5.4 Spring foundations
As the natural frequency of a low-tuned foundation support structure decreases, the isolation between the support structure and the sub-foundation increases, and the sub-foundation is called upon to absorb less of the energy resulting from dynamic and transient forces generated in the shaft line and static sections of the plant.
The closest practical approach to complete dynamic isolation results from mounting the turbine-generator plant on springs and this approach has been used successfully overseas. In the UK, condensers of the underslung type have been spring-mounted, as have smaller items of plant, but spring mounting has not yet been used for main turbine-generators.
Established overseas practice uses springs of the helical-coil or plate type supporting a reinforced concrete deck on which the turbine-generator is mounted.
Springs mounted in rows on structural concrete walls or in groups at the top of support columns have been used and viscous dampers are sometimes employed in conjunction with the springs. Natural frequencies of about 3 Hz for movement in the vertical direction are claimed for these systems. A hydraulic locking system is used to pre-load the springs to a set deflection for construction and plant erection purposes. The same locking system can be used to allow the insertion of packers, above a spring or spring-group, to correct for foundation column settlement: this also allows springs to be changed after erection for others having a different springrate, if it is necessary to change the characteristics of any part of the spring support system.
Such low natural frequencies are accompanied by the possibility of greater plant deflection under transient conditions. Under generator load rejection or short-circuit conditions, torsional rocking of the plant can impose vertical movements of ±1.5 mm at the springs, so pipe and electrical connections must be designed to accommodate this.
Because the sub-foundation is well isolated from dynamic loads, it is possible to reduce the mass of the sub-foundation towards that necessary to support the static load only, or even to dispense with a mass concrete sub-foundation and install individual foundations for each support column. The precise design, of course, depends on subsoil conditions at the particular site.
The spring support, concrete deck and support columns are no cheaper than other support structure types: the economic advantage claimed for spring foundations lies in the possible reduction in mass of the sub-foundation. Cost reductions of 25% are claimed in favourable circumstances.