Modeling of nonlinear structural response from explosion loads on offshore structures
Translated title
Modellering af ikke lineær strukturel respons fra eksplosionslaster på offshore strukturer
Author
Bendiksen, Bjarke
Term
4. term
Education
Publication year
2014
Submitted on
2014-09-10
Pages
77
Abstract
This thesis investigates the structural response of an offshore module subjected to explosion loads, with emphasis on material and geometric nonlinearities. Representative finite element models are developed in ANSYS and LS-DYNA, and the process of full nonlinear dynamic modeling is presented and verified. The results show that including nonlinearities is essential, as predicted maximum responses increase markedly compared with linear assumptions. Strain-rate effects are incorporated using the recommended Cowper–Symonds model; they significantly mitigate plasticity and, in the fully nonlinear model, almost halve the response at higher loads. Local strain measures from ANSYS are compared with code-based approaches, finding ANSYS least conservative (lowest plastic strains), and a convergence method for plastic strain in shell elements is found unreliable. Semi-analytical formulations for post-buckling of flat plates and for dynamic buckling are reviewed and shown to match ANSYS results up to about twice the critical load with similar accuracy. The work proposes representative modeling approaches for explosion-resistant assessment of offshore modules and clarifies the design implications of rate and nonlinear effects.
Dette speciale undersøger den strukturelle respons af et offshore modul udsat for eksplosionslaster med fokus på materielle og geometriske ikke-lineariteter. Repræsentative finit element-modeller opbygges i ANSYS og LS-DYNA, og processen for fuld ikke-lineær dynamik præsenteres og verificeres. Resultaterne viser, at inddragelse af ikke-linearitet er nødvendig, da de maksimale responser øges markant i forhold til lineære antagelser. Hastighedseffekter inddrages via den anbefalede Cowper–Symonds-model; de reducerer plastiske effekter væsentligt og halverer næsten responsen ved højere laster i den fuldt ikke-lineære model. Lokale tøjningsresultater fra ANSYS sammenlignes med normbaserede metoder; ANSYS er mindst konservativ (lavest plastiske tøjninger), og en konvergensmetode for plastisk tøjning i skal-elementer vurderes upålidelig. Halv-analytiske metoder for efterknækning af plane plader og for dynamisk knækning gennemgås og stemmer med ANSYS op til cirka det dobbelte af den kritiske last med tilsvarende nøjagtighed. Arbejdet foreslår repræsentative modeller til vurdering af eksplosionsmodstand i offshore moduler og tydeliggør betydningen af hastigheds- og ikke-lineære effekter for design.
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Keywords