NAVAL ARCHITECTS &
What Is MOSIS?
Tymor MOSIS (Measurement of Stability in Service) is a system for determining the stability of a floating structure that is comparable to the traditional inclining test...
It is valuable for vessels where Variable Deck Load (VDL) is important or those working at the limits of regulatory compliance and also provides a database of actual VCG suitable for a safety management system and for certification/verification purposes – the aim of which is to avoid coming off station to perform an inshore inclining.
MOSIS: A Simple Concept : It is relatively easy to check the weight of a vessel by reading draughts and comparing these with the known properties. It is not so easy to check the position of the vertical centre of gravity, yet this dominates stability …
To control something you need to be able to measure it, but if the VCG cannot be regularly measured – how can it be controlled?
The VCG position can be estimated by moving a weight to produce an overturning moment. Knowing the restoring properties (buoyancy) of the vessel and measuring the equilibrium angle that the vessel reaches, the VCG position can be calculated. This is an Inclining Test and it is done inshore in still water and free of mooring restraints to achieve accuracy. It costs money and the unit is not generating revenue.
With MOSIS, Inclining Tests can be performed at sea with the mean angle of heel measured by averaging over a period of time. Allowances can be made for the moorings and other forces. In any one MOSIS test, any inaccuracies can be smoothed out by performing multiple weight shifts and by using both the longitudinal and transverse rotations to get independent estimates of VCG. After that a whole series of MOSIS tests will give statistical estimates of the centre of gravity coordinates – as with industrial statistical process control.
MOSIS and the conventional inclining have one thing in common in that they both require a formal and complete Deadweight Check at regular intervals. This is done at the same time as a standard inclining test and takes at least 2 days, but with MOSIS this can be performed while the vessel is on location and doesn’t affect operations.
The system allows for sinkage when ballast is added, for rotational effects on mooring tension and corrects tank soundings for gauge positions and heel/trim. Although the programming is complex to treat all uncertainties exhaustively, the ideas are simple.
Application Of MOSIS : The stability regulations are based on knowing the fixed weight of the vessel (from an inclining) and estimating the working (deck) loads to get the vessel condition. This is checked against the allowable VCG ...
Directly measuring the property of interest (vessel VCG) and comparing with a base value to plot changes gives the means of controlling deck load in the true sense of the word. If things change you do not have to wait 5 years for an unpleasant surprise at the Inclining and you can do something about it now. The figure below shows how this information can be presented.
UK HSE Validation
The United Kingdom Health and Safety Executive commissioned a review of MOSIS in 2000 by Global Maritime (GM-44070-0500-47051 – available on request) that concluded that the MOSIS method of predicting GM in offshore conditions is tolerant of realistic values of both uncertainties in the measurements and of hidden unmeasured moments. It was judged to be, at least equivalent to a conventional Inclining Test.
DNV-RP-H104 (SEPT 2011)
This DnV Recommended Practice document considers Ballast, Stability and Watertight Integrity as a guide to planning and operating systems essential to the vessel’s safety.
MOSIS fits perfectly within this framework – not just in terms of validating lightship, but also because it is part of the ‘Verification of Technical Capability Against Operational Requirements’.
Evidence for VCG Growth
There is a strong body of evidence – supported by our own observations that the mean unaccounted growth in VCG will be about a quarter of a per cent per year unless controlled. This figure agrees reasonably well with records of weight growth obtained from merchant and naval ships.
A conventional Inshore Inclining is sometimes claimed to be accurate to about 0.05 metres at best.
MOSIS achieves this by
• Multiple weight shifts in several directions in a Test
• Multiple Tests over time to get trends and averages