Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

Evaluation of the Effect of Contact between Risers and Guide Frames on Offshore Spar Platform Motions

Prinicipal Investigators:

Robert Randall and M. H. Kim

Sponsor:

Minerals Management Service and Industry Consortium

Completion Date:

August, 2003

Final Report ID#

A133(Click to view final report abstract)

Despite the considerable amount of analytical and experimental studies conducted on the spar platform, there is still a need to compare and correlate the results obtained by various approaches. The main objectives of this study are to investigate the differences and increase the accuracy of the coupled analysis to predict for the global spar platform motions. The first part of this study discusses buoyancy-can effects on the global spar motions. The newly developed numerical analysis program includes:

 

• multi-contact coupling effect for modeling the buoyancy-can effects on the global spar motions.
• a series of guide frames are modeled by four different spring connections (i.e. connected spring, piecewise-linear gap-contact spring, piecewise-quadratic gap-contact spring, and cubic spring) to evaluate the gap contact effects on the global spar motions as well as the buoyancy-cans
• Coulomb friction effects are modeled between buoyancy-cans and riser guide frames to evaluate the Coulomb damping effects on the global spar motions.

To capture the buoyancy-can effects (i.e. multi-contact coupling between buoyancy-can and guide frames which includes gap boundary condition and Coulomb friction) on the global spar motion, four comparison studies are conducted as follows:

 

• extended riser model (i.e. consider buoyancy-can effects) versus truncated
riser model.
• four different guide frame models (i.e. connected spring, piecewise-linear gap-contact spring, piecewise-quadratic gap-contact spring, and cubic spring) are compared case by case.
• Coulomb damping effects versus without Coulomb damping effects.
• compare the results with existing experimental and simulation results.

From these comparison studies, the newly developed numerical analysis program that includes buoyancy-can effects will be verified.

In the second part of this study, an alternative riser support system (i.e. pneumatic cylinder) is modeled in the newly developed numerical analysis program. The comparison study is conducted between buoyancy-can riser support system and pneumatic riser support system. Their different dynamic characteristics are extensively studied, and the feasibility of the pneumatic riser support system is discussed in this part of study.

In the third part of this study, the Mathieu’s instability for a spar platform is discussed. In the newly developed numerical analysis program, the pitch/roll hydrostatic coefficients are recalculated for heave motion at each time step. As mentioned in section 1.2, the pitch damping effects on the Mathieu instability are extensively studied by following procedure:

 

• develop damped Mathieu stability diagram
• regular wave simulations are conducted for a spar platform without mooring lines and risers with different spar hull CD (drag coefficient) values
• regular wave simulations are conducted for a spar platform with mooring lines and risers

Based on the regular wave simulation results, the Mathieu instability in the swell wave condition is checked.

In the last part of this study, the vortex-induced vibration (VIV) effects on a spar platform are studied. In the newly developed numerical program, the VIV is modeled as a harmonic lift force. The comparison study between with- and without-VIV effects are conducted for loop current condition in Gulf of Mexico, and a parametric study is conducted to evaluate the effect of different Strouhal number and lift coefficients.

Related Publications: Koo, B.J., Kim, M.H., Randall, R, “Mathieu instability of spar platform with mooring and risers” Journal of Ocean Engineering, 2004, vol. 31, no. 17-18, pp. 2175-2208.

Koo, B.J., Kim, M.H., Randall, R.,“The effects of nonlinear multi-contact coupling with gap between risers and guide frame on global spar motion analysis” Journal of Ocean Engineering, 2004, vol. 31, no. 11-12, pp. 1469-1502 

©2013 All Rights reserved Offshore Technology Research Center