A method is described whereby the hydrostatic stability of a floating structure such as a drilling semi-submersible or floating production system can be accurately and conveniently measured while the vessel is in service. Measurements are made using a small instrumentation package which may be permanently installed on the vessel. Operation of the system does not significantly affect the vessel's operation and may be conveniently performed by the vessel's own marine staff.
It is a requirement on operational and legal grounds that the Margin of Stability against capsize and extreme motions of Semi-submersible and other floating structures can be assessed at any given point in time whilst the vessel is in service.
This paper describes the methodology and rationale behind the development of an on-board instrumentation and analysis package which permits accurate measurement of the current magnitude of the stability lever arm of the vessel against capsize. It is a practical paper which directly addresses the Engineering solutions to a practical problem.
In section 2 the conventional means for establishing the margin of stability against capsize for a floating structure are discussed and the potential problems and sources of error identified.
This section also considers the advantages to the vessel operator of a direct means of establishing the vessel stability conditions in service, and the way in which such a capability eases the vessel management function.
Section 3 provides a critical summary of the main methods proposed in recent years to either check or to monitor vessel stability. Some of these proposals involve great mathematical complexity in their formulation and development; it is also true to say that these are the systems which have demonstrated the least ability to perform reliably when implemented.
The salient points of the final stability measuring system are described in section 4 and the successful implementation on differing types of semisubmersible are shown in section 5. The results of several In-Service Stability Measuring exercises are given in detail and compared with the expected values.
The present requirements for establishing the measure of a particular vessel's stability against capsize are based on ship practice, and thus on the use of a formal Inclining Experiment to define the basic vessel weight characteristics.
For a ship, the "base point" is termed the vessel Lightweight and comprises the vessel structural steelwork and those miscellaneous items sufficient to allow the vessel to operate. It specifically excludes cargo and fuel, but does include service fluids and the crew and their effects. For a merchant ship this Lightweight is substantially constant over the whole of the vessel's life. For a semi-submersible however, the establishment items required to operate the vessel will vary greatly from one operating mode to another, leading to difficulty in defining the lightweight. The most satisfactory practice is to remove as much accountable weight from the vessel lightweight measurements as possible: in this way the availability of the vessel for different duties is more apparent, although the work involved with the management of the on-board weight inventory, and the accuracy required, greatly increases.
