Wednesday 30th Sep, 2020

Designing modular process plant buildings

Process plant design and installation involves a number of special loading conditions not covered by Australian Standards. David Arnold from Aspec Engineering explains how advanced structural analysis is required for such a task.

Process plant design and installation involves a number of special loading conditions not covered by Australian Standards. David Arnold from Aspec Engineering explains how advanced structural analysis is required for such a task.

Port Pirie, a multi-metals plant located in South Australia, is one of the largest primary lead smelting facilities and silver producers in the world.

To reduce emission levels at the plant, the site required a new furnace building, which would house a new top submerged lance (TSL) furnace and waste heat boiler. Measuring in at 75 metres high on a 30- by 30-metre base, the building would comprise of 4000 tonnes of structural steel, 2000 tonnes of mechanical and process equipment, nine major modules, interconnecting flat-pack floor panels and modular exterior cladding panels.

The furnace building itself was constructed in a module yard in China, with each module fitted out with mechanical equipment, process piping and electrical equipment. At this stage the 200-tonne waste heat boiler was installed, which was delivered in six sub-assemblies across three building modules. The boiler’s ancillary equipment such as feed water pumps and steam drum were also fitted, as was the coal injection equipment, which included a coal silo and pneumatic powder handling equipment.

Related stories:

Assembling the modules in China was an important method of avoiding interferences with the smelter’s operation. The Port Pirie smelter is a crowded, brown-field site, with infrastructure buildings and operational process plant surrounding the site of the TSL building. This also meant the logistics of unloading the modules at the smelter wharf was key, as transporting them through the existing plant to the building site would require strict design limits for module dimensions and mass.

Australian design standards are written primarily for the requirements of designing conventional buildings for human occupation. Design loads for buildings are typically limited to a small number of simply defined action types. The complexities of process plant design require that these load types be expanded to separate dead loads and imposed loads into sub-actions.

For transport and erection of a modular building, additional actions must be considered.

Sea transport

The furnace building modules were transported from China to Australia on heavy lift ships. Wind and wave actions on the ship imposed large inertia loads on the modules. These loads were often governed the design of columns and bracing, particularly for modules located higher in the building. Additional temporary bracing was usually necessary to resist the lateral inertia loads.

To secure the modules for sea transport, they had to be securely lashed to the deck of the ship.

Self-propelled modular transporters

The modules were transported on land using self-propelled modular transporters (SPMTs). Each module was provided with a temporary grillage to support the modules on the SPMTs. SPMTs are used around the world for moving large and heavy loads. By arranging transporter modules together in the required configuration, modules of almost any size could be handled.

The design actions imposed during SPMT transport are similar to those described for sea transport. However, the slow, controlled movement of the SPMT group imposes much smaller design accelerations.

Lifting

The furnace building modules were designed for a four-point crane lift that involved loading and unloading from the ship for installation. A four-point lift was the basis for the design, with vertical slings attached to nominated lifting points on the modules. Standard, 200-tonne, working load limit pad eyes were bolted to the nominated lifting points while slings and spreader bars were arranged to distribute and share the load between them.

Additional temporary bracing was necessary to transfer the module loads to the lifting points.

The process equipment was a large proportion of the live loads imposed on the TSL furnace building structure. The loads were assessed and applied to allow for the range of operating scenarios that may occur during the life of the plant.

An area live load of five kilopascals usually provides an adequate allowance for personnel access, material laydown, and process spillage on the operating floors of a process plant. To cater for the special maintenance requirements of the TSL furnace, floor live loads of up to 25 kilopascals were applied in specifically nominated areas of the furnace building.

Wind loads, including allowance for dynamic effects, were applied to the TSL furnace building in accordance with AS1170.2. Different design wind speeds were used for the various phases of the building life.

The TSL building is supported on deep piles bored into deep soft clays. The soils, in combination with a relatively high site hazard factor, resulted in relatively high seismic loads by Australian standards. However, for overall design of the building, wind loads exceeded the seismic loads.

Analysis of the different structural configurations, boundary conditions, and transport loading scenarios would typically require breaking up the building analysis model into separate models for each module, with individual models modified to suit the shipping and transport loading requirements.

This approach is both time consuming and a potential source of design error. With suitable software, the analysis and design of the individual modules, and of the completed building, can all be performed within the one model.

Software

Analysis of the building was performed using Strand7 software. Strand7 provides a number of advanced modelling features which were particularly useful for the analysis of the furnace building, such as definition and analysis of multiple boundary conditions, definitions of analysis stages and the ability to graphically copy and paste models into other models.

Design of the steel framing members was completed using BCDsteel software. BCDsteel accesses the Strand7 analysis results to perform code checks in accordance with AS4100, and provides graphical output of member capacity utilisation ratios.

Modelling strategy

The magnitude of the design task required the workload to be shared across a team of structural engineers. To ensure efficient coordination of the team effort, a modelling strategy was implemented.

This included the creation of a master model template that incorporated a standardised section library, primary framing to define building grids, floor levels, columns and module splits, and primary load cases for all loads. In addition, the master model included element groups, analysis stages, load combinations and freedom cases for all design scenarios.

Individual engineers developed detailed designs for individual building floors using the master model template, which they then copied into the master model. Following this, designs for transport bracing and grillages for the individual modules were also developed and included in the master model.

A column and bracing design was then created before a final code check for all design scenarios was performed in the master model. Any subsequent design changes were implemented only in the master model.

Load application

The transporting and handling of the modules involved inertia loads in various directions. By using non-structural masses for all structure and equipment dead loads, in conjunction with relevant accelerations, the application of these inertia loads was greatly simplified.

The use of non-structural masses also enabled the mass and centre of gravity of the modules to be directly extracted from the model.

Analysis stages were defined to enable the building and modules to be analysed using the one model. For each stage, only the relevant parts of the model, together with the appropriate freedom case, were activated.

The use of stages also simplified the application of moving loads to the building. For each moving load, a single primary load case was defined, with the loads attached to the model via links. Stages were defined to activate or suppress the links for the different load positions.

Analysis of the furnace building for all scenarios required 94 separate analysis stages and 1171 load combinations. This applied to transport, lifting, operating loads, equipment moving loads, wind loads, and seismic loads.

The load combinations defined in AS1170.0 do not adequately cover the complexities of the loading scenarios applicable to process plant design. Imposed loads for peak operating and extreme operating conditions in a process plant have a lower probability of exceedance than the normal operating loads.

For example, it is virtually impossible to exceed a flooded process vessel load that is based on upper bound values for both filled volume and contents density.

Weight control

The logistics of each phase of construction, from module yard to final erection, require careful planning to ensure that the rigging and handling of modules is carried out in a safe and stable manner at all times. It is essential that the mass and centre gravity of each module is accurately determined.

To implement weight control for the furnace building, a register of all mechanical and process equipment in the building was maintained. This included a record of equipment name and number, equipment loads, floor level and module in which the equipment was located, vendor drawing numbers, and equipment data status.

In addition, the equipment register was checked and updated as each issue of vendor data was received and the equipment loads in the analysis model were cross checked against the equipment register and vendor data. The mass and centre of gravity for each module were reported directly from the analysis model.

The structural design of modular buildings for process plant requires the consideration of complex loading scenarios for the construction and operation of the plant. For the TSL furnace building, this challenge was met by the application of well-defined project design criteria, use of structural analysis software with advanced modelling features, and leveraging the software capabilities through application of modelling strategies.