Progress Reports: December 2005 June 2005 December 2004 June 2004 December 2003 June 2003 December 2002 June 2002 December 2001Polyester Rope Analysis Tool
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.
ANTICIPATED PROJECT DURATION: 6 years
PROJECT PLAN FOR YEAR 6 (2004-2005):
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 2004-2005:
1. Continued development, validation, and completion of our computational model for representing the response of damaged polyester rope under both static and cyclic loads.
2. Acquire full-scale test data from MMS JIP on Damaged Polyester Rope.
3. Verification of results using test data from the MMS JIP on Damaged Polyester Rope.
4. 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: The following results are expected for 2004-2005:
1. Computational model that accounts for strain localization.
2. Analysis results of full-scale damaged ropes.
3. Documentation of analyses.
PRINCIPAL INVESTIGATOR & OTHERS INVOLVED IN PROJECT:
PI: Eric Williamson, Ph. D. Others: Juan Felipe Beltran (Graduate Student)
Date: December 9, 2005Project Title: Polyester Mooring Line Damage Model
MMS Project: 369 TO Numbers: 17019/35981
PI: Eric Williamson
COTR: S. Buffington
Estimated Completion Date: December 31, 2005Project Description: As exploration and production of petroleum moves to deeper and deeper waters, 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. Due to these limitations, 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 such mooring systems provide sufficient reliability and safety over an expected design life of 20-30 years.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 experimental testing.
Progress: During the last six months, additional verification studies have been conducted using the computational modeling software of rope behavior that has been developed under this research. These verification studies have in turn led to further refinement of the software. As described in previous reports, the software is capable of predicting the response of synthetic-fiber ropes under both monotonic and cyclic loads, accounting for the effects of damage accumulation and strain localization. Results to date have demonstrated that the software provides good predictions of rope capacity and can be used effectively as a design tool in studying how mooring systems perform when damage occurs to one of the lines. Predictions of rope behavior for both small-scale and large-scale ropes have been carried out under this research, and the computational models developed as part of the current study have been validated with experimental test data for ropes of various cross-sectional dimensions, length-to-diameter ratios, and levels of initial damage.
Reports & Publications: Below is a list of reports and publications that have been developed from the research conducted on this project:
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.
Date: June 2005
Project Title: Polyester Mooring Line Damage Model
MMS Project: 369 TO Numbers: 17019/35981
PI: Eric Williamson, Ph.D., P.E.
COTR: A. Konczvald
Estimated Completion Date: December 2005
Project Description: As exploration and production of petroleum moves to deeper and deeper waters, 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. Due to these limitations, 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 such mooring systems provide sufficient reliability and safety over an expected design life of 20-30 years.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 experimental testing.
Progress: Research efforts over the past six months have continued to focus on the refinement of our computational model to account for the “length effects” observed in tests conducted on large-scale rope specimens. As discussed in previous reports, strain localization near the site of damage becomes more pronounced in longer specimens in comparison to shorter ones. With shorter ropes, termination behavior and splice efficiency tend to control the response. The modeling efforts to account for strain localization have required significant time to implement due to the complexity of the mechanics involved and also the need to reorganize the software to accommodate this new feature. These efforts are now essentially complete, and the graduate student on the project has begun to write first drafts of the chapters that will appear in his dissertation. These chapters will form the basis of the final project report. In addition to these activities, initial efforts have begun on developing detailed three-dimensional finite element models of different ropes so that comparisons can be made to the results predicted by our current software. While it is unexpected that the finite element modeling will mature to the level that validation can be adequately carried out, it is proving useful for exploring some of the assumptions used in the models that we have developed as part of this research.
Reports & Publications: Below is a list of reports and publications that have been developed from the research conducted on this project:
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.
