Modern designs for naval vessels are increasingly concerned with weight reduction in order to improve performance, manoeuvrability and fuel efficiency.
The complex light weight panels used in construction are prone to distortion and thus extensive re-work could be envisaged. TWI worked with two of its Member companies, the Ministry of Defence and BVT Surface Fleet, to address this problem.
The sequence of fabrication starts with welding stiffeners onto base plates to make panels. These panels are welded into assemblies, which in turn are welded into units. The units are then finally welded together to complete the structure of the ship.
A typical bulkhead panel was selected for this work. The photograph of the panel shows it bolted down onto a pallet for fabrication. The pallet provides an effective heat sink and mechanical restraint during welding, which are beneficial techniques in reducing distortion due to welding. The total number of welds to fabricate the panel was 194.
The panel is a difficult structure to model. The structure is large having approximate dimensions of 10m in width and 5m in height, and requires over 100m of welding during fabrication of the 194 weld passes. The size and the number of weld passes makes this problem challenging. Although finite element analysis has been used for many years to model welding distortion, its application to large fabrications has been limited by the size of the model required and the necessary computing capacity. Advanced techniques used by TWI enabled this complex panel to be analysed much more quickly than was previously possible and this allowed the effect of welding variables and weld sequence to be studied.
The first phase of the work programme involved manufacture of the ship panel under production conditions using the current build strategy, taking distortion measurements at various stages of the fabrication process. This was carried out by BVT. The predictive model was then validated with the distortion measurements. The aim of the second phase of the work programme was to demonstrate the ability of finite element weld modeling to assist in the reduction of distortion and rework. The model was solved several times to investigate different fabrication procedures systematically in an attempt to minimise distortion in the panel due to welding.
As a result of the programme, the weld distortion in subsequent production panels was significantly reduced.
TWI's insight into predicting and controlling the distortion in large structures is applicable across many industry sectors. For further discussion with an expert please contact us.