In the existing procedures for assessing stability of ships and floating structures, the qualities of the vessel-are represented by righting arms curves in calm water. As a result, dynamic effects of the interaction of the vessel with the environment cannot be rationally taken into account It is possible to say that m general, no sufficiently comprehensive and coherent conceptual framework exists at present for the establishing of fully rational procedures for stability assessment.

In the chapter a new procedure is described which relies on a direct application of stability criteria to the generalized configuration (defined m terms of absolute and relative motions) of the vessel The procedure relates the generalized configuration to a set of control parameters which are regulated by the designer and operator of the vessel, and provides a unified approach to problems of intact and damage stability Dynamic effects of the interaction between the vessel and environment are explicitly taken Into account and can be evaluated either by means of model tests or through a numerical simulation. The procedure is therefore entirely rational and can be applied at various levels of advancement.


The need to rationalize the existing procedures for assessing stability of ships and floating offshore structures appears to be widely recognized1-"'. Reviewing what has recently been written on the subject three main factors can be identified as contributing to that recognition.

  1. new types of ships and structures do not easily fit in the format of existing rules and regulations;

  2. losses of floating units, involving tragic losses of human lives, challenge the adequacy of existing rules and regulations,

  3. the substance of existing rules and regulations has become incommensurable with available knowledge of dynamics of floating bodies

In spite of the tendency to introduce appropriate rationalizations, attempts to this effect are being found difficult to succeed. It can be argued that the difficulty arises from the nominality of current procedures, as in fact regulating the GZ curve of a The evaluation of stability according to these procedures is nominal since the GZ curve does not directly represent the behaviour of a vessel in the seaway, see refs 7 and 8 for relevant comments with respect to rules applicable to offshore structures. The nominality of the procedures hampers rationalizations motivated by (a) and (c) listed above because, with respect to (a). without gathering an appropriate experience it is virtually impossible to judge d nominal criteria apply to a new type of vessel, and with respect to (c). there is no clear cut relation between the dynamics of vessel motions and nominal criteria of stability Considering factor (b), and assuming the nominal criteria to be adequate, it should be observed that stability assessments according to nominal criteria cannot be sufficiently related with other design considerations to provide efficient assessment procedures. This aspect of the present discussion will be further elaborated.

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