Probability density functions of extreme wave- and current forces on fixed spaceframe offshore structures

Abstract

Random wave and current forces acting on an offshore structure, cause random base shear forces in time. The peak base shear is the maximum value of base shear that occurs during the passage of a wave. The extreme base shear is the maximum value of base shear that occurs within a period of time, say one hour. So far, the distributions of extreme base shear forces acting on jacket type structures were calculated numerically. Time domain simulations of base shear were generated for the jacket type offshore structure by means of a computer program, using the Morison equation on every member segment. Extreme base shears could be sampled from these time records of base shear forces. By simulating a storm over a period significantly greater than one hour, say sixty hours, histograms of extreme base shear could be developed. These computer simulations cause high computer costs. This study investigates a method to develop probability density functions of extreme base shears acting on a jacket-type offshore structure. A probability density function of extreme base shears acting on a single vertical cylindrical column structure was derived analytically in the past. The probability density function for a single column is used to obtain the probability density functions for a jacket-type offshore structure. The complex jacket-type offshore structure is represented by a number of vertical cylindrical columns which cause equivalent wave and current forces.