Summary Arctic Engineering 20122013

Summary Arctic Engineering 2012/2013

Arctic Engineering (Technische Universiteit Delft)

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Arctic Engineering Summary, V.H.R.I. Doedée

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Abstract—This short paper is a summary of the Arctic Engineering lectures.

I. BASICS OF ARCTIC OFFSHORE ENGINEERING

I. Introduction & Overview

An IEEE

II. Classification of Offshore Structures

Offshore structures can be classified as follows;

1) Artificial Islands

Artificial islands were first built in the Beaufort Sea in the early 1970’s, both in Canada and Alaska. Most often used in shallow coastal zones. They have great resistance to ice loads, but may need protection from wave and ice scour, and hence require more maintenance.

A short ice-free season can be used to advantage since fill can be transported over ice roads. Artificial islands can be further categorized into:

The best performing in Canada are the caisson retained

artificial islands. They could be built quickly and needed less sand. They could also be more easily installed in (deeper) waters plus they had better wave and ice protection. An overview of the different structures can be seen below.

Slopes of an artificial island vary from 1:5 – 1:20, and a

sacrificial beach is sometimes used to protect drilling

OE4680 Arctic Engineering Summary

Author: V.H.R.I. Doedée




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structures. Typical water depths are 5-20m, with rock berms it could be extended to 50-90m. These depths however have not been reached.

2) Bottom-Mounted

There are numerous bottom-mounted structures in the offshore industry, as can be seen below. Bottom-mounted structures first came to the scene in Cook Inlet, Alaska, in the 1960’s. Main types are gravity based structures (GBS), piled-based structures (PBS) and mixed base structures (MBS).

A GBS platform resists lateral load solely by large mass and friction or shear at base. PBS platforms develop shear resistance through use of piles, and assume monopod or jacket forms. MBS are a combination of both.

Generally platforms are designed to minimize ice loads through reducing diameter at waterline, and introducing sloped surfaces which fail ice in bending (lower loads than vertical faces which cause failure in crushing).

A nice example of a GBS is the Hibernia. The Hibernia GBS (offshore Newfoundland, Canada) is 111 m high and has storage capacity for 1.3 million barrels of crude oil in its 85 m high caisson. It is still the world's largest oil platform in terms of weight, at a total of 1.2 million tonnes. This consists of a 37,000 tonnes integrated topsides facility mounted on a 600,000 tonne gravity base structure along with 450,000 tonnes of solid ballast. The GBS is specially designed to withstand the impact of sea ice and icebergs to allow for year-round production.

Figure 1. The Hibernia.

3) Floating Both steel and concrete floating structures have been

proposed – steel structures, (barges, ships and semi-subs) have actually been used. Ice platforms also fit this category. Moored barges, drillships and semisubmersibles, have been used in the Arctic to date. Moored caissons remain on the drawing boards. Dynamically positioned vessels have been used where ice forces are moderate, or where there may be a need to move off quickly (for example in case of ice bergs). Ice platforms have been used where there is a stable ice cover for much of the year. Below is an overview of the different floating structures.



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Ice conditions and water depths are two major factors affecting the choice of structural configuration. Operational requirements, foundation conditions, and available infrastructure are other important constraints.

III. Ice Features & Ice Regimes

` This section covers ice regimes, sea ice and iceberg ice, ice conditions in various parts of the world, arctic locations and bathymetry, general arctic features and operating limitations.

There are three general classifications for ice; First Year Ice (FY), Multi Year Ice (MY) and Glacial Ice.

Ice types depend very much on region, distance from shore and water depth. Below is the ice regime typical for the

Canadian (and US) Beaufort Sea. Further below are the different ice regimes with some elaboration.

First Year & Multi Year Ice

a) Level Ice Level ice is Sea ice of fairly uniform ice thickness, usually

land fast. Depending on location, level ice can grow up to 2.5m or more.

b) Ice Floes Any relatively flat piece of sea ice 20 m or more across

(individual feature). Ice concentration is measured as the relative amount of water with respect to sea ice, measured in tens.

Floes are subdivided according to horizontal extent as follows:

• Giant: over 10 km across • Vast: 2-10 km across • Big: 500-2000 m across • Medium: 100-500 m • Small: 20-100 m




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c) Rafted Ice Deformed ice occurs when one piece of ice overrides

another.

d) Ice Ridges Ice formation consisting of ice blocks formed as a result of

compression or shear of pack ice. First year ridges are not nice and linear, but very irregular. There are five different types of ridges.

