Feasibility study on Subsea Power Generation from Wellstream Heat using a Binary Organic Rankine Cycle
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Geothermal heat from the subsurface can potentially be used as a clean energy source for offshore oil and gas producing facilities in the future. This thesis investigates the feasibility of thermal energy conversion by the use of a binary organic Rankine cycle offshore, with the aim to see if on-site power generation based on waste heat may be viable for utilization. This study uses wellstream data from the Tordis and Midgard fields to evaluate the energy conversion potential from oil- and gas-based reservoirs, respectively. A comparison of the raw power potential between the two reservoirs at different wellhead temperatures have been quantified, to investigate which reservoir type is more suitable for geothermal energy exploitation. It was found that the liquid-based Tordis wellstream has larger thermal potential than Midgard saturated gas for any wellhead temperature when production mass rates and other conditions were equalized. A large amount of fluids were screened as potential working fluids for a thermodynamic cycle operating at the Tordis field. The working fluid selection criteria were based on performance, accessibility and environmental impact. The fluids R134a and butane were found to be the most suitable for an organic Rankine cycle placed topside at Tordis. For a cycle placed on the sea floor, a binary mixture of 10 mol-% propane and 90 mol-% ethane performed well at the high-pressure subsea ambient. The necessary operating conditions for optimal net power output have been defined using a subsea cycle at Tordis as a base case. This involved estimating the pressure and temperature requirements for each stage of the cycle along with optimizing the working fluid mass flow rate, which was performed using the thermodynamic software Aspen HYSYS. The simulation of the subsea power generation system involved using a rigorous model for the shell and tube evaporator heat exchanger, to get a realistic estimate for the heat transfer potential from wellstream to cycle. The highest thermal energy transfer was achieved using a series of five dual-pass heat exchangers totalling an effective heat transfer area of 1397.8 m2, which would transmit 23.2 MW of heat to the cycle. The cycle thermal efficiency was 9%, effectively producing approximately 2.1 MW worth of power. Rough basic design features of the cooling system and turbine were also determined. It was opted to use a passive system for working fluid cooling by making use of natural convection with the surrounding sea water, and the required effective heat transfer area of the passive cooling manifold was estimated to a minimum of 950.0 m2. Sizing of the turbine was performed based on the energy state of the working fluid to calculate the optimum lengths of the rotor blades to produce electricity at the Norwegian grid frequency of 50 Hz. The unit was modelled as a simple axial turbine, and it was found that with inlet binary working fluid velocity at 260.8 m/s, using initial rotor blade lengths of 0.19 meters, grid frequency is achieved. The outlet velocity was calculated to 98.1 m/s, yielding an overall maximum energy utilization efficiency of 85.8% for the proposed turbine. It was investigated which control requirements are necessary to run a subsea organic Rankine cycle, and the following elements were found as the most efficient actuators for output optimization and cycle supervision. Installation of a by-pass valve circumventing the heating train for pressure control, a turbine by-pass that can be opened to route the working fluid in case of condensate droplets occurring, pump speed control and a gear box for the generator to ensure energy production at constant frequency. The thermodynamic feasibility of a power generation unit at the Tordis field has been assessed qualitatively after an extensive literature review, and is deemed as plausible. The energy requirement for subsea boosting at the field was used as evaluation criteria. Tordis requires approximately 4 MW of power supply for its boosters, and the results from the literature study suggested that a net energy output of up to 4.65 MW is achievable based on a realistic cycle thermal efficiency factor. When the thermodynamic feasibility was assessed quantitatively based on the proposed dynamic model for subsea cycle, it is deemed non-viable. The calculated net energy output of 2.1 MW from the rigorous model does not suffice to cover the boosting power requirement.