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Synthetic ropes

Introduction

"Synthetic rope" is a generic term that encompasses different types of ropes made of synthetic fibers. The mechanical behaviour of these ropes depends both on the fiber material and the rope construction.

Note that this is also true for classical steel wires but they have been used for ages assuming an elastic behavior.

With the growing interest for new mooring configurations, a lot of work has been done to better capture the complex mechanical behavior of synthetic ropes and different approaches have been implemented into DEEPLINES.

So far, the synthetic ropes are always treated like bar elements, that means that their bending stiffness is ignored.

The mooring model is defined as a classical mooring pattern in DEEPLINES. A mooring line may be composed of segments of different types referring to dedicated properties depending on the type of rope.

Polyamide ropes

Within the PolyaMoor collaborative project conducted by France Energies Marine (FEM), a specific visco-elasto-plastic behavior law applicable to polyamide ropes has been developed.

Following this approach, a constitutive behaviour law is defined at the stress/strain level by several parameters derived from experimental tests.

This feature is still protected by a specific license key and, therefore, is not part of DeepLines commercial version. However, it can be used upon request after prior agreement of FEM.

Dyneema ropes

A specific type of properties for Dyneema DM20 ropes has been implemented into DEEPLINES V5R7 without specific license key. To use this feature, you need a dedicated encrypted file provided by DSM.

This file contains information which allows an automatic update of the Dyneema rope dynamic stiffness and structural damping during a time domain simulation accounting for the temperature, the loading frequency and the average axial strain experienced by the rope.

For more details please refer to the document "DWL_Dyneema_ropes_guidance note" provided in ./Doc folder.

Generic ropes

This modelisation is initially based on experimental results for polyester ropes. These experiments have shown that these synthetic ropes present a different stiffness under a static or a dynamic loading.

A formula has been withdrawn from the measurments to determine the dynamic stiffness. This formula is a linear function of the ratio between the mean static axial force and the breaking load.

Whatever the synthetic rope used, it is possible to define up to three axial stiffness's, namely a static, quasi-static and dynamic stiffness.

Either three values are directly entered by the user or the parameters defining the dynamic stiffness as a function of the mean load are introduced (see keyword *SYNTHEPRO).

In all cases, the calculations are similar to a bar element with the following successive steps :

Analysis	Axial force \(N_x\)	Axial stiffness
Static	\(EA_s\) \(\epsilon_s\)	\(EA_S\)
Quasi-static	\(EA_s\) \(\epsilon_s\) \(+\) \(EA_{QS}\) \((\epsilon_{QS} -\epsilon_S)\)	\(EA_{QS}\)
Dynamic	\(EA_s\) \(\epsilon_s\) \(+\) \(EA_{QS}\) \((\epsilon_{QS} -\epsilon_S)\) \(+\) \(EA_t\) \((\epsilon_t-\epsilon_{QS})\)	\(EA_t\)