· · · Repository Hydraulic Engineering Reports

In m.any design problem.s the sea state is specified as part of the design criteria. For offshore structures it is important to know the maximum. probable wave height and the maximum. probable wave force which might be experienced during the life expectancy of the structure, for example the 50 or 100 year wave height and period and the 50 or 100 year wave force. Based on previous analysis of wave and wave forced measurements, practically no correlation was found between the measured "apparent wave height" and "apparent wave force." Several authors have proposed statistical distribution of drag and m.as s coefficients, with recommendations for use in design. The statistical distribution of the calculated drag (or mass) coeefficients has no relationship to the statistical distribution of the measured wave drag forces (or inertial forces). Borgman (1964) has considered the statistical distribution of wave forces. This paper presents methods for predicting the probability distributions of peak wave drag and inertial forces, and a method is proposed which might be used to predict the most probable maximum wave force once the sea state is specified. It is found that there is a good correlation between the probability distributions of wave heights and the probability distributions of peak drag forces. For example, if the most probable maximum. wave height is given from. a statistical distribution of wave height, then it is possible to predict the most probable maximum drag (or inertial) force, and the most probable maximum force (combined drag and inertial force).

A computational formula is developed for determining, from the sea surface spectral density, the spectral density function of the force per unit length at a point on a vertical pile. A surprisingly accurate and simple approximation for the formula is presented and used to explain the near proportionality between the spectral densities of force and sea surface measured in the ocean near Davenport, California. The computational formula and its approximation are extended to provide procedures for the determination of the spectral density of either total force or total overturning moment on a structure consisting of an array of vertical cylinders. The approximation leads to a particularly simple relation in which the spectral density for the total structure is the spectral density for a single pile times a function which characterizes the geometry of the array. As an illustration of the procedure, the total force spectral density is computed for a four-pile instrument platform in 49 feet of water.

Drag and mass coefficients corresponding to Stokes' fifth - order theory have been determined from previously published wave and wave - force data. Comparisons between statistical distributions for fifth-order and corresponding first order coefficients have been made for varying wave steepnesses. For other existing wave data, from which the fifth order drag coefficients could not be determined , the statistical distributions of first-order coefficients have been investigated varying wave steepnesses. Statistically based fifth- order and first-order coefficient design values for varying Reynolds numbers are presented. Statistically based fifth and first - order mass coefficient design values are suggested . Comparisons between drag-coefficient distributions . and corresponding mass-coefficient distributions seem to provide some insight into the reasons for the scatter on the coefficients.

Experimental research to the influence of the air pressure on the impact pressure cause by breaking waves. The research shows that there are three factors which load the structure during the breaking of waves: the water layer over the structure, the shock due to the impact of the water jet from breaking and the air pocket entrapped in the water mass. The influence of all of the factors is determined.