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Ice Loads and Dynamic Response of Offshore Structures
Transcript of Ice Loads and Dynamic Response of Offshore Structures
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Ice Loads and Dynamic Response of Offshore
StructuresGesa Ziemer, HSVA
Sea Ice – Structure Interaction WorkshopBritish Antarctic Survey, November 13th, 2017
THE HAMBURG SHIP MODEL BASINSetting the Standard in Ship Optimisation
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Introduction
• Ice loads on offshore structures are hard topredict
• If ice is present, limiting design case is usuallyice load
• Structures in ice-infested waters becomeincreasingly complex (multi-legged, flexible, ..)
• Ice-structure interaction is in many aspects not well understood yet
THE HAMBURG SHIP MODEL BASINSetting the Standard in Ship Optimisation
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Structuralresponse
Ice Ice Loads
Offshore Structure
Thickness, strength, drift speed, feature…
Geometry, stiffness, natural frequency, …
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Challenges
Ice
Ice Loads
Structuralresponse
• Good forecast needed, but limited data available• Ice properties hard to predict for specific sites• risk of over- or underestimation at high costs• worst case scenario often inhomogeneous condition
(e.g. pressure ridges, rafted ice)
• Rules inaccurate / oversimplified (e.g. ISO 19906)• Complex interaction mechanisms not fully understood• Prediction and numerical simulation especially difficult
for complex structures and at transition conditions
• Not yet reliably predictable in dynamic ice-structureinteraction
• mechanism of ice-induced vibration not fullyunderstood
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Recent advances in model testing• Project BRICE „breaking the ice“• Fraunhofer IWES, VTT, HSVA• Aid numerical models with physical modelling of complex
ice-structure interaction scenarios:– Transition to non-linear response (ice-induced resonant
vibrations)– Transition of failure modes
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Example: Norströmsgrund Lighthouse
• Wide, cylindrical structureclose to Swedish Coast
• Instrumented in LOLEIF / STRICE projects
• Measurement of forces, accelerations, ice properties
• Physical model at 1:8.7
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• Video!!!!!!!
THE HAMBURG SHIP MODEL BASINSetting the Standard in Ship Optimisation
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Model test results• Different interaction types
captured well• Good agreement with full
scale data• Only few data points
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Example: Cone angle variation• Crushing on vertical
structures; risk of ice-induced resonant vibrations
• Flexural failure on conicalstructures (ice cones); lowbreaking frequency
• Transition?
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Model test results• Maximum ice loads decrease
on conical structures, but velocity effect increases withincreasing slope
• Structural displacementincreases with increasingslope
• Oscillations can arise on all cones when ice drift speed ishigh enough
• 80° cone experiences periodsof crushing
THE HAMBURG SHIP MODEL BASINSetting the Standard in Ship Optimisation
CFD CAD Office Resistance & Propulsion Propellers & Cavitation Seakeeping, Manoeuvring & Offshore Arctic Technology www.hsva.de
Model test results• Maximum ice loads decrease
on conical structures, but velocity effect increases withincreasing slope
• Structural displacementincreases with increasingslope
• Oscillations can arise on all cones when ice drift speed ishigh enough
• 80° cone experiences periodsof crushing
THE HAMBURG SHIP MODEL BASINSetting the Standard in Ship Optimisation
CFD CAD Office Resistance & Propulsion Propellers & Cavitation Seakeeping, Manoeuvring & Offshore Arctic Technology www.hsva.de
Closing remarks• Ice loads and structural response of offshore structures
are subject to extensive research, but knowledge isinsufficient
• Validated numerical models required for designers toassess risks and define realistic design cases
• Physical model tests needed to monitor ice behaviourand provide validation data
• Model tests attractive for complex structures to get a feeling for their behaviour in ice