Abstract

This paper describes a laboratory investigation and methods for analysis of vortex induced vibrations (VIV) of catenary risers. Most VIV models are based on frequency domain analyses where the stiffness properties of the riser must be kept constant. A catenary riser will, however, experience a strong non-linear behavior close to the touchdown point caused by the varying contact condition and friction forces between pipe and seafloor. The most accurate way of modeling such effects is to apply a non-linear finite elementmodel, meaning that the analysis must be performed as a time domain simulation.

The paper describes how results from a conventional VIV analysis can be applied in a non-linear time domain simulation and thereby improve the prediction of bending stresses in the touchdown zone. The method is based on the assumption that boundary conditions in the touchdown zone do not influence the vibrations at the excitation zone. Presented results indicatethat this will be the case under most realistic conditions.

Response frequencies and amplitudes of a catenary riser subjected to uniform current profiles have been measured in a laboratory. The measurements are compared to results from a newly developed computer program employing finite elements and a frequency domain method. In general a good agreement is found, but some discrepancies between theory and experiments are also seen. Attempts have been made to explain the disagreements and thereby improve future theoretical predictions.

Introduction

All floating production systems must rely on some kind of marine risers for transport of the well-stream from the seafloor to the platform, and in many cases also for transport of processed oil and gas down to a pipeline. Among the many proposed riser concepts, the steel catenary riser is in particular promising due to its simplicity and low costs. This is in particular true if the heave motions of the floater are moderate, like for tension leg platforms, SPAR buoys and deep draught floaters.

A key issue in design of catenary risers is to control stresses and fatigue damage in the touchdown zone. Vessel motionsand waves will cause time varying stresses, and because of the boundary conditions such stresses will in most cases have the largest values within this zone. An another phenomenon that may contribute significantly to fatigue is vortex inducedvibrations (VIV) in constant current. Although amplitudes are small compared to wave and motion induced stresses, fatigue damage can be high because of the large number of stresscycles that may occur. Reliable prediction of bending stresses in the touchdown zone from VIV is hence desired.

A catenary riser is different from a tensioned riser in that sense that current will have a flow direction relative to the pipe axis different from 90 degrees. The basis for almost all empirical VIV models is experiments with incoming flow perpen-dicular to the cylinder. Hence, there are many uncertainties related to hydrodynamics of catenary risers, like Strouhal number, added mass and lift coefficients. Comparing tests and analyses is therefore of large interest.

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