LD compressors are used to supply natural gas as fuel for the main boiler.

When designed to supply all the fuel gas required by the main boiler through a LD compressor, the capacity is approximately 8,500 m3/h. The inlet pressure of the designed single stage centrifugal compressor is 1.06 bar (a), the inlet temperature is −60°C, and the discharge pressure is 1.96 bar (a). Owing to the large difference between the maximum and minimum amounts of fuel gas required by the main boiler, the operation range of the LD compressor is very wide. In order to meet the turn-down ratio of the main boiler, a VFD must be installed as well as the inlet guide vane adjustment to be configured to control the speed of the LD compressor.

When the LD compressor is selected based on its capability of treating only NBOG, it has a capacity of approximately 4,000 m3/h. In this case, it is designed to supply insufficient fuel gas by using a forcing vaporizer, because it cannot supply all the fuel gas required by the main boiler with the LD compressor. The VFD can be eliminated, because the capacity of the LD compressor has been reduced and the operating range of the machine can be narrowed. The inlet pressure of the designed single-stage centrifugal compressor is 1.06 bar (a), the inlet temperature is −140°C, and the discharge pressure is 1.96 bar (a). In this design, the amount and capacity of equipment are reduced, thereby reducing the CAPEX requirements. However, because the LD compressor and forcing vaporizer must be operated in parallel, it is necessary to establish operation concepts and develop control logic.

The types of vaporizers used in LNG carriers are LNG vaporizers and forcing vaporizers. An LNG vaporizer is mainly used to make gas and supply it to the tank when vapor cannot be returned from the shore while unloading cargo and purging the cargo tank. The forcing vaporizer considered in this study is used to supplement and supply FBOG when the NBOG is less than the fuel demand of the main boiler during laden and ballast voyages.

A GMS manages NBOG generated in the cargo tanks and FBOG produced in the forcing vaporizer. Therefore, the main function of the GMS is to supply fuel gas to the main boiler that produces the steam necessary to achieve the propulsion of the LNG carrier. The equipment components of the GMS include LD compressors, a forcing vaporizer, and a gas heater. In this study, we investigate the GMS optimization design process and consider the new GMS operation method and control system research and development contents that have been optimized.

Figure 1: 
Performance curve for conventional LD compressor

2.1.3.1 Conventional GMS

A conventional GMS supplies all the fuel gas required by the main boiler by compressing and supplying NBOG from the cargo tank to the required pressure using a LD compressor, and the required temperature conditions are properly maintained using a gas heater. In addition, when the fuel gas required by the main boiler is more than the NBOG, it is configured to produce FBOG through the forcing vaporizer and supply it to the main boiler, supplementing the insufficient gas by supplying it to the inlet side of the LD compressor, as shown in Figure 2.

Figure 2: 
Conventional GMS [1]

From the perspective of the equipment piping connections, it is easy to configure the control logic as there is no interference between the equipment, with a series connection system from the stripping/spray pump in the cargo tank to the main boiler via the forcing vaporizer, LD compressor, and gas heater. However, from the viewpoint of selecting the capacity of the LD compressor, it must be designed to supply all the gas required by the main boiler through the LD compressor. This requires a large LD compressor, which consumes more energy for operation and additionally requires the speed control of the LD compressor by installing a VFD to match the turn-down ratio required by the boiler. Therefore, the initial investment cost increases. Figure 1 shows the performance curves according to the inlet guide vane control range and the speed control range of the LD compressor.

In order to supply the total amount of fuel gas to the main boiler of the 138,000 m3 LNG carrier, the LD compressor capacity is approximately 8,500 m3/h when it is selected considering the range of the inlet temperature change. Equation (1) shows the relationship between the boiler gas consumption and the fuel gas supply between equipment.

2.1.3.2 New GMS with Manual Operation

The new GMS was developed to complement the conventional GMS. The FBOG produced by the forcing vaporizer is connected to the outlet side of the LD compressor to supply fuel gas to the main boiler. Because of this, the LD compressor can supply only NBOG generated in the cargo tank to the main boiler, and thus it is possible to select a smaller capacity and the VFD for speed control, because the capacity is reduced. This eliminates the need to install the system, which reduces initial investment costs, as well as operating and maintenance costs. Figure 5 shows the equipment arrangement of the new GMS system.

