An Improved Linearization Technique For Frequency Domain Riser Analysis
Authors: Krolikowski Lawrence P. , Gay Tom A. 
Published: October 2008

Frequency domain riser analysis is widely used in industry because it includes dynamic effects at reasonable solution cost. The linearized dynamic equations that are used can be solved almost as inexpensively as static riser equations. The drawback is in the loss of accuracy due to linearization of the nonlinear hydrodynamic drag term. If the linearization is carefully carried out, the loss of accuracy can be limited. In frequency domain techniques described in previous publications, the linearization gave satisfactory results for a regular wave environment but was not adequate when a steady current .was included.

In this paper an improved linearization technique is applied to the hydrodynamic drag term. The result is greater accuracy for regular waves with current. The same technique is also applied to random waves and random waves with current. Example cases are presented comparing results from this method with those from static and time domain dynamic analysis techniques. Both regular and random wave cases are included. The comparisons show that with the new linearization method, frequency domain results agree well with nonlinear time domain results. The speed and convenience of the new frequency domain technique offer the designer the opportunity to check a larger number of cases with different environments and riser parameters.

INTRODUCTION

There is increasing incentive to operating drilling and production risers in severe environmental conditions. This places greater emphasis on the reliability of the design procedure. The method described here offers an economical means of increasing the accuracy generally achieved in riser design analysis.

Numerical analysis is used to predict riser response so that failure or wear can be avoided. Possible failure modes for a riser include buckling due to insufficient tension, yielding or parting due to excessive bending or tensile stresses, and fatigue due to alternating stresses. Wear caused by drill pipe rubbing near the upper or lower ends of the riser may result from large riser angles.

Environmental and operational conditions affecting the riser include waves, current, mean vessel offset, dynamic vessel motion, top tension and mud weight. In dynamic riser response analysis, the dynamic vessel motion is determined from the wave excitation using a frequency-dependent linear operator. Also, the wave, current, vessel, and riser motions are commonly taken to occur in the same plane (see Figure 1). This assumption greatly simplifies the analysis and is believed to be conservative in that it over predicts peak values of riser response as compared with having the environmental factors coming from multiple directions.

The three most common ways of dealing with the time varying aspect of the problem are:Static analysis (riser does not move).Time domain dynamic analysis (nonlinear with numerical integration in the time domain).Frequency domain dynamic analysis (linearized drag allowing closed-form solution in the time domain).

Static analysis is the simplest of the three techniques and is probably the most common for routine design. Steady current forces and the static effects of vessel offset can be modeled.