Installation of the Subsea Buoyancy Gravity Energy Storage System

Abstract

To meet the targets set at the Paris climate convention, a rapid decrease in the emissions caused by the process of extraction of fossil fuels is required. Especially offshore, where a grid-connection is often non-existent, having power supplied by local renewable energy sources with a variable power generation can potentially lead to problems in the energy balance. To avoid the use of back-up diesel generators, Novgorodcev Jr. et al came up with the idea of implementing an energy storage unit based on the principles of potential energy, namely the SBGESS (Figure 1). The objective of this thesis was: To assess the dynamic behavior of the SBGESS system during installation and provide suggestions to increase workability by reducing the dynamic response. To this end, the SBGESS was preliminary dimensioned based on simulations of the energy balance and the lowering operation was modelled using these dimensions. A 10.32MWh SBGESS, with three power supply modes (including a power saving mode), is able to operate a 4.34MW water injection system with power being generated by two 12MW wind turbines. Simulations were run to assess the sensitivity of the system to the energy storage capacity and the ranges in which a certain mode was active. Based on historical wind data, this system is able to meet the empirical reliability and performance criteria for a water injection system. Using the energy storage capacity and power demand as input, the size and number of buoyancy- and gravity units were scaled to minimize overall dimensions as to reduce installation complexity. The vertical displacement of the SBGESS during the lowering phase was modelled using non-linear mass - dashpot - spring models (Figure 2). It was concluded that using polyester rope for the lowering cable leads to relatively low dynamic forces during the lowering and is able to meet the DNV criteria for an offshore operation in more sea states than other alternatives. Taking precautions such as heave compensation and a lower payout velocity are however necessary to avoid slack conditions and large dynamic forces during the beginning of the lowering operation, as this is where resonance predominantly may occur. A Sensitivity analysis showed that the hydrodynamic coefficients require extensive lab testing as their values have a large impact on the dynamic force. The added mass coefficient has a significant influence on the overall dynamic force and location of the resonance zone, while the drag coefficient mostly impacts the oscillation magnitudes in the early parts of the lowering operation.