Steel Modules — Practical Approach of Engineering and Design

Background

Large industrial projects have historically proven difficult to complete on time and schedule. Field construction is predominantly risky execution. As projects progress and fast tracking becomes the norm, it becomes increasingly important to take steps to minimize field work. In recent years, modularization has been used as a risk management technique. This improves productivity, quality and reduces indirect costs.

The field construction portion of a major project accounts for approximately 40–50% of the total cost. The construction phase is also subject to some of the highest schedule risk and is heavily impacted by labor availability and efficiency, as well as weather conditions. By adopting a strategy to relocate field hours into fabrication shops, some of the mechanical, electrical and instrumentation work can be pulled ahead in the schedule and executed in parallel with the civil. Furthermore, most modularization facilities are located close to reliable sources of qualified labor.

Additionally, the productivity and progress for stick-built field construction makes the project cost 1.5 times costlier, especially in harsh weather and remote area. There is additional expense and to fly the labor to construction sites. Also planning the modularization early on the project saves engineering cost as well, as it tends to standardize the modules. Entire modularization strategy needs to be communicated with client and all stakeholders.

Modularization increases the direct costs when transportation and other factors are considered. Modules will typically use a larger amount of structural steel to accommodate shipping and lifting requirements. Furthermore, more engineering is required. However, the reduction in indirect costs easily offsets these factors, especially where a remote site requires a camp for labor. Modularization increases cost and schedule certainty in the field.

Engineering Execution During Modularization

Many of the decisions surrounding modularization will have to be made using preliminary and historical information from other projects. When vendor information is received, the assumptions around modularization will have to be validated. Until bids are received and some equipment is sized, all modules are preliminary. It is easier to remove a module later than to add one, however.

To meet the early project milestone dates required to assemble modules, engineering have to be released at an earlier stage than usual. An investigation into detailed schedule requirements becomes essential at feed phase. Estimated risks may have to be taken, including potentially advancing portions of the 3D model reviews before all conditions are met. This may result in rework and material waste. Finalizing changes at an agreed point in the project is critical to a successful module program.

Pros of Modularization

· Better quality control can be accomplished in the module shops.

· When the construction site is at remote location with no permanent infrastructure in place.

· It reduces interference with the construction contractors onsite.

· Improved Safe execution and module building.

· The controlled environment of a module shop combined with less elevated work means less incidents on a per-hour basis as well.

· Testing and Engineering verifications become easy. testing and checkout can be completed in the module yard. This saves time at the end of construction, when the schedule is the most critical. The ease of access at the module yard makes frequent visits less expensive and time consuming.

· Access into module yards is very good, so there is less risk delivering materials there than at site. The site material management program is much simpler and a cost savings in laydown and warehousing space can be realized.

· The module building saves the scaffolding significantly since the large modules are being built at grade in module yards.

Cons of Modularization

The time and cost savings due to modularization also has some risks which needs to be carefully evaluated.

· Engineering is required much earlier in the project. It requires pulling in other mechanical/piping/electrical/Instrumentation/tracing work while modules are being designed is critical to be managed. Experienced and high caliber engineers are required to make right decisions early on the project, as there are very less chances of redesign.

· Modularization necessitates extra engineering than traditional stick-built construction. Much better interdisciplinary collaboration is required to ensure that everything fits together in the field, and more drawings are required as well.

· Long lead material and equipment commitments must be finalized very early and on preliminary engineering information.

· Layout and module key plans are developed early on without detailed equipment and piping information. Information becoming available later in a project may affect the modularization program, which can be very expensive.

· A higher standard is required of the module yard operator than would be if pipe racks alone were being modularized. The contractor must organize and coordinate all subtrades (structural, mechanical, insulation, electrical, instrumentation). Markup on subcontracts will apply as well.

· Modules demand additional steel and support during transportation due to longitudinal and transverse acceleration loads from transporting vehicle eg. Ship, Rail or Road.

· Maximum module weights and dimensions limit the possibilities of optimizing, and exceeding weight is not an option. Regulatory permits are extremely expensive and are often not available. These can be deal breaker, hence the approval with the municipalities required.

· Components can be damaged in transport. Thorough inspections in the field are critical to finding problems when the module arrives instead of later in the project.

· When work is done by several vendors, careful coordination is required to ensure consistency. Unless clear expectations are set for specifications and standards, rework may be required in the field.

· Modularized equipment will change hands several times before returning to the owner’s care and custody. A clear, concise preservation program will be critical to ensuring that the equipment is in good condition with appropriate documentation when it is time for startup.

· Transportation of modules requires close coordination between shops, permitting, and site. Ideally, the modules are set directly on their foundations when they arrive at site. If this is not carefully planned, expensive module setting rental equipment can end up sitting idle.

Design Parameters (inputs) for Analysis.

· Applicable Building Codes

· Standards Council for Design

· Design Handbooks

· Client Standards and Specifications

· Geotechnical Reports

· Computer Software for Analysis

· Meteorological Data & Site Specific Climatic & Seismic Information

· Material to be used for Piles, Concrete, Anchors, Support Structural Steel

Engineering considerations for software inputs.

