The load level on a jack-up platform is typically governed by the drag term of the Morison equation. Thus, the variabilities related to the extreme wave induced particle speed are very much amplified by the non-linearity of the loading mechanism. The total variability of the wave induced speed consists of both aleatory uncertainties and epistemic uncertainties. The first category is caused by the inherent randomness of e.g. the annual largest wave, while the latter are introduced by the imperfect modelling of this quantity and the calculation of the corresponding particle speed. Subsequently, the various sources of aleatory and epistemic uncertainties related to the calculation of the base shear of a jackup are identified. Uncertainties related to the wave climate description are established from data for two North Sea locations, while uncertainties related to the calculation of kinematics are quantified based on available literature. The relative importance of the epistemic uncertainties of the kinematics are assessed using a first order reliability method. Finally, the adequacy of the commonly adopted load factor of 1.3 for environmental load is discussed in view of the properties of and the loading on an idealized jack-up platform.
The overall uncertainty related to the design of marine structures is usually dominated by the uncertainties related to the load calculation. Uncertainties may be divided into two main categories; aleatory (inherent) - and epistemic uncertainties. The aleatory variability of the extreme forces is mainly produced by the inherent randomness of the environmental conditions e.g. the largest wave and the simultaneously occurring wave period, current, water level and wind. The aleatory uncertainty is accounted for by introducing the primary environmental parameters as random variables described by a joint probability density function.