Offshore Energy Storage Research Articles

Structural strength and fatigue analyses of large-scale underwater compressed hydrogen energy storage accumulator

Underwater compressed hydrogen energy storage (UWCHES) is a potential solution for offshore energy storage. By taking advantage of the hydrostatic pressure of deep seawater, the compressed hydrogen can be isobarically stored in underwater artificial energy storage accumulators. The accumulator should withstand high pressure and large buoyancy and possess reliable anchoring to the seabed. In this study, the structural strength analysis and fatigue life of the large-scale accumulator is conducted employing the finite element method (FEM). The dimensionless stress prediction model and dimensionless fatigue life prediction model are developed through dimensional analysis and multivariate regression analysis. The performance of the accumulator with operating water depth of 100∼300 m, gas storage volume of 1081∼10128 m³, and concrete wall thickness of 0.1∼0.63 m is investigated. The results show that with an operating water depth of 100 m, gas storage capacity of 10,128 m3, and concrete wall thickness of 0.63 m, the maximum compressive stress is 1.43 MPa (yield strength is 60 MPa) and the maximum tensile stress of the accumulator is 2.55 MPa (yield strength is 6 MPa). The design fatigue life is 106 cycles which is larger than the expected service life of 104 cycles. Therefore, the accumulator structure meets the static strength and fatigue life. As the operating water depth increases with a consistent gas storage capacity, a transition in the stress state shifts from primarily tensile stress to predominantly compressive stress. The accuracy of the dimensionless stress prediction model and the dimensionless fatigue life prediction model were verified, with maximum deviations of 10.3 % and 13.7 %, respectively. Furthermore, the anchoring factor of safety of 1.12 is achieved.

Exploring agenda-setting of offshore energy innovations: Niche-regime interactions in Dutch Marine Spatial Planning processes

The foreseen large-scale growth of offshore wind energy towards 2050 in pursuit of an energy transition obliges scholars and policymakers to start considering its integration in the wider offshore energy system. Both technological innovations and advances in spatial policy are necessary to facilitate offshore system integration. This study draws from agenda-setting theory to explore barriers and opportunities affecting the prioritization processes surrounding three offshore energy storage and transport concepts in Dutch marine spatial planning practice. The findings demonstrate that although various arenas for agenda-setting exist, they are geared to the input of established stakeholders, including the oil and gas and offshore wind sectors. Also, prioritization is hindered by a short-term (2030) governmental fixation and long-term institutional ambiguity. Therefore, supportive institutions are needed, providing regulatory certainty and reliable incentive mechanisms, whilst remaining adaptive to address imminent uncertainties, in pursuit of system integration needed for an energy transition.

ROPES Offers an Energy Transition Solution to Pipeline Decommissioning

Oil and gas companies around the world are repositioning themselves to face the energy transition—whether that means embracing the fact that the green economy of the future calls for less hydrocarbon use or something more radical, like Lundin Energy’s (now called Orrön Energy) recent move to sell off its oil and gas assets to become a pure renewables player. One certainty is that as companies trend away from oil and gas and toward a lower-carbon frontier there will still be plenty of “cleanup” to do while players figure out exactly what to do with more than 100 years of oil and gas infrastructure. Offshore, decommissioning of platforms and pipelines is a capital-intensive business and one that is expected to grow in the coming years. Last year, IHS Markit released a forecast predicting global offshore decommissioning spending to reach almost $100 billion for the 2021–2030 period, up by more than 200% compared to the previous 10-year period. According to the forecast, nearly 2,800 fixed platforms and 160 floating platforms could be decommissioned. That represents 33% of fixed platforms and 43% of floating platforms currently in operation. Additionally, more than 18,500 wellheads, 2,850 subsea trees, and 83000 km of offshore pipelines and umbilicals currently in operation are subject to decommissioning during the same period. More than 50% of the expected activity is spread across four countries: the UK, US, Brazil, and Norway. The financial burden and safety liability of marine decommissioning have prompted some to look at the potential for repurposing the hardware and using it in moving toward the drive to net-zero emissions: platforms that could be used to host wind turbines, vessels that could be redesigned to collect hydroenergy, and pipelines that could be repurposed for potential battery storage. Repurposing offshore pipeline as energy storage (ROPES) is a concept that is being investigated by a partnership of offshore projects and services specialists Subsea 7 and offshore energy storage startup Flasc. Flasc was founded as a spinoff from the University of Malta in 2019 and is based in the Netherlands. The concept was described in a paper presented at the 2022 Offshore Technology Conference in Houston (OTC 31703). Subsea 7 and Flasc signed a cooperative exclusivity agreement in late 2020 to work toward the commercialization of Flasc’s patented hydro-pneumatic energy storage (HPES) concept offshore. HPES combines pressurized seawater with compressed air to create an efficient, large-scale energy storage device that can be applied across a wide range of offshore applications. Energy is stored by pumping seawater into a closed chamber to compress a fixed volume of precharged inert gas. The energy can be recovered by allowing the compressed gas to push the water back out through a hydraulic turbine generator (Fig. 1). The technology leverages existing infrastructure and supply chains, along with the marine environment itself as a natural heatsink. The first working prototype was successfully tested in 2018, and DNV has granted the technology a Statement of Feasibility based on a technical and commercial assessment. “The hydro-pneumatic technology is at the core of the ROPES concept, but can also be applied to other embodiments,” said Daniel Buhagiar, co-founder and chief executive of Flasc. “Within this collaboration, we’ve looked at doing some different designs and different products, and ROPES emerged as a very interesting opportunity. Typically, we’re looking at doing new infrastructure, [for instance] installing a bundle or a new piece of kit to store the pressurized fluids. ROPES, we thought, was really the low-hanging fruit because the pipeline infrastructure is already there, and we can create a use case for it beyond the typical applications, such as hydrogen and carbon capture, which are not always possible.”

