This paper presents laboratory testing results to explore the experimental and fundamental dynamic axial stiffness behaviour of polyester ropes for deepwater mooring systems. Effects of mean load, load range, load or stroke control, extensometer versus crosshead, peak trough versus linear regression, cyclic period and scale effect (60 to 600kN) have been studied.
Understanding the axial stiffness behaviour is essential for calculating peak line tension and platform offset for riser excursion and fatigue life. Also, the effect of bedding-in of the rope structure, which increases stiffness, is required to establish the test matrix for certification.
A thorough review of the methods of data collection, interpretation of data and calculation of stiffness has been conducted. For those methods reviewed, no significant differences in stiffness have been found except for crosshead, which tends to be higher than measured directly on the rope between splices.
The effect of cyclic period and scale effect upon dynamic axial stiffness has shown to be insignificant and for dynamic mooring system response can be ignored, which reinforces previous work. For drift and longer term effects, the effect is very small and needs to be considered whether any allowance is needed. The effect of bedding-in on axial stiffness has shown that to reach peak axial stiffness, very large numbers of cycles are required. Typically, however, around 80-90% of the peak axial stiffness can be reached within several hundred cycles. For full-scale certification, the costs and time of large numbers cycles maybe prohibitive, so a technique has been suggested for conducting these tests on scale samples since there is no scale effect of stiffness.
These results will add to the knowledge base for understanding axial stiffness behaviour from testing, data collection, stiffness calculation method and mooring/riser system design. It will also provide valuable data for the future rewrite of API 2SM, to ensure that certification test methods collect the appropriate axial stiffness data for polyester ropes. The information will also be of use to the regulatory bodies.
The main purpose of the work presented in this paper was to investigate the experimental factors that can influence the measurement of dynamic axial stiffness of polyester rope when subjected to sinusoidal loading.
The factors addressed in this paper are:
Testing medium (air or water).
Constants and variables, in the calculation of dynamic axial stiffness.
Linear regression stiffness versus peak trough stiffness.
Extensometer derived stiffness v crosshead based stiffness.
Machine load control versus stroke control.
Effect of cyclic period upon axial stiffness.
60 kN sub-rope to 600 kN rope stiffness scaling.
A 60 kN sub-rope together with 600 and 2,500 kN ropes of parallel strand construction (PSC) were tested during this investigation.
Four servo-hydraulically controlled rope fatigue machines were used to generate the test data. The specifications of these machines are provided in Table 1. Testing was carried out with the rope samples fully immersed in tap water.
Table 1: Specifications of the Fatigue Test Machines (Available in full paper)