I am interested in designing ship-to-shore link and ESD arrangement in LNG vessel. I have involved in about three LNG projects, but I did not design or paid more attention in this field (because all the projects were front-end engineering design). In this post, I want to share you practical information about ESD arrangement as suggested by SIGTTO (Society of International Gas Tanker and Terminal Operators).
Function of Emergency Shut Down System
Function of cargo ESD system is to stop cargo liquid and vapor flow in the event of emergency and to bring the cargo handling system to safe condition.
International Gas Carrier code (IGC code) has several requirement of ESD systems. This includes:
- Manual trip points
- Automatic fire sensors that can initiate:
- ESD valve closure of liquid transfer between shore and ship
- ESD valve closure of vapor transfer between shore and ship
- Cargo pump stoppage
- Compressor stoppage
IGC does not explain ESD activation in shore area, however it is recommended to install linked system so that ESD activation on the ship will be sent to shore, and vice versa.
SIGGTO defined cargo ESD as ESD-1 and ESD-2.
- ESD-1 : emergency shut down of cargo transfer during loading or unloading.
- ESD-2: emergency shut down the transfer operation (ESD-1) and uncouples the loading arms after closure of both ERS isolation valves.
The cargo transfer emergency shut down process at majority of terminal is implemented in two stages:
- 1st stage – shut down the cargo transfer process in control manner (ESD-1)
- 2nd stage – activates emergency release system (ERS) installed on transfer arm (ESD-2)
The function of second stage is to protect transfer arm and ship’s manifold.
Emergency Shut Down System
SIGTTO has several requirement about ESD system in cargo. I made the list so that it can be easier to understand:
- Gas carrier shall be provided with high level alarm (LAH) and overfill protection, each system independent of the other. Overfill protection system closed valve a valve to prevent the tank becoming full.
- High level alarm (LAH) must also be independent of other liquid indicators
- High level alarm is commonly set at a level that will warn operator 15-20 minutes before target finishing level is reach at full loading rate. In a ship with normal filling ratio 98%, alarm will be set around 95% tank volume
The ship/shore link (SSL) is used to transmit, without delay, a signal from one party to the other, ie ship to shore or vice versa. There are several technologies of ship/shore link:
- Fiber optic
- Radio telemetry
- Pneumatic. First generation of SSL system used this technology. In many terminals, it is used as back up shut down signal.