LNG Loading Emergency Shutdown Research Project

Question

With growing climate concerns, the world looks to natural gas to as a clean way to meet energy demands. Liquefied natural gas (LNG) is liquefied by cooling the gas to -160°C or below (<-256°F), loaded into special tankers for transport, then turned back into gas at its next terminal.

The LNG loading process occurs through loading arms, constructed of expensive low-temperature alloys for cryogenic operation and designed to consider the moving vessel the arm is attached to. An LNG terminal might have two to five loading arms for different purposes. In an emergency shutdown, flow through these arms must be suddenly halted by closing a sequence of valves, opening vent valves, and tripping transfer pumps.

The sudden deceleration of flow results in pressure surge, potentially exceeding pipe design pressures and causing extreme pipe stress, further worsening an emergency.

Analysis

A research team at Lamar University led by Xinyu Liu evaluated the surge results from an LNG loading emergency shutdown. The system, as constructed in Datacor Impulse, is found in Figure 1. In addition to a scenario with the emergency system working as intended, the team considered five cases where the system malfunctioned:

1. Emergency system working properly
2.Pumps fail to trip
3.Vent valves fail to open
4.Pumps fail to trip and vent valves fail to open
5.1 of 4 pumps fail to trip
6.1 of 3 vent valves fail to open

The team also evaluated valve characteristics for the closing and opening valves. These characteristics can drastically impact a surge response, so the team performed sensitivity analysis for comparison.

Results

The analysis demonstrated that the system meets all operational, regulatory, and OEM requirements for both 5-turbine and 7-turbine configurations. No transient event exceeded the GNI slam-shut limit of 38.01 barg. Pressure drops during ramp-ups stayed above the turbine low-pressure alarm threshold of 29 barg. Pressure variations in all trip scenarios were within the allowable transient limit of 1.5 bar.