R&D Sloshing in Membrane Tanks Full Scale Measurements of Sloshing in Membrane LNG Tanks

Purpose

The size of LNGCs is steadily increasing, leading to larger tanks. In addition, the ships operate more frequently on trades that have rough weather for considerable parts of the year. Therefore the current practice, where the design of LNG membrane tanks is mainly based on comparative model tests, needs to be improved. In January 2007, DNV launched a Joint Industry Project (JIP) to develop more reliable methods for the design of LNG membrane tanks. The first phase of the project aims to acquire high quality data from full-scale measurements of responses to sloshing loads in LNG tanks.

Benefits

The results from this phase of the JIP consist of measured data, presented as time histories and statistics of sloshing events. In the next phase, the scaled time series of recorded ship motions for selected periods will be used to control the motions of a 6-degree of freedom test rig with a scale model of the actual tank mounted. Hence, the full-scale conditions with respect to motions and sloshing pressures may be simulated on a model scale. The results from model tests will be used to verify DNV’s guidelines on sloshing assessments.

Features

A fibre optic measuring system has been developed in a collaboration between DNV and a professional manufacturer of hull stress monitoring systems. In order to meet the project requirements, a specially designed pressure sensor for installation and safe operation in LNG membrane tanks has been developed and qualified. Several pressure sensors are mounted in an NO96 containment system in the four top corners of tank no.2. Both the primary and secondary containment boxes, including the supporting hull structure, are also fitted with fibre optic strain gauges to measure the sloshing response. In addition to the recordings directly related to the sloshing response, parameters such as sea-state, ship motions and ship operational parameters are recorded.