• New ridge: Ridge with sharp peaks and slopes of sides

usually about 40° to the horizontal. • Weathered ridge: Ridge with peaks slightly rounded and

slope of sides usually 30-40°. Individual fragments not visible.

• Very weathered ridge: Ridge with peaks very rounded, slope of sides usually 20-30°.

• Aged ridge: Ridge which has undergone considerable weathering.

• Consolidated ridge: A ridge in which the upper parts of the ridge has frozen together.

e) Rubble Pile Floating or grounded accumulation of broken ice blocks of

first-year ice, generally caused by natural or man-made obstruction.

f) Rubble Fields Accumulation of floating or grounded rubble that forms in

same way as an ice ridge, but covers large expanse of sea surface.

Glacial Ice

a) Ice Islands Large tabular ice features also originating from glaciers,

these can be very large in size. From 1983 till 1989 there even was a research centre on an ice island near Elliesmere Island in Canada.

b) Icebergs



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Floating remnants of glacial ice broken away from glaciers and ice shelves. Icebergs can be classified by weight or by shape.

Iceberg classification:

• Growlers (sail < 1.5 m). • Bergy bits (sail 1.5 to 5 m, mass < 5400 t). • Small bergs (sail 5 to 15 m, mass 5400 to 180,000 t). • Medium bergs (sail 15-45 m, mass 180,000 to 2,000,000 t). • Large bergs (mass > 2,000,000 t).

Or by shape:

• Tabular; • Blocky; • Dome; • Drydock; • Pinnacle; • Wedge.

After calving usually on the west coast of Greenland, icebergs drift to the coast of Newfoundland, which is about 1,800 nautical miles. The average iceberg drift speed is about 0.7 km/hr, but this is influenced by factors including iceberg size, shape, currents,waves and wind. They travel in the Baffin Current, then the Labrador Current and finally reach the Grand Banks of Newfoundland. Once they reach the Grand Banks, icebergs drift either eastward – north of the Flemish Cap – or Southward between the Flemish Cap and the Grand Banks – also known as Iceberg Alley.

Arctic Bathymetry & Areas of Interest The Arctic can be very deep in places and the bathymetry

greatly influences flows in and around the arctic. A figure of the bathymetry is given below. Note locations of extreme depths, and features such as the Lomonosov ridge running diagonally across the North Pole.

The ice conditions in the most interesting regions are stated here below.




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Note that in most of these regions there is an absence of icebergs due to the lack of gletsjers. Also, due to the Atlantic current, in the Barents sea there can be no ice at all during the year.

IV. Ice physics & Ice Mechanics

In this section, the aim is to obtain the necessary understanding of ice and ice-structure interaction to be able to



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calculate loads due to ice molecular properties & crystallography, ice growth & formation, mechanics and mechanical properties and finally ice load reduction methods. These different aspects are also elaborated in the sections below. First the material, molecular and formation properties of ice and water and water.

The density of water is about 1000 kg/m3, where the density

of pure ice is 916,7 kg/m3 (theoretically). The solid phase of ice is less dense because of the hydrogen bonds forming in ice. A hydrogen bond is a chemical bonding that occurs when a hydrogen atom finds itself between 2 highly electronegative atoms and is mainly an electrostatic bond. During the process of freezing, the average distance between adjacent water molecules grows as the crystallographic structure of ice is formed. The density of pure ice as a function of temperature is:

( ) 916, 7 0,13T Tρ = − (1)

With T the temperature in degrees Celsius. Average density

of sea ice is 910 kg/m3 (±6) and of glacial ice 900 kg/m3. Sea ice density is lighter due to salinity and porosity, and the glacial ice is less dense than pure ice due to air enclosed in the compacted snow. For icebergs, it is sometimes important to calculate the freeboard; the height of the ice above the water. This can be calculated with:

Seawater Glacial IceFreeboard Iceberg

Seawater

h hρ ρ

ρ−

= (2)

On Earth, only crystalline ice can from. The crystalline

phase is the situation where the oxygen atoms are in a fixed position relative to each other, while the hydrogen atoms may or may not be proton-ordered but are always obeying the so-called ice rules.

