A complete ship FE model incorporates all the various forces acting on it. Theoretical and empirical data must be used to verify and constantly improve the model.
Interacting forces and loads
The first and most important tool to analyze, predict and prevent structural issues in an offshore installation in service is a complete ship FE model. This complete global ship FE model must take into consideration all the interacting forces and loads as well as combinations of these.
One significant contribution to fatigue loads could arise from the interaction of certain cargo and ballast volume combination and a specific hindcast metocean data (wind, wave and current directional combinations).
Finite element update
Normally an operator possess certain ship FE model with varying degree of accuracy in relation to f. ex. structural modifications, new equipment or even structural repairs. Therefore a normal part of a life extension program is to update any existing FE ship model to include all relevant structural changes.
This structural review should include an increased accuracy of the structural drawings of critical details, which would need to be verified by offshore inspections. This FE model update will allow to identify and quantify weld connections with high stress concentrations and in need increased fatigue resistance which can be achieved by the ultrasonic peening treatment.
Finite element verification, theoretical and empirical
Probably the most relevant part of a complete ship FE model is the verification of its accuracy. This must be done theoretical and empirical. The theoretical or numerical accuracy is normally carried out by means of convergence and energy norms.
The empirical way of checking the accuracy of the model is by accelerometers and strain gages applied on specific parts of the hull to provide ongoing record of the hull behavior. The measurements must then necessarily be correlated to a specific hindcast metocean data and the ship’s exactly coordinates including any offset. These measurements can then be compared to the by the model predicted stresses.
Understanding upcoming structural issues
Strength and spectral fatigue analysis are carried out for a number of relevant cargo-ballast- hindcast metocean load cases in order not to just understand where structural issues will occur in the future but also to quantify them. The extension of locations with high stresses can then be exactly determined and therefore the application of fatigue life improvement techniques as ultrasonic peening would be of full benefit.
One important derivative of this analysis is the understanding of possible causes to these future structural issues. The understanding of the causes will give the opportunity to operators to modify certain operational steps in order to prevent unforeseen structural damages.