Page 446 - Pipelines and Risers

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Chapter 22

Fatigue of Risers

22.1 General

Fatigue damage of risers is mainly due to (see API RP 2RD, 1998):
- 1'' order wave loading and associated floater motion
- 2"d order floater motion
- Vortex induced vibrations (VIV) due to current

Fatigue analysis of risers may be satisfactorily conducted considering the effects of VIV, 1"
order loads and motions and 2"d order effects independently. Catenary riser tension
distribution and therefore response are far more dependent on floater position and the
interaction of environmental conditions and vessel motions is more important. Consequently,
the effect of drift motions must be carefully considered in all aspects of fatigue analysis, and
is unlikely to be dealt with satisfactorily in isolation.

22.2 Fatigue Causes

22.2.1 lSt Order Wave Loading and Floater Motion Induced Fatigue

As a minimum, wave loading shall be defined by a Hs-Tp (or Hs-Tz) scatter diagram.
Definition of wave loading by individual waves is not satisfactory for catenary riser response
due to the dependency of floater position on seastate period. The parameters, which define the
seastate spectra, should be provided based on observed data. This may take the form of
Pierson-Moskovitz or JONSWAP single peak spectra or a Bi-modal spectrum. Further
definition of wave loading conditions should consist of a spreading parameter, which gives
the directional distribution of wave loading about the predominant direction. This is a cosine
function, the power of which varies according to environmental location.
The directional probability of wave loading should be specified for each of at least 8 compass
points. These probabilities are used to avoid undue conservatism in estimation of riser fatigue
damage that may result from assuming loading from one or two directions.

1. Linearisation