The way polyester ropes change their length in response to tension is often represented using a static-dynamic model with parameters for dual linear stiffness, based on testing according to defined procedures. This may not be sufficient for the mooring analysis since the stretch response to tension of fiber ropes is nonlinear. In the Syrope Joint Industry Project, a model to be used in mooring analyses was developed together with a test procedure to obtain the parameters for this model. Based on these parameters and experience from the testing, an equivalent non-linear spring-dashpot model was developed. This model can predict the response in the rope also for other test procedures than those used for establishing the parameters in the analysis model. This demonstrates the validity of this Syrope analysis model.