Direct drive wave energy array with offshore energy storage supplying off‐grid residential load

Current developments in wave energy conversion have focused on locations where the wave energy resource is the highest; using large devices to generate hundreds of kilowatts of power. However, it is possible to generate power from low power waves using smaller wave energy devices. These lower rated wave energy converters can form arrays to supply power to remote coastal or island communities which are off-grid. This study introduces wave-to-wire modelling of wave energy arrays for off-grid systems using low power permanent-magnet linear generators (PMLGs). Offshore energy storage at the DC link is added to keep the voltage constant along with a current controller for the inverter in order to supply constant low harmonic power to the residential load connected off-grid. Simulation results produced in MATLAB/Simulink environment show that the wave energy array can generate power independently from the residential side by keeping the system stable using offshore storage. In addition, two different types of controllers for wave energy devices that use PMLGs are compared based on the power captured from the waves.

Modelling of a novel hydro-pneumatic accumulator for large-scale offshore energy storage applications

Despite the ability of accumulators to smooth out fluctuations in small-scale hydraulic circuits, their use in multi-megawatt power transmission systems remains limited. This is due to the large pressure variations that they experience as their state-of-charge changes when their energy capacity is large. The present work highlights an approach whereby the pressure fluctuations are absorbed by a larger external volume of compressed air. This system has been integrated into a novel floating platform for offshore applications. A thermodynamic model of the gas compression process is developed in order to observe temperature and pressure fluctuations. A brief parametric analysis is undertaken to illustrate the effect of critical system dimensions. This comprises the effect of the external volume with respect to the accumulator volume and the diameter of the umbilical connecting the two components. The system is also simulated in different climates to observe the interaction between the external seawater temperature and the internal gas thermodynamics. A full charge-discharge cycle is simulated and results indicate that around 95% of the energy can be recovered after being stored for a 24-h period. The operational efficiency for a stochastic energy input was also computed and found to be relatively high. Electrical round-trip efficiency was found to be comparable to adiabatic and near-isothermal CAES, but the system can be more advantageous when integrated into the generation-side. The key attribute is the minimization of pressure fluctuations, which results in minimal deviations from the equilibrium temperature. This reduces thermal losses to the surroundings and results in a highly efficient energy storage system.

Influence of Technical Limitations and Operation on Sizing of an Offshore Energy Storage Connected to an Offshore Wind Farm

This paper aims to investigate the sizing of an offshore energy storage unit operated in conjunction with an offshore wind farm. The storage unit is evaluated with technical limitations to reveal the parameter sensitivities when coupled with a wind farm. The main interest is the sizing of the storage unit for capacity firming purposes.A storage unit in combination with a large offshore wind farm has been simulated in time domain over the course of one year. The storage unit has been operated with the goal of firming the wind power capacity within each bid period. Different constraints have been introduced to show parameter sensitivity of the storage unit. The constraints were both technical and control oriented. An important prerequisite for the simulation was that the combined wind power plant and the storage operate in a market where the imbalance between bid and delivered energy is measured and penalized.Results show that there are several important parameters regarding storage sizing. Storage sizing is shown to be very dependent on the production forecast error and market bid length. Furthermore, technical constraints in the shape of ramping rates and power reversal dead time can be countered by choosing an appropriate control strategy. No control strategy gives significantly more reduction in grid power imbalance than the constant, fixed mode control strategy. The same reduction can however be obtained, with somewhat less energy routed through the storage by applying an alternative control strategy.