Non-linear block load corrections for dry docking

Abstract

Damen is designing more and more slender hull forms, which leads to a small dock block contact area, and therefore a high load on the block bed. Dry docking is achieved by using multiple blocks located along the longitudinal length of the ship. Dock blocks are constituted by several layers, normally three or more. The configuration of the block bed (concrete in combination with hard­ and softwood) creates the support of the vessel with non­linear behaviour. This study focuses on a prediction of the blockload in the dry dock, taking into account non­linear material behaviour of the timber layers. Nowadays, the approach to predict the dock block load makes the assumption of linear elastic behaviour of the timber layers. The loads produced can cause a non­linear behaviour of each block, which might lead to an over­stress failure of one or more blocks. The assumption of linear elastic behaviour possibly results in wrong load distributions, which eventually leads to a redistribution of the loads on the other blocks. In turn this can force other block failures and ultimately this produces damages of the hull forms. The consequences of a poor dry docking analysis are potentially catastrophic. A docking failure can lead to extensive ship and dock damage, disruption of docking schedules, and loss of the ship to active duty until repairs can be made. Two set of models are conducted to predict the load on a dock block and evaluate the influence of the non­linear behaviour. The Timoshenko beam theory with multiple springs is used to calculate the load on every single dock block location. The load on every particular location is used in the model for a single dock block formed by elastically connected beams. A double Winkler foundation system represents the interaction of the stacked block layers and dictates the non­linear behaviour. Non­linear material properties are captured by the secant modulus of the top layer. A test campaign pointed out that for each layer of nominal identical specimens a large variability in material properties is noticed. Material properties found in the literature were different from those obtained during experiments. Moisture content has a significant influence on linear and non­linear effect, with the Young’s modulus and yield point being shifted to lower values. The variability in material behaviour and moisture content must be considered when predicting the block load. Moisture content variability lead to changing block bed load distributions. Underestimation of the moisture content can lead to large differences in the single dock block analysis. With the good matching in the validation work, this research confirms the reliability of a double Winkler system to prescribe non­linear material behaviour of a single dock block. Although the analysis of a single dock block matches results obtained from finite element models, further research is still needed. The model of a Timoshenko beam on spring supports gives reasonable results, but underestimates the load in case of local increased stiffness caused by a transverse bulkhead. Optimization on this particular section is needed to get more realistic results.