Accurately Predicting The Service Life Of Mooring Lines Starts With First Understanding Their Creep Behavior

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December 15, 2022

Know the FACTs

Creep is the third essential rope behavior you need to understand for safe and reliable mooring. Of the four silent saboteurs that degrade performance and safety margins: Fatigue, Abrasion, Creep, and extreme Temperatures, Creep is hard to see, and often misunderstood. Creep behavior is essential to determining service lifetime and therefore it is a critical parameter to understand when selecting the right mooring lines for your vessels. In this article we will explain Creep, and how the most advanced synthetic fibers are mitigating it. 

A quick explanation of creep.

Simply put, creep is the continued extension of a material when subjected to constant, long-term static loading. For a rope, creep is the length or rate at which the fibers in the rope stretch irreversibly over time. However, it should not be confused with constructional elongation as creep relates to the fiber in the rope, not to rope construction.

In mooring ropes creep is dangerous because it happens over months or years and is hard to visibly notice. It occurs when a rope is holding a heavy static load for a long period of time, and there are three stages a rope goes through before it fails. In the first stage under load, the fibers in the rope are elastic and recover when stretched. This stage only lasts for days. In the second stage the load stretches the fibers but they don’t recover. This can start happening after a few months and can take hundreds of years. Finally, creep rupture occurs in stage 3 when the fibers are stretched beyond their ability to hold the load.

How the right fibers prevent creep.

Temperature, loading level, time, and the type of fiber used all affect creep lifetime of a rope. The first two are important as high temperatures and very high loads are accelerating factors. Fiber choice is critical as the one factor you control that most influences the service life of the rope.

Dyneema® SK78 fiber has been engineered at a molecular level to deliver up to four times longer creep lifetime than generic HMPE. If you study the molecular chains of Dyneema® SK78 fiber you would see longer molecule chains, and you’ll notice anchors placed along the chains. All HMPE fibers show viscoelastic behavior, where the chains slide under load. However the anchors in the unique design of Dyneema® SK78 prevent the molecular chain sliding, dramatically extending the second phase of creep.  

Data driven design

Every mooring rope made with Dyneema® SK78 can be designed with accurate service life based on creep performance. The mooring experts with Dyneema® have built a specialized creep testing facility for a deep understanding of fiber and rope performance and enabled the team to develop performance models that can be used to predict rope lifetime. Independently certified by DNV,  these performance models can forecast the service lifetime of a rope design in defined conditions.  The models reinforce the significant advantages of selecting Dyneema® SK78 fiber, which performs reliably for longer and in harsher conditions without the fear of premature failure. 

Ask the experts about creep

Selecting the right material with the right rope design is essential when looking for more durable breast, head, spring, and stern mooring line systems. Dyneema® SK78 fiber is engineered to deliver trusted strength, longer service life, and reliable performance. Check out the video below to dive deeper into creep resistance. And to continue the conversation, scan the QR code below or contact the mooring experts with Dyneema® by using this link Connect to an expert (dsm.com).

Dyneema®, with you when it matters. Always.

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