While it was technically feasible to strengthen the boiler pressure parts that take the loads, it was not practically possible. Base isolation of the boiler using lead-rubber bearings under each of the four support legs was a potential solution, although more commonly found in buildings and bridges than in industrial plants.
Beca AMEC carried out a series of increasingly sophisticated analyses including non-linear time-history analysis simulating the effect of seven likely earthquake types that could affect the site.
A base-isolated structure has to be allowed to move around in an earthquake, and much of the cost of the project related to ensuring that the surrounding structure, walkways, and connections would safely allow more than 100 mm of horizontal movement.
While the boiler would be base isolated and therefore free to move in an earthquake, the biomass grate would remain fixed to the foundations. Beca AMEC developed a ‘plastic’ hinge methodology and railway buffer style system that would allow the boiler to move the grate in an earthquake, limiting the potential damage to both the boiler and the biomass grade.
The previous methodology with a fixed corner and controlled expansion on the three other corners was considered to be key in retaining existing boiler to grate air and ash seals. Beca AMEC designed a ‘breakaway’ thermal guiding system that replicated the existing fixed system but broke away during a service level seismic event.