Ice Loads and Dynamic Response of Offshore Structures

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

Ice Loads and Dynamic Response of Offshore

StructuresGesa Ziemer, HSVA

Sea Ice – Structure Interaction WorkshopBritish Antarctic Survey, November 13th, 2017



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



Structuralresponse

Ice Ice Loads

Offshore Structure

Thickness, strength, drift speed, feature…

Geometry, stiffness, natural frequency, …



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



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



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



• Video!!!!!!!



Model test results• Different interaction types

captured well• Good agreement with full

scale data• Only few data points



Example: Cone angle variation• Crushing on vertical

structures; risk of ice-induced resonant vibrations

• Flexural failure on conicalstructures (ice cones); lowbreaking frequency

• Transition?



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



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



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

Ice Loads and Dynamic Response of Offshore Structures

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