VIV Responses of Riser With Buoyancy Elements: Forced Motion Test and Numerical Prediction

Abstract

Steel Lazy Wave Riser (SLWR) is an attractive deep water riser concept. When subjected to vortex induced vibrations (VIV), the vortex shedding process of the buoyancy element and the bare riser section will be different due to the difference in diameter. VIV responses can be strongly influenced by the dimension of the buoyancy element and its arrangement. Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are widely used by the industry for design against VIV loads. However, there is lack of hydrodynamic data to be used in these programs when buoyancy elements are present. Experiment to obtain hydrodynamic data for riser with staggered buoyancy elements was carried out in the towing tank in SINTEF Ocean. A rigid cylinder section with three staggered buoyancy elements was subjected to harmonic forced cross-flow (CF) motions. Hydrodynamic forces on one of the buoyancy elements were directly measured in addition to the measured forces at both ends of the test section. Two buoyancy element configurations were tested and the corresponding hydrodynamic data are compared with that of a bare cylinder. The obtained hydrodynamic data was also used in VIV prediction software and good prediction against existing flexible cylinder staggered buoyancy element VIV test data was achieved. A roadmap to achieve an optimal SLWR design by combining different experimental and numerical methods is suggested.

Hydrodynamic Coefficients of a Flexible Pipe With Staggered Buoyancy Elements and Strakes Under VIV Conditions

“…The added mass on the buoyancy segments is always be negative identified from the flexible model testing, while in positive from the forces oscillation test. Furthermore, the added mass coefficients of 10-15 identified are found close with those around 9 obtained from the corresponding forced oscillation tests on the corresponding rigid bare-buoyancy segment, where the same frequency and close amplitude was applied [14], while around -0.5 identified on the buoyancy elements and 1.9 from the forced oscillation tests. The identified force coefficients are further divided into two groups: coefficients associated with displacement phase angle 0-180 degrees corresponded with counterclockwise orbits are marked by blue circles, and the ones with 180-360 degrees phase angles corresponded with clockwise are marked by red circles.…”

“…This is though because the present identifications are from the bare segments vibrations dominating frequency, where the vibrations of the buoyancy elements was a kind of "forced motions" and kept in the same exciting/damping regions as bared segments, but couldn't get enough energy to reach their resonate conditions. The guess of the reason will be further confirmed by more comparisons between the identified coefficients from configuration C5 and those from the forced oscillation test on the corresponding rigid segments of C5 [14].…”

“…The coefficients in phase between 180-360 degrees are mostly in negative, which is in anti-phases with those of the forced oscillation tests. bare-buoyancy segments, where the same frequency and close amplitude was applied [14]. And the exciting force coefficient are around 0 identified on the buoyancy elements and -0.5 from the forced oscillation tests [14].…”

Numerical Investigation of Vortex-Induced Vibrations of a Flexible Riser with Staggered Buoyancy Elements

“…The non-dimensional frequency for the buoyancy element was generally lower than that for the bare riser section. Significantly lower non-dimensional frequency (f CF ≈ 0.1) could be found in model tests for shorter buoyancy element (L B /D B = 1) configuration [18]. When comparing with the measured responses, the response frequencies are, in general, over-predicted by the VIVANA model with the default set of excitation coefficients.…”

“…Large differences were observed while predicting VIV responses of the riser with staggered buoyancy elements using the default hydrodynamic coefficients for both bare and the buoyancy elements [17]. A forced motion test of a rigid cylinder with staggered buoyancy elements was carried out to extract hydrodynamic coefficients on buoyancy elements and bare pipe sections by Wu et al [18]. In their tests, hydrodynamic force coefficients subjected to pure CF motions were obtained.…”