Reliability of Mooring Systems for Floating Production Systems

Summary

Objective

Mooring systems for floating production systems, including the mooring lines and anchors, are currently designed on the basis of individual components. The most heavily loaded line and anchor are checked under extreme loading conditions with the system of lines intact and with one line removed. However, the performance of the floating production system depends more directly on the performance of the system of lines and anchors rather than on the performance of a single line or anchor.

This project will assess and compare the component and system reliabilities for the mooring system of a deepwater Spar. Results will allow evaluation and possible improvement of the design codes for FPS mooring lines and anchors.

Approach

The reliability of mooring and foundation components and the overall mooring system for deepwater Spars are being analyzed and compared as follows:

1. Designs for a representative Spar and mooring system for several depths in the Gulf of Mexico were selected for study.
2. Models describing the performance of individual components and for the mooring system are being used in the analysis. The performance models include models describing (1)the time-varying loads on individual mooring lines and foundations during hurricane and loop current events, and (2) the time-varying capacities of the mooring lines and foundations. Existing models that represent the current state of practice are generally being adopted, although some modifications have been necessary to suit the purposes of this study.
3. Uncertainties in the performance models, including those due to (1) temporal variations in loads, (2) spatial variations in seafloor properties, and (3) limited data sets to calibrate the load and capacity models are being identified and quantified.
4. These performance models are being used to assess and compare the component and system reliabilities and develop measures of system performance, such as robustness and redundancy. These measures will also be compared to those for conventional steel jackets for reference..

Deployment of Results

Results will be documented in a final report and published in journals and conference proceedings. Results should provide helpful reference material for industry to use in evaluating and possibly improving existing design codes.

Project Plan

Previous Results: In the first year, we adapted a hydrodynamic model to calculate time-varying mooring line loads on a Spar, and identified and modeled Spar designs for 3000-ft water depth with wire rope moorings and 6000-ft and 10000-ft water depths with polyester moorings. The model compared favorably to model tests for hurricane and loop current environments, and was used to evaluate the distribution of maximum lifetime loads. We also developed a model for the foundation pullout under time-varying load.

In the second year, we analyzed the reliability for components in the system (lines and foundations) and for the system itself considering hurricane events for the 3000-ft water depth. The hydrodynamic model was modified to analyze performance when a foundation fails (the model accounts for the residual capacity in the line due to the weight of the anchor) versus when a line breaks (the model assumes no residual capacity). Results are presented in a de-coupled format so that the response of the mooring system is expressed in terms of metocean variables (wave heights, wave periods, and current and wind profiles), hydrodynamic variables that affect the loads (drag coefficients, added mass coefficients and VIV coefficients), and design variables that affect the system capacity (mooring line strength, connection strength, and foundation capacity). This allows the reliability to be easily assessed and results to be extended to different metocean conditions and different design capacities (such as different factors of safety in Working Stress Design).

Scope of Work In the third year, the study focused on finalizing the reliability analyses through:

• including loop current design events
• extending the analyses to the 6000 and 10000-ft water depths
• conducting sensitivity studies
• engaging an industry review team to transfer the results to practice

Anticipated Results Expected results include:

• comparison of individual component reliabilities (e.g., line breakage versus foundation failure)
• comparison of the system reliability to reliabilities of the weakest component (redundancy)
• the impact on system and component reliabilities due to
  • water depth
  • mooring line configuration
  • metocean conditions
  • factors of safety

Related Publications

Yu Ding, Minsuk Kim, Xiaohong Chen and Jun Zhang (2003) ‘A Numerical Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’, Proceedings of International symposium on deep-water mooring system, Houston, TX, 2003, p168-182.

Yu Ding, B. Theckum purath, Jun Zhang, R. B. Gilbert, S. Dangyach and Y.J. Choi (2005), “Reliability of Mooring Systems for A Spar”, Proceedings of OMAE 2005, paper No. 67290.

Gilbert, R.B., Choi, Y.J., Dangyach, S. and Najjar, S.S., “Reliability-Based Design Considerations for Deepwater Mooring System Foundations,” Proceedings, ISFOG 2005, Frontiers in Offshore Geotechnics Perth, Western Australia, 317-324, 2005.