Engineering analysis and design software
Civil and Structural engineering

Case Study

LUSAS used to prove nuclear safety case at Devonport Royal Dockyard

The Consultancy Services department at LUSAS worked in partnership with Babtie Group on the nonlinear dynamic analysis of a variety of major structures. The seismic qualification analysis was required as part of the multi-million pound modernisation programme of nuclear submarine refit facilities for DML at Devonport Royal Dockyard.

As part of the work, nonlinear dynamic analysis with LUSAS Civil & Structural was carried out to predict the stability and structural integrity of massive cellular, reinforced concrete caissons at numbers 9, 10, 14 and 15 docks during a severe seismic event. These multi-celled gravity structures are used to seal-off the open ends of dry-docks. After construction, each caisson is floated into place at the entrance of the dock, ballasted with water, and sunk into position prior to the draining of the dock.

On number 15 dock caisson, a preliminary analysis of caisson response using the prescribed seismic data led to the adoption of a full-width 0.3m deep shear-key and stop-block to restrain caisson movement. A further 9 analyses were performed with a more detailed LUSAS model to give an initial assessment of the effect of friction coefficient, foundation stiffness and ground acceleration upon both caisson movement and internal forces and moments. Finally the LUSAS model was transferred to Babtie Group for further sensitivity analysis runs.

Modelling

The caisson was idealised in LUSAS Civil & Structural by using 4-noded thin shell elements for the cell walls and 4-noded thick-shell elements for the caisson base. Nonlinear contact joint elements were used to model the lift-off and frictional sliding at the foundation, stop-block and shear-key interfaces. Additional joint elements were used to provide hydrodynamic mass and damping actions on the walls and base-interface respectively.

Ground acceleration history from BNFL Engineering Ltd. for a UK hard site provided the seismic input with increments of 0.005 second being used for each time step. Hydrostatic pressure and self-weight were applied as initial static loads. Hydrodynamic forces from the water enclosed in the cells were simulated by locating joint elements at each node on each wall and assigning directional masses calculated using the Westergaard model. Acceleration histories were applied to the foundation to drive the ensuing dynamic analysis. Values of frictional damping at the contact interface of 3%; structural damping of 5%; and interface damping of 2% to simulate the effect of the fluid between the base and the dock floor were used in the analyses.

Results processing

After running the analyses, full use was made of the extensive results processing features in LUSAS. Time-history plots were obtained showing total reactions on the caisson base and reactions at the stop-block and shear-key. Envelopes of maximum and minimum shear force and bending moment were obtained for each caisson wall and the base for all 9 analyses. To assist with steel design, envelopes of Clark-Neilson forces were also produced for all of the walls. This method combines in-plane and out-of-plane actions to derive the tensile force to be carried in the plane of the steel. Time histories of displacement, velocity and acceleration were also produced for all 8 corner nodes of the caisson. Secondary response spectra were obtained from these motions for use by equipment designers. Animations of each simulation were created to give a complete overview of the response of the caisson to the seismic event.

Relative displacements and Clark Neilson forces

In Summary 

Using the comprehensive results obtained with LUSAS Civil & Structural, Babtie Group were able to prove a safety case for the caisson. The analysis clearly demonstrated that the caisson had adequate structural capacity to withstand a seismic event and that the seals could accomodate the displacements expected. Independent technical assessment and peer review has been carried out by leading UK nuclear engineering consultants who have accepted the use of LUSAS finite element analysis for these advanced analyses in the nuclear industry.


 

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