Page 456 - Pipelines and Risers

Fatigue ofRisers 423

Fatigue of SCR was investigated by, e.g. Hatton and Willis (1998), Jensen (1999), Martins
(1999) and others.

22.7 Vortex-Induced Vibration Suppression Devices

Often, a deepwater riser will fail to meet the fatigue design criteria due to VIV. The designer
may choose to:

- Redesign the riser either by changing the mass (e.g. subtracting buoyancy), increasing the
tension, modifying the riser design (e.g. changing the type of top termination) or radically
changing the riser design (e.g. using a top tensioned riser instead of a catenary riser); or
- Add VIV suppression devices to reduce the vibration.

Changing the riser structural design is usually expensive relative to using suppression devices
(Howells and Lim, 1999). For example, since the natural frequencies of the riser, in bending,
are proportional to the square root of the tension, pulling harder on the riser gives only a
fractional effect. In addition, making the riser heavier or lighter, at best, only slightly alters
the tension to mass ratio in the natural frequency equation. The natural frequencies for the
modes of interest are, usually dominated by the tension, and not the structural bending
stiffness of the risers.
The addition of VIV suppression strakes increases the hydrodynamic drag loading on the
riser. This impacts all aspects of the riser response as well as riser hardware, materials,
fabrication and installation methods. This effect is particularly important for production risers
where service lives in excess of 25 years are often required.

22.8 Fatigue of Deepwater Metallic Risers

22.8.1 General
The industry has met today's challenge of developing reserves in 4,000 ft water depth, and
there are now plans for developments in depths of 6,000 ft and even 10,000 ft. The industry is
meeting the challenge of finding technical solutions and is going through the process of being
more cost effective. See Langford et al. (2000).
Specific technical challenges include:
Now assurance and insulation;
High top tensions for fully suspended systems;
Fatigue and touch down uncertainties in suspended systems;
0 High costs for hybrid riser and flexible systems.
Tremendous effort has been expended in the determination of the global (and local) response
of these systems, increasing the confidence in the industry of the optimum approaches for the
specific applications. However, the local strength of these systems has comparatively not
undergone such a detailed review.