PERSPECTIVE METHODS OF PIPELINE CONSTRUCTION IN ARCTIC CONDITION ON AN EXAMPLE OF TRANS ALASKA PIPELINE SYSTEM

PERSPECTIVE METHODS OF PIPELINE CONSTRUCTION IN ARCTIC CONDITION ON AN EXAMPLE OF TRANS ALASKA PIPELINE SYSTEM

Scientific article

Chukhareva N.V.¹, Ermolaeva A.V.²

^{1, 2} National Research Tomsk Polytechnic University, Tomsk, Russia

Abstract 

The exploitation of the Arctic region depends on meeting the challenges of pipeline design, construction and operation which to be solved by modern science and petroleum industry.  The objective of this research is to identify the most perspective technologies to be applied in Arctic pipeline construction. Arctic pipelines are those which are located northward of 60th parallel. The main peculiarities of such pipelines are defined by climatic and soil conditions of the Arctic region. Sever pipeline conditions are the following:

• deep water (up to360 m) high pressure main pipeline challenges;
• necessity of pipe-burying up to 2-3 mbecause of intensive frost penetration which can cause pipeline damage;
• soil instability;
• considerable temperature difference (+2 -60 °C);

As there are severe environmental requirements for Arctic pipelines, which are determined by significant challenges in leak detection and accident response, the main criterion in determination of pipeline construction technology is its reliability.

Let us consider perspective technologies of an inshore pipeline construction. The main pipeline construction challenges will be discussed in terms of Trans-Alaska Pipeline System. The Trans-Alaska Pipeline System lies from oil field Prudhoe-Bay at coast of  Beaufort Sea (Arctic Ocean) to portValdezlocated in the south ofAlaska.

It is intended for oil delivery, which is extracted in Prudhoe-Bay field in the north ofAlaska, and then it is transported southward to the port of the city ofValdiz. It crosses the State ofAlaskafrom the north to the south, its length being1288 kmand diameter being1220 mm. It is made from steel pipes, with yield point being 422-492 МPа and pipe thickness ranging from11,7 mmin the level land to14,3 mmin hilly regions  where it is required to increase pipeline working pressure.

Most part of pipeline (676 km) is located above the ground sitting on the top of above-ground supports which are located in 123 sections of pipeline system where soil and ice sheets are unstable because of summer thawing or operation temperature impact.

There are some sections with total length being605 kmwhere traditional construction methods such as underground laying are used. There are also three sections southward from the riverYukon, with total length being about6 km, and several shorter sections in northern part where pipeline lies underground. In these sections, soil was deliberately frozen. Transarctic pipeline is characterized by following specific construction features: crossing of 20 big rivers, 300 streams, three mountain ridges (the Atigun pass, elevation1440 m, the Alaska chain, elevation990 mand the Thompson pass, elevation840 m), tundra areas and regions characterized by wood vegetation and seismic activity (Denali-Folt). Under such conditions, the cost of transporting construction materials, equipment and people is 25 % of all expenses for pipeline construction.  An overall cost of pipeline construction reached 8 billion dollars.

The underground sections of the discussed pipeline are supported on box beams in order to cope with pipeline expansion and contraction by sliding horizontally and longitudinally on the support beam. There are pipe expansions in the middle of pipe beam span. The distance between the earth surface and pipe is usually0,9 m.  It make possible to prevent snow banks just under the pipeline and pipe temperature influence on snow sheet.  

Because of a season thawing, it was necessary to frozen the soil in order to support its bearing capacity. It was reached by using so-called "thermal tubes", which operation principle is used on heat removal from soil during the winter period so that make soil frozen around a pile which will not thaw even during the summer period. These thermal tubes made from heat-treated steel are 10,8 —19,8 min length, 37,5 mmin internal diameter, with  wall thickness being 6,25 or18,75 mm. The thermal tube is filled with coolant, i.e.  anhydrous ammonia. There are two thermal tubes in each pile. When the temperature of external air is lower than soil temperature, the liquid ammonia being in the lower part of a thermal tube starts to evaporate. Vapor rises to the top part of a tube where it is condensed. The cooled ammonia condensate vapor flows down the tube walls, cooling a pile and the soil located around it. The described cycle of evaporation and condensation of ammonia is repeated until the temperature of the lower part of a thermal tube becomes lower than temperature of its top part.

The main advantage of the mentioned technology is that there is no need in external power sources, as well as the lack of any mobile details excludes the need of repair and control. However, it is necessary to protect radiators from impact forces which can occur during pipeline construction. In accordance with rough estimates, service life of thermal tubes is not less than 30 years. In Denali-Folt area, which is characterized by high seismic activity, it was required to use the supports which would provide more significant pipeline movements. Therefore, so called «soil supports», i.e. long gravel embankment, have been used. This embankment is covered by concrete bed plates with steel sheets. The pipeline is based on these plates by means of sliding poles. Such technology makes it possible for pipeline to move horizontally up to6 mand vertically up to 1,5. During the Trans-Alaska Pipeline System construction, trench excavation was mainly performed by blasting workings followed by traditional excavation activities.  Before the pipe strings were lowered into the ditch, selected padding (15 cm), characterized by definite hardness and fineness (not more than25 mm) was placed on the ditch bottom.

Approximately 70 rivers and streams crossed by the discussed pipeline are considered to be large water barriers. Majority of such obstacles as streams and wetlands are crossed applying traditional method, i.e. underground crossing. It is necessary to note that in most cases welding was done on the river ice cover. It made possible to use the same technologies which were used in line pipe construction.  Because of special soil conditions and river features, underground crossing was applied in thirteen cases.

Pipeline crossing at the Yukon River (600 min width) spanning theYukonis of particular interest. There are 6 spans, with the end span being  96 mand central span being , the width of the carriageway being9 m. Bridge load-bearing structure is box-shaped and the pipeline is located sideways from a bridge beam in the special consoles welded to the beam. Pipeline sections were welded on the bridge carriageway then the welded strings of the pipe were installed in accordance with pipeline system design.

Thus, it is possible to make a conclusion that there is an extensive experience in construction of the Arctic pipelines which requires the application of various technical decisions and technologies.

References

1. By the materials of official site: http://www.bibliotekar.ru/spravochnik-119-konstrukcii/33.htm
2. Construction regulations 2.05.06-85* Main pipelines
3. E.V. Ivanitskaya. Expirience of  Trans-Alaska pipeline system monitoring/ Science and technology of oil and oil products, 2011, № 1, page 96-101