Fatigue and Environmental Loading of Large-Bore Manifold Piping

Abstract

Large-bore piping inside manifold structures are designed to be flexible. The piping must absorb thermal and pressure loading, as well as installation tolerances, well growth and in some cases stroking and/or other temporarily imposed displacement-controlled conditions. In order for branch pipes, flex-loops or other jumper type configurations to cope with the functional, fabrication and installation loads they are designed to be highly flexible. Depending on well conditions and functional requirements the piping is commonly highly utilised statically. Vortex-induced vibrations (VIV) and wave-induced loading on the other hand are directly related to the Eigen-frequencies of the piping systems, and the more flexible a system is, the more prone it is to respond to cyclic loading induced by waves and current. In order to mitigate environmental loading effects, it is favourable to heighten the harmonic frequency of a system. Increasing the harmonic frequency is however done by making the system more rigid, which generally is undesirable for flexible large-bore piping systems in manifolds, since this in turn increases the level of static utilisation. There is to DNV’s knowledge currently no standard industry practice on VIV and environmental loading for large-bore manifold piping, since these normally are sufficiently rigid to prevent VIV. Recently, however, DNV has encountered systems where VIV and environmental loading have presented significant challenges for flexible large-bore systems, and this paper proposes a series of methodologies to estimate fatigue and extreme environmental loading of such systems. Advanced applications of DNV-RP-F105 and DNV-RP-C205 are detailed from recent project experience.