BIM Approach on a Nuclear Project: Sellafield Maintenance Facility
The project, being completed for Sellafield Ltd. the company responsible for safely delivering decommissioning, reprocessing and nuclear waste management activities on behalf of the Nuclear Decommissioning Authority (NDA), is an integral part of the Sellafield site waste retrieval and hazard reduction programme, which is of strategic importance to the NDA and the UK Government. The facility will provide mechanical handling plant for operational maintenance support during waste retrieval operations, the retrieval of the waste being part of the Nuclear Installations Inspectorate (NII) Licence Instrument Specification 326(a), which requires retrieval of a substantial proportion of the bulk intermediate level waste sludge on the Sellafield site to interim safe storage by August 2020.
The site of the multi-million pound facility is bounded by existing facilities and a stream to the south of the site, these constraints have influenced the final facility location and orientation. Although there are a number of exclusion zones that need to be considered in construction operations, the site constraints were not critical when considering the structural forms adopted.
The structural form is predominantly influenced by the spatial and operational requirements of the plant. There is a requirement within the building to move packages (weighing up to 55t) throughout the plant, which is achieved through the use of an overhead building crane supported off the main building frame structure. The layout of the building has been set-out to enable the most efficient operation for transferring the packages to their areas of service and storage.
The plant can be divided into three separate structures; the main Process Building structure, the Administration building and the Services building.
The Process Building structure is 35m wide x 112m long x 22m high, and is formed from a series of mono-pitch portal frames that create an open plan internal space. The portal frames are designed with nominally-rigid base connections into the supporting raft foundations to provide stability and to limit serviceability deflections so as not to affect the operation of the overhead building crane. The structure is stabilised along the length of the building through lateral bracing in the roof, which transfers the lateral load to vertical braced bays located along the exterior elevations of the building.
Within the main building there are numerous concrete cells and steel structures with blockwork infill panel walls and concrete roof slabs that form process rooms (Figure 1). The concrete cells are independently stable concrete boxes, where the supporting walls act in shear to transfer any lateral forces into the supporting foundations. Internal steel frame structures are stabilised by being tied back to either the main frame steelwork or to the concrete cells. Concrete slabs that form the roofs to these structures are constructed within the depths of the supporting steel beams, with tie reinforcement provided through the beams to ensure that diaphragm action occurs. These diaphragms effectively tie the structures back to the concrete cells and main frame steelwork, providing lateral stability, as previously described.
Figure 1 - Facility Overview
The Services building will be a two-storey steel frame structure located adjacent to the main building, but with an independent stability system. The steel frame is braced in two directions with a concrete diaphragm slab at first floor level and roof bracing.
The Administration Building adopts modular build construction and is also located adjacent to the main building and, as with the ventilation building, is independent of the surrounding structures. The modular build has been designed by a specialist manufacturer, and the modular units have a self-contained stability system.
The structures are all supported from concrete raft foundations. The main raft was designed to simplify site excavation and construction works, by maintaining a constant formation level while accommodating the many depressions and steps required to the top surface of the raft.
The raft is designed to distribute substantial localised plant loadings in the heavily loaded process areas, and high local point loads and moments beneath edge columns, caused by the support to the overhead building crane. The design has been based on minimising any anticipated settlements to less than 25mm, and eliminating the requirement to introduce multiple layers of reinforcement.
Many structures on the Sellafield site are designed for seismic loads. This building, however, is a non-seismic facility and as such is generally designed for normal operating loads. However, there are a number of fault or accident conditions considered, such as the loading induced by a package being dropped from the main overhead crane.
The project has passed through the detail design gate (a formal review process), and the structural information has been progressed and issued for construction. Sellafield Ltd led the design through the concept and preliminary design phases, and worked in conjunction with the Cavendish Nuclear and Balfour Beatty joint venture team to develop the design to a level of maturity for costing. The civil, structural and architectural design was undertaken by URS, supporting the joint venture. As the project moved into the detailed design stage, Sellafield Ltd withdrew from the day-to-day design and moved into the role of the intelligent customer, required by the Office of Nuclear Regulation to ensure Sellafield Ltd, as the site licensee, maintains knowledge, control and oversight of the design completed by its supply chain.
