CGR was asked to undertake the preliminary design of a concrete caisson for the outer breakwater construction at Peterhead Bay, Scotland. The purpose of the design was to carry out a cost estimate for comparison with a rubble mound breakwater. The outer breakwater was required to protect the local harbour against significant wave heights of 18m and to provide an all weather berth for a 1 in 10 year storm event.
The design was carried out to BS 6349 Maritime Structures and BS 8110 Structural Use of Concrete, utilising finite element analysis to obtain an efficient section.
The caisson was to be in 22m of water and needed to minimise wave activity in the harbour. The dynamic forces due to the wave impact and the wave overtopping loads needed to be considered as well as the ballasting requirements for towing the caissons into position. After determining the wave forces, the finite element package, LUSAS, was used to model the caisson and to produce the force fields in the structure.
CGR provided a standard rectangular caisson solution and then further developed an innovative cylindrical faced caisson for consideration. This curved face helped to reduce wave reflection, reduced the wave loads and gave added structural strength advantages. Various methods for stiffening the caisson faces were investigated in order to ensure the most economic section was provided.
After detailed financial and technical comparisons, the traditional rubble mound breakwater was selected by the consultant as the most appropriate method of construction, based on overall economics at this particular site.