In determining the motion and structural response characteristics of offshore platforms, designers, due mainly to lack of well proven alternative methods, make predictions by combining maximum loads resulting from waves. winds and currents, by assuming they occur from the same direction together at their peaks Usually the effects of wave directionality are also neglected In design calculations Design calculations based on a combination of extreme events with a correlation factor of unity and on a uni-directionally wave approach can result In unintentional and implicit conservatism The amount of conservatism built Into design calculations depends on the type of structure and the environmental conditions where the structure is to be Installed While it may be assumed that the uni-directional wave approach yields conservative design values, In the case of extreme motion and structural response of certain types and configurations of offshore platforms, one has to question the application of this assumption to directionally sensitive structures and fatigue load calculations There are a number of recognised advantages to including the joint probability approach and wave directionality design calculations(see also Ref 8)
It serves us as a sound bass for combining environmental events in a rational manner
It provides a framework for incorporating Improved knowledge of the environment
It provides greater accuracy In design calculations
It allow a more consistent design approach In moving from one area to another
It avoids unintentional and implicit conservatism
In this paper the results of motion and structural response predictions for a directionally sensitive compliant articulated tower-ship system (SALM) are presented to illustrate the effect of short-crestedness and of extreme wind, wave and current forces as they occur with different assumed correlation factors both In magnitude and In direction In the last part of the paper the results obtained from the analysis of full scale measurements and the use In deriving joint probabilities of extreme wave, wind and current parameters are discussed
In order to predict extreme motion or structural response parameters In multi-directional or short-crested waves one needs to define a directional wave spectrum A directional wave spectrum is generally represented by the following equation Equation (1) (Available in full paper)
where G(¿, a) IS the directional spreading function and represents the directional distribution of the total wave energy Our knowledge of directional wave energy is limited due to lack of field measurements However, there are a number of proposed equations, as discussed by Goda In Ref 2, which can be utilized to model short-crested waves A review of directional wave energy models n also given by Prince-Wright and Percival In Ref 10
In this study, the articulated tower-ship term shown In Fig 1 IS considered in studying the effect of wave directionality and of joint probability of occurrence of wave, wind and current forces
