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 andthus 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 thestructure of the ship.
A typical bulkhead panel was selected for this work. For fabrication, such panels are bolted down onto a purpose-made metal pallet. The pallet provides an effective heat sink and mechanical restraint during welding, which arebeneficial 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 numberof 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 thenecessary 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 bestudied.
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 carriedout 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 modelling to assist in the reduction ofdistortion and rework. The model was solved several times to investigate systematically different fabrication procedures in an attempt to minimise distortion in the panel due to welding.
As a result of the programme, weld distortion in subsequent production panels was significantly reduced.
TWI's insight into predicting and controlling distortion in large structures is applicable across many industry sectors. For further discussion with an expert please contact nmo@twi.co.uk.
