Fatigue and Environmental Loading of Small-Bore Manifold Piping

Abstract

Small-bore pipes 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. Depending on well conditions and functional requirements the piping is often highly utilised statically. Vortex-induced vibrations (VIV) are on the other hand directly related to the frequency of the piping, and the more flexible a system is, the more prone it is to respond to cyclic loading induced by waves and current. Generally small-bore piping responds easily to VIV since the VIV onset flow velocity is directly proportional to the diameter. Small-bore piping have by default very small diameters, therefore the onset value for VIV is lowered compared to systems with larger diameters. In order to mitigate this environmental loading effect, it is favourable to increase the frequency of the system. Increasing the frequency is however normally done by making the system stiffer, more rigid, which generally is not desirable since this in turn increases the level of static utilisation. The current industry practice for VIV design of small-bore piping systems is to apply onset criteria. Onset criteria are in themselves applicable for small-bore systems, but they are very conservative, and DNV has in projects encountered several systems where a balance between onset of VIV and static utilisation was not obtainable. As temperatures, well growth and operational pressures increase with more complicated well conditions, the challenges with flexibility versus rigidity in small-bore piping will only increase, and previous design methodologies using typical onset criteria will be more difficult to apply successfully. There are many possible solutions to this problem; such as shielding, clever layout design, and/or direct VIV mitigation measures mounted on the pipes. This paper proposes a general methodology for advanced fatigue calculations to decrease the level of conservatism, and introduces some mitigation techniques which have been successfully applied in projects for systems where both onset criteria and fatigue utilisation criteria fail, and where fulfilling them is not possible within the confines of acceptable static utilisation.