Floe Ice – Sloping Structure Interactions
Abstract
Several interaction processes can be identified during floe ice - sloping structure
interactions. However, fierce processes around the waterline are the most conspicuous
phenomena. These violent processes feature ice fracturing and the potential rotation of
subsequent ice blocks. This thesis focuses primarily on the fracture of ice floes. The
exact failure pattern of an ice floe is complicated by the actual ice conditions in which
the sloping structure is positioned. Regarding the ice condition, we do not predefine it
as a ‘level ice’ or a ‘broken ice’ field. Instead, considering an ice field composed of ice
floes of varying sizes is a more realistic approach. Studying the interaction between an
ice floe and a sloping structure (i.e., floe ice - sloping structure interactions) can yield
more general results because an ice floe has a spatial scale that typically ranges from 10
m to 10 km. The ice floe can be large enough to be regarded as level ice or can be small
enough to be treated as a member of a broken ice field.
Currently, Arctic exploration and exploitation are expanding into a relatively ‘open’ ice
condition, within which, a knowledge gap exists regarding the fracture of a finite size
ice floe in the context of ice-structure interactions. To advance our understanding, this
thesis studied the following observed failure modes of a finite size ice floe within such
an ‘open’ ice condition:
1) In-plane splitting failure;
2) Out-of-plane flexural failure; and
3) Competition between different failure modes in the context of ice - sloping
structure interactions.
This thesis addresses influential factors, such as an ice floe’s size, geometry,
confinement, and ice-structure contact properties. In a decoupled manner, these
different failure modes were studied primarily using the concept of fracture mechanics.
The major contributions of this study are the following:
1) Analytical solutions for each failure mode were proposed for ice floes of varying
sizes, which range over a large spatial scale (i.e., from approximately 1 m to 10
km); and
2) Each analytical solution within the established analytical framework was
rigorously verified, either by numerical simulations, existing idealised analytical
results, experimental measurements, or all of the above.
Theoretically, through derivations, implementations and validations of all these
analytical solutions, many interesting results were obtained, such as the following:
1) The validity of using Linear Elastic Fracture Mechanics (LEFM) to study an ice
floe’s splitting failure on an engineering scale was confirmed;
2) An ice floe’s confinement has a much more profound effect on increasing the
force required to split an ice floe in comparison with the influence of floe
geometry;
3) Three out-of-plane flexural failure scenarios were further conservatively
identified and analytically studied; and
4) A floe size requirement (i.e., a square floe’s size L <27(ice thickness)3/4 ) was
suggested for the radial-crack-initiation-controlled fracture of an ice floe.
From a practical point of view, with all these analytical solutions of failure modes at our
disposal, we are thus able to construct a failure map that relates an ice floe’s dominant
failure mode to its geometry, relevant contact and material properties. In addition, these
analytical formulae can be effectively incorporated with multi-body dynamic simulators
to assess the performance of Arctic offshore structures in ice over large temporal and
spatial scales.
In addition, as opposed to relatively ‘open’ ice conditions, we have also studied ice -
sloping structure interaction in ‘tight’ ice conditions. One extreme interaction scenario
within these ice conditions is when a large amount of ice rubble accumulates in front of
a sloping structure. This scenario has long been recognised as one of the controlling
design conditions and has previously been under thorough investigation.
In this thesis, a novel approach that combines both physical model tests (i.e., measured
by tactile sensors and load cells) and a theoretical model was employed to study ice
load’s spatial and temporal variations on a sloping structure under the influence of
rubble accumulation. The following results were found:
1) The presence of rubble accumulation increases the global ice resistance;
2) The maximum value of ice resistance occurs in a location below the waterline,
which signifies the importance of the ice rotating process and rubble
accumulation effect; and
3) The developed theoretical model, which was validated by both physical model
tests and existing theoretical models, can serve as a preliminary tool to study ice
load’s temporal and spatial distribution under the influence of rubble
accumulation.
As an extension, we also explored a seemingly promising numerical method’s (i.e., the
Cohesive Element Method (CEM)) applicability in studying ice - sloping structure
interactions under the influence of rubble accumulation. Preliminary results demonstrate
that there is still a substantial knowledge/computational gap in using this numerical
method to simulate ice and sloping structure interactions in a three-dimensional setting.
The primary deliverable contributions of this thesis to the scientific and engineering
community are the proposed analytical solutions and the framework for different failure
modes under two different ice conditions. Such analytical treatment prepared us to
simulate loads related to floe ice - sloping structure interactions, which are important for
Arctic exploration and exploitation on large temporal and spatial scales.
Has parts
Paper 1: Lu, Wenjun; Lubbad, Raed; Løset, Sveinung. In-plane fracture of an ice floe: A theoretical study on the splitting failure mode. Cold Regions Science and Technology 2015 ;Volum 110. s. 77-101 http://dx.doi.org/10.1016/j.coldregions.2014.11.007Paper 2: Lu, W., Lubbad, R. and Løset, S., Out-of-plane failure of an ice floe: radial-crackinitiation- controlled fracture
Paper 3: Lu, W., Lubbad, R. and Løset, S.,. Fracture of an ice floe: Local out-of-plane flexural failures versus Global in-plane splitting failure
Paper 4: Lu, Wenjun; Lubbad, Raed; Høyland, Knut Vilhelm; Løset, Sveinung. Physical model and theoretical model study of level ice and wide sloping structure interactions. Cold Regions Science and Technology 2014 ;Volum 101. s. 40-72 http://dx.doi.org/10.1016/j.coldregions.2014.01.007 This article is reprinted with kind permission from Elsevier, sciencedirect.com
Paper 5: Lu, Wenjun; Lubbad, Raed; Løset, Sveinung. Simulating Ice-Sloping Structure Interactions With the Cohesive Element Method. Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme 2014 ;Volum 136.(3) Is not included due to copyright. Available at http://dx.doi.org/10.1115/1.4026959
Paper 6: Lu, Wenjun; Serré, Nicolas; Høyland, Knut Vilhelm; Evers, Karl-ulrich. Rubble Ice Transport on Arctic Offshore Structures (RITAS), part IV: Tactile sensor measurement of the level ice load on inclined plate. I: The proceedings of the 22nd International Conference on Port and Ocean Engineering under Arctic Conditions. : Port and Ocean Engineering under Arctic Conditions 2013