Page 451 - Pipelines and Risers

418 Chapter 22

Normal operational shutdown and start-up of the oil transport will introduce load cycles
giving stress range for risers. Stress ranges calculated from stress variation between cold
unpressurised to normal operating condition. Stress concentration for welds and corrosion
allowance should be included in the stress ranges calculation.

- Effect of installation

The effect of reeled installation and plastic deformation of riser welds should be included in
the fatigue life estimation.

- Effect of floater

The hull flexure (springing) may have effect on the fatigue life of risers. This should be
considered by taking into account of springing numbers.

- Effect of soiYriser interaction
The effect of soil/riser interaction is investigated by Carisma JIP (2000).

22.3 Riser VIV Analysis Program
The preliminary design of fatigue resistant risers requires relatively easy-to-use structural
dynamic models, which have the capability to estimate dynamic stress levels in the riser as a
function of the properties of the structure and imposed velocity profiles (Allen, 1998). The
programs must lend themselves to easy parametric variations of current profiles, tension and
structural properties. The user must understand the assumptions and program limitations. The
most widely used program at the present time is the MIT program SHEAR7 (MIT, 1995 and
1996). More information on SHEAR7 may be available from Vandiver and Li (1998) and
Vandiver (1998).

SHEAR7 combines easy to mn features with a reasonably sophisticated, but invisible to the
user, non-linear, fluid-structure, interaction model. The interaction model allows for the local
lift coefficient and local hydrodynamic damping coefficient to depend on the response
amplitude. SHEAR7 does not, as yet, include a means of allowing the response of one mode
to influence the excitation of other modes.

SHEAR7 is based on mode-superposition 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 super-critical Reynolds numbers limits the absolute accuracy of all
programs currently available. In many straight riser scenarios in sheared currents, common to