The Naval Architect: October 2016
Global demand for LNG is increasing. LNG is an environmentally friendly source of energy and the number of shipbuilding orders for large-sized LNG carriers for transportation is on the rise.
At the same time, the use of LNG for marine fuel has emerged as one of the most feasible methods of complying with the regulations in MARPOL Annex VI regarding air pollutants in ships’ exhaust gas.
As a result, the construction of LNG-fuelled ships is increasing while LNG vessels and offshore structures, including LNG bunkering ships, small and medium sized LNG carriers and LNG bunkering terminals continue to be developed.
New ways to increase safety levels are needed in terms of system design and cryogenic materials, which will improve the economic feasibility for cargo tanks, fuel tanks and piping systems of LNG carriers and LNG-fuelled ships.
The Korean Register (KR) – an International Association of Classification Societies – member – has been investigating the properties of a newly developed, high manganese (Mn) steel which possesses mechanical properties comparable to those materials for cryogenic service listed in both the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) and the International Gas Fuel Code (IGF).
The new Mn steel which has been developed by the Korean steel conglomerate, POSCO, could be used for the cargo tanks, fuel tanks and the piping systems of LNG carriers and LNG-fuelled ships. The toughness level of the high Mn steel easily meets the requirements of the IGC and IGF Codes, being similar to that of 9% nickel steel.
High Mn steel has also shown superiority over existing materials in terms of ultimate tensile strength and elongation. In addition, the relatively small thermal expansion coefficient of high Mn steel offers the advantage that the displacement caused by temperature change is minimal when designing cryogenic equipment and devices.
Consequently, high Mn steel can be used to strengthen the structure of cargo tanks, fuel tanks and piping systems of LNG carriers and LNG-fuelled ships.
One of the benefits of high Mn steel is its economic feasibility. The continuous rise in demand for LNG-fuelled ships, related equipment and devices is leading to a shortage of nickel. High Mn steel is manganese-based steel and does not contain any nickel elements, potentially offering a solution to ease the issue of nickel scarcity.
KR has also investigated the mechanical properties of the base metal of high Mn steel, specifically its tensile strength, yield strength and charpy impact energy. KR has assessed the metal’s weldability to check if the mechanical properties of weldment of high Mn steel satisfy the minimum requirements.
As a result of these investigations, KR has approved the base metal and welding consumables.
Following on from this, KR has participated in the certification of several applications of the high Mn steel.
A cylindrical pressure tank with a volume of 24m3 was manufactured for LNG storage. The cryogenic test was omitted and the hydrostatic test with the maximum pressure of 15bar for 50minutes was conducted in the presence of a surveyor. KR approved the pressure test record.
A prismatic pressure tank with the dimensions 11.8m × 2.1m × 2.5m with a volume of 52m3 and design pressure of 10bar was fabricated. KR surveyed the prismatic pressure tank as if it was manufactured for LNG storage. The tank went through the cryogenic test shown in Figure 1 and its hydrostatic test and structural integrity were assessed. A hydrostatic test with a pressure of 15.1bar was carried out in the presence of a KR surveyor and was found to satisfy the requirements.
In the cryogenic test, liquefied nitrogen was used and the temperature measured throughout the test by thermocouple, monitored by KR’s surveyors. KR approved the pressure tank for LNG storage.
A track record is needed to demonstrate that high Mn steel can be applied to give the structural integrity required to demonstrate the stable nature of the material and its failure mechanism.
For this purpose, the construction of a bulk carrier with LNG fuel tanks made of high Mn steel is planned. The shipowner signed the construction contract in the middle of June 2016 and plans to take delivery of the vessel in the second half of 2017.
The ship will be a 50,000dwt bulk carrier built at the Hyundai Mipo shipyard and will be 191m in length, 32.26m wide with a 12m draught. The vessel will be used to transfer limestone between Donghae harbour and Gwangyang harbour in South Korea.
The ship will have an eco-friendly and highly-efficient dual fuel engine installed, with an output of 7,550kW, which will be able to use bulker C oil and LNG as fuel. The vessel’s pressure tank for LNG fuel will have a volume of 500m3 and is planned to be constructed of high Mn steel.
This is the first time a bulk carrier will have been constructed to include the LNG-fuelled system and a fuel tank made of high Mn steel, under the application of the CSR-H (Harmonized Common Structural Rules), anywhere in the world.
KR has signed a direct contract with the shipyard to class this LNG bulk carrier with high Mn steel fuel tank, the very first of its type.
Applying CSR-H to the bulk carrier
The contract agreement to construct the bulk carrier with LNG fuel tanks made of high Mn steel was made between the owner and shipyard in the middle of June 2016. As a result, the vessel will be the first bulk carrier in the world to be built under the CSR-H.
CSR-H requires a comprehensive analysis in order to verify the structural soundness of the entire cargo hold region. This means a significant increase in the man-hours required for the design and approval process, even though the ultimate deadline is not changed.
KR will be collaborating with the shipyard and using its own-developed powerful structural analysis software SeaTrust-HullScan as shown in Figure 2, for the design and approval stages of the bulk carrier.
This software uses auto fine mesh modelling to support the work undertaken in the vessel’s structural analysis. This highly regarded software is rated as ‘excellent’ and used by the top Korean shipyards and design companies, with over 200 copies distributed. Following its launch, KR used feedback received for over a year to continually improve SeaTrust-HullScan. Now, this software is evaluated as being superior amongst all class societies’ software.
The specification development of high Mn steel
High Mn steel has only been recently developed and is not generally available for the construction of international vessels. To resolve this issue, a specification for high Mn steel must be developed in order for it to be listed in the IMO IGC and IGF Code.
With this in mind, KR is now working to develop these clear specifications. These will verify the safety of high Mn so that users can confidently trust and use the material in construction projects.
The specification will evaluate the base metal safety and weldment safety characteristics of high Mn steel. Tensile tests will evaluate the yield strength, tensile strength and the charpy impact test will evaluate the degree of impact energy absorption in cryogenic conditions.
The assessment of weldment safety will evaluate the high Mn steel weldment. Similar to the base metal evaluation, it will include tensile tests, charpy impact tests and a CTOD test in order to assess the precise fracture strength of any weldment.
In addition to these tests, if deemed necessary, additional tests will be included in order to fully evaluate the safety of high Mn steel.
Anticipating effect and future works
This bulk carrier will be the first case where CSR-H and a dual fuel engine are applied. The full impact of CSR-H on the design and approval stage will be analysed and the usefulness of a dual fuel engine will be evaluated. Throughout this process, KR will work to make significant and meaningful improvements.
Through the fabrication and operation of this bulk carrier with its LNG fuel tank made of high Mn steel, the vessel’s structural integrity in cryogenic conditions of service will be fully investigated, helping the high Mn steel to be listed in the IMO IGC and IGF Code. Moving forward KR will attend all stages of the construction and will contribute towards expanding the application of high Mn steel.