Date: December 2004Project Title: Polyester Mooring Line Damage Model
MMS Project: 369 TO Numbers: 17019/35981
PI: Eric Williamson
COTR: A. Konczvald
Estimated Completion Date: October 31, 2005Project Description: As exploration and production of petroleum moves to deeper waters, the use of steel mooring systems for floating structures becomes very expensive and introduces operational challenges. Steel mooring systems require a large anchor footprint and the need to support its large self-weight. Due to these limitations, alternative mooring systems are being sought to help reduce costs and improve efficiency. One alternative that has attracted significant attention 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 such mooring systems provide sufficient reliability and safety over an expected design life of 20-30 years.
To address these concerns, 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 conditions. Such software is needed to interpret and extend test data and to develop design 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 experimental testing.
Progress: As discussed in previous project reports, early research efforts focused on conducting a literature review of prior research and evaluating the capabilities and limitations of the commercial software “Gen-Rope.” Based on the results of this early research, a decision was reached in conjunction with the MMS to begin development of new software.
The software currently under development is capable of simulating the behavior of damaged and undamaged ropes under a variety of loading conditions. It includes many of the assumptions used in Gen-Rope, but it offers several features that are not available in the commercial software. Specifically, simulation of rope response can be done using both load and displacement control, and the deterioration of rope properties under both monotonic and cyclic loads can be modeled. The damage routine in the software is founded on continuum damage mechanics principles and accounts for the effects of strain history, stress range, and friction between rope components.
The model has been shown to provide accurate predictions of rope capacity for a wide variety of damage levels and distributions. In a previous report, we summarized the validation work that was carried out using data from the testing program on small-scale components conducted at the Composites Engineering and Applications Center (CEAC) at the University of Houston.
Currently, we are conducting simulations of ropes that are much larger than those tested in Houston. Ropes with capacities of 35 tonnes and 700 tonnes have been tested for several different rope manufacturers, and we are evaluating how well our current model is able to predict capacity. Initial results have been promising in that the prediction of breaking load matches measured data reasonably well. Estimates of the failure strain, however, have not been as accurate, and we are currently studying this effect.
With regard to enhancing the computational model, our recent efforts have focused on addressing the “length effects” observed in tests conducted within the last year. For some of the longer rope specimens, experimental observations suggested a failure mechanism in which strain becomes localized around the site of initial damage. For shorter ropes, effects of termination efficiency make isolating this effect more difficult to characterize. Because the issue of strain localization was not identified in previous tests of shorter rope specimens, all previous computational models, including Gen-Rope, did not account for variation in properties over a rope length greater than a representative pitch distance. Measured results suggest that such an assumption to characterize the response of a mooring rope may not be accurate. Accordingly, our current work has focused on extending the previously developed models to account for the effects of strain and damage localization at the site of damage initiation. This work represents a major extension over other computational models, and this effort has been the focus of our research over the last several months. Work has also continued on the development of computational models that more accurately address the loss of symmetry that occurs immediately following the failure of a component within the body of a rope.
Reports & Publications: Recent publications that have resulted from the research are listed below. Additionally, the P.I. and his students have made several presentations describing the ongoing research at a variety of workshops and conferences.
Beltran, J. F. and Williamson, E. B. “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. “Degradation of Rope Properties under Increasing Monotonic Load.” Journal of Ocean Engineering (accepted for publication June 10, 2004).
Date: June 2004
Project Name: Polyester Rope Analysis Tool
Project Number: 369 Task Order: 17013
Principal Investigator: Eric B. Williamson
Estimated Completion Date: September 2005
Project Description:
As exploration and production of petroleum moves to deeper and deeper waters, the use of steel mooring systems for floating structures becomes very expensive and introduces operational complexities due to the large self-weight and anchor footprint. Alternative mooring systems using polyester mooring lines in a taut mooring configuration are being used to help reduce costs and improve efficiency.
The primary goal of this research project is develop and validate a numerical model to predict the response of undamaged and damaged polyester ropes under a variety of loading conditions. Such a tool is needed to better understand rope performance, interpret and extend test data, and to develop guidelines to mitigate observed damage in mooring lines. Laboratory testing of large and full-scale ropes, while essential for gauging performance, is expensive and time consuming. The availability of a reliable computer model has the potential to significantly reduce the costs and time needed for experimental testing.
