is assumed to be in flow state and the core sand moves up with caisson. The sand flow around the caisson is generated to fill the emerging gap at the caisson's base. Bingham plastic flow model is used to express the sand flow behavior, in which the shear stress is a function of shear strain rate. Accordingly the pullout capacity estimation based on sand flow model is dependent on the pullout rate and shows a good agreement with experimental data reported in literature. The parametric study indicates that the pullout capacity is independent of the water depth, but is positively linearly correlated to the pullout rate, viscosity and yield stress of sand flow.;Failure Mode III is similar to Failure Mode II, except that a cavity with zero pressure occurs at the caisson's base. A modified sand flow model with penetration-depth-dependent partial slip boundary, which can capture the transition of sand in non-flow state with perfect slip boundary at the beginning of pullout process to sand flow state with no slip boundary at the end of pullout process, is proposed to estimate the pullout capacity for Failure Mode III. The estimation shows good agreement with experimental data in literature as well.