Page 166 - Pipelines and Risers
P. 166
Force Model and Wave Fatigue 139
Theoretically, it is necessary to add the fatigue damage due to all of the above. The
accumulated fatigue is obtained by accounting for all sea-states and the joint probability of
sea-state combined with current. Since vortex shedding has been thoroughly discussed in
DNV (1998), this chapter shall focus on the wave-induced fatigue.
- Current Conditions
The current velocity is statistically described by a Weibull distribution as:
Where y,,,,p,,a, are Weibull parameters. The current velocity at a given depth U(z& is
transferred to current velocity at pipe level.
- Long-term Wave Statistics
Long term statistics are to bc applied in the fatigue damage assessment, whereby the wave
climate is represented by a scatter diagram of the joint probability of the sea state vector
Q=\H,,T,.~,~ and the wave spectrum, defined by significant wave height Hs, peak period Tp,
and main wave direction 8,.
- Short-term Wave Conditions
An irregular sea-state is assumed to be a short-term stationary process represented by a wave
spectrum,
s,, (f.S) = s,, (f)WS)
The directional properties are usually modeled as:
The non-directional spectrum s,,(f) adopted in this chapter is the JONSWAP spectrum. The
velocity and acceleration spectra at pipe level are derived from the directional wave spectrum
through a transformation, using Airy wave theory:
Sf,f,(f.g) =G;(f)s,,,(w.q), S,(f,s) =G:(f)S,(d)
where:
