Containment Integrity Monitoring Solutions for LNG Carriers – Cryogenic tests for the validation of a cargo containment system for LNG ships

INTRODUCTION
This report shows the results of cryogenic test for confirming the KC-1 cargo containment system which will be applied to LNG ship. Open mock-up tank has already been built to prove the fabrication, assemble and construction procedures of KC-1 membrane system for ship application. A LNG carrier closed type mock-up tank as recently built, hereafter CMT, of the KC-1 type membrane system.

Experimental stress analysis systems equipped with strain gages and fibre optical sensors have been reflected in the design to perform a cryogenic test. CMT cryogenic tests were conducted by filling and discharging LNG 5 times and all the tests have been successfully completed in October 2009.

The purpose of construction and cryogenic tests of CMT are as follows

(1) Confirming the stability of KC-1 membrane system during cryogenic test

(2) Improving the quality control system of membrane and insulation system of LNG tank which assures the reliability of LNG carrier

(3) Evaluating the applicability of automatic welding machine on KC-1 membrane system

(4) Intensifying the techniques of design, material, fabrication and inspection required for the design of LNG carriers

SPECIFICATION OF CMT
This size of the tank is 12m long, 14 m wide, 7m high and inner volume of 250m3. Its construction was commenced in September 2008 and brought to completion in April 2009.

CMT is connected to DCS (Distributed Control System) of Pilot LNG Plant and operations are controlled at remote control room. CMT’s cryogenic tests were performed by repeated charging and discharging LNG for 5 times. Before operation, pre-cooldown test were also performed to lower the temperature of primary membrane to LNG temperature by spraying liquid nitrogen. Construction and cryogenic tests of CMT adopted in KC-1 membrane system were made with the purpose of consolidating the design/fabrication technology, establishing the building system for LNG carriers, establishing the quality control for construction procedure and proving the KC-1 membrane system experimentally.

CMT OPERATIONS
The following figure shows the variation of liquid level and primary membrane’s temperature during operation. IB and IBS spaces filled with nitrogen were sampled regular intervals to check the leakage of LNG during operation. After the operation of 1st and 5th pre cool down, CMT was opened and primary membrane system was checked through visual inspection.

TEMPERATURE MEASUREMENT SYSTEM
Liquefied natural gas (LNG) normally has a temperature of -162°C, so all the storage facilities such as insulation and hull systems are subject to temperature decrease caused by long-term storage. RTD sensors were installed at several points of CMT for the purpose of monitoring CMT only for the period of operation. The LIOS fibre optic temperature measurement for CMT structure has been developed to investigate the temperature distribution of KC-1 membrane system.

In case of KC-1 type cargo containment system, cone type membrane anchor structures are fixed in hull and support the reaction force of membrane during operation. Since the fibre optic sensor cable is coiled around cone anchor and measures the temperature every 50cm interval, it is possible to investigate the temperature distribution of CMT.

Two fibre optic cables were installed in the insulation and a third one was included on the cone anchor to investigate the distribution of temperature during operation. The following figure shows the whole temperature distribution of CMT during operation. All the measured data were plotted at the maximum LNG level.

The LIOS DTS system as well as the design of the applied fibre optic sensor cable could be successfully approved as an effective and reliable integrity monitoring system for the KC-1 cargo containment system. The cryogenic temperature measurements are precise and stable in course of the entire test period.