Introduction

For true probabilistic design of load-carrying members of offshore structures that have to withstand the effects of the environmental forces generated by wind, waves, and current, it would be necessary to provide a simultaneous probabilistic model of all three load-generating sources. As an alternative, a simplified conservative approach may be applied. According to NORSOK N-003 (2007), the 100-year environmental condition can be represented by the 100-year wave and mean wind speed, and the 10-year current.

In the following sections, the focus is on the three commonly used approaches for calculating characteristic values of extreme load effects due to wave forces for ULS design checks, viz.,

• the design wave approach

• the design sea state approach

• the full long-term approach

Design Wave Approach

For the design of offshore structures where the load effects to be considered are primarily of a quasistatic nature, the extreme load effect for ULS design check can be estimated by the so-called 100-year wave approach. This is understood to mean a wave with a wave height H(100) being exceeded on the average only once every 100 years. Equivalently, H(100) can be interpreted as the wave height that is exceeded during a period of one year with a probability of 10−2, cf. Section 10.8.

Estimation of load effects according to this format would in practice proceed as follows. (1) The design wave height is established on the basis of available data relevant for the offshore location in question.