Proton-ordered ice is ice with a regular arrangement and placement of the hydrogen atoms; i.e. there is a sequence in the placement of hydrogen atoms. In proton-disordered ice, the hydrogen atoms are NOT arranged regularly. Their only ordering is given by the satisfaction of the Bernal-Fowler rules. Since the break-up of proton-ordering does not take lots of energy, protonordered ice can only exist at temperatures < -80 C (193 K). Hexagonal ice is proton-disordered.

The freezing point of sea water is lower than the freezing point of pure water (0�C), due to the presence of compounds other than water; this is known as freezing-point depression. As the air above an ocean starts cooling down the sea-surface below its freezing point, the upper layer of the sea becomes (slightly) supercooled and the first molecules in the sea water start forming hexagonal crystals of ice. A supercooled liquid is a liquid at a temperature below its freezing point without it becoming a solid (yet). When sea water freezes, the salt is expelled completely from the first flat ice platelets that form. These platelets therefore exist of almost pure ice. Due to their

size, NaCl- or salt-ions cannot be incorporated into the hexagonal crystal lattice of the ice and neither do they fit into the ice crystal as interstitial molecules. Therefore, the salt crystals are rejected from the ice while the ice crystals are forming. Consequently, the forming of ice on the water surface is accompanied by an increase of salinity in the surrounding water.

As the individual ice platelets join together some gas and so-called brine becomes entrapped: Brine is water that is supersaturated with salt. Salt is expelled from the first platelets that form, increasing the salinity of the surrounding water. As the ice platelets take in H2O-molecules from the seawater on growth, the seawater salinity increases further. Then, due to the growth process in the SK-layer, the salt in sea ice accumulates and is included along the platelet boundaries in the form of liquid or solid inclusions. The now isolated brine inclusions are called brine pockets. The brine in the brine pockets remains liquid because much lower temperatures would be required to freeze the highly saline brine.

The structure of first-year level ice is depicted below.




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As the ice sheet continues to grow and becomes colder, the brine between the ice plates drains out leaving behind air pockets, this is due to freezing of water in the brine, expulsion and drainage processes aided by gravity. Brine expulsion and drainage always occur along a flow route that resembles the trunk and branches of a tree; respectively called the drainage tube and drainage channels. As a result of this, the salinity of sea ice decreases as it becomes older. In sea ice, the salinity varies with depth; the salinity is lower in the middle, while the salinity at the top and bottom is higher. The growth of ice thickness is influenced by the air temperature and the height of the snow on top of the ice sheet. An empirical equation for ice thickness growth was found by Doronin & Kheisin in 1975 as:

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0( ) 405 ah m m h T= − + + − ∑ (3)

With Ta the mean daily air temperature, m the empirical

coefficient depending on snow height, h0 the initial ice thickness and h he resulting ice thickness. The sum is taken over the number of days in the period. The maximum thickness of undisturbed level ice grown in 1 winter can be found as:

bFDDh aC= (3)

With h ice thickness, a site specific constant, b the heat

conduction component (b=0.5 for linear heat conduction) and CFDD the accumulated freezing degree days. The maximum thermally grown level sea ice thicknesses are in the range of 2 meter for the Arctic region. The heat flux in water has a significant influence on ice thickness growth; when the heat flux through the sea is equal to the flux through the ice, the ice thickness growth stops. Accumulated freezing degree days (CFDD) for a winter is a means of

characterizing the general severity of ice and weather conditions.

The number of freezing degree days is the number of °C that the mean air

temperature is below the freezing point of water.

V. Ice Loads, Ice Actions & Action Effects

VI. Scaled Ice Tank Model Tests & Scale Effect

VII. ISO19906

II. DYNAMICS OF ICE-STRUCTURE INTERACTION

I. Introduction & Overview

II. Frequency Lock-in

III. Models for Ice Structure Interaction

4) Beam Theory 5) Plate Theory

IV. Industrial Experience; Shell

V. Numerical Modelling

III. SPECIAL TOPICS

I. Arctic Oceanography

II. Ship Design for Arctic Conditions

III. Assignment & Exam

IV. FORMULAS AND EQUATIONS


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Summary Arctic Engineering 20122013