Physical Component Testing to Simulate Dynamic Marine Load Conditions

Abstract

The reliability and integrity of components used in the marine offshore environment is paramount for the safety and viability of offshore installations. The engineering challenge is to design components that are robust enough to meet reliability targets whilst lean enough to minimise cost. This is particularly the case for offshore marine renewable installations which operate in the same, possibly harsher, environment as offshore oil and gas installations, and are subjected to highly cyclic and dynamic wave, wind and operational load conditions. The cost of electricity produced has to compete with other means of electricity generation and does thus not offer the same profit margins available as oil and gas commodities. As a result, components for marine renewable installations have to meet the target reliability, without the application of costly safety factors to account for load and environmental uncertainties. Industries with similar design tasks such as the aviation or automotive industry have successfully used a service simulation test approach to develop robust yet lean designs. This paper builds on an approach to establish and validate the reliability of floating renewable energy devices in which dedicated component testing using the purpose built Dynamic Marine Component test rig (DMaC) plays a pivotal role to assess, validate and predict the reliability of components in the marine environment. This paper presents a test rig for both static and fatigue tests of marine components such as mooring lines and mooring shackles under simulated or measured load conditions and provides two case studies from recently conducted mooring component tests. This includes an investigation into the load behaviour of synthetic mooring ropes and the ageing of mooring shackles.

Synthetic rope responses in the context of load history: The influence of aging

“…less than 14% MBL) lower stiness values were reported for aged 8-strand plaited ropes in [6]. In the current study the trend of increasing axial stiness with the application of higher mean load levels is noted for both new and aged samples (Figure 7a), as observed in other studies [16,24,44,47]. A single degree-of-freedom line tted to mean load and axial stiness values (both normalised by the rope MBL) can be used to quantify the dynamic stiness of a sample [7].…”

“…For this study a 4.5m long sample was taken from the SWMTF rope with the original splice and eye retained at one end and three u-bolts used to terminate the other end. A second sample was taken from the SWMTF rope and used for preliminary tests at L'Institut français de recherche pour l'exploitation de la mer (IFREMER) and the University of Exeter [44]. Yarn assemblies, yarns and bres were extracted from the remaining central portion of the SWMTF rope for detailed analysis and testing.…”

Synthetic rope responses in the context of load history: Operational performance

“…It was also shown in [20] that there was an e ect of harmonic oscillation period (and hence load rate) on both average axial sti ness and damping. The axial sti ness values calculated for the bedding-in cycles contained within load sets LS1 and LS2 are in agreement with this, in that the lower axial sti ness values correspond to bedding-in cycles with lower load rates (Figure 6b).…”

“…Previous tests conducted by the authors on a section of aged rope of the same construction demonstrated that the application of higher mean load levels resulted in steeper hysteresis loops, leading to higher average axial sti ness values [20], re ecting studies conducted on HMPE and aramid ropes [6,11]. For load sets LS1 and LS2 the same e ect is demonstrated with the samples subjected to bedding-in cycles (Samples 2 and 3, subjected to BI2 and BI3 respectively) compared to Sample 1 (Figure 6a).…”

A novel mooring tether for peak load mitigation: Initial performance and service simulation testing

“…Performance and survival in extreme sea states 4. Indication of fatigue performance All tests have been carried on the Dynamic Marine Component test rig (DMaC), a unique facility to replicate the forces and motions that components and sub-systems are subjected to in (floating) marine applications [23][24][25]. It comprises of two hinged gimbles with a backplate, termed 'moving headstock' which is capable to replicate motions in 3°of freedom and a linear actuator to provide the axial loading on the specimen.…”