Summary
Objective
As exploration and production of petroleum moves to deeper and deeper water, the use of steel mooring systems for floating structures becomes very expensive and introduces operational complexities. Steel mooring systems require a large anchor footprint and the need to support its large self-weight. Alternative mooring systems are being sought to help reduce costs and improve efficiency. One alternative that has received a great deal of attention from the oil industry is the use of polyester rope in a taut mooring configuration. Of major concern, both to the oil industry and the Minerals Management Service (MMS), is that polyester taut mooring systems provide sufficient reliability and safety over an expected design life of 20-30 years.
The primary goal of this research project is to ensure the availability of a validated software tool that can be used to predict the response of polyester ropes under a variety of loading and damage conditions. Such a tool is needed to interpret and extend test data and to develop design and operational guidelines. Laboratory testing of large and full-scale ropes, while essential for gauging performance, is expensive and time consuming. Development of a reliable computer model has the potential to significantly reduce the costs and time needed for physical testing.
Approach
In recent years, there has been an increasing amount of research activity investigating how polyester ropes behave in the marine environment. Test results have indicated a variety of different failure mechanisms that depend upon the nature of the applied loads as well as the characteristics of the test specimen. In addition, deformation characteristics have been shown to be a complex function of the load, load rate, and load history.
The general approach to the current research project is to extend existing computation models of synthetic-fiber rope behavior to allow for the prediction of performance under a variety of loading conditions. In particular, because little information exists on the response of synthetic-fiber ropes that have been damaged, and because the long-term safety of structures utilizing such mooring ropes is unknown, the primary goal of the project is to develop a validated software tool that can be used to predict the capacity of ropes that have been damaged. A review of the literature has shown that most prior work on modeling has focused on steel wire rope. Little information exists on the response of synthetic fiber ropes that, unlike steel, may experience significant deformation in its cross-sectional shape under applied loads.
Deployment of Results
Results from this research will be distributed through periodic meetings with the project sponsor, presentations at meetings of the learned societies, progress reports, and a final report. In addition, software developed for this research will be made available to the sponsor as requested. The tool can also be useful in developing strategies and guidelines for dealing with damaged polyester rope.
Project Plan
Previous Results: A review of the research literature on the modeling of synthetic-fiber ropes indicated that none of the previous work in this area addresses the behavior of ropes that are damaged. All existing models rely heavily on assumptions of symmetry and uniform properties over a rope cross-section; these assumptions are not valid if damage occurs. As a result, research to date has focused on the development of modeling software that incorporates the features of previous models but allows for greater capabilities in simulating the response of ropes.
A significant advancement made in the current research is the development and implementation of a load-control methodology for analyzing rope behavior. This capability did not exist in other available rope modeling tools, and its incorporation into the current software was of great importance for our model on damage propagation. Because other software only allows for displacement control, a user is somewhat restricted in comparing computed output with measured test data as most tests on rope capacity are done using load-control. Aside from providing a user with greater flexibility in modeling rope response, the load control algorithm is essential for redistributing rope stresses from a failed component to the remainder of the intact rope. Thus, the incorporation of load control in the software marks an important step for characterizing the response of damaged ropes.
Limited test data have been available to test and validate the model. Comparison of the model predictions with recent tests of small-scale rope components (Composites Engineering and Applications Center (CEAC), University of Houston) showed good agreement for all damage levels. In addition to studying the results from these small-scale rope tests, analyses have begun that consider the response of large-scale ropes.
A related MMS JIP project has tested moderate-scale rope to address the effects of specimen length in affecting the measured rope response. Preliminary results suggest a failure mechanism in which strain becomes localized around the site of initial damage. Accordingly, recent work has focused on extending the current computational models to account for variation in rope properties along its length in order to include the effects of strain and damage localization. This work has represented a major extension over previous computational models, and this effort has been the focus of our research over the last several months.
Currently, the MMS JIP is completing large-scale tests on damaged ropes. These tests address issues related to length-effects, test scale, and different damage states. Once results from these tests become available, analyses will be conducted to validate the numerical model’s capability to predict the residual strength of damaged polyester rope.
Scope of Work: The following work is planned for the remainder of the project
- Continued development, validation, and completion of our computational model for representing the response of damaged polyester rope under both static and cyclic loads.
- Acquire full-scale test data from MMS JIP on Damaged Polyester Rope.
- Verification of results using test data from the MMS JIP on Damaged Polyester Rope.
- Support for the development of mitigation strategies and guidelines for polyester moorings damaged either in-service or during installation. 5. Prepare a final report
Anticipated Results:
- Computational model that accounts for strain localization.
- Analysis results of full-scale damaged ropes.
- Documentation of analyses.
Related Publications
Rungamornrat, J., Beltran, J. F., and Williamson, E. B. (2002). “Computational Model for Synthetic Fiber Rope Response.” Proceedings, Fifteenth Engineering Mechanics Conference, American Society of Civil Engineers, Columbia University, New York, NY, June 2-5, 2002.
Beltran, J. F, Rungamornrat, J., and Williamson, E. B. (2003). “Computational Model for the Analysis of Damaged Ropes.” Proceedings, ISOPE (International Society of Offshore and Polar Engineers) 2003 Annual Meeting, Honolulu, HI, May 25-31, 2003.
Beltran, J. F. and Williamson, Eric. B. (2003). “Degradation of Rope Properties under Increasing Monotonic Load.” Proceedings, 2003 International Symposium on Deepwater Mooring Systems: Concepts, Design, Analysis and Materials, American Society of Civil Engineers, Houston, TX, October 2-3, 2003.
Beltran, J. F. and Williamson, E. B. (2004). “Investigation of the Damage-Dependent Response of Mooring Ropes.” Proceedings, ISOPE (International Society of Offshore and Polar Engineers) 2004 Annual Meeting, Toulon, France, May 22-28, 2004.
Beltran, J. F. and Williamson, Eric. B. (2005). “Degradation of Rope Properties under Increasing Monotonic Load.” Journal of Ocean Engineering, Vol. 32, Issue 7, pp. 826-844.
