The ethane market and the concept of the VLEC

According to the study, the US' ethane production capability is under-utilised. In 2013, 200,000 barrels per day of ethane were left in the natural gas stream or 'rejected', owing to the difficulties involved in storing the chemical. The rate of rejection is set to rise as US ethane supply outpaces domestic demand and export options remain limited.

The ethane surplus could be 6m tonnes a year by 2015 and 12m tonnes by 2020, although the development of US ethylene production capacity could soak up significant amounts of ethane towards the end of the decade. The annual export of 1m tonnes of ethane represents employment for 180,000 cu m of ship capacity if transported on the North Europe trade, or 320,000 cu m on a South Asia/China route.

Projects are underway to build bigger ships for the ethane market, but orders are generally limited to cases where a firm employment contract is in place for the vessel. Hartmann recently released details of a trio of 36,000 cu m ships it has under construction in China.

The vessels will be the largest liquefied ethane/ethylene carriers (LEC) in the world once completed, and capable of running its MAN ME-GI engines on ethane. An approval in principle has been issued for the engines, which are due to be supplied in Q4 2015.

"The window of opportunity to tie up ethane exports and secure tonnage to serve this trade is now open to feed potential markets in Europe and Asia," said Tim Protheroe, president, Lloyd's Register North America, "Lloyd's Register has identified the technical risks and best technical pathways to help ensure that near term demand for large VLECs can be met by shipyards and gas containment system suppliers. Our job is to help anyone looking at trading ethane to make the best commercial decisions based on the best technical insight as well as working with regulatory bodies such as the US Coast Guard and flag administrations to ensure that the risks are understood ."

The challenge in building larger ethane carriers, or very large ethane carriers (VLECs) is in the containment of the chemical on board.

Leonidas Karistios, global gas technology manager at Lloyd's Register commented: "We have been studying the potential for ethane for over a year and we asked the question, 'What would a safe and efficient 80,000 cu m ethane carrier look like?' The answer is that to transport larger quantities in a single hull will almost certainly require the adoption of alternatives to Type 'C' gas containment systems."

Type C tanks are pressure vessels designed to carry ethylene, with its -104 degree Celsius boiling point, as well as other LPG cargoes. The estimated feasible size of a ship using such a system is around 40,000 cu m although advances are not out of the question.

Considering all available tank technology, the LR study finds that for an ethane carrier of 80,000 cu m, either prismatic type 'B' tanks or membrane systems are the main contenders, with ships carrying three or four tanks.

The class society continues to work with containment designers such as GTT, which has agreed a methodology with LR to assess its tank technologies' suitability for carrying ethane. GTT's system were originally designed for use with LNG which is less dense than ethane, and so the tanks must be proved capable of carrying the denser liquid through sloshing assessments and other tests.

For the VLEC engine burning ethane, modifications will need to be made to accommodate the higher pressure required for ethane and although the changes are not insignificant, there will be a certain amount of conceptual commonality with MAN Diesel's ME-LGI engine, which LR has already approved.

"We qualify the application of existing technology for new purposes and the introduction of new technologies. Our goal is to help ensure that all involved in the ethane transportation chain have assurance that their investments are as safe as possible and do what is expected of them," added Karistios.

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