DNV Improving Fiber-Mooring Design Practices

Fiber ropes are commonly and successfully used as mooring lines on deepwater drilling and production platforms, but there has been a need for better design data. DNV and Tension Technology International have now conducted a joint industry research project to develop a new testing standard which will further facilitate the use of such ropes through more secure design and enhanced cost effectiveness.

The joint industry project was sponsored by leading oil companies and contractors, with broad support from the rope-making industry.

The need for data

"There is a great need for a simple standardized test method to determine the fiber-rope visco-elastic change-in-length characteristics," explains Vidar Ahjem, DNV's representative on the project.

"Accurate data on rope stiffness and stretch are important when designing deepwater mooring systems. Stiffness affects the required rope break strength, and permanent stretch affects the required rope length. Further, stiffness and stretch influence the maximum platform offset and thus required riser length," elaborates John Flory, who represents TTI in the project.

Designers of polyester mooring systems generally use the results of only a few special tests on how the rope will stretch during installation and storms. Such tests are expensive and time consuming, and are applicable to only one project. The newly developed standardized test method will more efficiently determine the change-in-length performance of large polyester ropes, independent of the loading scenario. So now the test needs to be run only once for a particular rope design, and the test results can then be used on any project.

Use of the data

The mooring system designer can now use the test results to predict the change-in-length properties of the rope after any loading history and in response to any change in load. It will no longer be necessary to carry out the customary series of tests for each deepwater mooring system design.

The required mooring line length and take-up can be calculated based on an assumed installation procedure. If the installation procedure changes, for example as a result of installation vessel availability, the calculation can be rerun without having to retest the rope.

Peak mooring tensions and maximum mooring line stretches in response to hurricanes and loop currents can readily be calculated. The total permanent stretch over the service life of the mooring can be accurately predicted, and the required length take-up can be determined.

Cost savings

The main benefits are:

--This 'standardized' test method provides data which are applicable for any installation or service situation

--Accurate rope change-in-length data are available at the beginning of the design process, as well as throughout the phases of a project, from concept study to operation.

--The fiber rope can be optimized by selecting the proper strength, stiffness, and length of rope.

Implementation of the new test method

The new test method will be released in the next edition of the DNV Offshore Standard DNV-OS-E303 "Certification of Fiber Ropes for Offshore Mooring". Before that time, an updated version will be issued internally to the JIP sponsors so that they can benefit from this new method immediately.