The Background.

A Dead Weight Support (DWS) for a firewater caisson was found to have developed a number of cracks in the stiffener plates and around welds to the deck structure in a relatively short period of time since installation. The client therefore asked Optimus to develop a design for a repair that mitigated the risk of further cracking and also to provide FEA and fatigue analysis to estimate the life of all replacement fabrications.

Figure 1: The DWS and surrounding deck structure.

Finite Element Analysis and Fatigue Life Analysis.

The caisson, the Dead Weight Support and the localised deck structure of the existing design were modelled in Abaqus as a dynamic analysis and wave loading was applied using Abaqus Aqua. Guide clearances and thinned wall regions of the caisson were taken into account, as were estimates of the marine growth on the drag and inertia.

The caisson was modelled as a beam, while the dead weight support and deck structure were modelled as shells. This was necessary in order to capture the localised stresses around the plated structure and in order to apply wave loadings to the caisson.

Each FE analysis case was run as a dynamic step, with the stresses throughout the caisson and structure calculated throughout a whole wave cycle. A series of individual analysis cases were carried out, with one case for each wave height and direction that was expected within a 1 year return period.

A fatigue analysis was carried out using FESafe. Every FEA case was taken into the software, which then performed a fatigue analysis using the full stress tensor throughout the whole model on an element by element basis for one wave cycle at each wave height and direction expected in a one year period. The resulting damage estimate was factored by the number of occurrences to give an estimate of the accumulated damage and thus the fatigue life.

In collaboration with our brownfield detail design team , a number of design options were analysed and benchmarked against the life estimate for the existing design. The final design omitted all welds between the DWS and the deck structure and a significantly improved fatigue life.

Figure 2: Contours of life on the caisson and the DWS.