Why adopt BIM technology?
The team have been continually challenged by Sellafield Ltd to introduce efficiencies within the design process. From the outset, there was a push to adopt BIM technology and methodology, which was driven both by individual experiences from within the team and also an appreciation of the advances in technology being used outside of the nuclear sector.
This project was an ideal opportunity for Sellafield Ltd to trial BIM technologies and by co-locating in a single office, the team were able to work off a single server, simplifying the transfer of information between different models over a secure network. The co-location also generated increased collaboration and discussion around the model, which eased many of the troubles inherent in implementing BIM technology.
How was this done?
At the outset of the project, BIM execution and Computer Aided Engineering (CAE) plans were created. These documents mapped out the BIM workflow, identifying the protocol and naming conventions that were to be adopted during the modelling. It was recognised that when using BIM, it was essential to plan ahead, because identifying the correct software and protocols to be adopted are imperative for ensuring that the process runs smoothly. Along with the correct software, having suitable, experienced and qualified people is important, so a plan was put in place identifying the most suitable people to work on the project. The project had up to 100 people working in the design team, again emphasising the requirement for clarity and the need to have clear procedures in place.
Figure 2 - How BIM technology was implemented.
All structural and architectural building modelling was undertaken within the Autodesk Revit environment. Initially all plant and equipment, CE&I, pipework and HVAC was done within the Autodesk Inventor environment but as the project progressed to detailed design, the CE&I, pipework and HVAC were transferred to Autodesk Revit MEP, which enabled these disciplines to assign intelligence to the object modelling. The individual discipline models were then combined in an Autodesk Navisworks environment, which enabled clash detection to be completed, space management and a viewer to be used in reviews. The 3D modelling was always undertaken by trained technicians from the appropriate discipline. The collaboration of models was the responsibility of the appointed BIM Manager, who regularly synchronised the Revit and Inventor models within Navisworks.
The BIM Manager was also responsible for managing the clash detection, and co-ordination meetings on a weekly basis. Although Navisworks was used for clash detection, there was still a requirement to share information between the Revit and Inventor models, so that the accurate co-ordination and positioning of items was achieved. To allow this sharing of information, it was important to divide the models into manageable sub-models, to minimise the amount of data that needed to be transferred between them. This approach helped to prevent any operating problems, while enabling different sections to be suppressed (or have their visibility turned off) for ease of viewing or for drafting purposes.
The transition from a 2D to a 3D environment can create challenges; the first being how the engineer reviews the modelling work. The project experience has shown that there is a reliance on the 2D drawings for review, as this is often still the primary deliverable. Despite this, there were many hours spent around a computer screen reviewing details within the 3D environment of the Revit model. Until the end of the detail design stage, when there was a requirement to produce 2D drawings, all of the design progress was recorded within the Revit model. Although clash detection meetings were held weekly to review co-ordination between disciplines, there was no need for the transfer of 2D drawings between disciplines for coordination purposes that would be typical in a project not carried out within a BIM environment.
This worked well and saved significant time. However, by not having the formal transfer of 2D information, there was not the same audit trail to verify that information had been passed between the different disciplines. This left the process very reliant on people communicating verbally within the team, and promoted true collaboration and trust. In producing structural drawings from the Revit model, it was required to ‘freeze’ the model at least two weeks before the issuing of 2D drawings. This was due to a significant amount of work being required in the 2D drawing views to ensure that the information was suitable for issue. Minor changes in the model can impact on multiple 2D drawings, which could lead to significant reworking and rechecking.
It is important to emphasise that within the nuclear industry, the design is led by the plant layout and process engineer, as opposed to a normal commercial build that may typically be led by an architect. The building is heavily congested with services and plant, which need to be co-ordinated and supported. Without the aid of the 3D environment, it would be very difficult to understand the routing of these services and the support and positioning of this plant, especially given the large number of technicians and engineers working on the many different process areas of the building. The clash detection system has saved significant time in checking drawings, and more importantly has enabled clashes to be detected in the office as opposed to on site. From a structural perspective, there was a strategy not to link the Revit model with the structural analytical models.
