Foundations on permafrost: an engineering challenge at Yamal LNG
The foundations of the Yamal LNG plant called for an engineering solution that was specifically designed for permafrost conditions. The bearing capacity of pile foundations must withstand extreme loads over the long term in order to guarantee the stability of industrial facilities. An overview of the technical solutions implemented.
Energy with a future
The Yamal LNG project was launched at the end of 2013 by Total, the operator Novatek and CNPC. The purpose of this project is to develop the South Tambey gas field in a remote site located on the Yamal Peninsula in the Russian Arctic. The construction of industrial facilities in this area, which is very close to the polar circle, is subject to particularly difficult conditions. In winter, the temperature can fall as low as -50°C. The ground consists of permafrost, a permanently frozen subsurface layer of soil whose the thickness varies and can reach more than 400 meters. When permafrost thaws, the first two meters of the top layer (known as the active layer) turn into unstable mud.
Pile foundations for extreme loads
The construction of the Yamal LNG gas liquefaction plant on this unusual type of ground requires the engineering of very specific foundations. The technical solution chosen for the foundations involved driving piles of various diameters and depths into the permafrost. These foundations are also designed to make up for unforeseen bearing and volume variations in the active layer and to thereby ensure the stability of the plant throughout the site’s lifetime, namely at least 50 years.
These solutions, adapted to the Russian Arctic, have been successfully applied by Novatek and its partners for decades. However, in the case of Yamal LNG, it is the first time that they have been used on such a scale, given the large size of the site’s facilities and the weight of the loads to be borne. A total of 65,000 piles, including 38,000 primary piles, are currently being installed to guarantee the stability of three liquefaction trains, a number of natural gas processing units and four 140,000-metric-ton storage tanks (maximum operating weight).
The design of these foundations was based on an advanced geotechnical and geophysical analysis of the soil, whose purpose was to identify the characteristics of the soil in as much detail as possible and to predict its long-term behavior. An initial campaign, carried out in situ between November 2010 and June 2012, was completed in 2013 by undertaking several other observational, sampling and instrumentation studies. Their aim was to accurately characterize the different types of soil, salinities, temperatures and ice contents, to produce reliable parameters describing soil behavior over time.
The foundations of the Yamal LNG plant called for an engineering solution that was specifically designed for permafrost conditions.
65,000 piles, including 38,300 primary piles, are in the process of being installed to ensure the stability of the whole liquefaction facility.
38,300 primary support piles with a diameter of 273 or 530 millimeters will be sunk to depths of between 10 and 28 meters.
The two essential requirements when performing the work are the optimal positioning of the piles and continuous temperature control.
The quality of the installation of the primary piles will help to ensure the bearing capacity of the foundations.
Soil flowage studies
On the Yamal LNG site, the soil is heterogeneous and comprises clay, silt and sand. The soil’s salinity increases close to the banks of the Ob river, reaching 20 ppt (parts per thousand or ‰). The soil’s ice content is greater in the first five meters under the surface, a layer which also contains unfrozen saline pockets known as “cryopegs”.
All these factors play a decisive role in the study of soil flowage, a physical phenomenon characterized by the delayed and irreversible deformation of material that is subject to constant stress. While permafrost flowage values can be found in scientific literature, not all of these values have been validated, or even estimated in some instances, particularly in the case of saline soils. More than 130 uniaxial deformation laboratory tests have therefore been carried out by Total and its partners on 11 different types of soil. They were complemented by in situ static load, compression, traction and shear tests on piles. These laboratory and in situ tests provided compaction and flowage parameters for all of the existing types of soil under the LNG plant.
Determining the bearing capacity and dimensions of the piles
The soil’s characterization was used as the basis for determining the bearing capacity, the long-term deformation potential and the positioning of the piles in the permafrost in accordance with the plant’s specifications. The choice of calculation method was of primary importance in ensuring the various parameters would be taken into account as exhaustively as possible.
To carry out this work, Yamal LNG (Novatek, Total, CNPC and Silk Road Fund) and its partners synthesized the methods and standards used by countries known for their geomechanical permafrost modeling expertise (Russia, Canada and the United States). Calculations were governed by a stress and flowage distribution law that includes the rate of displacement and stress for a pile adhering to frozen soil that contains variable amounts of ice. The specific deformation rate of piles in a saline environment follows American recommendations due to the absence of Russian directives on this subject and is based on parameters established from laboratory test results. The dimensioning of the special pile foundation system was completed in June 2014.
The Yamal LNG industrial facilities will stand on a forest of 38,300 primary piles driven to 10 to 28 meters deep, with a diameter of either 273 or 530 millimeters and a bearing capacity of up to 3,100 kN at -4°C, for the heaviest structures and equipment, and 26,700 secondary piles driven to 10 to 14 meters deep, with a diameter of either 159 or 273 millimeters.
Foundations adapted to a modular design
10,000 pile caps will connect groups of piles.
To limit the number of operations carried out in very cold weather conditions, the plant is built using prefabricated modules that are delivered directly by sea. Assembling the modules on the foundations led to noteworthy technical engineering solutions. Once installed, certain groups of piles are interconnected by a concrete “pile cap”. A total of 10,000 prefabricated pile caps (or pile heads) are attached to a group of piles to support the columns of the corresponding modules and to spread their weight over the piles that serve as their foundations. Certain pile caps can group up to ten piles.
The prefabricated pile caps comprise reentrants that allow the beams to be slotted onto the previously installed pile heads. The pile caps are connected to the piles by pouring very low shrinkage concrete into the reservation opening. The pile caps are designed to sit flush with the finished level of the platforms, thereby permitting the circulation of Self-Propelled Modular Transporters (SPMT) that are used for transporting the modules and installing them on their foundations.
An essential soil refrigeration system
The thermosyphon maintains the soil temperature below the -4°C threshold.
A total of 28,000 thermosyphon systems are positioned on the primary piles as close to the pile caps as possible.
The installation of thermosyphon systems helps refreeze permafrost, which could be affected by boring activities and the setting of concrete during the installation of the piles. The thermosyphon systems also compensate for the transfer of heat attributable to the plant’s construction and operating activities by maintaining a temperature that guarantees the full bearing capacity calculated for the piles, set at a maximum of -4°C for the plant’s operating lifetime.
In fact, the performance of the thermosyphon systems installed for the plant’s primary foundations should maintain the temperature of the soil below the -4°C threshold, as shown by the temperature curve of the LNG tank’s foundations. This curve highlights the essential contribution made by the thermosyphon systems to strong soil freezing: a reduction of up to -14°C over a year and a half, which should be compared with the temperature recorded during the construction of the foundations and prior to their installation, namely -3°C/-4°C.
This safety margin accounts for the future effects of climate change, which is expected to result in a 50% increase in the active layer of the permafrost under natural conditions (excluding insulating backfill and without thermostabilization) by 2050, per Anisimov’s application of the UKTR model (1997).
Compliance with strict tolerance values during the construction process
The driving of the primary piles into the permafrost is a delicate process. There are two principles that guide the activities, from boring to monitoring: the optimal positioning of the pile and the continuous monitoring of the temperature of the piles and cement, in order to limit the risk of thawing caused by heat transfer.