TRANS ALASKA PIPELINE

800 mile Trans Alaska Pipeline system (TAPS) stretches from Prudhoe Bay to Valdez in North America

Pipeline Transports crude oil

Constructed between 1975 and 1977

Cost $8 billion for entire system

Crosses three mountain ranges and over 800 rivers

Extreme geological conditions including permafrost and arctic tundra formations

BACKGROUND:

Pipeline location:

TRANS ALASKA PIPELINE

GEOLOGICAL CONSIDERATION:

Presence of PERMAFROST both Continuous and Discontinuous

PERMAFROST: Soil or Rock, with or without included water, which has remained at or below 0°c for 2 or more years

Presence of permafrost along the Trans Alaska Pipeline

Permafrost cross section

TRANS ALASKA PIPELINE

STRUCTURE OF PERMAFROST:

Varying depths of permafrost presence up to 200m depth

The Active layer: Fluctuates below freezing during the year

Ground water movement within the active layer effecting stability

Thick snow layer will act as and insulator

Formation of Ice wedges

Important to classify type of ground ice present in permafrost as this will effect ground strength

TRANS ALASKA PIPELINE

PROBLEMS CAUSED BY PERMAFROST GEOLOGY:

Thaw settlement:

•Thermal change in the ground caused by the construction of pipeline.

Frost Heave:

•Ground heaves during freezing periods

Heterogeneity:

•Unstable formation of permafrost Thaw settlement on pipe section

TRANS ALASKA PIPELINE

Pipeline sensitive to movement – ruptures or leaks lead toEnvironmental Problems

ISSUES CAUSED BY THE PRESENCE OF PERMAFROST GEOLOGY:

Instability:

Instability of the ground within the active layer during periods of freezing

and unfreezing. Surface ground settlement, subsidence

Construction:

•Construction problems in frozen ground, unpredictable ground ice structures

•Ground ice is always present in this climate

Support settlement

ENGINEERING SOLUTIONS FOR PIPELINES IN PERMAFROST:

TRANS ALASKA PIPELINE

POSSIBLE ENGINEERING SOLUTION:

Control Thaw Settlement:

Run the pipeline hot or cold?

Areas of discontinuous permafrost this is a problem.

Thaw bulb created around the pipeline where thermal transfer warms ground

Methods to control thermal transfer and reduce settlement

Thaw bulb created around buried pipe

Insulation:Cover ground with material

such as wood chip, slow thawing process.

TRANS ALASKA PIPELINE

Passive cooling:

Installation of pipes through areas around pipeline to cool ground and remove thermal heat

Air is lower thermal conductivity and so use wind to remove heat

Thermosyphon:

Heat exchange system

Keep the ground Frozen:

Ground Freezing Techniques:

Refrigeration plants on surface

Aware of the impact of frost heave

Frost heave damage

Passive cooling installation

TRANS ALASKA PIPELINE

SOLUTIONS ADOPTED ON TRANS ALASKA PIPELINE:

Used a combination of techniques,

Above ground

Below ground, thaw settlement control.

Above ground:

Where thaw-stable soils were identified pipeline was buried in a conventional manner, thaw settlement was managed with the use of woodchip insulators

Conventional below-ground — 376 miles

Unstable thaw areas where thermal transfer may cause thawing and instability in the ground

Refrigerated below-ground — 4 miles

Cross section of conventional pipe

Below ground section

TRANS ALASKA PIPELINE

Above Ground:

Supported Pipeline above the ground

Above-ground — 420 miles

Consider settlement of supports: specially design 2 inch heat pipes act as a thermosyphon to remove heat from the system

Design of supports

Section of pipeline running above ground

TRANS ALASKA PIPELINE

ANY QUESTIONS ?