The structural analysis was undertaken using Staad Pro software supplemented by hand calculations. Staad Pro was used for assessing the stress distribution in both the steel and concrete elements. The concrete raft and concrete cells were assessed using finite element design, while the finite element models were generated independently of the Revit models. The benefits of linking the Revit and analytical models were not considered necessary, because the geometry of the structure was fairly regular, which enabled analytical models to be built quickly. Changes in geometry and form were implemented separately in both the Revit and analytical models by relying on communication between the technician and engineer, this provides an example where BIM methodology was not fully followed.
The major benefit of using BIM has been seen in the multi-disciplinary co-ordination aspects of this project. The ability to generate a 3D visual model of the plant has simplified the spatial co-ordination of the different disciplines. Not only will this reduce clashes on site, but significant time savings have been made in internal and external stakeholder review of drawings and information (Figure 3 shows examples of collaborative models).
In particular the 3D visualisations have enabled a high degree of engagement of all parties in the design. As well as the basics of BIM and the 3D modelling, further benefits were seen as the project progressed. Database specifications (using NBS Create) have been linked to the 3D models to further advance the information stored within the models. As the project moves towards construction there is an intention to build contractor information into the model. This will enable the model to be used as a programming tool. The structure within the models will be updated to reflect splice connections and concrete pour sizes, which will enable the contractor to assign their associated programme information to the relevant elements.
With the appointment of the steel fabricator, it is envisaged that the structural steel model will also be issued to them, so that they can utilise the information in the production of the fabrication information. Similarly with the concrete works, the benefits of using object modelling will be derived when carrying out the reinforcement detailing. The Revit model has been linked with a reinforced concrete 3D detailing package, which will enable detailing of the reinforcement in 3D, while still producing 2D output drawings and schedules. There are also thoughts that the 3D model can be shared with the site operatives, to be used as a visual aid to simplify the on-site placement of the steel reinforcement.
Although Sellafield Ltd is still in the early stages of deploying BIM technology, this project has realised significant benefits. The timescales of projects, the co-location of project teams, and the employment of the design team throughout the design phases of the projects, mean that the company is well placed for using BIM technology. The main challenge for the company is to ensure that their current standards, procedures and software relating to CAE, are updated to provide consistency in the processes used and deliverables produced, throughout all projects. The update will incorporate the lessons from this, and other projects, as well as the development and uptake of BIM within the construction, chemical process and nuclear industries.
On future Sellafield Ltd projects, there would be significant benefits to linking the analytical and drawings models, thereby enabling the two-way sharing of information. This could offer reductions in analytical modelling time, and will ensure that there is an accuracy and consistency across the design models. However, many nuclear facilities require dynamic assessments under seismic loads, and Ansys is currently Sellafield’s preferred analysis software. The links between Revit and Ansys would need to be developed and confirmed to be feasible before such a strategy can be progressed. This is just one example of the learning curve faced by the nuclear industry in a bid to successfully implement BIM, and Sellafield Ltd now need to determine how the project BIM model is used throughout the lifespan of the facility. There have been discussions on using the model as an as-built representation of the final building, being updated to include sub-contractor information and details.
It is envisaged that in the not too distant future, the models could be used as building manuals containing all of the relevant information required to be able to operate and maintain the building. Further developments that will offer significant benefits to the nuclear industry include developing the software into an interactive model, which will allow ‘virtual’ on-the-job training as well as ad hoc planning for emergency operations that are required to be carried out in areas not normally accessed by operatives.
In conclusion, it is evident that this project emphasises that the nuclear industry is beginning to embrace the use of BIM technology within the design process. Substantial benefits have already been identified and experienced, which promote its future use. The opportunity to further maximise the benefits of BIM technology is one that is being explored, and will be challenged on future Sellafield Ltd projects.