Any calculation of semi-submersible stability against capsize in waves implies some theoretical perspective of the hydrodynamic hull loads. Current stability criteria are based on a perception of the loads as purely hydrostatic this paper shows how a new theory of hydrodynamic loading on lattice structures is both simple enough to be applied in regulations, and powerful enough to explain wave-induced tilt phenomena. These steady tilt effects have long been observed and are symptomatic of the malaise in existing stability regulations, because they cannot be explained hydrostatically. The relation of the new theory to the established ‘Froude-Krilov’, ‘Stokes’ expansion' and ‘Monson's equation’ hydrodynamic theories is explained (particularly the inadequacy of those theories for studying tilt effects), and model test results are presented.


Although it is possible to devise stability criteria based entirely on physical measurements on rigs at sea, and it is also possible to devise criteria based on physical model experiments (single-point mooring systems, for example, are classed by Certifying Authorities on the basis of model tests), much the most useful are criteria which can be applied purely by calculations at the design stage. These avoid the need for physical models, but require some theoretical formulation of the stability problem Such, of course, is the existing hydrostatic theory (Section 3 below) that forms the basis of present stability criteria, which leads to calculations that are simple enough to carry out by hand on the drawing board

The motivation for this chapter is effectively the advent of cheap digital computing power, which allows much more complicated criteria to be applied at the design stage The prototypes for this type of computer-applied criteria are the wave loading design rules applied to fried jacket-type offshore structures using ‘Monson's equation’ (see Section 5 below). These can only be applied to modem deep-water jacket designs with the aid of computer programs, but the results of such computer runs are accepted by certifying Authorities as proof of compliance with the various wave loading design rules.

The problem is therefore to devise a more realistic theory of vessel behaviour which overcomes the criticisms of the present hydrostatic theory cited in Section 3 below, and leads to stability criteria which can be readily applied by computer program, although they may be impractical to apply by hand.


The truth about engineering computer programs is very simple most computer programs are thought to work only because they have not been run often enough to reveal the bugs. That select group of engineering computer programs whose track record with many users over the years has proved that they really do work, almost all have the following three characteristics.

  1. They perform a function which can be specified with great mathematical precision (e.g find a stress distribution or a heat flux with well-defined boundary conditions) In particular, if any empirical coefficients are incorporated, they are exactly defined (e g the inertia and drag coefficients in ‘Monson's equation’)

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