## 6 The Case Study and Simulation Results  
### The Case Study  
The model has been applied to simulate the loading and the unloading processes of the CNG tanker that is being designed under the GAVESSEL project; see Table 2.  
The on-board compression station consists of two double-stage centrifugal compressors that can work in parallel. The polytropic efficiency varies from 0.5 near the choke point up to the maximum value, which is supposed to be 0.73. The outlet temperature of both the intercooler and cooler is set to 40 \({}^{\circ}\)C.  
Regulation is carried out by means of a throttling valve and variation of speed. The objective is to optimize the mass flow rate, keeping the absorbed power below the maximum design value of 10 MW. The loading process is carried out from a storage terminal constituted of a Floating Production Storage and Offloading (FPSO) unit, whose water volume is supposed to be twice the ship water volume. The FPSO initial pressure is 240 bar. The initial pressure in the ship gas cylinders is 30 bar, and the loading process ends when it reaches the design value of 300 bar. The forced loading phase starts when the mass flow rate due to the free loading falls below 20 kg/s. The unloading process is carried out in a pipeline at a constant pressure of 120 bar. The initial pressure in the ship gas cylinders is supposed to be the same reached at the end of the loading process, 300 bar, neglecting the possible variations due to the ship engines consumption and to the heat exchange with the surroundings. The unloading process ends when it reaches the final value of 30 bar. Such a final value has been imposed considering that the remaining gas must be sufficient to guarantee the fuel for the return journey. The forced unloading phase starts when the mass flow rate due to the free unloading falls below 20 kg/s. In these conditions, the pressure in the gas cylinders remains for a while higher than that in the pipeline: this requires acting on the lamination valve to make the suction pressure lower than 120 bar, so allowing the normal operating conditions of the compressors. Two calculation hypotheses have been taken into account: i.e., adiabatic or not adiabatic pressure vessels. The first is useful as a term of comparison, while the second is more realistic. In the not adiabatic case, the environmental temperature is another variable of the model: the temperature of nitrogen filling the holds has been set to 40 \({}^{\circ}\)C and 5 \({}^{\circ}\)C in summer and winter conditions, respectively. Figure 5: Comparison between experimental [20] and numerical results. Figure 8: CNG flow rate (adiabatic loading). Figure 10: Compression ratio (adiabatic loading). Figure 9: Mass of CNG in the pressure vessels (adiabatic loading). The entire loading phase needs about 35 h. The singular point that can be observed at time 11 h in the flow rate, compression ratio, and compressor power curves corresponds to the passage from the free loading to the forced loading phase with the start of the two compressors. Therefore, the forced loading phase takes 24 h. The singularities visible during this phase, between 11 h and 25 h, are due to the regulation of the compressor with variation of the rotational speed, starting at 10,000 rpm up to 11,500 rpm, to maintain the flow rate within the range between the pumping and flooding limits. The last singularity, at about 28 h, corresponds to the switching-off of one of the two compressors, due to the reduction of mass flow rate that occurs toward the end of the process. The loaded mass is about 6750 ton. The overall mean flow rate is about 192 ton/h but, when compressors are running, the mean value is 133 ton/h. The maximum compression ratio is 3.3, obviously at the end of the forced loading phase. The fuel consumption is 36 ton (=0.5% of the total cargo), and the specific energy consumption is 96 kJ/kg. Particular attention requires, for the reasons discussed in paragraph 1, the temperature trend in the pressure vessels.