Engineering for Arctic Environments

This introductory lecture sets out the reasons for outside interest. It covers petroleum, minerals and fish, and sets them into the context of the world economy as a whole. Some studies have concluded that one-third of all the petroleum that remains to be discovered will be found in the Arctic, and that they will become increasingly important as more easily reached reserves elsewhere become depleted or politically inaccessible.

Much of the Arctic is already populated, both in Siberia and in Arctic North America. Some of the people are indigenous groups who have been there for thousands of years, and have evolved ways of maintaining themselves in a challenging environment. Others are recent 'incomers,' who have arrived as explorers, prospectors, traders and miners. All have legitimate interests and concerns, sometimes conflicting, and development has to take them into account. The political framework is extremely complicated, and reflects history and shifting balances of power and interest.

Linked to the human context is the economic context. Development can be beneficial and profitable to everyone concerned, but costs in the Arctic are often very high and potentially uncertain. The balance between cost and risk has to make economic sense to everyone involved.

This is an introduction to the geography, geology and climatology of the Arctic regions, which cover a huge area with many different environments: it would make no more sense to think of 'the Arctic' as a single environment than it would to think of 'the Tropics' in the same way. The lecture will give examples of different environments in which developments have taken place.

The offshore environment varies in the same way as the onshore environment. In some locations the sea is ice-covered for much of the year, and the ice can be used as a stable working platform, but the ice contains deep multi-year pressure ridges and is almost inaccessible to ships. At the other extreme, some locations have a long open-water season, interrupted by periods in which the sea is covered by loose pack ice through which ships can navigate, but there are occasional extremely large icebergs that threaten the safety of any structure that cannot move out of their way. Ice is far from being the only difficulty: there are sometimes severe storms that create large waves, and sometimes long periods of dense fog.

Sea ice is very variable in form. This lectures examines the formation of sea ice, and the different stages it goes through, a progression from the first development of small crystals ('grease ice'), thorough larger accumulations into 'pancake ice' and extended ice floes. It continues to pressure ridges and rafting, and ultimately to thick multi-year ridges, which have a complex structure and from which much of the original salt has drained away, so that the mechanical properties have changed significantly.

In the first phase of the design exercise, participants will work in groups. They first choose a location and an engineering objective for a project, either offshore or onshore. They then begin by identifying and assembling the data they will need to carry out the conceptual design, using the resources of the Internet.

Ice forces govern much of the design of many offshore projects. Consider, for example, a jack-up or a SPAR in a sea covered by moving ice driven by winds and currents. The ice pushes against the structure, and can exert large forces. A typical laboratory compression test on ice finds its compressive "strength" to be 450 psi (roughly 3 MPa). If the ice is 6 feet (2 m) thick, and the width of the contact between the ice and the structure is 30 feet (10 m), then the projected contact area is 180^ft2, and a first estimate of the maximum ice force is the ice strength multiplied by the contact area, which is 11,600,000 pounds (5800 tons, 52 MN). That calculation is badly oversimplified, and fortunately overconservative, but it indicates that ice forces need to be taken seriously. Even if the ice were thinner and the structure narrower, the structural implications would still be significant.

This lecture begins by considering the mechanical properties of ice, which behaves differently from the materials such as steel and concrete that structural engineers generally deal with. Ice deforms in creep at low stresses, but at high stresses fractures, as we know from everyday experience (such as the cracking of an ice cube when water is poured over it). It considers simple fracture mechanics, and its surprising practical implications for structures in ice, and goes on to examine the evidence from field measurements and from laboratory tests in ice tanks.  

This continues the theme of ice mechanics I, and goes on to examine the evidence from field measurements and from laboratory tests in ice tanks. The subject remains immature and far from completely understood, and the lecture examines competing approaches to it.

Most of the land surface in the Arctic is underlain by permanently frozen ground, which extends as far south as western China. This section of the course considers the conditions under which from ground forms, the seasonally-thawed active layer at the surface, and what happens when the temperature regime changes.

Construction on permafrost is difficult because, if inadvertent changes in the thermal regime cause the soil to thaw, it often becomes a geotechnically incompetent material on which structures sink and deform. The guiding principle of permafrost construction, propounded long ago by Tsytovich, is to keep the ground frozen. This section of the course studies the behavior of roads, pipelines, airstrips and high-rise buildings, and considers the design measures that need to be taken.

Drifting sea ice in shallow water runs aground on the seabed. Driven by the wind and by other ice pushing behind it, it cuts deep gouges into the bottom. If it strikes a marine pipeline, or if it crosses just above a pipeline, it can drag the pipeline and create severe damage. The problem can be avoided by trenching the pipeline, but that is extremely expensive if a deep trench is required. This section of the course reviews the state of knowledge, develops a rational procedure for deciding on the trench depth, and examines alternative measures to protect a pipeline.

Many different structures have been considered, both for offshore petroleum production and for other applications such as lighthouses. They include artificial islands, concrete gravity structures, steel gravity structures, piled structures, jack-ups, floating semi-submersibles and spars, as well as seabed systems located below the deepest ice. The choice depends on the functional requirements, the ice climate, the wave climate, and the seabed geotechnics.

There are now many years of experience of structures in ice. This section considers the design choices that were made, the field experience with them, and the influence of factors such as safety and evacuation in case of mishaps.

The new ISO 19906 is to appear early in 2010. It represents several years of work by a large group of specialists, from North America, Europe, Russia, Japan and China. It represents the best consensus currently available, though there remain substantial areas of disagreement. This section examines the processes applied to reach a consensus, and the outstanding areas of uncertainty. DNV has recently initiated a JIP, which will examine a number of case studies and will prepare a systematic approach consistent with ISO 19906.

Marine transportation is the most economical way of shipping cargo to and from the Arctic and is part of many construction projects. The Northern Sea route through the Northeast Passage north of Asia has been in use for more than 80 years, and there is renewed interest in the Northwest Passage, which potentially offers an economical route to Asia from the east coast of North America. Global climate change will make both routes accessible for longer periods, and ultimately they may be open the whole year round. This segment of the course examines the constraints on the operation of ships in ice-covered waters, and the rules applied by classification societies.

This continues the design project begun on Day 2. Participants prepare a conceptual design, calculate ice forces following the methods put forward in the intervening two days, and identify outstanding questions that would have to be addressed in further studies.

Most offshore petroleum production schemes require seabed pipelines. This segment of the course describes the problems and their solutions. It  is illustrated by completed and current projects in the Canadian Arctic (Drake), the Alaskan Arctic (North Star), Sakhalin (Sakhalin II) and Kazakhstan (Kashagan).

Much remains to be done to learn how to construct economically and safely in the Arctic. This discussion will identify outstanding problem areas, and will attempt to set priorities.