Nowadays the price of natural gas has become high and the efficiency of propulsion systems of LNG (Liquefied Natural Gas) carriers has increased. Due to these trends, a lot of effort has been made to develop an insulator for a LNG carrier with high thermal insulating performance and a low boil-off rate (BOR). A typical BOR specification for a 170K LNG carrier has been 0.15 vol.%/day. But the above mentioned reasons have resulted in pressure to lower this BOR specification to 0.12~0.10 vol.%/day.

Polyurethane foam (PUF) blown by trichlorofluoromethane (CFC-11) has been widely used as an insulator in various industries. PUF consists of polymer skeleton and closed cells filled with blowing gas. But since CFC-11 depletes the ozone layer, many attempts have been made to develop new alternatives. Due to its low Ozone Depletion Potential (ODP), 1,1-dichloro-1-fluoroethane (HCFC-141b) has replaced CFC-11. Polyurethane foam has been used as the main insulator for LNG carriers with corrugated membrane type cargo containment systems. 270 mm thickness of insulation panel with PUF as the main insulator has typically been used to meet the 0.15 vol.%/day BOR specification. But HCFC-141b also depletes the ozone layer although its effect is small compared to CFC-11 (a tenth of CFC-11). Nowadays the third generation blowing agents, hydroflurocarbons (HFCs) have begun to replace HCFC-141b.

Since the thermal conductivity of HFCs is higher than that of HCFC-141b, thicker insulation panels should be used to show the same insulation performance when adopting HFCs as a blowing agent. Because of a requirement for LNG carrier with a lower BOR specification, thicker insulation panels should be used. This solution could cause two problems. One is the increase in insulation panel price. The other one is the increase in the LNG ship size.

In this study, many attempts have been tried to increase the insulating performance of polyurethane foam blown by HFCs. These attempts include the use of new raw materials, and blending of well known polyether polyols, polyester polyols and various additives such as surfactants, catalysts, and so on. As a result, the same thickness of the new insulation panel using PUF blown by HFCs as a main insulator meets lower BOR specification for a LNG carrier.

0.15 vol. %/day (typically with 270 mm thick insulation panel) has been used as the Boil-Off Rate (BOR) specification of LNG carriers for over 20 years. But the energy price, especially liquefied natural gas price and the enhancement of propulsion systems add much pressure for the development of Low BOR LNG carriers. To meet this need the designers of LNG CCS(Cargo Containment System) have been making a lot of effort to develop an insulator with high thermal performance. Contrary to these trends, the use of traditional blowing agents such as CFC-11, HCFC-141b has been banned or reduced due to ODP (Ozone Depletion Potential). A blowing agent is a material that gives the organic material cell structure and thermal insulating performance. From the beginning of 2010s, new blowing agents such as 1,1,1,3,3-pentafluorobutane (HFC-365mfc), 1,1,1,3,3-pentafluoropropane (HFC-245fa) with no ODP have begun to be used as blowing agents. Organic cellular foam such as polystyrene foam and polyurethane foam have been used as a main component of insulation panels.

This content is only available via PDF.
You can access this article if you purchase or spend a download.