Progress:
The numerical model and software currently under development is capable of simulating the behavior of damaged and undamaged ropes under a variety of loading conditions. It includes several features that are not available in commercially available software. Specifically, simulation of rope response can be done using both load and displacement control, and the deterioration of rope properties under both monotonic and cyclic loads can be modeled. The damage description is founded on continuum damage mechanics principles and accounts for the effects of strain history, stress range, and friction between rope components.
Recent efforts have focused on extending the computational model to address the “length effects” observed in small-scale Length Effect Tests completed by the MMS JIP in 2003. Test results from longer rope samples suggested a failure mechanism in which strain becomes localized around the site of initial damage. This effect could be important for long mooring lines. Previous rope models have not accounted for variations in properties over a rope length greater than a representative pitch distance and thus could not address this mechanism. (Perhaps this resulted from the more common practice of testing shorter rope samples, and the difficulty in isolating this length effect from termination effects.) Current work is focused on extending the rope model to account for the effects of strain and damage localization at the site of damage initiation, which is a major extension to the present model.
Work has also continued on developing computational models that more accurately address the loss of symmetry that occurs immediately following the failure of a component within the body of a rope. Finally, numerical simulations have been conducted to determine the form of the damage model that most accurately captures the response of ropes under monotonically increasing tensile loads.
Tests sponsored by the MMS JIP (reported elsewhere) are providing data on the response of near full-scale damaged rope specimens. These data will be used to validate the predictions made by the computational software being developed under the current study.
Reports & Publications: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. “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.
Date: December 2003Project Name: Polyester Rope Analysis Tool
TEES Project Number: 32558-58878 MMS Task Order: 17019 MMS Project Number: 369
Principal Investigator: Eric B. Williamson
Estimated Completion Date: June 2004
Project Description:
As exploration and production of petroleum moves to deeper and deeper waters, 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. Due to these limitations, 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 such 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 experimental testing.
Early research efforts focused on conducting a literature review of prior research and evaluating the capabilities and limitations of the commercial software “Gen-Rope.” Based on the results of this early research, a decision was reached in conjunction with the MMS to begin development of a new software tool.
The software currently under development is capable of simulating the behavior of damaged and undamaged ropes under a variety of loading conditions. The new software includes many of the assumptions found in Gen-Rope, but it offers several features that are not available in the commercial software. Specifically, simulation of rope response can be done using both load and displacement control, and the deterioration of rope properties under both monotonic and cyclic loads can be modeled. The damage routine in the software is founded on continuum damage mechanics principles and accounts for the effects of strain history, stress range, and friction between rope components.
Progress:
Significant progress has been made on the validation of the new software. Data obtained from the Composites Engineering and Applications Center (CEAC) at the University of Houston for both intact and partially damaged rope components have been used to calibrate our computational model. Predictions made by our software show excellent agreement with test data reported for damaged ropes (refer to the papers below and the 22 April 2003 Project Status Report for additional information). At present, work is continuing in this area to further refine the model and establish appropriate limits over the parameter space for which the software provides accurate results.
Currently, our efforts are focused on extending the computational model to address the “length effects” observed in experiments. Test results have demonstrated that similar levels of damage on identical cross-sections affect longer segments of rope more significantly than shorter segments. The original formulation using the Gen-Rope methodology does not account for this feature, and progress is being made under the current research to include this capability.
Work has also continued on the development of computational models that more accurately address the loss of symmetry that occurs immediately following the failure of a component within the body of a rope.
Finally, numerical simulations have been conducted to determine the form of the damage model that most accurately captures the response of ropes under monotonically increasing tensile loads. A complete description of this aspect of our current research is included in a paper that was recently submitted for publication in the 2004 ISOPE conference proceedings.
