Sloshing in Partially Filled LNG Tanks - An Experimental Survey
Authors: Pastoor Wouter , Tveitnes Trym , Valsgård Sverre , Sele Hans 
Published: October 2008

Sloshing in LNG tanks has gained increasing attention over the past period of time. This is mainly caused by developments in the LNG market, changes in the design and operation of LNG ships and an increasing interest in floating gas field exploitation.

The issue of sloshing in partially filled tanks is relevant for spot trading and offshore loading/off loading of LNG ships as well as for FPSOs with LNG capacity. DNV has developed a step-by-step experimental procedure to determine sloshing loads for structural analysis of the insulation system and tank support structure. Of key importance for a reliable evaluation is the step-by-step approach, putting emphasis on an accurate treatment of every step. This means careful modelling of operational and environmental conditions, accurate ship motion calculations, a well-defined procedure for identifying design sea states, a proper experimental set-up and an accurate treatment of the statistics involved in every step in order to determine reliable and realistic design sloshing pressures.

To study sloshing loads in partially filled LNG tanks irregular sloshing experiments have been conducted for head and beam seas for different filling levels and sea severities. A 1/20 scale model of a tank from a 138.000 m3 membrane type LNG ship was used for the tests. Measurements have been conducted using pressure transducers and pressure transducers mounted in clusters. An overview of the tests is given with an analysis of the impact pressure statistics, the pressure pulses and the associated subjected area. From these analyses a discussion is presented on the effect of different filling levels, sea severity, ship speed and heading.

Introduction

The gas market is in an upswing and will in the future provide an increasing part of the world energy demand. A large part of the natural gas reserves will be transported by sea from well to customer. Several changes are seen in the offshore production of gas and the sea-borne transportation of LNG. Floating installations to produce offshore gas is an upcoming market. In case of liquefied storage, present filling restrictions will be violated implicitly. In the LNG shipping industry three main changes are seen or expected:The LNG market is expected to develop more into a spot-market instead of long-term contracts. This view is supported by the fact that LNG carriers have beenordered without having a first transportation contract. As a consequence of spot-market trading shipowners prefer to increase their trading flexibility by having the possibility to operate with not-fully loaded tanks, which would imply a reduction of the upper filling restriction.A second change is foreseen in the maximum size of LNG carriers. By a market-push to reduce transportation costs the maximum size of LNG vessels will increase.A third change is foreseen in the location of loading and offloading. With an increasing focus on safety, supported by the threat of terrorism, it is becoming difficult to build land-based terminals for loading and off-loading, especially in the US. Offshore terminals, far away from dense populated areas are the logical solution, but with the implication of more severe environmental conditions when loading or discharging.