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100% accurate. Hence, the curvature standard deviations are from FEA output to analyze the percentage and sensitivity. In also compared for different top tensions. The top tension of time domain, accelerations with and without g-contamination about 1800kips is increased and decreased 50kips and the are compared and shown in Figure 12. A maximum g-contam- corresponding curvature change is shown in Figure 11. For a ination of 14% of the acceleration standard deviation is found constant tension change along the riser length, the curvature along the riser length. The g-contamination is found negligible. standard deviation shifts by a constant. Every 1% of tension Note however that for applications where higher dynamic change in the riser leads to about 1% of change in curvature riser angles are expected, the g-contamination can be removed standard deviation. through the use of angular rate measurements.

Field measured accelerations contain the portion due to

Conclusions gravity (g-contamination), and it may affect the curvature accu- racy if not well understood. By retrieving the riser tilt angle To take full advantage of the accumulated monitoring data, a at any given location from the FEA model, the acceleration of new fatigue monitoring methodology was developed using ana- gravity, g, can be projected into the accelerations as controlled lytical acceleration to curvature transfer function to account for g-contamination, which is compared with the accelerations the fatigue damage due to both wave and VIV effects.

This new methodology has been validated very well with a fnite element analysis (FEA) method, by comparing curvature distribution. The advantage of using FEA results to validate the methodology is that there is no noise, g-contamination, and added mass and tension uncertainty in the accelerations and curvature time traces.

Comparing with feld measured data, the results show that the calculated fatigue is sensitive to added mass and drag diam- eter, but not g-contamination. With a 54in-diameter drag and an added mass of 1.8, the proposed method matches well with the measured feld data. A standardized approach for selecting the added mass coeffcient and hydrodynamic diameter is the subject of ongoing work.

For future work, both acceleration and strain measurements from a riser system with continuous buoyancy or slick joints are

Figure 10: Sensitivity of Curvature Standard Deviation to preferred to better understand the effect of the total added mass,

Added Mass Coefcient which is dependent on the added mass coeffcient, Ca and the drag diameter. In addition, a more detailed and complicated

CFD simulation may be conducted to investigate the actual drag affect and added mass effect. Extra strain sensors on different locations and a non-staggered riser confguration would also assist in further validation of this methodology.

Acknowledgments

The authors thank the management of BP for permission to publish this paper. In addition, the authors wish to express thanks to 2H Offshore engineers who did the data analyses documented in this paper.

Michael Long Ge is subsea riser engineer.

He has worked on a wide range of projects

Figure 11: Variation of Curvature Standard Deviation with

Tension for BP in the Gulf of Mexico, including engineering, design, inspection, mainte- nance, monitoring, and integrity manage- ment. He holds a MS in mechanical engineering from University of Florida and a BS in engineering mechanics from

Shanghai Jiao Tong University.

Himanshu Maheshwari is a senior engineering specialist at 2H Offshore in

Houston. He has extensive experience in delivering riser integrity management and monitoring programs including project management, system engineering, instru- mentation, installation and data manage- ment. Maheshwari has a MS in mechanical

Figure 12 G-Contamination Efect on Acceleration Standard engineering from Texas A&M University.

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