Abstract

Vortex-induced vibration (VIV) of tubings and pipings is of practical interest to oil industry. When VIV occurs to pipes, it induces alternative stress thatwhich would results in the reduction of pipe fatigue life. The objective of the paper is to investigate VIVs in tubings or pipings with non-linear geometry.

This paper categorizesd the occurrence of cross-flow and in-line VIVs of pipes based on flow directions and pipe orientations. The complexity of VIV for pipes with non-linear geometry, as compared to straight pipes, is demonstrated. We devised a scheme to incorporate power balance principal into finite element method (FEM) to analyze the cross-flow VIV for tubings and pipings with non-linear geometry. A main feature in our approach is the modeling of lifting forces (excitations) and hydrodynamic damping in FEM. At present stage, most commercial VIV packages mainly deal with risers or pipelines that are either straight or with large radii, and they only considered curvature component associated with the cross-flow direction of the flow (d.o.f.), that is the (i.e., the direction perpendicular to the flow velocity). Stress calculation near the bends and elbows of the pipes, is not accurate. Our approaches, on the other hand, consider the responses in all 6 d.o.f of each element along the pipes and it is a truly three dimensional vibration analysis. Our approach provides an alternative to complement the existing method for solving the VIV problem for tubings and pipings with highly non-linear geometry.

1. Introduction

The highly specialized subject of vortex-induced vibrations (VIVs) is part of a number of disciplines, incorporating fluid mechanics, structural mechanics, vibrations, computational fluid dynamics (CFD), acoustics, wavelet transforms, complex demodulation analysis, statistics, and smart materials. They occur in many engineering situations, such as bridges, stacks, transmission lines, aircraft control surfaces, offshore structures, thermowells, engines, heat exchangers, marine cables, towed cables, drilling and production risers in petroleum production, mooring cables, moored structures, tethered structures, buoyancy and spar hulls, pipelines, cable-laying, members of jacketed structures, and other hydrodynamic and hydroacoustic applications. One stimulus for a resurgence of interest in VIV came from the oil service companies foras the requirement of sustainable drilling equipments for use in to sustain in a deeper ocean and hurricane environments. and as the sea state along the depth of the ocean was found to be vehement in a hurricane.

Previous efforts in VIV in the offshore systems are mostly devoted to risers or free spanned pipelines with large radii of curvature. In contrast to previous work, this paper analyzes VIV for tubings or pipings with non-linear geometry. The tubings and pipings analyzed in this paper consists of horizontal, inclined, and vertical sections which are in the same plane. This paper categorizesd the occurrence of cross-flow and in-line VIVs of pipes based on flow directions and pipe orientations. The complexity of VIV for pipes with non-linear geometry, as compared to straight pipes, is demonstrated. We incorporated power balance principal into FEA to analyze the vibrational amplitude and stress for the tubings and pipings. A main feature in our approach is the modeling of lifting forces (excitations) and hydrodynamic damping in FEM. What distinguishes our approach from that of commercial VIV packages [1] is the absence of using piping modal curvatures in the prediction of the stress along the pipings. Most commercial VIV packages mainly deal with risers or pipelines that are either straight or with large radii, they only considered curvature component associated with the cross-flow direction of the flow (i.e., the direction perpendicular to the flow velocity). Stress calculation near the bends and elbows of the pipes, is not accurate. Our approaches, on the other hand, consider the responses in all 6 d.o.f of each element along the pipes and it is a truly three dimensional vibration analysis.

The structure of this paper is as follows. Section 2 discusses the occurrence of VIV to pipes with nonlinear geometry. The occurrence of cross-flow and in-line VIV is categorized based on flow directions and pipeline orientations. In section 3, the amplitude prediction scheme of power-balance, previously used in riser VIV analysis, is applied to pipings with nonlinear geometry. In section 4, the incorporation of the power-balance principal in FEM is devised and demonstrated using an example. Results from the amplitude prediction scheme and FEM are compared. The analysis is preceded in section 5 with SHEAR7 implementations of VIV in nonlinear pipings together with comparisons and discussions of results from FEM and SHEAR7. Conclusions are drawn in section 6.

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