Naval Architects &
What Is MOSIS?
Tymor Marine’s 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. The system consists of software and hardware used to measure the hydrostatic stability of semisubmersibles, ships and other floating units while they remain in service.
MOSIS is a unique system for carrying out an inclining experiment without taking a vessel out of service. It differs from a traditional inclining experiment in that it is carried out on a more regular basis and monitors and tracks a vessel’s vertical centre of gravity (“VCG”) over time allowing action to be taken to prevent small issues from growing. It has proven HSE and cost benefits as well as alignment with commercial and operational goals. There is, to our knowledge, no other proven and accepted system in operation.
In more detail…
In the current economic climate, the ability of a vessel to continue earning revenue instead of incurring the cost of mobilising to a sheltered port for a traditional inclining is a major cost benefit. This is particularly true for production units that are almost impossible to bring inshore once they are deployed but is also true for drilling units where there are contract pressures to reduce costs. In addition, the safety benefits of using accurate and up to date stability information cannot be underestimated.
Accepted by Classification Societies and Flag States, MOSIS monitors, analyses and tracks stability data and provides owners and clients with information to increase operability, particularly optimising variable deck load (“VDL”). The ability of a drilling rig’s client to maximise the variable deck load, benefits their entire project. Whether it is running casing and loading as much as possible onto the pipe deck or transiting to other well locations while back loading as little client equipment as possible; this can all be achieved with an accurate knowledge of VCG.
Contact our Business Development Manager for a detailed comparison between the cost of MOSIS and a standard inclining if it should be required at any time over a 5 year period. Potential cost savings on an inclining alone are clear, without considering the operational benefits.
A Technical Overview
The two factors which critically affect the stability of a floating vessel are their displacement (hydrostatic characteristics) and their vertical centre of gravity. The displacement is straightforward to measure by reading the draft. It is conventionally split into deadweight and lightweight – that is, weight that changes in the short term and weight that remains stable over long periods. The split is determined by carrying out a deadweight survey, but the breakdown can be different for different organisations and type of vessel.
The rules are straightforward. Every year, Class will require a check on the lightweight additions log. Every five years the lightweight may be verified by a check on the vessel’s displacement. If this shows a discrepancy greater than 1% between the calculated draft and the actual draft, a deadweight survey should be carried out.
If the lightweight resulting from the deadweight survey shows a difference of greater than 1% in operating displacement for a MODU (2% lightship change for SOLAS vessels), an inclining should be carried out. Some class societies recommend an inclining experiment where the total accumulated weight change exceeds 3%.
As an alternative to an inclining, the weight difference may be included in the calculated lightship, but the ‘unknown’ weight figure must be placed at an indisputably conservative position on the vessel. Currently the vessel operator has two commercial choices. They can either take the penalty weight or go inshore for a traditional inclining experiment. Neither of these are particularly attractive. In addition, there is no cast iron guarantee that the results of the traditional inclining will be completely accurate.
MOSIS provides a third alternative. It allows the vessel operator the opportunity to estimate VCG accurately over time. Statistically, a number of MOSIS tests will be as accurate as, or better than the estimate of lightship VCG from a traditional inclining.
For the vessel owner, there are significant cost savings from utilising MOSIS. From a regulatory perspective, the stability of the vessel is being monitored much more closely and regular use of the system can detect undocumented change so that other comparative techniques can be used to isolate the causes.
It is important to understand vessel weight. As noted a vessel is made up of estimated weights that vary over time. Although the Lightweight is considered a fixed weight, errors may have been made in the ‘formal’ Lightweight. Every inclining test has uncertainty and the reported result is very unlikely to be the true value.
Tracking of weights on board a vessel can be difficult. However, assuming that a thorough deadweight survey is carried out at or near the time of MOSIS installation, this gives a good baseline to go forward with. It is recommended that a deadweight survey is carried out regularly and no more than 5 years apart.
In addition to the deadweight surveys, every MOSIS test includes a weight check because drafts are read and displacement checked against the assumed value. If there’s a difference, then it will be obvious – even if it’s not that easy to find out where it is.
Like full load displacement, the actual VCG is estimated from each MOSIS test for that particular full load condition.
There are two checks. The MOSIS estimate of displacement times the MOSIS VCG can be compared with the vessel estimate. Separately, the vessels estimate of the deadweight can be ‘backed out’ of the overall vessel condition and a longer-term comparative and consistent measure of Lightweight and Lightweight VCG can be tracked.
These are checks with different, but related purposes – one looks at the overall floating condition of the vessel which is a comparator of the sum of deadweight and lightweight, the other looks at the ‘Lightweight’ as if the Deadweight is correct. Taken over time and several tests, it is possible to start to isolate where a discrepancy might be – or at least where the largest discrepancy might be.
The Transverse and Longitudinal centres of gravity (TCG and LCG) are simply determined from the heel and trim for a given loading condition.
MOSIS uses ballast as a means of getting an angle of around 1 degree. The weight shifted, and the resultant mean angle are used to determine the VCG in exactly the same way as a traditional inclining experiment.
Four critical elements required for an accurate MOSIS result are:
1. Accurate draft measurements
2. Accurate tension measurement of any moorings and risers
3. Accurate content measurement of the tanks used for ballast transfer during the MOSIS tests
4. Accurate recent deadweight survey