3719. Modernising Ship Stability: Lightship Evolution Diagnostics with In-Service Stability

Paper

Abstract

Lightship mass and center of gravity are the basis for assessing ship regulatory stability and the maximum payload that the ship can load results from this assessment. Knowing the ship mass and centre of gravity is therefore of utmost importance for both commercial and safety reasons.It is known that, over time, both these quantity change. At present, changes in the lightship are addressed by five-yearly audits that may lead to an inclining experiment – the traditional way to measure ship mass and centre of gravity. The time gaps are filled with estimates based on weight control which can be shown to be a ‘random walk’ process. This means that, temporarily, undetected worsening of the ship stability might occur.Draught measurement provides immediate feedback of the accuracy of the estimate of weight change, provided draught sensors are adequately maintained. Evidence of change in the vertical position of the lightship center of gravity is not, however, obvious.In-service stability measurements, integrated into the vessel’s operational routine, directly estimate the vessel VCG and can diagnose changes in the lightship vertical moment using statistical process control techniques. Changes in the progression of mean values of Deadweight and Lightship vertical moment are used instead of records of weight changes to build a model of ship stability over time with uncertainty on the mean value decreasing with increasing number of measurements. Weight control remains important to characterize the changes and discrepancies from the loading program can be used to identify sensor failures, defective estimates of cargo deadweight and Lightship changes.This paper briefly reviews conventional techniques (referring to previous Conference papers). It then discusses attempts to perform conventional inclinings at sea and the difficulties in obtaining precision, before setting out the methods of in-service stability assessment, techniques for analysis of the results and finally the control limits that can be used to trigger further investigation. The technology is suitable for autonomous vessels.