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Apr-2015

Modular construction for harsh climates

Severe Arctic conditions were overcome by the construction of small scale gas processing units in prefabricated modules.

THIBAULT HEIM, Benoît MARèS and Vincent SIMONNEAU
Prosernat
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Article Summary
Estimates show that in 2013 Russia flared more than 17 billion  cubic metres of associated petroleum gas, about 25% of the federation’s total associated gas production.1 Reducing flaring requires investment in associated gas treating and recovery as sales gas. The resulting benefits are a reduction in greenhouse gas emission as well as financial reward from associated gas marketing. A gas recovery rate of 95% (gas flaring rate 5%) is commonly set by Russian political authorities as a policy target for flaring. Associated petroleum gas generally requires sweetening and sulphur recovery facilities among other treatments (dehydratation, dew point control, compression, liquefaction) before it is marketed.

More than half of Russia’s oil and gas production is concentrated in the West Siberia region, characterised by its remoteness and particularly harsh climate. Achieving a new gas treating project in these areas involves challenges in design, logistics and construction that can be solved by a modularisation strategy.

The concept of modularisation

The purpose of modularisation is to provide a completely prefabricated, skid-mounted unit, thus minimising site works. Each individual skid is generally designed as the largest transportable component of the facility. The assembly of several skids results in the building of one module. One unit may be composed of several interconnected modules.

Module construction is performed in a workshop or yard remote from the final project site and equipped with adapted manufacturing and personnel capacities.

The extent of skid pre-fabrication is typically:
• Structural steel works: base frames and secondary structure are assembled
• All equipment is installed and connected
• All piping and valves (manual and automatic) and piping accessories (supports) are installed
• Skid interconnecting piping spools are adjusted during a workshop trial assembly – as extended as possible – and delivered loose
• Depending on transport limitations, tertiary structure – such as stairs, ladders, support gratings – are delivered installed in the skids or loose but ready for site erection. All instruments are fully installed and connected with instrument air and cables up to skid junction boxes. Cables are supplied laid down on their cable trays with extra length to allow final connection to the junction boxes when necessary
• Power and lighting cable trays are installed; remaining site works consist of connection to the electrical network
• Final painting and insulation are carried out and installed.

Each individual skid is fully assembled and tested (equipment hydrotest, piping hydrotest and cable continuity tests). Inter-connected piping spools and structural steel are adjusted during trial assembly.

Why choose a modularisation strategy?

A new plant erection requires skilled manpower and adapted construction means. Depending on the project context, a modularisation strategy offers numerous advantages versus on-site manufacturing.

The delivery of a complete skid-mounted module is a key choice in the following contexts:
•   Remote areas, with complex access for construction means and lack of manpower
• Extreme climates, where working possibilities are restricted by seasonal constraints
• Lack of labour quality and availability
• Offshore sites, where on-site manufacturing is hardly possible and plot area optimisation is compulsory
• Existing plants, where on-going operations are an obstacle to safe on-site manufacturing.

A modularisation strategy is an asset with the following project drivers:
• Shortened schedules, with requirements to market the products rapidly
• Cost savings
• Improved safety requirements
• Tense project location or unstable political context
• Limited work at site.

A successful modular project involves the contractor’s ability to manage simultaneously various interfaces with the client, the detail engineering subcontractor and the skid manufacturer. Each project is unique and has to be tailored according to specifications and availability of transportation. A modular construction offers a potential reduction in schedule and investment during project execution, as well as a product with HSE advantages. The contractor’s experience and know-how are crucial to take full advantage of a modularisation strategy.

According to transportation constraints and unit capacity, a sulphur recovery unit can potentially be delivered fully modularised. The equipment dimensions will reasonably allow a modularisation strategy of up to about 100 t/d of sulphur per train, and even above if transportation is feasible. Modularisation feasibility should be studied case by case according to project context and drivers.

Reduced schedule
The modularisation strategy offers schedule reduction possibilities during the front end design phase as well as during project execution. During the front end design phase, the schedule normally allowed for the execution of detailed studies, preparation of technical requisitions, requests for quotation of equipment suppliers and EPC contractor, as well as evaluation of offers and placement of orders. When the technology is supplied by the contractor, layout and modularisation studies are already anticipated during basic engineering studies. Combined licensor and contractor know-how allows earlier start-up of detail engineering studies, limits the rework and results in several weeks‘ schedule reduction.

A 25% schedule reduction is usually achievable during project execution when modular construction is considered (see Figure 1). Most of the labour intensive tasks are achieved in a workshop or on a yard chosen for its favourable and efficient working conditions. Labour can be adapted according to the requirements of the execution phase. Construction permits are 
not necessary and there is no interaction with on-going site operations.

A major reduction in the schedule is obtained because construction can be performed at the same time as the construction phase on site (civil works for accessibility, foundations, piling). By the time the modular unit reaches the owner’s site, the field is ready for module installation and connection with existing facilities.

Investment cost reduction
By maximising workshop construction and assembly, full advantage is taken from an efficient and organised work progression. Higher labour productivity is achieved and supervision time with a trained and skilled workforce is reduced. With fewer site activities during construction, savings are also observed regarding campsite installation and manpower logistics.
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