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Planning and operating a new freeze plant system for EuroChem’s Gremyachinski shaft sinking project

The freeze plant for EuroChem’s Gremyachinski shaft project has now been in operation since the end of December 2009. The freezing process could only begin after the planning and assembly work had been completed and all the permits and licences had been obtained for the sinking project.

Planning phase

After analysing the results of the geological and hydrological investigations and identifying the most appropriate sinking method it was decided that the freeze zone should be established to a depth of 520 m. This meant carrying out a detailed programme of FEM calculations that took account of the stress and deformation behaviour of the frozen and non-frozen ground for three different rates of sinking advance per working day. On the basis of these findings Thyssen Schachtbau GmbH (TS) established that a freezing performance of 4,500 kW would be required at a brine temperature of -38 °C. TS also opted to employ a high-performance refrigerant for this particular project so that, if operating conditions and the client’s remit required it, the coolant temperature could be adjusted without interrupting the freezing process.

Work commenced at the same time on the design and planning of the freeze plant. Because of the geographical location and the tight construction schedule it was decided early on that a modular system would be used so that on-site installation costs would be kept to a minimum. TS therefore planned to use container-housed freezing machines and as many prefabricated pipeline assemblies as possible. The planning team eventually opted for ten containerised freeze units and a 1,400 m-long pipework circuit that was to be pre-assembled and pre-insulated on the surface. The required refrigeration output was also to be further reduced by optimising the brine circuit routing. This meant that the expensive control systems used on earlier freeze-shaft projects would not be needed in this case. The result was an even freeze-wall buildup across the entire array of freeze pipes, which in turn resulted in a shortened freezing phase.

The decision to use a modular system with a high degree of prefabrication meant that the freeze plant was ultimately able to start operating just three months after the completion of the freeze holes and in parallel with the foundation construction work and the preparations for the foreshaft sinking.

Freeze-plant operation, control and monitoring

TS is using the latest measurement and control technology for the operation of the freeze plant, which actually allows the installation to be remotely monitored from Germany. All the data are collected and processed in the on-site control room. Brine leakage into the strata – which can result from unexpectedly high convergence movements that in turn can lead to freeze-pipe failure – can now be diagnosed promptly thanks to the use of state-of-the-art technology. Immediate steps can therefore be taken at any time to correct malfunctions at the freeze plant.
 

TS is also employing innovative methods for measuring the freeze-wall buildup in the strata. This involves using laser technology – a technique never before tried on a freeze shaft sinking anywhere in the world – with a resolution power of 1 metre (also an industry first) to measure the ground temperature over the entire depth of the freeze shaft. This temperature survey, combined with other measurements taken during the freezing process, enables the current thickness of the freeze-wall to be determined on a simultaneous basis over the entire depth range – information that is available to the freeze-shaft sinking team at any time. TS has used its extensive knowledge and expertise in freeze-shaft sinking to develop special software that can provide numerical and graphical information on the shape and size of the freeze-wall as and when required. Voids in the freeze-wall are therefore almost completely eliminated. The software can also predict the further development of the freeze-wall so that the ongoing freezing process can be adjusted and optimised if necessary. If the flow rate or temperature in any of the individual freeze pipes deviates from the corresponding mean value the digital measurement system ensures that this information is relayed immediately to the freeze-plant control panel.

EuroChem, the Russian authorities and Thyssen Schachtbau worked closely together to ensure that the operating permit required for this state-of-the-art freeze plant was obtained without delay.

This innovative freeze-plant system, which is the first of its kind to be employed in a EuroChem shaft sinking project, has given TS another technical lead in sinking mine shafts under cover of a protective freeze wall. The successful application and control of the new freeze-shaft sinking technology, where identifying irregularities in the freezing and sinking process plays a key role, depends on continuous and precise coordination of the different operations being carried out by the respective shaft sinking and ground freezing teams.

 

Mülheim an der Ruhr, May 2010

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