ClassNK to fore in gas containment classification
ClassNK focuses on updating its classification rules and developing standards in new areas.
Mr. Hayato Suga, Corporate Officer, Director of Plan Approval and Technical Solution Division at Nippon Kaiji Kyokai (ClassNK), spoke to the Motorship in London in early February about the classification society’s recent work in the field of gas containment regulation, and broader developments in the field of alternative fuels.
Question: ClassNK released a number of updates on guidance around on prismatic systems in July 2018. One of the points referred to revisions to strength assessment methods against fatigue cracks caused to vessels by prolonged and repeated loads. What implications does this have for filling constraints for self-supporting container?
Hayato Suga: We decided to establish (self-supporting) Prismatic tank guidelines because there was a gap in practical guidelines around the technology, which previously depended upon the know-how of the designers. The key item to be reviewed was the specific assumption to deflect fatigue strength phenomenon. We clarified in our guidelines that the filling level was not particularly connected with fatigue strength, unless a partial filling condition is planned as a standard loading condition. We have also taken the opportunity to update the fatigue strength assumptions, which were overly conservative.
Question: In your guidelines, how will revising fatigue strength assessment methods affect the frequency of inspections?
Hayato Suga: We have improved our understanding of which areas of the containment tank stresses are likely to concentrate, and are focusing our attention on these likely areas, such as tank supporting structures.
However, these changes are purely during the design review and construction stages. The revised guidance does not imply any change to inspection schemes post-delivery. The possible changes have been generally mentioned in our Rules and need further consideration. In addition, there have been discussions at the AIS level about potentially extending the interval between inspections. The increasing availability of data from sensors might make real-time monitoring of the condition of tanks possible: after all, a number of commercial software providers are currently providing detailed information about the condition of gas contained inside containment tanks, from a sloshing-monitoring perspective.
Question: What scope do you see for increased data availability to play in developing LNG containment monitoring guidelines?
We do think that the increased availability of data from sensors has potential applicability, although there is a need to verify the accuracy of sensor data, and to cleanse the data to ensure it is reliable. We need to distinguish between the development of more accurate monitoring and the requirements for monitoring the containment vessel, which is a regulatory requirement. The condition monitoring will provide evidence of the reliability of the Ship inspection, but it will not replace the ship survey. The measurements from the survey have to meet class standards and will remain the gold standard.
Question: In your guidelines, how will revised guidelines for prismatic tanks impact filling constraints?
We don’t see any changes to filling constraints at the moment.
Question: What is your opinion of sloshing and in operation monitoring systems?
Sloshing issues are usually critical for membrane tank systems. There are many related R&D projects giving higher reliance on this issue, but attention must continue to be paid to the issue. Our colleagues from ClassNK’s commercial arm collaborate with commercial software developers (such as NAPA) for improving the management of LNG cargoes, to reduce boil-off rates, for example. Other software suppliers offer other systems, and GTT has developed a sloshing monitoring solution for membrane containment systems.
We do think that the increased availability of data from sensors has potential applicability, whether that is in terms of monitoring boil-off rates, sloshing or the ageing of the contents of the LNG containment vessel, which is more relevant from a commercial point of view, or pressure control issues and more accurate condition monitoring, which has potential safety advantages from a class perspective.
Question: You have conducted research into the integrity of in-tank cargo pump towers. Do you have any views on their performance when subjected to higher loads in partially filled tank conditions?
The tower design varies between Moss, independent prismatic tank Type B and membrane containment systems. There have not been many operational problems reported on cargo pump towers so far, but we have conducted some research into the issue. Our findings identified potential issues around the pump with supporting structures. Although different systems use different designs, they all seem to perform satisfactorily.
Question: Does the degree of deformation have any effect on the safety or longevity of containment systems?
The degree of deformation varies depending on the tank type, and permissible tolerance is normally included and controlled in the strength review. Under special circumstances, we have to pay extra attention to verify comprehensive safety with the tank test, model test etc. It is through these steps that we believe sufficient safety and longevity can be confirmed.
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