Page 458 - Pipelines and Risers

Fatigue of Risers 425

and therefore has a practical limit of about one hundred participating modes. The program
was initially written to model straight risers with constant diameter with spatially varying
tension. It has been extended to model structures such as catenaries, by hybrid techniques in
conjunction with finite element models. As with all existing VIV design programs for risers,
SHEAR7 requires calibration with measured data.

The relative lack of data at supr-critical Reynolds numbers limits the absolute accuracy of all
programs currently available. In many straight riser scenarios in sheared currents, common to
the industry today, the likely error in the response amplitude prediction may be as high as a
factor of two. Much of the reason for this lack of accuracy is to be found in the complexity to
model the hydrodynamics and in the lack of calibration data at high Reynolds numbers.

The conclusion from this review of determination of VIV is that the level of uncertainty in
analysis is relatively large, this alone will result in conservative, or inappropriate (un-
conservative), factors of safety being applied which in turn could mean unnecessary VIV
mitigation measures are adopted. The industry is addressing this issue, the most notable being
the STRIDE Joint Industry Project (Willis, 1999).

Slugging
If the hydrocarbons being transported from the seabed is in a liquid phase then there will be
no slugging. However a large proportion of developments either have condensate (a mixture
of gas and liquid hydrocarbons) or require gas lift to get the hydrocarbon to the surface (due
to low well pressure - shallow reservoirs). In the case of both the condensate and gas lift there
will be a tendency for the gadliquid to separate, which will result in a change in momentum.

The effect of two and three phase flow in the riser should be included in the fatigue life
estimation, but the software available to the industry can not handle this effect yet. The
slugging inside the riser makes the riser to move with large deflections. The stress induced by
the deflection should be included in the fatigue analysis.

With present design practices it is not normal to include slugging effects in the fatigue
analysis, and when it is performed there are large question marks about how representative the
analysis is. This is an area that requires attention from the industry.

Seabed Touchdown Point (TDP)
Riser TDP varies due to vessel 1st and 2nd order motions, current drag, VIV and effects due
to slugging. The change in the TDP changes the natural frequency of the riser, and in turn
affects the response and so the loadings generated by VIV, current drag et al. With the
uncertainties of the loads generated by VIV and slugging for a quasi-static situation then the
loads for a dynamic situation are questionable. Bearing mind the uncertainty of analysis, there
is potentially interaction between the riser and seabed. Should the seabed act in a rheotic way
i.e. the seabed will increase in stiffness when the riser is pulled out from an embedded
position. This effect will have a dramatic increase in local stresses and will have a direct
impact on the fatigue life of the riser. This concern is being addressed by the industry.