Structural system includes moment frame for piping modules in transverse direction, braced frame along longitudinal direction. Horizontal & longitudinal vertical bracings are provided to transfer lateral loads to the foundation. Column bases are considered to be pinned connection. Module structural system, dimensions, location of horizontal / vertical bracing & anchor bays shall be sketched before inputs in the software.

Basic Load Cases and Combinations.

In-Situ Analysis and Design Load Cases

a) Self Weight (SW)

No fire-proofing consideration

b) Pipe Empty Load

Apply empty pipes/equipment loads are applied as per design criteria at each pipe supporting level.

c) Cable tray Load

Weight of cable tray including cables — 1.0 kPa uniformly distributed loadover full module at each cable tray supporting level.

d) Pipe Operating Load

For pipes operating 2 kPa uniformly distributed load over full pipe rack per design criteria at each pipe supporting level.

e) Pipe Friction Load

Friction loads consist of longitudinal forces due to temperature effects. Following minimum longitudinal forces are applied on the pipe rack:

• 30% of the total weight (dead + content) of one heaviest pipe
• 20% of the total weight (dead + content) of 3 large diameter pipes
• 10% of the total weight (dead + content) of more than 3 pipes

f) Wind load on structure and Pipes (WL)

Modules are designed for site specific wind effects. No wind load on pipe rack is considered along longitudinal direction. Tributary area for pipes is considered per ASCE petrochemical facilities. Shielding is not considered on pipes and structures.

g) Stress load: Anchor (AL) & Guide load (GL)

Stress loads consist of anchor and guide loads. These loads are applied on the modules as per inputs from other disciplines.

Please note the following:

• Anchor loads and friction loads shall be applied in same direction.
• Wind loads and Pipe Guide loads shall be applied in same direction.

h) Thermal loads (TL)

Thermal loads on structure, due to environmental conditions, are not considered on module. These need to be considered for very long modules only.

j) Hydro test Load (HL)

All pipes are considered to be filled with water during operating onditions. Hydrotest loading shall be ≤ equivalent operating load

k) Snow Load (SL)

Snow load is calculated in accordance with ASCE.Snow load on modules is calculated based on gross plan area. 100% of calculated snow load is applied at top level.

l) Platform Live load (LL)

Apply the live loads as per agreed design criteria with the Client.

m) Special Loads (If applicable)

Also apply the additional special loads as required by design.

Transportation Analysis and Design:

Module are generally transported by 2-file trailers with transport beam under columns. The transport envelope is finalized at this point.

a) Vertical Impact Load (IV) — 50% of (self-weight of structure + Empty weight of pipe + Cable tray)

IV = 0.5 X (SW+DL1+DL2) acting vertically downward

b) Horizontal Impact in Longitudinal direction (IL) — 25% of (self-weight of structure + Empty weight of pipe + Cable tray)

IL = 0.25 X (SW+DL1+DL2) acting in longitudinal direction of module

c) Horizontal Impact in Transverse direction (IT) — 20% of (self-weight of structure + Empty weight of pipe + Cable tray)

IT = 0.2 X (SW+DL1+DL2) acting in Transverse direction of module

d) Wind load (WLT) — Wind load is applied on pipe rack module structure, pipe and cable tray, in Transverse direction.

Factor of safety against overturning during transportation shall be > 1.5

Module Lifting Analysis and Design:

Modules are usually be lifted at 4/6/8 column points depending of size, weight and number of cranes to be used.

Vertical Hoisting impact factor load = 1.5

Vertical Hoisting impact factor load = 1.5

Additional Considerations for Computer Model.

· Prepare computer model for analysis / design using Authorized computer programs. Verify the inputs and confirm Boundary conditions, Member restraints / releases, Units, Young’s / shear moduli and ensure the system is in static equilibrium.

· Review computer output and apply approximate manual checks to audit accuracy.

· Check Structural stability to ensure that all loads have been transferred to foundation.

· Review section sizes ensuring sections are suitable for connection design.

· Update computer input to reflect revised sizes and run code check program.

· Design column bases and size anchor bolts.

· Design critical connections for Fabricator.

· Produce engineering sketch C/W connection loads if loads exceed standard connection loads

· Finally, Self-check calculations and obtain required checks and approvals before issuing to Customer.

Key Considerations for Structural Drawing preparation and drafting instructions.

· Use Authorized computer programs and draft the drawings with suitable scale, title block, location, north direction etc.

· Call out the Bill of Materials.

· Review and thoroughly check the member size and dimensions.

· Submit for Customer Review and Fabrication.

· Review section sizes ensuring sections are suitable for connection design.

· Update computer input to reflect revised sizes and run code check program.

· Design column bases and size anchor bolts.

· Design critical connections for Fabricator.

· Produce engineering sketch C/W connection loads if loads exceed standard connection loads

· Finally, Self-check calculations and obtain required checks and approvals before issuing to Customer.

Modularization Planning and Management

Below Stakeholders have a very critical role for modularization success. Inputs and review from all parties ensures clear, accurate and timely completion of the project.

Project Director | Construction Manager | Interface Manager | Fabrication Manager | Project Controls Manager | Project Scheduler | Materials Manager |Cost Analyst | Quality Manager | Inspector | Procurement Manager | Contract Administrator| Transportation and Logistics Specialist |

All strategies and opportunities with respect to fabrication and module installation must be reviewed periodically. The defined processes, practices and protocols established for the management plan needs to be updated as well.