Simplified VIV Response of Partially Strake-Covered Pipe Spans

Abstract

A simplified method for the fatigue assessment for partially strake-covered pipeline spans is provided and calibrated so that for the bare pipe case it is consistent with the current DNVGL Recommended Practice (DNVGL-RP-F105). It is based on a Rayleigh-Ritz approximation using the first undamped mode shape. In that sense it is equivalent to a single-mode analysis using Shear7, except that it accounts for frequency-dependent added mass, and the formulation is presented in a compact and convenient non-dimensional form. The method presented may also be considered a simplification of that in Vivana in that it does not account for propagating wave effects, and is therefore named “Sivana”. Like any frequency-domain method Sivana is based on the assumption of harmonic response. This requires excitation and added mass functions that define a harmonic hydrodynamic force as a function of the harmonic motion. On the other hand, current practice by DNVGL-RP-F105 is based on a response function, which gives VIV amplitude as function of reduced velocity and the Scruton mass-damping parameter Ks. To bridge this gap, an inversion technique is developed whereby the excitation function for bare pipe is derived from the DNVGL-RP-F105 response function, such that for bare pipe the Sivana calculation will lead to the same response amplitudes as DNVGL-RP-F105. The Sivana method is illustrated by an example involving a single span with different degrees of strake coverage centered at midspan. Although the Sivana method might be considered a natural and straight-forward extension of current span assessment practice to include partial strake coverage, caution is in order for certain approximations in the DNVGL-RP-F105 response function may be less appropriate for the use in this paper, than for the use envisioned by the developers of the DNVGL guidance.