New LNG sloshing guidelines

Lloyd’s Register has published a new guidance document for the design of membrane-technology liquid natural gas containment systems. The document is aimed at improving design procedures with respect to sloshing forces, and is the result of a long programme of research and development, including extensive consultation with industry, and an investment of over $1.1 million.

The document has been used as part of the appraisal process for the approval of the largest LNG carrier built to date, the Q-Max type ships, of which LR is the lead class.

The ‘Sloshing Assessment Guidance Document for Membrane Tank LNG Operations’ provides guidance and recommendations on the assessment of sloshing in membrane LNG tanks. It provides design teams with an overview of suitable procedures for assessing the strength of Gaztransport and Technigaz NO96, Mark III cargo containment systems (CCS) and also new containment systems.

“LNG sloshing is a very complex issue as there are many aspects that are difficult to address explicitly by calculation or testing,” said Nigel White, Technical Manager, Hydrodynamics, with LR?s Marine Product Development team. “Consequently, the assessment of the cargo containment system of membrane LNG ships for sloshing loads is very complicated and there is no single definitive assessment procedure that may be applied. The new document is written as guidance and it provides best engineering practice on the assessment of the CCS and the supporting hull structure.

“Several levels of assessment method are possible and these procedures reflect an increasing complexity of analysis which can provide better design assurance. This document gives an overview of these alternative assessment methods with recommended procedures that can be applied. It is then the responsibility of the designer to select appropriate procedures.”

The guidance mainly applies to membrane tank LNG ships with a barred fill range typical of the vast majority of membrane tank LNG ships in current operation. It can also be applied for the assessment of membrane tank LNG ships at offshore terminals or for LNG ships with no barred fill range. The document can also form the basis of a sloshing assessment procedure for in-depth investigation on other ship types that store or transport liquefied or liquid cargoes in large tanks and which are expected to experience significant sloshing loads. These include Moss spherical tank LNG ships, LPG ships and offshore storage or production vessels, for example, FLNG, FSRU applications using membrane (or independent) LNG tanks.

Recently there have been several incidents involving damage to LNG membrane tanks and the approach adopted in the document provides guidance on the processes necessary to ensure that these incidents will not recur. Hence, as part of the design process, the designer should undertake a risk assessment and a hazard review to determine all possible failure modes. Having done this, it is then possible to set suitable design appraisal methods and acceptance criteria to show that these potential hazards are clearly controlled.