Marine and Infrastructure

Fender Quality Framework


High quality fenders are essential to the safety and efficiency of port operations, protecting vessels and terminals alike.

The industry has historically undervalued this mission critical equipment, often looking for low cost solutions, and in the process, jeopardizing operations. However, the tide is beginning to turn. Port owners, operators, and specifiers, are beginning to understand the value of high performance fender systems, and the importance of specifying them with a keen eye on quality.

Trelleborg has committed significant time and resources in taking a smarter approach to fender specification, design and manufacture, and refocusing the industry on high quality solutions that will help to protect port infrastructure and investment. We are committed to leading the conversation in best practice fender design through a commitment to ongoing research and development.

The Fender Quality Framework details the latest research into rubber fenders to provide a central resource for smarter fender selection. In these pages, you will find an overview of our deep research library, with links to more in-depth resources such as relevant reports, webinars and specification clauses.

This framework will continue to be updated as Trelleborg’s research evolves.

  • Vessels’ abnormal berthing energy calculation
  • Selection of a suitable fender, type and size
  • Specification generation of all the components of a fender system
  • Design of all the components based on the specifications

Calculation of abnormal berthing energy requires a number of vessel-related inputs and a manual selection of safety factors. The selection of input parameters depends on the experience of the designer and is often subjective due to a lack of clear guidelines. The value of abnormal berthing energy may vary substantially depending on the parameters selected, which in turn has a significant impact on the fender selection process.

Fender selection is usually done by manually comparing the performance numbers of the fenders from the manufacturers’ catalogs with the required maximum abnormal berthing energy of vessels. Two designers may pick two different types and sizes of a fender depending on whether fender-related factors such as velocity, temperature and angulation (see later) have been considered during the selection process. Fender selection, therefore, is still a gray area and clear guidelines are not followed globally.

All the accessories in a fender system are designed based on the reaction force created by the compression of the fender during berthing operations. A suboptimal selection of a fender results in a suboptimal design of a fender system, which may not produce the most cost-effective solution. As with the berthing energy calculation and fender selection process, there are no globally standardized design guidelines for fender systems. The final design of a fender system depends on the designer’s experience, data input and the load cases considered during the design process.

A starter for specification and fender selection

Trelleborg’s berthing energy calculation tool and fender selection tool almost completely digitizes the fender selection process, removing the need to carry out complex and time-consuming manual calculations. The specification generator tool is intended to guide the industry towards a standardized engineering process, reducing subjectivity in the design process and helping to shape consistent industry best practice.

The new tools can complete a calculation process for up to ten types of fenders, cross referencing up to 23 grades of rubber, which would take several hours if prepared manually. The digitized process requires simple input sections, and generates a document detailing all potential fender systems suitable for the application and performance required. Results generated are compliant with both PIANC WG33 Guidelines for the Design of Fender Systems: 2002, and the latest British Standard Institution’s Code of Practice for Design of Fendering and Mooring Systems, BS6349-4:2014.

NB: Carbon Black is the high quality, reinforcing filler Ash contains calcium carbonate, a non-reinforcing white filler

Some fender suppliers have historically undercut reputable manufacturers by supplying lower cost, but lower quality fenders. They can pass on short-term cost savings to their customers in two ways:

  • By using a higher percentage of recycled rubber within the fenders, instead of virgin rubbers
  • By replacing carbon black fillers with non-reinforcing white fillers

Research conducted by Trelleborg found that fenders with recycled rubbers and white fillers are heavier (and denser) than virgin rubber fenders. This significant weight difference for the same size and grade of fender tells its own story. Users can evaluate whether a fender uses low cost recycled materials or is made with a high performance rubber compound, with the benefits of long life and superior resilience.

Trelleborg has developed two analytical tests to help buyers determine the quality of the fenders they procure. These chemical tests require only a small sample from the fender body, leaving the fender’s performance unaffected, but ensuring peace of mind for owners and operators.

For more information on the key differences between ingredient selection in high quality and low cost fenders, and how to determine the raw materials your supplier has used, read ‘Fenders: why it’s not so black and white’.

When a rubber fender is compressed, the resultant reaction force and energy absorption are greater when the compression occurs at a higher speed. Currently, performance data from most manufacturers is presented with a berthing velocity of 2-8 cm/min, and rarely is there guidance on the effects of temperature or velocity.

The difference between this and actual real life berthing conditions (those used for the design of fender systems and wharf structures) needs to be accounted for in the engineering design.

Velocity Factor (VF)

Typically, the normal berthing velocity of vessels is from 20mm/sec to 500mm/sec. Manufacturers should test at actual berthing velocities to determine the performance of the fenders. However, in practice this is exceptionally difficult given the level of investment in equipment and range of fenders to be tested.

For a given velocity, there are two factors that have the greatest influence on VF: strain rate (compression time) and the type of rubber used in the fender.

Strain rate

Compression time is a direct measure of strain rate. If all other parameters are kept constant, strain rate has a significant impact on VF.

For a given velocity, large fenders are slower to compress than smaller ones. Subsequently, at the same berthing velocity, larger fenders experience a lower strain rate and magnitude of VF than smaller fenders.

