Nylon fibre rope moorings for wave energy converters
The wave energy market has traditionally employed mooring concepts adopted from the mature technology of the oil and gas industry. However, this approach may not always be appropriate to satisfy the particular requirements of Wave Energy Converter (WEC) mooring systems, especially the need for very compliant moorings. Specific challenges are: the huge 1st order motion induced forces generated by storm waves in shallow water; the potential for slack-snap loading on a mooring; and, the fatigue performance of the mooring components. This paper describes the benefits of employing high compliance fibre rope in mooring systems of less than 100 m water depth. Nylon fibre ropes are a very attractive proposition because of their low modulus and cost but have previously been thought to have a poor wet fatigue performance. A specially designed long-lay-length Superline subrope has been tested both for compliance and for wet fatigue performance. The rope compliance is about twice that of parallel-subrope construction polyester rope. The fatigue performance is less than that of parallel-subrope polyester rope but is comparable to that of conventional construction polyester rope and to that of the chain and steel wire ropes which must also be used as mooring components. The high compliance of the nylon subrope construction together with the more than adequate fatigue performance demonstrated in this study qualify such nylon ropes as ideal for WEC mooring systems.
Fatigue durability of nylon rope for permanent mooring design
Over the last nine years three major research studies have led to the development of a new design of nylon rope suitable for permanent moorings with design lives in the region of at least 20 to 30 years. Prior to this research nylon rope designs had a fatigue performance which was only adequate for temporary moorings typically lasting six months to a few years. This paper reviews two pivotal very long duration tests (10 and 20 million cycles) which were inspected for damage following completion of the load cycling, before measuring the residual breaking strength. In addition to these very long duration tests which were undertaken with load ranges more representative of wave loading, to account for all the ranges experienced through life it is necessary to be able to calculate any strength loss. Hence, a series of “partial life” fatigue tests was conducted to determine the strength loss for a series of load ranges. In these tests rope samples were cycled to a representative number of cycles which the rope would experience in service before stopping the test to assess the accumulated damage by means of a series of rope yarn tests. The residual breaking strength data is presented for critical layers through the cross-section which clearly identifies how yarns in different positions in the rope are exposed to varying degrees of inter-strand abrasion. A preliminary strength loss relationship was developed which could then be used to calculate strength loss for the fatigue spectrum based on an example mooring analysis of a WEC device. This work was successfully undertaken under the umbrella of UK Government funded Research and Development test programmes driven by consultancy firm Tension Technology International (TTI), as part of a team with rope-maker, certification body and wave energy converter (WEC) developer. The results of these projects have created a new technology enabling improved mooring systems with the additional benefit of significant cost reduction for the renewables industry. This technology can also be applied to any offshore mooring system where first order motions create high peak line loads.
Defining, Measuring, and Calculating the Properties of Fiber Rope Deepwater Mooring Lines
This paper explains and defines the synthetic fiber rope stretch and stiffness change-in-length properties which are important for deepwater platform mooring system design. It introduces the concept of accumulated elastic stretch, which causes temporary stretching and stiffening of the rope during tension cycling. It proposes how these properties can be determined by rope testing and by computer modeling. It proposes how the change-in-length properties should be used in mooring system design and analysis. Background The offshore industry and the fiber rope manufacturers have now had some experience in using fiber ropes in deepwater platform moorings. Petrobras pioneered this in the mid 1990s. In the late 1990s several class societies 1 2 3 and then API 4 published guidelines for fiber rope deepwater moorings. Tension Technology International with others also published fiber rope design guidelines. Those guidelines were based on what was then perceived to be the best design methods and the best test methods for the rope properties needed to carry out those methods. Since those guides were published, several fiber rope mooring systems – Mad Dog 6 7 and Red Hawk – have been designed for the Gulf of Mexico. A number of large fiber ropes have been tested and analyzed for use in those and other projects. Also, findings are now available from several significant fiber rope research programs which were conducted while and after those guides were written. INTRODUCTION The unique stretch and stiffness characteristics of synthetic fiber ropes are important in the design and analysis of deepwater platform mooring systems. Mooring system designers are experienced in designing with wire rope and chain catenary mooring lines. The stiffness characteristics of wire rope and chain are essentially linear. These components do not experience permanent stretch until high tension. With these components, the principal mooring system restoring force is produced by the catenary effect. As mooring water depths increase, there are incentives for using fiber ropes, as discussed in other papers. But with fiber rope, the principal mooring system restoring force is produced by the tension vs. stretch (stiffness) characteristics of the rope instead of by the catenary effect. Mooring system designers are still trying to understand the unique stretch and stiffness characteristics of fiber ropes and to incorporate them into traditional mooring design procedures. The stretch and stiffness characteristics of typical synthetic fiber ropes are time dependent and non-linear. Fiber ropes experience both elastic and permanent stretch. Conflicting and confusing terminologies are sometimes used. Some important rope properties have only recently been adequately understood and explained. Here the term stretch refers to any change in rope length which is produced by tension. Stiffness is the relationship between stretch and applied tension. Other terminology is introduced in the text and summarized in Nomenclature at the end of this paper. The paper is divided into four parts: ! Part 1 uses a spring-and-dashpot analog model to describe the principal synthetic fiber rope stretch and stiffness characteristics. ! Part 2 illustrates some of these characteristics with results from several large rope test programs. ! Part 3 discusses how these characteristics should be used in a deepwater platform mooring analysis . ! Part 4 describes how these characteristics might be determined through rope testing and through computer modeling of the rope. Fiber Rope Deepwater Mooring Lines OTC 16151 Defining, Measuring, and Calculating the Properties of