(185b) Analysis of Process Dynamics during Transient Operations on LNG Vessel Gas Management System

Nobutaka Umeyama Japan Marine United Corp.

Kouji Kikuchi, Kenichi Matsuoka Azbil Corp.

David Hill, Aaron Herrick CHEMSTATIONS Corp.

During LNG transportation on an LNG vessel, a significant fraction of the LNG cargo evaporates as boil-off gas (BOG). The rate of BOG generation fluctuates based on several factors, such as weather and tank contents. The dual-fuel engines (DFE) that power an LNG carrier have fluctuating fuel demands based on factors such as vessel displacement, the speed of the ship, and power demand. Additional BOG can be generated by vaporizing LNG. It is undesirable to flare or vent excess BOG, and most vessels are not equipped to re-liquefy BOG. The control system including compressor and surge control attempts to match the BOG generation with the DFE fuel demand to operate the vessel.

During the LNG vessel design phase, compressor systems have to be examined carefully, in order to avoid setbacks such as compressor surges or start-up difficulties due to undersized / oversized capacity. Then, potential problems during plant start-up, emergency shutdown and various turn down conditions are able to be identified and eliminated in advance.

We developed a dynamic simulation model for LNG vessel gas management systems including compressor surge control, and conducted an analysis of process dynamics during transient operations (e.g. start-up, shutdown, trip and change of fuel gas compositions).

The dynamic simulator was used to model the gas management system (GMS) – compressor system with surge control etc., to handle BOG, along with a detailed BOG model from the LNG cargo tank. It was then used during the design of process control, validation of operating procedure controllability, and factory acceptance test (FAT) to validate distributed control system (DCS) functionality.

In this project, the use of dynamic simulation system has been summarized as follows:

(1)Process Study & Design of Control concept

(2)Validation & verification of control functions in DCS:

(3)Tune-up of plant control parameters, by integration with DCS during FAT

In this paper, we compare the simulation result with the actual process behavior, and show the effectiveness and accuracy of dynamic simulation. In addition, the paper shows that the period of time for the tune-up of plant control during start-up on site could be reduced by investigating the dynamic behavior of the process, initially using the optimum control parameters during FAT.