Combined Semi-analytical and Finite Element Approach for Hydro Structur e Interactions during Sloshing Impacts - 'Sloshel Project'

摘要

The paper discusses the development of the combined semianalyticalrn(fluid flow) and numerical FEM (structure) models forrnhydro structure interactions during the sloshing impacts in therntanks of the membrane type LNG carriers. This is a very challengingrnproblem and lot of work, both experimental and numerical,rnhas been done in the past. However, it is fair to say that no fullyrnconsistent solution exists up to now. Indeed, the small modelrntests, which are usually conducted in this context, suffer fromrnscale effects and numerical CFD calculations suffer from numerousrnnumerical problems and prohibitive CPU time requirements.rnThere is a clear necessity for full scale measurements with the realrnLNG and the real containment system. Unfortunately this is technicallyrnextremely difficult and, up to now, there is no documentedrnwork on this, at least not in open literature. In the absence ofrnthe real full scale measurements, some "quasi" full scale measurementsrnwere performed. These measurements consist in impactingrnthe real containment system structure through the drop testsrntechnique Kim et all (2008), or through the more sophisticatedrnwave generated impacts (Sloshel project - Malenica et al (2009),rnBrosset et al (2009)). The developments which are discussed inrnthis paper were conducted within the Sloshel project which concentratedrnon the structural response of the real (full scale) containmentrnsystem under the breaking wave impacts. Very rich andrnambitious project objectives have been defined, with the finalrngoal of improving the current methodologies used in the structuralrnassessment of the containment system and the supportingrnship structures. At the beginning of the project, it was decidedrnthat the effort in the numerical part would be concentrated onrnthe development of semi-analytical methods for the local fluidrnflow, which will be combined with the complex structural FEMrnmodels for containment systems. The use of the classical CFDrnmethods for the local analysis of extremely complex sloshing phenomenarnwas avoided at the beginning of the project, but was includedrnlater. The simplified semi-analytical models for fluid flow,rnkeeping the main physical parameters allow for the assessment ofrnthe phenomena within the reasonable computational effort. Therndrawback is that the impact situations should be highly simplified.rnIn that respect, the sloshing impacts were preliminary classifiedrninto 3 main groups:rn1. impact without inclusion of air (steep wave impact),rn2. impact with entrapped air pocket (Bagnold type impact),rn3. impact with aerated fluid.rnSome intermediate impact types were also identified but theyrnreceived less attention so far. For each impact type, the dedicatedrnsemi-analytical model is developed and the computational resultsrnare compared with the full scale Sloshel tests.rnDuring the project some additional modeling difficulties arosernand one of them is the modeling of the wave propagation whichrnis necessary for proper definition of the main flow parametersrnjust before the impact happens. Different approaches, basedrnon the potential flow modeling of the wave propagation, werernproposed by ECM and were shown to be very efficient in thernpresent context. These wave propagation models are also brieflyrndiscussed.