Are cyclic conditions impacting your marine fender performance?
There is growing recognition that challenging environments have particular impacts on the performance and durability of marine fenders, but how well are these scenarios understood and what impact can they have on fender resilience and longevity?
Condition-based selection
Fender systems installed on continuous quays are typically selected based on berthing energy. However, for island berths, like FPSO’s which usually involve vessels moored against two to four fenders, wind and wave impacts also have an effect. These forces should be calculated through Dynamic Mooring Analysis to determine whether they exceed the fender reaction forces.
Whilst useful, this analysis isn’t always sufficient for some conditions. For example, moored vessels are subject to wave actions that can induce cyclic motions to the vessel oscillating the fender body. This can mean that fenders can be compressed thousands of times a day, causing fatigue and potential failure within a short period of time if not properly considered during the fender selection process. Currently, fender manufacturers often lack published data or guidance on handling these conditions, and literature on the topic is limited.
The performance of fenders under cyclic conditions depends on the fatigue resistance of the fender, which in turn depends on the stresses developed in the fender body. These stresses are influenced by the type of rubber, design and geometry of the fender, and the quality of the rubber compound. Differences among suppliers mean that fender selection for cyclic conditions requires specific engagement with suppliers and ideally should be based on relevant test results.
This article focuses on the performance of fixed rubber fender systems, such as cone and cell fenders, with consideration of floating fenders like foam and pneumatic fenders. It is based on a combination of in-field use, our testing data and review of available research and literature.
Cyclic motions and fender impact
Wave forces on vessels can be significant, particularly long periodic waves which can excite the natural periods of surge, sway, and yaw of moored ships, leading to large ship motions and mooring loads. Under these conditions, the motions of the moored vessel can induce high-frequency compressions of fenders that can lead to fatigue and failure.
For permanently moored vessels, the impact of cyclic conditions is even more pronounced. An example from a recent Trelleborg project in Latin America showed that fenders on a permanently moored vessel were subject to millions of small compressions and hundreds of compressions up to and beyond their buckling point annually.
Guidelines for fender testing, such as PIANC WG211 (2024) consider 3000 full compressions in durability testing. These procedures are often performed on small fender samples under standardized conditions. However, real-world conditions show significantly higher compression frequencies, higher compression velocities and a wider range of full to partial compressions. . This suggests that more robust testing and data are needed to ensure fender suitability under such cyclic conditions.
Impact of fender quality
Fender quality, including rubber compound, fender thickness, and bonding agents, plays a crucial role in performance under cyclic conditions. The rubber compound's durability is influenced by its chemical composition, with non-reinforcement fillers and recycled rubber reducing fatigue resistance. The fender's thickness affects stress distribution, with thinner sections leading to higher stresses and reduced durability. The bonding quality between embedded steel plates and the rubber body is critical to prevent catastrophic failure.
Selecting the appropriate fender for cyclic conditions is critical and involves more than just considering berthing energy. The selection varies among suppliers, depending on their confidence in the performance of their fenders under cyclic conditions. This confidence is derived from extensive field experience and rigorous testing. The below factors are important to consider:
Cyclic Loading Profile
To assess the suitability of a fender under project-specific conditions, a detailed fatigue profile is essential. This profile outlines the number of deflections per fender compression range, which can be determined through dynamic mooring analyses or based on actual site data. Using the detailed fatigue profile, fender suppliers can then recommend appropriate fenders.
Fender Recovery
Fender recovery is crucial in cyclic conditions. A fender must return to its original height after compression before the next wave impact. Typically, buckling-type fenders recover their height almost instantly (95-98% recovery within three seconds after full compression), making them suitable for wave return periods of 50-200 seconds.
Fender Damping
Fender damping affects the moored ship system. The damping effect can be integrated into dynamic mooring analyses using a Velocity Factor and the published load-compression curve.
While this article focusses on fixed rubber buckling fender systems, it’s important to acknowledge that floating fenders are commonly used on berths around the world, and require a different set of considerations under cyclic conditions.
Foam fenders have a linear load-deflection curve, meaning that small wave forces already will deflect the fender. Compressions will be more significant than compared to a buckling fender. Also, unlike fixed rubber fenders, foam fenders take longer to recover to full height, leading to progressive creep and reduced energy capacity. Consequently, foam fenders are not recommended for permanently moored situations or cyclic conditions.
Pneumatic fenders absorb energy through air, which does not suffer from fatigue, maintaining their performance under cyclic conditions. However, the fender body, reinforced with nylon fabrics, is impacted by repeated compressions.
The quality of pneumatic fenders varies significantly. Mold-produced fenders generally have better fatigue resistance compared to those made using the wrapping method, which has weaker bond strength and lacks a bead-ring construction, increasing the risk of delamination and failure under fatigue conditions. High-quality pneumatic fenders should comply with ISO 17357-1 standards, ensuring proper manufacturing processes and compound properties. Regular pressure checks are necessary for pneumatic fenders, as performance can degrade over time. This maintenance may not be practical for fenders in cyclic conditions, particularly in permanently moored situations.
Knowledge is power
Understanding the impact of cyclic conditions on marine fenders is essential, particularly for permanently moored vessels exposed to harsh environments. The selection process must extend beyond traditional factors like berthing energy and wind forces to include comprehensive assessments of cyclic loading profiles, fender recovery, and damping characteristics. The quality and design of fenders, including material composition and bonding integrity, play a critical role in their durability under frequent wave-induced compressions. By integrating rigorous testing data and supplier expertise into fender selection, the industry can significantly mitigate the risk of premature fender failure, ensuring the sustained performance and safety of marine operations.