As shown in the system diagram, the forcing vaporizer and the LD compressor are connected in a parallel configuration, and it is very difficult to construct an automatic control system, because two different types of equipment must be operated in parallel. That is, if the LD compressor and the forcing vaporizer are not well matched, the system becomes unstable. In the early stages of development, proper control logic for stable operation of the system had not been developed, and the forcing vaporizer was actuated by manual operation. The LNG carrier could be operated manually because the price of oil was not much higher than the price of gas, and the forced vaporizer was not used frequently. However, as the price difference increased over time, the forcing vaporizer was frequently used, and its manual operation could not be the best solution. Figure 3 shows the system pulsation problem caused by the failure of proper adjustment of the load of the forcing vaporizer due to the reduction in the main boiler load during manual operation of the forcing vaporizer.

Figure 3: 
Problems of Forcing Vaporizer Manual Operation when Boiler Load Decreases

Reducing the speed of the turbine engine in order to reduce the ship speed in the absence of an engine room operator during a voyage will initially reduce the load on the LD compressor when the main boiler load is reduced. However, this decrease in the load stops occurring when the load on the LD compressor is lowered to the minimum. At this point, the forcing vaporizer encounters a situation in which FBOG is produced and supplied to the main boiler while maintaining a manually set value in a high load region. In this condition, if the operator does not intervene, the gas produced in the gas supply pipe accumulates because the main boiler uses less gas and the forcing vaporizer continues to produce gas. The pressure increases due to the accumulated gas, which causes a surge phenomenon in the LD compressor, thereby operating the surge prevention controller. The pressure in the pipe fluctuates, and the main boiler gas supply controller also fluctuates. As the pressure fluctuates, the flow controller of the forcing vaporizer also suffers from a severe pulsation, which leads to an emergency situation in which it is no longer possible to operate. To prevent this, if variations in the main boiler load are required, the operator must adjust the gas output of the forcing vaporizer accordingly.

The capacity selection of the LD compressor is shown in Figure 4 only NBOG generated in the cargo tank requires a capacity selection of approximately 4,000 m3/h, considering the range of the inlet temperature change. The additional fuel gas is supplied to the main boiler by the FBOG produced using the forcing vaporizer. Equation (2) shows the relationship between the boiler gas consumption and the fuel gas supply between equipment.

Figure 4: 
Performance Curve for New GMS LD Compressor

2.1.3.3 New GMS with Automatic Operation

The new GMS system developed for manual operation may violate certain classification regulations. The classification regulations state that the systems related to the main propulsion system must not be operated by the operator during unmanned engine room operation. Therefore, it is essential to develop a new GMS system to enable automatic operation. In addition, as shown in Figure 5, a high pressure prevention line and a control valve were installed to enable automatic operation of the new GMS system. The installed control valve prevents high pressure by recirculating excess gas during automatic operation when the main boiler load is operated at a load of approximately 24% or less, which is the minimum flow rate of a LD compressor, and when operating at a load of approximately 20% or less, which is the minimum flow rate of a forcing vaporizer. It can be operated safely by recirculating gas into the cargo tank to prevent abnormally high pressure in the fuel gas supply pipe when sudden load fluctuations of more than 5% per second or abnormal operation occur during the operation of the LD compressor and forcing vaporizer. In this way, the system is controlled by a high-pressure prevention controller.

Figure 5: 
New GMS with Unmanned GMS Control System [1]

Because the actual ship test was limited, simulation tests were used for verification. Various dynamic simulation software packages have been developed, but the package used for this simulation was a commercial process simulator, Aspen HYSYS V10 simulation package [2][3].

Modeling was performed using the Peng–Robinson state equation, which known to be relatively suitable for cryogenic gas processes [4].

A model was built for the detailed characteristics of the new GMS-related equipment.

