Modularization of Onshore Plants
Karthik Nurani
Deputy General Manager, Projects
Aker Solutions

Modularization is a plant design concept that is fast gaining acceptance across the construction industry as a means to reduce cost, improve quality and speed up construction. This article attempts to discuss the various factors that serves as drivers for modularization techniques, as well as the pros andcons of the concept.

What is a Modular Plant?
Simply put, a modular plant consists of a skid on which the various plant components such as equipment, piping, structures, supports, insulation, distribution boards, control consoles, fire alarm panels, cable trays and cabling and even light fittings are all mounted. Each module is thus a fully contained process unit and could represent an entire plant, or part of a plant. This skid is then transported to the project site and installed at the location with minimal effort.

Drivers behind the Modularization Concept
The traditional approach to design and construction of a plant involves in -situ construction of the facilities using locallysourced labor, resources and material. However, in specific cases, EPC companies nowadays are increasingly going for modularization.

The factors that influence the decision to go in for modularization are as follows:
  • Remote site locations: Remote site locations throw up a variety of challenges, especially in terms of availability of skilled labor, and the transport of materials to the job site. This is where the biggest advantage of module-based construction lies. A modular plant can be constructed in an altogether different location, far away from the job site, and then be transported to the site.
  • Hostile weather conditions: Vagaries of weather, such as extreme hot and cold conditions, pose a threat to productivity, worker safety and associated parameters such as quality. In such cases various parts of the plant could be modularized and fabricated at a location where more favourable conditions for construction exist, leaving only the bare minimum work and module interfaces to be carried out at site.
  • High field construction costs: Construction cost can form a major component of the overall project cost, especially if labor costs are high. It would therefore help reduce the project cost if field work is minimized and most of the work is modularized. The module could be fabricated in regions such as Asia or Eastern Europe where labor is relatively cost-efficient.
  • Crashed schedules: With modularization of a plant, work can go on in parallel on different modules. Even multi-storied structures can be handled in this manner. This can help in improving schedules as opposed to the linear construction sequence of stick-built designs.
  • Where repeatability or duplication is possible: Companies are already exploring the options of breaking up a large capacity plant into smaller modules of lower capacities. Thus an in-situ conventional natural gas liquefaction plant with a capacity of 2.5 million metric tons per year (MMtpy) could be broken into 10 typical modules, each of 0.25 MMtpy capacity,shaving off an entire 12 months from the construction schedule.
  • Where reusability is a factor: In a bid to be competitive in an already crowded market, companies are looking to reduce costs all the time. Modules designed to be reused across locations can greatly reduce capital investments.
Modularization as a concept has been used widely in the offshore industry, but is now making its way into the onshore industry for the reasons stated above.

At the same time, some projects may not lend themselves to modularization .

If the project site is in a location where infrastructure is good and skilled labor is abundantly available, there may be no point in using modularization.

It is to be noted however that while modularization can complement in-situ site work, it cannot totally replace it. Certain activities such as site grading, civil foundation work, large equipment erection and interfacing between the modules can only be performed on site.

Planning and Execution:
While a modularization strategy seems to offer all the solutions to typical in-situ construction problems, it is important that it is thoroughly planned out during the early and detailing phases of the project, and engineered efficiently.

A few important points to be considered are:
  • Evaluation of the strategy: While modularization can have major cost and schedule benefits, it could lead to complexities if not evaluated properly. The plant must be of a nature such that parts of it can be modularized. The added design and coordination costs along with the transportation costs must be carefully estimated. The modes of transportation that are likely to be employed (truck, rail, barge etc.) and the availability of a roll-on/roll-off (RORO) jetty in port or at site must be determined at the beginning of the project as they also have an impact on the way design is carried out. Ultimately the modularization must provide definite cost benefits over the in-situ method.
  • Deciding which part of the plant lends itself to modularization: The dimensions and weight of a module have a direct bearing on the shipping cost of the module, and must therefore be evaluated carefully. For example, very long or heavy equipment cannot be a part of modularization as they would result in large module sizes. These would be difficult to transport, and in fact may end up being more expensive than the in-situ method. In projects which involve extremely tight schedules, multiple module fabricators may be considered, but the coordination between them is critical.


