Pragmatic Riser VIV Analysis
Authors: Triantafyllou Michael , Triantafyllou George , Tein Y.S. David , Ambrose Bill D. 
Published: October 2008

Drilling and production risers experiencing vortex-induced vibrations (VIV) may adversely impact operations in two ways: by increasing deflection due to amplified drag and causing structural fatigue damage due to VIV induced dynamic stresses. Complex hydrodynamic mechanisms controlling correlation length and force dependence on motion and frequency of response are major obstacles in predicting the VIV response, except for a limited number of cases for which extensive experimental calibrations were carried out. A pragmatic VIV analysis methodology has been developed based on correlation length measurements, and riser specific experimental tests to establish basic CD and CL databases for realistic riser configurations with auxiliary lines and buoyancy modules. Correlation length properties are established using flow visualization methods, in particular Digital Particle Image Velocimetry (DPIV), which allows whole-field quantitative information on the velocity and vorticity fields to be obtained. Identification of the role of two basic VIV mechanisms, the formation of braid vortices and the impact of vortex splitting, has provided a robust correlation length model. A VIV analysis program has evolved from these experimental tests in conjunction with extensive research in structural and hydrodynamic riser responses.

Introduction

The design of drilling risers for mobile offshore drilling units (MODUs) is governed by operational considerations, specifically, the need to keep the riser deflection angle within 1° to 2° during normal drilling operations. This necessitates stringent measures:keep the rig within a tight watch radius of around 0.5% to 2.0% water depth,maintaining high riser top tension, andminimizing riser net in-water weight by the use of buoyancy modules.

The occurrence of VIV may adversely impact drilling operability because VIV causes an apparent increase in hydrodynamic drag, resulting in increased riser deflection angles. In addition, fatigue damages appear to be a second concern, however less so since it is feasible to frequently inspect risers. In the event of strong current, the need to keep riser deflection angles at a minimum may dictate the use of faired riser sections. This is, however, a very costly option and must be avoided if at all possible. For example, increased tensioner capability may prove to be just as effective without encountering the same handling difficulties.

As for production rigid risers and steel catenary risers for permanently installed platforms, they are much more tolerant of riser deflections and require minimal riser tensions. As a result of low top tension these risers become much more compliant and VIV responsive than rigid drilling risers. Fatigue considerations may dictate that these risers be protected with VIV suppression devices, such as strakes, in ections exposed to strong current.

This paper highlights a pragmatic VIV analysis tool (VIVA) based on riser-specific experimental tests and mathematical models. This is followed by a review of VIV analyses for representative rigid drilling and production risers for 3000-foot water depth. VIV response sensitivity is investigated for 1.5-knot slab, linear, and reverse current profiles that are indicative of the 1-year extremes West of Shetland and Brazil.

A Pragmatic VIV Analysis Methodology

VIVA is a computational tool developed to predict the vortex-induced vibrations and fatigue damage of marine risers in shear flows based on strip theory and an extensive database of hydrodynamic data on sectional force coefficients and correlation length (Reference 1). The program is generically written to allow the input of additional hydrodynam