Over the past few years, the volume of shaft/bearing damage has increased. Such damage may be very costly due to delayed delivery, high repair costs and expensive off-hire due to repairs.
In extreme cases, damage to a bearing may even lead to a loss of propulsion, which in turn could lead to the loss of lives and property. In response to this, DNV has launched a research programme to perform full-scale measurements on different vessel types.
The programme is motivated by the significant volume of bearing damage that occurs, and the project will seek to identify why this damage occurs and what measures can be taken to prevent it.
The current way to optimise the shaft/bearing interaction is to carry out a shaft alignment calculation and then to ensure that the shafting is installed according to the specification obtained from the calculation. The main limitation with this method is that such calculations usually do not take into account a number of factors affecting the shaft alignment. Such factors include propeller forces, hull deflections and the influence of adverse weather conditions. Failure to consider the potential influence of such effects may lead to bearing damage. The Joint Industry Project (JIP) therefore aims to identify and quantify such factors for a number of different operational conditions for the test vessels, and to develop analysis methods capable of accurately predicting these effects.
The propeller loads are measured by a new methodology developed by DNV. It involves the instrumentation of the shaft aft of the aftmost bearing. The propeller loads will be measured in continuous and transient conditions during a sea trial of the vessel. In addition, measurements have also to be carried out during the operational phase in order to investigate the effect of weather conditions on the shaft/bearing interaction.
The hull deflections are monitored by installing sensors along the entire shaftline. The sensors are able to monitor the effect of both static and dynamic hull deflections on the shaft/bearing interaction. The local deflection effects of the main engine will also be monitored.
This comprehensive measurement programme enables the entire shaft line to be monitored for all feasible operational conditions that the vessel will experience. The aim of this is two-fold:
To clearly identify the potential hazards which may lead to alignment-related damage.
To implement state-of-the-art remedies for these hazards into the DNV Rules as an active measure to prevent such damage from occurring in the future.
In addition to the measurement programmes, detailed finite element (FE) models are produced incorporating the main propulsion line in a fine-mesh model of the hull. These models will be calibrated against the measurement results. Based on this, a best-practice guide will be prepared. The guide will document how to produce such FE models so that analyses of other vessels may be carried out, accurately predicting the impact of critical parameters on the shaft/bearing interaction without the need for a full measurement programme. For a critical installation with highly loaded bearings, such an updated analysis method could be vital in achieving an alignment which ensures safe operation in all feasible vessel conditions.
The calibrated FE model will also be used in an investigation into the effect of aligning the shaft in dry dock versus in an afloat condition. For alignment sensitive installations, the installation tolerances are extremely small and hull deflections occurring when the vessel is launched may cause these tolerances to be exceeded. If this happens the alignment will no longer be according to the specification and this may lead to damage. By combining the FE analysis with measurements, the adverse effects of vessel launches on shaft alignment will be investigated.
For further information on this research project or queries related to shaft alignment consultancy services, please contact us at machinery@dnv.com