Modularization involves prefabrication, installation and precommissioning of a complete plant unit at a location remote from the construction site with the objective of reducing on-site construction, cost and schedule. However, modular construction introduces certain risks in the project which need to be appropriately addressed to ensure that the benefits of modularization outweigh the risks.
Potential Benefits and Opportunities
Modularization may not necessarily yield reduction in engineering design effort, fabrication effort, quality assurance and quality control effort or project management costs.
However potential benefits may accrue in other areas such as reduced on-site work, improved safety management and compressed project schedule. Following potential benefits may be realised by adopting modularization on the project:
- Improvement in construction schedule due to deployment of multiple construction contractors working in parallel.
- Flexibility due to multiple work fronts and multiple construction locations.
- Reduction in construction effort at site due to reduced on-site fabrication and installation.
- Improvement in safety and safety management due to reduced site personnel and construction personnel at a single location.
- Improvement in construction quality and productivity due to fabrication being executed in a controlled fabrication yard/workshop environment.
- Reduction in commissioning schedule due to pre-commissioning of individual modules being carried out off-site to the extent possible.
- Potential to take advantage of low cost centers and specialised skills base.
- Site civil works are not required to be in the construction critical path.
Potential Risks and Constraints
Following are potential risks and disadvantages of modular construction:
- Due to multiple work sites, additional resources are required to be mobilised for project management, contract administration, quality control, inspection, expediting and precommissioning.
- Increase in structural cost due to modification of design to suit modularization and transportation of modules.
- Increase in cost of transportation and logistics due to multiple site locations.
- Increase in procurement effort due to tracking, receiving and storage of materials at multiple locations.
- Increase in material management effort as material take-offs have to be broken down by module.
- Design changes at a late stage can have a significant adverse impact on project cost and schedule.
- Since transportation barges have to be booked in advance, there is less flexibility in absorbing schedule delays. Consequently, shipping of incomplete modules and transferring of remaining works at site can have significant adverse impact on project cost and schedule.
Modular Design Assessment
The overall plant layout and equipment layout will be different for a stick-built design as against modular design. Hence it is critical to conduct a modularisation study at an early stage of the project to decide on the type and extent of modularization to be adopted and to identify any impediments in adopting modularization for the project. An initial screening for modularization should be made early in the project during the conceptual phase (FEL-1). The modularization strategy should be subjected to further assessment at DBSP (FEL-2) phase. A modularisation assessment and implementation report should be prepared to document the project strategy for adopting modularization and to ensure that it is in alignment with the overall business objective. Conversion from stick-built design to modular design should not be made during the FEED (FEL-3) stage of the project.
Assessment for modularization should cover the following aspects:
Process Design
Modularization may require revisiting the process design to optimize the equipment sizes to fit the module size/weight limitations without loss of any functionality.
Transportation and Logistics
The maximum size of module that can be transported is governed by the following factors:
- Existing site constraints apply when adopting modularization on brownfield projects. Access clearances within the site are crucial. Thus even if the module gets transported to the site location, if getting access within the site turns out to be a bottleneck, the module simply cannot be installed at the intended location. This requires a careful study of the maximum module size that can be safely transported in a brownfield site. Greenfield projects have lower degree of constraints with regards to shipment of modules within the site depending on the sequence of installation of the modules.
- Whether access to the site is by road or sea and limitations of the transportation equipment. If access to site is by road, the maximum acceptable size and loads along the transportation route will determine the maximum size and weight of the module. The transportation route may have restrictions on the height or weight limitations on bridge crossings. If access to the site is by sea the shipment of module will done by barge and allowable shipping weight on the barge can be a limiting factor.
- The available crane capacity at both site and fabrication locations can also contribute to limitations in modularization.
- The allowable width and height of the module can be constrained due to limitation of the transportation carrier (barge or truck).
Early Engagement with Fabricators and Vendors
An early engagement with fabricators as regards their fabrication capacity and modularization experience will facilitate the decision to proceed with modularization or otherwise. Modularization approach may also necessitate early engagement with vendors to ensure that procurement of equipments is done in a timely manner to allow modularization execution to be carried out without minimum hindrance.
Interface between Modules
Proper interface between the different modules with respect to various items related to piping, structural, electrical, instrumentation, mechanical is important to ensure that they fit-up properly at site. Attempt should be made to keep the interfaces between modules as simple as possible with provision for installation tolerances. It is recommended to make adjacent structural modules structurally independent instead of obtaining a fit between the two modules at site.
Critical Success Factors
- Screening and Assessment of Modularization strategy should be conducted at an early stage of the project.
- The full potential of modularisation can be realised by maximizing the extent of modularization rather than adopting partial modularization.
- Sufficient float should be allowed for activities at the fabrication yard. Any delay in the schedule may require modules to be shipped incomplete and remainder work to be transferred to on-site location.
- Freeze design at an early stage and minimize changes. Expedite vendor data to finalize design at early stage.
- Engage with vendors who have successfully executed modular design projects and implement their lessons learned.
- Ensure fabrication of modules with a high level of dimensional accuracy to assure proper fit-up on site. This can be achieved by using tight tolerances and taking measurements from a single reference point to avoid build up of tolerances.
- Continuously monitor dimensional and weight control to ensure they stay within limits.
Piping Design Considerations
It is suggested to have a work break down structure by module. It is recommended to prepare the 3D plant model and engineering deliverables by module. This strategy will enable the extraction of isometrics, general arrangement drawings, bill of materials and procurement related activities to be carried out by module.
Dimensional and weight control are critical to module fabrication and delivery. It is suggested to plot the shipping envelope as a volume in the 3D model to ensure that the overall module dimensions are not exceeded. All valves and special items should be modeled as accurately as possible.
Fabricate piping to the maximum extent possible including small bore piping to avoid any on-site fabrication. Field routing of small bore piping will defeat the modularization intent.
All piping dimension should be measured from a single reference to avoid tolerance build up.
While designing pipe supports consider the stability of the piping system during transportation. Identify all temporary supports required only for shipping. All supports should be designed for dynamic loads that the system may incur during transportation. Maintain a separate log of the temporary transportation supports so that this log can be used a checklist for dismantling the temporary supports after module installation on site.