Page 450 - Pipelines and Risers

Fatigue of Risers 417

Deepwater risers are usually so long that significant currents will excite a natural bending
mode that is much higher than the fundamental bending mode. Since deepwater currents
usually change in magnitude (and direction) with depth, it is therefore possible that multiple
modes of the riser can be excited into VIV. This makes deepwater riser VIV prediction much
more complex than that for short riser spans typical of fixed platforms in shallow water.

The VIV response of deepwater risers is further complicated by the presence of adjacent
tubulars such as risers and tendons. When all, or part, of a riser is in the wake of an upstream
tubular, the VIV of the riser can be substantially altered and often worsened. Furthermore, the
presence of adjacent tubulars can cause changes in the drag forces acting on a riser, resulting
in the possibility of damaging collisions between tubulars.

Analysis Methods
VIV may be generated by waves or currents and may occur either in-line or normal to the
direction of current flow. The most severe form of VIV, in terms of riser fatigue damage, is
cross-flow vibration due to steady current.
The analytical methods used for calculation of VIV response are based on empirical
observations. Until recently, much of the guidance on VIV behavior only considered lock-on
vibration in uniform flow. This can give conservative predictions of fatigue damage. The
methods must therefore consider the sheared flow regime along the riser length and
interaction of vibration modes excited at different points along the riser.

Modeling Approach
Definition of current velocity profile is an important factor. The current velocity component
normal to the riser must be calculated which is dependent on the angle variation along the
riser and the incident angle of the current.
TDP at the seabed may be modeled using a pinned end restraint. Consideration should also be
given to the damping effect of the seabed.

Analytical Approach
Analyses are first conducted assuming no suppression devices are attached to the riser. The
fatigue damage incurred from VrV of each profile analyzed is then factored accordion the
frequency of Occurrence of the profile is calculated and the total fatigue damage due to VIV is
then given by the sum of the factored damage for each profile. Final analyses are conducted
using the specified arrangement, which incorporates VIV suppression devices as required to
achieve the desired fatigue life. As directionality of current and riser orientation is not
specified, analyses are conducted for currents flowing in the plane of the riser and normal to
the riser. For application of the currents in the plane of the riser, the velocity profile is
resolved normal to the nominal riser position.
22.2.4 Other Fatigue Causes

The following causes should be considered for fatigue evaluation as appropriate.

- Shutdown and start-up