The technical background and modeling methods for major component modeling is based on the thermal/fluid and mathematical models provided by HYSYS. Data such as performance curves for LD compressors, heat transfer coefficients for forcing vaporizer, specifications of various equipment, and flow coefficients for control valves are based on actual data provided by suppliers. Figure 8 shows the entire dynamic model of the new GMS.

Figure 8: 
Overall Dynamic Model of New GMS

If the speed of the steam turbine propulsion system is increased to increase the ship's speed on the bridge, the steam consumption of the steam turbine increases and the steam pressure of the main boiler decreases. The main boiler operates the system to maintain pressure by burning more fuel gas to maintain the pressure. When the fuel gas consumption increases, in order to increase the fuel gas supply, the fuel gas supply system first reacts with an LD compressor to increase and supply the required gas. The gas supply flow controller of the forcing vaporizer checks the fuel supply flow of the LD compressor, and automatically calculates the available NBOG in the cargo tank to perform control. Accordingly, the gas production amount FBOG of the forcing vaporizer is controlled such that the fuel gas supply amount of the LD compressor can be supplied only with the available NBOG in the cargo tank. The remote setting value of the forcing vaporizer flow controller is sent from a separate LD compressor flow controller. Therefore, in the transitional state, the LD compressor operates preemptively to the load variation, and the forcing vaporizer operates to supplement it. This parallel operation control system is designed to automatically and stably supply the fuel gas required by the boiler [8].

4.2.1.1 Increase and Decrease Boiler Load (Four-Step Change 4,000 kg/h 6,000 kg/h)

Dynamic simulations were performed to confirm that the new GMS control systems were properly controlled as the boiler load increased and decreased in four steps. The results are shown in Figure 9 to Figure 12. In order to confirm the controllability and responsiveness of the new GMS control system, a step response test was conducted, and the load of the LD compressor and forcing vaporizer was increased and decreased by increasing and decreasing the load of the main boiler in four steps from 4,000 to 6,000 kg/h vice versa. The GMS system was well controlled by the new GMS control system.

Figure 9: 
LD Compressor Performance Curve with Operating Point for Four-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)

Figure 10: 
Flow Trend of Parallel Operation between LD Compressor and FV for Four-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)

Figure 11: 
Controllers Trend of Parallel Operation between LD Compressor and FV for Four-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)

Figure 12: 
Controllers Trend of Parallel Operation between LD Compressor and FV for Four-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)

4.2.1.2 Increase and Decrease Boiler Load (1 Step Change 5,000 kg/h 6,000 kg/h)

Dynamic simulations were performed to confirm that the new GMS control systems were properly controlled as the boiler load increased and decreased in one step. The results are shown in Figure 13 and Figure 14. A step response test was conducted, whereby the load of the LD compressor and forcing vaporizer was increased and decreased by increasing and decreasing the load of the main boiler in one step from 5,000 to 6,000 kg/h and vice versa. The GMS system was well controlled by the new GMS control system.

Figure 13: 
Flow Trend of Parallel Operation between LD Compressor and FV for One-Step Boiler Load Variation (5,000 kg/h to/from 6,000 kg/h)

Figure 14: 
Controllers Trend of Parallel Operation between LD Compressor and FV for One-Step Boiler Load Variation (5,000 kg/h to/from 6,000 kg/h)

4.2.1.3 Increase and Decrease Boiler Load (Two-Step Change 4,000 kg/h 6,000 kg/h)

Dynamic simulations were performed to confirm that the new GMS control systems were properly controlled as the boiler load increased and decreased in two steps. The results are shown in Figure 15 and Figure 16. A step response test was conducted, whereby the load of the LD compressor and forcing vaporizer was increased and decreased by increasing and decreasing the load of the main boiler in two steps from 4,000 to 6,000 kg/h and vice versa.

Figure 15: 
Flow Trend of Parallel Operation between LD Compressor and FV for Two-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)

Figure 16: 
Controllers Trend of Parallel Operation between LD Compressor and FV for Two-Step Boiler Load Variation (4,000 kg/h to/from 6,000 kg/h)