Reports & Publications:
Currently, research efforts are focused on the development and refinement of a validated software tool for modeling polyester rope behavior. Listed below are the publications that have resulted from the research that have been completed to date. In addition to these publications, the P.I. and his students have made several presentations describing the ongoing research at a variety of workshops and conferences.
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. “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. (paper under review).
Date: June, 2003
Project Name: Polyester Rope Analysis Tool
Project Number: 32558-58878 Task Order: 17019
Principal Investigator: Eric B. Williamson, Ph.D.
Estimated Completion Date: 6/30/04
Project Description: As exploration and production of petroleum moves to deeper and deeper waters, 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. Due to these limitations, 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 such mooring systems provide sufficient reliability and safety over an expected design life of 20-30 years.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 experimental testing.
Progress: As discussed in previous project reports, early work focused on conducting a literature review of prior research and evaluating the capabilities and limitations of the commercial software “Gen-Rope.” Based on the results of this early research, a decision was reached in conjunction with the MMS to begin development of a new software tool.
The software currently under development is capable of simulating the behavior of damaged and undamaged ropes under a variety of loading conditions. The new software includes many of the assumptions included in Gen-Rope, but it offers several features that are not available in the commercial software. Specifically, simulation of rope response can be done using both load and displacement control, and the deterioration of rope properties under both monotonic and cyclic loads can be modeled. The damage routine in the software is founded on continuum damage mechanics principles and accounts for the effects of strain history, stress range, and friction between rope components.
Since the last submitted project status report, significant progress has been made on the validation of the new software. Data obtained from the Composites Engineering and Applications Center (CEAC) at the University of Houston for both intact and partially damaged rope components have been used to calibrate our computational model. At present, work is continuing in this area to further refine the model and establish appropriate limits over the parameter space for which the program provides accurate results.
Statistical analyses of the data were carried out to obtain the parameters needed to define the damage model in the software. Figs. 1-3 below show a comparison of the measured data along with the predicted response. It is important to note that the software model was calibrated for data at the stress–strain level corresponding to the strand. Figs. 1-3 represent the response calculated at the element and sub-rope levels. Thus, both the constitutive and geometric effects of damage have been accounted for by the model. In interpreting the significance of these results, it is important to recall that a rope is modeled in a hierarchical fashion in which the behavior is computed by summing the contributions from lower levels to obtain the response at higher levels. For a complete description of the rope modeling process, refer to the documents presented below in the “Reports & Publications” section of this progress report. While the results computed to date compare well with the measured data and are quite promising, additional experimental data are needed to gain a better understanding of how damage propagates throughout a rope up to the point of failure. Such information could then be used to enhance the current computational model to account for the sequence of component failures that must occur prior to complete rupture of a rope cross section.
Reports & Publications: Currently, research efforts are focused on the development and refinement of a validated software tool for modeling polyester rope behavior. Listed below are the publications that have resulted from the research that has been completed to date. In addition to these publications, the P.I. and his students have made several presentations describing the ongoing research at a variety of workshops and conferences.
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.
Date: November 15, 2002
Project Name: Polyester Rope Analysis Tool
Project Number: 32558-58878 Task Order: 17019
Principal Investigator: Eric B. Williamson, Ph.D.
Estimated Completion Date: August 31, 2004
Project Description: 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. Due to these limitations, 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 taught 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.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 reduce significantly the costs and time needed for experimental testing.
Progress: A review of previous research related to the behavior of synthetic fiber ropes revealed that the U.S. Naval Civil Engineering Laboratory (NCEL) was one of the primary supporters of work in this field. Previous research sponsored by the NCEL in the late 1980s and early 1990s addressed both experimental testing as well as the development of a modeling tool called Gen-Rope. The Gen-Rope software was developed by Chris Leech, Ph.D., of Tension Technology Incorporated (TTI) in Preston, England.
