ABSTRACT The development of a system which allows accurate definition of the stability of a semi-submersible in service is described The system is easily deployed and operated by the marine crew and allows the build-up of a statistical statement of vessel Lightweight and associated. Vertical Centre of Gravity Numerical checks of the system accuracy have been performed using Monte-Carlo simulation techniques and practical checks have been performed, firstly by direct sheltered water comparison with a conventional inclining experiment and secondly, in-service, against the weight records of a range of semi-submersibles. Alternative methods of monitoring the stability of semi-submersibles in-service are discussed and shown to be inapplicable. INTRODUCTION The structure of this chapter broadly follows the stages of development actually experienced in the design of In-Service Stability Measuring Equipment, In that the problem identification and the necessity for in-service measurements are first identified followed by an evaluation of the currently available and proposed methodologies. For specific reasons these available or proposed techniques were rejected and development resources focussed upon emulating a formal inclining experiment in the service environment. To this end, a specific characteristic of the floating body was identified, namely that such a body exhibits orthogonal major and minor rotational stiffness axis. It is this physical fact which is used to optimise the results of a series of inclining tests performed in what is technically known as a ‘noisy’ environment The ‘noise’ referred to in this case arises from environmental wind and wave action which perturbate the vessel mean position in addition to the more normal concept of observational error, and its effect must be minimised if a high degree of accuracy is to be achieved in the final results. The combined process of filtering the vessel responses and of then fitting the results optimally to an elliptic rotational stiffness model of the vessel restoring characteristic has been termed 'Model Optimisation'. The initial development of the methodology and algorithms was performed using synthetic data, and the level of accuracy established using Monte-Carlo simulation techniques. Subsequently the equipment has been installed offshore on several semi-submersible vessels and sample results from these are given for comparative purposes THE NEED FOR IN-SERVICE STABILITY MEASUREMENT It is a legal requirement that the stability of all marine vessels, including Offshore Floating Production Systems and semi-submersibles engaged offshore, should have their stability against capsize estimated by some contemporary means, normally based on a known vessel reference condition. Conventionally ths obligation has been achieved firstly by establishing a datum point, known as the Lightship, and by then calculating the current vessel condition by extrapolation from ths lightship using the currently recorded vessel weight inventory Traditional marine practice has normally required the establishment of this datum point only once in a vessel's lifetime, (1 e after completion and before handing over for service) unless significant modifications have been made to the vessel.