The Mechanics of Ice: Developing Design Standards

The Mechanics of Ice: Developing Design Standards - Arctic Engineering & Offshore Technology

Ice mechanics is a relatively new field, providing information for building structures in the Arctic and other places where there is moving ice. Studying “ice interactions,” such as ships literally ramming into icebergs, in order to understand loading and failure mechanisms helps ice engineers understand forces and consequences to building in these regions.

Ice-loading models can be integrated into design rules so structures such as ships, offshore platforms, bridge and light piers, and wharves don’t come apart during global, cyclical, or local loading events with moving ice. These studies also help protect cables, pipelines, wellheads, or other structures at the bottom of the sea against gouging or scarring from ridges in multiyear ice. To date, the studies have mainly served fixed oil and gas structures and newer floating systems, according to Canadian researcher and consultant Ian Jordaan, of Ian Jordaan and Associates, St John’s, Newfoundland, Canada.

The latest empirical results of ice-ramming tests have helped create models that have been useful for defining a newly adopted international design standard. Still, there are many aspects of ice loading that are not yet understood.

HPZs and Microstructure Analysis

Like glass, ice is extremely brittle, and is a viscoelastic solid close to its melting point while in our oceans. Its nature can change quickly, with its strength being a function of its strain rate. It is lighter than water so it floats at atmospheric pressure and exhibits creep.

The Mechanics of Ice: Developing Design Standards - Arctic Engineering & Offshore Technology

Its complex failure mechanism can include shear, microcracking, recrystallization, load oscillation and geometry changes, and time-dependent behaviors. The different responses to different loading conditions make ice a fascinating material to study, Jordaan says. The forces applied to a structure are transmitted though zones of intense high pressure, termed high-pressure zones (HPZs).

Glacier Impact Effects

Jordaan examined results of recent ice ramming and field-indentation tests, as well as tests in which icebergs were towed and impacted against instrument panels. The ice’s impact zone and areas surrounding it were sectioned and analyzed to understand ice loading and impact effects of glaciers to offshore structures.

He found that when ice undergoes rapid, nonuniform compression, like during the ramming testing, regions that contain many random flaws could “spall” or flake off and break away upon impact. Regions that don’t break away often contain HPZs in which the intense pressure of the impact is localized and can be dangerous to a structure.

Other Failure Modes

Ken Croasdale from K.R. Croasdale and Associates, Ltd., Calgary, Alberta, Canada, says more work is needed to understand failure processes, since varying types and sizes of ice features can impart different loads on different structures. Jordaan agrees with this approach.

Thinner ice can raft and form ridges. Multiyear ice that is several years old can include ridges that present a severe hazard. Croasdale says it is important to know when and how to predict limit-force ice loads when ridge building occurs, especially for floating structures surrounded by “unmanaged ice” that has not been broken up by ship before it reaches the structure.

A Worldwide Design Standard

Many years worth of analytical, numerical, and empirical models, tests, field measurements, and case studies have recently culminated in a new design standard that allows the same ice load calculation methodologies to be used for designing floating or fixed offshore structures, such as from steel or concrete, man-made gravel, or rock islands.

The international standard ISO 19906 was published in December 2010 titled “Petroleum and Natural Gas Industries - Arctic Offshore Structures.” It provides guidance for design, construction, transportation, and installation of oil or gas offshore structures in arctic and cold region environments that are subject to iceberg and icing conditions, whether partially or wholly covered with ice, seasonally or year-round.

As the research continues on ice mechanics, expect more useful real-world benefits like this to emerge.

Debbie Sniderman is a consultant, an engineer, and a contributing writer to Mechanical Engineering.

More work is needed to understand failure processes, since varying types and sizes of ice features can impart different loads on different structures.

Ken Croasdale, K.R. Croasdale and Associates Ltd., Calgary, Canada


January 2012

by Debbie Sniderman,