An early objective of the research that has been completed is the evaluation of Gen-Rope's capability to model the response of polyester ropes under a variety of loading scenarios. For the purposes of the current project, it was essential that the software be able to represent several different types of rope construction. In accord with the concerns of the MMS, particular emphasis was placed on determining whether or not Gen-Rope could model the response of damaged ropes. In order to make this assessment, a thorough review of the theoretical development and implementation of the software was conducted. During the early stages of this project, several months were invested in reviewing previous literature and evaluating the capabilities and limitations of Gen-Rope. Based on our findings and our understanding of the sponsor’s long-term goals for this project, a decision was reached, in conjunction with the MMS, to begin development of a new software tool rather than modifying Gen-Rope to achieve the desired capabilities. The new software being developed as part of the current research includes many of the fundamental assumptions on which Gen-Rope is based and which have been shown to provide good correlation to test data obtained from tests done for the U.S. Navy. To date, approximately 13,000 lines of software have been written, and progress is continuing on the development of a validated software tool.
Aside from providing similar capabilities as Gen-Rope, major features of the new software include the ability to model the response of damaged ropes and the ability to define very general loading scenarios that compliment experimental studies being conducted by other researchers supported by the MMS. The software being developed incorporates a damage degradation algorithm to represent the deterioration of component properties under loading, and it is able to account for failure of various components throughout the hierarchy of a given rope geometry. Another major advancement that has been completed is the implementation of a load-control algorithm to simulate rope response. Gen-Rope does not offer this capability, but many experimental results are based on tests that were conducted using load-control. Thus, the software being developed is able to simulate the load-displacement history of experiments more accurately than Gen-Rope. Current work is focused on the development of algorithms to allow for braided rope constructions and propagation of damage following component failure.
The work described above has been conducted with extensive feedback from the MMS, representatives from the oil industry, and rope manufacturers. The P.I. has attended the workshop entitled “Advanced Fibre Rope Technology” hosted by Tension Technology Incorporated (developers of the Gen-Rope software), has visited with Marlow and Whitehill rope manufacturers, has met with Chris Leech, Ph.D. (developer of Gen-Rope), and has toured the testing facilities at the National Engineering Laboratory in Scotland. Further involvement with industry representatives is being coordinated through the OTRC.
Reports & Publications: Currently, research efforts are focused on the development and refinement of a validated software tool for modeling polyester rope behavior. Because this work is still in progress, no reports have yet been produced. However, the P.I. and his students have made several presentations describing the ongoing research at a variety of workshops and conferences. In addition, an overview of the computational framework that has been developed to model rope response was discussed in a paper presented at the ASCE Engineering Mechanics conference.
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.
Date: June 2002
Project Name: Polyester Rope Analysis Tool
Task Order: 17019 Project Number: 58878
Principal Investigator: Eric B. Williamson, Ph.D.
Estimated Completion Date: February 28, 2003
Project Description: 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. Due to these limitations, 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 taught 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.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 experimental testing.
Progress: As described in the previous project report, after reviewing previous literature and evaluating the capabilities and limitations of the modeling tool called Gen-Rope, a decision was reached in conjunction with the MMS to begin development of a new software tool rather than modifying Gen-Rope to achieve the desired capabilities.
In order to be cost effective, synthetic fiber ropes must perform acceptably well in the marine environment for an extended period of time, perhaps twenty years or greater. Consequently, rope durability is an important concern. Both under service loads and during the installation process, various researchers have demonstrated that synthetic fiber ropes are prone to being damaged, and the ability to predict the remaining life of a previously damaged rope is needed. Because the software being developed for the current research includes a simple damage model that can evaluate the deterioration of rope properties under general loading scenarios, it can simulate the response of damaged ropes. To compliment the current analytical work, other researchers have tested previously damaged ropes to determine their capacity. This data is being used to calibrate the software model.
Initially, the software tool being developed for the current research was formulated using displacement control in which the deformed configuration of the rope is assumed. Currently, a loading control scheme is being implemented in order to include an extensive progressive damage model to account for redistribution of component stresses that result after the failure of a rope component.
