Page 200 - Dynamic Loading and Design of Structures

Page 175

Chapter 5

Wave loading

Torgeir Moan

5.1 INTRODUCTION

Wave loading is important for ships, fixed and compliant offshore structures, floating bridges
and airports. The focus here will be on marine civil engineering structures such as fixed and
compliant platforms and buoyant bridges, as shown in Figure 5.1.
Wave load effects are required for design checks of ultimate, accidental and fatigue limit
states (ISO, 1994). Ultimate and accidental limit states are often governing and are based on
extreme load effects. For structures in extratropical climates, fatigue may also be an important
design criterion and require an estimate of the number of stress ranges at different magnitudes
for the service life. It is noted that the main contribution to fatigue damage is caused by load
effects which are of the order of 15–25 per cent of the extreme load effects in the service life
and hence by waves with periods in the range 2 to 8 sec.
Wave load effects for design are commonly determined by quasi-static analysis methods
when the structures or structural modes have natural periods in the lower range of wave
periods, while an intrinsically dynamic analysis approach is required for structures with
natural period above the wave period.
Typical ranges of natural periods for some marine structures are displayed in Figure 5.2.
However, besides wave loads with period equal to the wave period, in the range of 2–3 to
20 sec, the presence of certain non-linear features of the loading may cause steady state loads
with a period which is a fraction , ,…or a multiple 2, 3, …of the wave period. Such steady
state loads, as well as wave impact loads and other transient loads, may cause inertia and
damping effects which need to be accounted for by using a proper dynamic analysis
methodology, also for platforms with natural periods below the wave period. For such
structures dynamic effects on fatigue loads will be most important.
For situations where dynamic effects need to be considered, the long-term stochastic
character of the wave loading and its effects is important to recognize. At the same time,
design analyses should be made as simple as possible, not least to avoid errors.