    Figure 1: 3D Model of a process module for an onshore plant. Note the extent of detailing that has been carried out.
  • Advancement of engineering activities: One of the success factors in employing modularization is to have the modules delivered to the construction site as soon as the foundations or structures to receive them are ready. Modules therefore have to be engineered in advance so as to extract the full advantage of the concept.
  • Minimizing revisions to documents: As mentioned above, the construction of modules should start early in order to have them delivered early. Engineering revisions can have a negative impact on the module progress, and must be avoided. A project that is likely to see concurrent engineering may be a bad candidate for modularization.
  • Procurement: When using modularization, it may be a good idea to let the module fabricator procure the majority of the module components so as to reduce interfaces within the module. This will also give the fabricator a freer hand in developing the module.
  • Monitoring: Adequate monitoring of the module constructor's progress is essential, though too much interference can be counter-productive. In case the purchase of materials is not in the module builder's scope, it is important to monitor the supply of materials to the module minutely to ensure that module construction is not affected.
Design Considerations:
While modular designs do not differ greatly from conventional design methods , they need some special design considerations. Some of these are listed below.
  • A clear scope split between the module vendor and the contractor is essential. The scope document must clearly define the contents of the module as well as the termination details. Since the module vendor will be working with limited space, it may not be possible to accommodate scope additions at a late stage without causing a major impact on cost and delivery. The termination details such as pipe ends, flanges and junction boxes should be located towards the edges of the module so that field construction has minimal (or preferably no) interface inside the module.
  • The module size and weight are the governing factors and will drive the layout, rather than the layout considerations driving the module size. The size of the transporting truck or the capacity of the barge that is used to transport the module will usually drive the design. The module vendor will therefore work within a very limited space.
  • However modularization does not provide any concessions on the basic safety distances to be maintained between facilities within the module, access and egress requirements, operability and maintenance requirements . All these are to be met within the given size, and this poses the biggest challenge to design.
  • Modules may be oriented differently during module construction and transportation as compared to the final position in the field. It is therefore extremely important for the design team to be aware of the fabrication and transport details and account for any additional forces in design.
  • Modules will be subject to lift forces as they are being lifted for transportation and erection. Such lifting points must be clearly identified and the corresponding forces determined. The modules will also encounter acceleration forces during transport, including pitching, surge, sway and rolling if they are being transported to the job site by ship or barge. The structures must thus be designed not only for design and operating considerations, but also for lifting and transport.
  • In other words, modular structures need to be designed to be safe in the assembled module as well as in the final erected form. Additional strengthening may be provided to avoid damage to the structure under different circumstances. On account of the additional considerations of orientation, lifting and transport, modular structures usually use more steel than in-situ ones.


    Figure 2: Modular pipe racks being erected at site. The alignment of the foundations is extremely important in such cases as a mismatch would directly result in misaligned pipes at the interface. Note the gaps between the pipes in the module. A make-up piece is field-welded in the gap and takes care of any site tolerances.
  • Weight management is an important aspect that often gets overlooked while designing modules. Determination of the weight and the center of gravity (COG) is a key component in the weight management process. The weight and COG are also important factors for transportation as they will proportionately increase acceleration forces. Some allowances should be made in the weight and COG calculations initially so as to provide a degree of flexibility during design. These allowances can be gradually reduced during the course of detailing when more definitive information is available.
Pros and Cons:
Based on the above, the pros and cons for the modularization concept can be summarized as follows:
Pros:
  • Higher productivity
  • Shorter construction schedules
  • Heavy construction equipment such as cranes need to be deployed for a shorter duration
  • Minimum field inventory
  • Better quality
  • Increased safety
  • Cost benefits
Cons:
  • High module transportation costs
  • Heavier cranes and machineries to be deployed for construction to handle complete modules as compared to individual components
  • Heavier structures since transportation and lifting must be factored into design
  • Detailed planning is required upfront
  • Increased coordination required between multiple agencies
In short, modularization adheres to the ageold principle that it is prudent to spend extra efforts and resources in the engineering office rather than spending more time at construction sites!

Modularization can be a cost and time saving alternative to the conventional insitu construction methods. With good initial planning, a clear scope split and proper follow-up, it can be a winning strategy.