Advanced Frequency Domain Analysis Techniques for Steel Catenary Risers
Authors: Lane Michael , O&apos;Sullivan Tim , Grealish Frank , Kavanagh Kieran , Thompson Hugh 
Published: October 2008

Steel catenary risers (SCRs) represent one of the main solution alternatives for production and export in deepwater floating production developments. Key advantages relative to other options such as flexible risers or tower systems are their simplicity and lower procurement costs. However reliable prediction of the extreme and fatigue response of floating production SCRs is a complex issue, critically dependent on the methods and assumptions associated with their analysis. This paper addresses some key design issues related to the analysis of SCRs, and presents an assessment of frequency domain compared to time domain analysis methods for fatigue life prediction. Results from an advanced frequency domain analysis tool are presented and evaluated relative to corresponding time domain results.

Introduction

As the offshore industry continues to push into deeper waters, the current limits of flexible pipe technology may militate against the continued deployment of unbonded flexible risers for floating production systems. These limits include maximum diameter (currently 19"), external pressure (3,000psi), and production fluid temperature (130°C). Increasingly SCRs are being seen as enabling technology for deep and ultra-deep floating production applications. Already SCRs have been used in the Gulf of Mexico for TLP and Spar applications, and offshore Brazil for semi-submersible floating production, and are proposed for more onerous applications such as FPSO riser systems.

A key issue relating to SCR design for floating production is the fatigue design of the riser. Because of the criticality of fatigue performance, there is typically a requirement to analyse a large number of fatigue load case combinations consisting, in many cases, of between 10 and 30 wave classes examined for 8 to 16 environmental directions. Each condition may need to be analysed several times as a design evolves, or to assess the sensitivity of predicted fatigue lives to key input parameters. The computational effort for large multiple riser projects is potentially very resource-consuming.

This paper examines the feasibility, advantages and limitations of SCR analysis in the frequency domain, particularly for fatigue calculations. Finite element analysis in the frequency domain has a long history in the design of rigid top tensioned risers, where riser rotations rarely exceed 10° and, in the case of a drilling riser for example, must be maintained below 4° for continued operation. Frequency domain analysis is less common for flexible risers, particularly for extreme response prediction, because of the geometric nonlinearity of the response and the importance of nonlinear effects such as intermittent seabed contact and friction. These same limitations also prevent the systematic use of frequency domain analysis for SCR extreme response prediction.

Fatigue response prediction in the frequency domain may however be an attractive option. Fatigue is of course determined mainly by the response to routine rather than extreme conditions, and a suitably configured frequency domain tool may be able to overcome the usual drawbacks to provide a fast and efficient facility for computing fatigue damage.