To date, the simple damage model currently included in the software has been used to study the reduction in capacity following the failure of different components within a given rope geometry. Assumptions regarding the change in geometry following component failure have been required in order to facilitate the computational procedure. The trend shown by the computations demonstrates the capability of the software to model damaged rope, but further research is needed to validate the computed response and incorporate propagation effects and changes in rope geometry.
Reports & Publications: Currently, research efforts are focused on the development and refinement of a validated software tool for modeling polyester rope behavior. Since the basic formulation of the computational model has already been established and some preliminary results have been obtained, a paper was submitted to the 15th ASCE Engineering Mechanics Conference (June 2 – 5, 2002, Columbia University, New York, NY) with the title Computational Model for Synthetic – Fiber Rope Response. This paper summarizes the formulation of the computational model to simulate rope response using displacement control.
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Date: May 22, 2001
Project Name: Polyester Rope Analysis Tool
Task Order: 17019 Project Number: 58878
Principal Investigator: Eric B. Williamson, Ph.D.
Estimated Completion Date: August 31, 2002
Project Description: 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. Due to these limitations, 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 taught 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.
To address these concerns, 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 conditions. Such a tool is needed to interpret and extend test data and to develop design 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 reduce significantly the costs and time needed for experimental testing.
Progress: A review of previous research related to the behavior of synthetic fiber ropes revealed that the U.S. Naval Civil Engineering Laboratory (NCEL) was one of the primary supporters of work in this field. Previous research sponsored by the NCEL in the late 1980s and early 1990s addressed both experimental testing as well as the development of a modeling tool called Gen-Rope. The Gen-Rope software was developed by Chris Leech, Ph.D., of Tension Technology Incorporated (TTI) in Preston, England. Because of the lack of reliable experimental data to validate the assumptions incorporated in the software, there are limits on the parameter space over which the program provides accurate results.
An early objective of the research that has been completed is the evaluation of Gen-Rope's capability to model the response of polyester ropes under a variety of loading scenarios. For the purposes of the current project, it was essential that the software be able to represent several different types of rope construction. In accord with the concerns of the MMS, particular emphasis was placed on determining whether or not Gen-Rope could model the response of damaged ropes. In order to make this assessment, a thorough review of the theoretical development and implementation of the software was conducted. During the early stages of this project, several months were invested in reviewing previous literature and evaluating the capabilities and limitations of Gen-Rope. Based on our findings and our understanding of the sponsor’s long-term goals for this project, a decision was reached in conjunction with the MMS to begin development of a new software tool rather than modifying Gen-Rope to achieve the desired capabilities. The new software being developed as part of the current research includes many of the fundamental assumptions on which Gen-Rope is based and which have been shown to provide good correlation to test data obtained from tests done for the U.S. Navy. Major features of the new software include the ability to model the response of damaged ropes and the ability to define very general loading scenarios that compliment experimental studies being conducted by other researchers supported by the MMS. To date, approximately 10,000 lines of software have been written, and progress is continuing on the development of a validated software tool.
The work described above has been conducted with extensive feedback from the MMS, representatives from the oil industry, and rope manufacturers. The P.I. has attended the workshop entitled “Advanced Fiber Rope Technology” hosted by Tension Technology Incorporated (developers of the Gen-Rope software), has visited with Marlow and Whitehill rope manufacturers, has met with Chris Leech, Ph.D. (developer of Gen-Rope), and has toured the testing facilities at the National Engineering Laboratory in Scotland. Further involvement with industry representatives is being coordinated through the OTRC.
Reports & Publications: Currently, research efforts are focused on the development and refinement of a validated software tool for modeling polyester rope behavior. Because this work is still in progress, no reports or publications have yet been produced. However, in January of this year, a presentation discussing the current status of the research was made at an MMS-sponsored workshop. The workshop was attended by over 70 participants and included representatives from the oil industry, rope manufacturers, and drilling contractors. A copy of the presentation was included in the meeting minutes that were later distributed to the participants.
