XII Международная студенческая научная конференция Студенческий научный форум - 2020


Текст работы размещён без изображений и формул.
Полная версия работы доступна во вкладке "Файлы работы" в формате PDF

When construction started on the Yamal LNG facility at Sabetta in 2011, the site had no access route by land or sea. The area is isolated – located in a remote wilderness on the banks of the Ob, a river that is ice-bound for eight months of the year – and far from any town or any oil industry infrastructure. It is pitch black for two months of the year. In winter, the temperature can drop to -50°C.

The problems posed by the environment on the Yamal Peninsula make logistics an unusual challenge, entailing as they do building a natural gas liquefaction plant with a capacity of 16.5 Mt per year, four 160,000 m3 storage tanks and an LNG export shipping terminal, all in just six years.

To achieve this, transportation and local accommodations for up to 20,000 people had to be provided, 150 prefabricated LNG plant modules each weighing between 200 and 8,000 metric tons had to be transported from Asian shipyards, and materials and equipment had to be supplied to the site, including more than 5 million metric tons of bulk sand, gravel, cement, concrete slabs, and more. One additional difficulty was that the bulk materials and modules had to be shipped within the short period when the Ob is not ice-bound, spanning four months of the year between mid-June and mid-October.

Large-scale marine logistics

Constructing Yamal LNG within the deadline required proper maritime access. Module deliveries began in September 2015 with a fleet of module carriers that was expected to be twenty-strong by the summer of 2016. Two of these carriers are ice class Arc4 and are thus able to reach the site until November.

Two ice class Arc7 module carriers with reinforced hulls were built that were unrestricted by the ice and able to deliver modules to Sabetta during the winters of 2014, 2015 and 2016.

Air transport and housing

The construction of an international airport in Sabetta was a key factor in opening up the Yamal Peninsula, a region with considerable gas potential. As operations increased at the site, it became impossible to continue transporting staff by helicopter given the size of the workforce. Construction work on the airport began as early as 2011. The airport, which opened in February 2015, now has a 2.6-kilometer-long and 47-meter-wide runway suitable for a Boeing 767. We are expecting approximately 150,000 passengers in 2016 – Yamal LNG staff or subcontractors, travelling via direct flights from Moscow, Samara and even Beijing.

Some 20,000 people worked on the construction site during the summer of 2016, housed in several specially built camps on the site. Thanks to Russia’s experience in Siberia and the Arctic, staff support logistics are running smoothly in this quintessential boom town, located in an area that had been essentially deserted until five years ago. The site now has its own power station, worker transport system, housing and recreational facilities, including traditional Russian banyas (saunas).

In 2018, the imposing port and airport hub, built from the ground up by logisticians for the Yamal LNG construction project, will continue to evolve while playing the same role of transporting LNG and providing transport for staff rotations. Fifteen Arc7 LNG ice-breakers, eleven conventional LNG carriers and two Arc7 Arctic oil carriers will then take over from module carriers on the sea routes of the Far North region. Around half of the cargo will be delivered to Asia, via the Northern Sea Route East in summer and via Zeebrugge in winter.

The Yamal LNG Foundations on Permafrost

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.

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 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.

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).

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. 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

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

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

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).

Thermosyphon system used for the foundations of the LNG tanks

A thermosyphon system is a cooling device that lowers the temperature of the soil by circulating a heat transfer fluid contained in a pipe inserted into the ground.

There are several different thermosyphon systems, known as active or passive systems depending on whether an external energy source is needed.

The passive thermosyphon systems chosen for the project are both cost-effective, as they function without the need for external energy, and reliable, as they require almost no maintenance.

Their operating principle is based on the natural circulation of a biphasic heat transfer fluid that is contained in the thermosyphon system. When the temperature difference between the ambient air and the ground is sufficient, the gaseous phase condenses and is brought down by gravity to the lower part of the thermosyphon system while the warmer liquid phase vaporizes.

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.

The high standard of execution achieved during the driving of the primary piles into the permafrost is key to guaranteeing the bearing capacity of the foundations, which is vital for the plant and an on-time completion of the construction schedule.

Drilling depth: must not exceed the required depth by more than 300 millimeters.

Maximum diameter of the hole: must not exceed the pile’s diameter by more than 150 millimeters.

Axis deviation must be less than 2% of pile height.

Excavation work: only carried out during the cold season to limit the heating of the soil, which is protected by a platform of sand at least one meter thick, and to facilitate drilling operations.

Temperature of the cement and sand mixture when poured to anchor the pile in the permafrost: a minimum of +10°C and a maximum of +15°C in the summer, a minimum of +10°C and a maximum of +20°C in other seasons. The temperature of the soil must then be checked before the modules are installed and the facilities are built.

Просмотров работы: 8