Modeling and Estimation of Fatigue Damage Accumulation in Marine Risers of Fixed Platforms

Offshore gas and oil fields are being discovered and exploited nowadays in water depths of more than
2000 m. The operation in depths increases the external forces so that the equipments should be more
advanced and expensive. Fatigue damage accumulation is one of the most important problems about
marine structures that are exposed to the alternative loads. Riser is one of these structures. The
surrounding environment of riser causes alternative stresses and fatigue in riser. To obtaini these
stresses, the analysis of the vortex induced vibration in two direction, in-line and cross flow, is
necessary. The risers are long relative to their cross sections so that the Euler-Bernoulli theory is
applicable for description of the pipe dynamic bending. The flow and riser motion in x direction forms
a vortex shedding in front of riser. These vortices cause the drag force in x direction and the lift force
in y direction. These forces are time dependent. The drag force is obtained from Morison’s formula.
The lift force excites the vibration in y direction. After obtaining the equation of motion and solving it
by using numerical methods such as Galerkin, finite difference and Runga-Kutta fourth order method,
the riser deflection in two directions will be obtained and the stress at the outer diameter of riser will
be calculated. The fatigue damage accumulation in the riser can be obtained by using Shigley theory
and Palmgren-Miner rule.