SAMCoT - Annual Report 2015

SAMC

• ANNUAL REPORT 2015

To check whether or not the fluid-structure-interaction

(FSI) technique of LS-DYNA (an advanced general-purpose

multiphysics simulation software package) can be applied

to the analysis of vessel-ice collisions, numerical simula-

tions of collision experiments at the Aalto Ice Basin in

Finland have been carried out. The input parameters for the

ice model and the fluid model were set independent from

the test used in order to validate the overall performance

of the FSI model. To verify the fluid modelling in LS-DYNA,

analyses were performed to determine the frequency-

dependent added-mass coefficients for a spherical body

and a rectangular block. The coefficients for different

frequencies were compared to the added mass coefficients

calculated in Wadam (software for marine hydrodynamics

and wave structure analysis) with the same geometry. In

the POAC15 paper entitled “Fluid-structure-interaction

analysis of an ice block structure collision” we address

verification issues arising from fluid modelling that uses

an equation of state. To build credibility in the constitutive

ice model and to validate ice input parameters, experimen-

tal data from indentation and impact tests were used. The

FSI simulation results were compared with the laboratory

experiments where a floating structure was impacted with

an approximately one tonne ice block at a speed of 2.0 m/s

(see Figure WP4_12). In short, the FSI method with verified

ice and water models is able to predict accurately the colli-

sion response of the floater as far as sway accelerations

are concerned. A comparison between the conventional

constant added mass approach and the FSI analysis is

currently in progress. Our preliminary results indicate that

for collision problems in which the hydrodynamic inter-

action effects are important, the FSI method can provide

more realistic and reliable predictions of the floater accel-

eration history than the conventional constant added mass

approach.

Validation of Smooth Particle Hydrodynamics

(SPH ) based approach to fracture of ice

Within this topic SAMCoT’s research efforts focused on

understanding which experimental data sources are better

suited for validation of the SPH based approach, and what

are the possible size and scale effects. In collaboration with

WP2, we searched for scale- and size-invariant ice param-

eters that can be used for the validation of SPH based

numerical and analytical models of ice-structure interac-

tions (including the SPH basedmodel) by re-analysing avail-

able in-situ and laboratory indentation test data at different

scales. Currently, we are investigating a crushing specific

energy index of ice and the possibility of its implementation

into numerical and/or theoretical models developed within

WP4. As a part of this study and in collaboration with the

National Research Council of Canada (Bob Gagnon), data

from indentation experiments conducted on iceberg ice

at Pond Inlet in 1984 were re-analysed for three different

spherically-terminated indentor sizes. Preliminary results

indicate that for any given test the crushing specific energy

of the ice shows little, if any, dependency on the volume

of the displaced ice and tends towards a constant value.

Furthermore, there is no apparent correlation between

the crushing specific energy of the ice and indentor size.

Possible reasons for these observations will be discussed

in the 23rd IAHR International Symposiumon Ice 2016 paper

(“A preliminary analysis of the crushing specific energy of

iceberg ice under rapid compressive loading”)

theoretically different fracture patterns (see the POAC’15 paper “Toward a holistic load model for

structures in broken ice”). The updated ice failure map is shown in Figure WP4_21 and is based on

observations of ice failure in contact with floating ship-shaped structures in level ice and in low ice

concentrations.

Figure WP4_21. Updated ice failure map and corresponding fracture patterns.

Application to accidental limit state design (significant plastic deformations)

Within the context of local ice loads due to an abnorm l ice event, our group h s be n addressing

two effects: first –an effect of structural deformations (

coupled ice-structure interaction during an

impact event

) and second – an effect of surrou ding water (

hydr dyna ic int raction effects

), see

more below.

Coupled ic -structure int raction

Results of ice and structure collision experiments where both the ice and the impacted structure

undergo permanent amage have be n presented at POAC’15, for detailed information refer to the

paper entitled “Pilot study of ice-structure interaction in a pendulum accelerator”. We highlight that

further i vestigations of th s coupled in eraction are vital to improve the understanding of ice loads

in a realistic impact scenario, and to establish additional requirements to limit catastrophic damage

on vessels with design loads wit igh probabilities (less than 100 year return period).

Hydrodynamic interaction effects

In the analysis of ice-vessel collisions, hydrodynamic effects from the surrounding water may also be

important because they affect the motions of the ice and the vessel before and after the collision

(e.g. se Figure WP4_22).

To check whether or not the fluid-structure-interaction technique (FSI) of LS-DYNA can be applied for

the analysis of vessel-ice collisions, numerical simulations of collision experiments at the Aalto Ice

Figure WP4_11. Updated ice failure map and the corresponding fracture patterns.

Figure WP4_12. Measured and calculated accelerations of the

floater versus time.