Type of rubber used

Trelleborg has undertaken extensive testing using actual high speed compression. Results show that given the same compression time, a fender comprised of 100% natural rubber (NR) will have a lower velocity factor than a fender comprised of 100% synthetic based rubber (SBR).

This is due to differing rates of stress relaxation between NR and SBR, and relates to differences in the microstructure in the respective polymer chains. SBR is commonly used in marine fenders in conjunction with, or as a replacement for, NR. This improves longevity, boosts high temperature performance and enhances certain physical properties.

Testing has shown that VF is highly dependent on the NR/SBR blend ratio and the overall rubber compound formulation. Therefore, it is necessary that manufacturers and designers understand the factors that affect VF and are able to provide competent commentary in relation to the application of VF in their rubber compounds and fender designs.

It’s imperative that manufacturers incorporate guidance on the effects of VF on their fenders to ensure that strain rate and material grade is properly considered. When comparing catalog figures from different manufacturers, it’s essential to check VFs are applied to ensure a fair and accurate comparison is made.

For more information on the impact of VF on fender performance characteristics, read ‘Applying the Right Correction Factors’.

Temperature Factor (TF)

For a fender, reaction force and energy absorption are directly proportional to rubber stiffness. This changes dramatically with temperature and has a huge effect on fender performance.

Any factor that has an impact on the stiffness of the rubber compound must be taken into consideration during the design of fender systems.

In general, increasing temperature has the same impact on stiffness as decreasing strain rate, i.e. it effectively makes rubber softer. To accommodate the variations in temperature that fenders will be exposed to under actual operating conditions, it’s essential to apply TF during the fender design and selection process.

Similar to VF, TF is highly sensitive to the type of rubber used – NR or SBR, or a blend of the two, as well as the inclusion of recycled rubber. TF is a characteristic that varies between fender types and manufacturers.

TF is vital in understanding changes to reaction force and energy absorption of fenders in normal operating conditions, and plays an important role in both the design of berthing structures and the selection of fender systems.

Despite PIANC recommending the application of TF and VF, these are really only the starting point in the design and selection of rubber fenders. Many suppliers (trading houses) and manufacturers either do not make the necessary investment in research and development to underpin their claims, or appear to artificially boost the fenders’ performance on paper.

Lifecycle determination

The life expectancy of fender systems is highly dependent on the critical rubber component, more than any other component or accessory. The durability and subsequent lifecycle of rubber fenders depends on many factors. These include compound formulation, environmental conditions in the field, heat, ozone, operational use, mechanical damage, and the type of rubber used.

Oxidative aging, a process described as the change in rubber properties over time, is one of the main issues impairing the functionality of rubber fenders over their lifecycle. The mechanisms that produce oxidative aging depend not only on the degradation agent (oxygen, ozone, UV radiation etc.) present in the environment, but also the type of rubber used (virgin or recycled), and the types of additives used in the compound formulation.

Hear from our materials expert, Mishra Kumar in ‘The Rubber Quality Webinar’ for a more in depth explanation of the impact of compound composition on VF and TF.

Trelleborg has devised a free to use specification data sheet to ensure quality compounds can be specified and substantiated, to drive standards up across the industry.

Jebel Ali Port

Jebel Ali Port

DP World, UAE Region, built Trelleborg’s rubber quality standards into their fender specification for the Quay four refurbishment project at Terminal 1 of Jebel Ali Port in Dubai. After working on the project with DP World for years, Trelleborg supplied 60 super cone SCN 1300 fenders to protect berths 18 and 19, with installation completed in April 2014.

DP World wanted to ensure they could verify the quality of the rubber that would be used in the fender systems for the berths – and, as such, built rubber quality parameters into the specification to protect themselves from the risk of poor quality substitutes.

Trelleborg developed an analytical test and advised DP World to test the final fenders when they arrived on site or before shipping from the production facility. They also recommended that this test be built into all specifications. In this case, DP World randomly selected one or two fenders while visiting Trelleborg’s manufacturing plant in Qingdao, China for further TGA testing to ensure continuity of quality throughout the production process.

Hesham Abdulla, Container Terminal 1 Director, DP World, UAE Region, says: “Trelleborg was able to offer technical support across all parts of the system, from ensuring the rubber element would precisely meet specification, to chains and accessories. Their support and local presence meant that they were a natural choice to supply the project and thanks to their in-house manufacturing capabilities we could even have the solution personalized.”

Trelleborg fenders stand the test of time at Hong Kong International Terminal

Trelleborg fenders stand the test of time at Hong Kong International Terminal

In 2004, Trelleborg manufactured and supplied 55 sets of its SCK Cell Fenders to the Hong Kong International Terminal (HIT) as part of the upgrade of its Container Terminal 8 (CT8). As a multi-purpose berth within the busy HIT ports of Hong Kong, CT8 experiences high volumes of traffic and following the expiry of the warranty for the rubber fenders in 2015, it was time to inspect their current quality. This was a precautionary measure – even with 12 years of continuous use CT8 had not experienced any port downtime due to issues with the fenders.

A joint visual inspection by HIT and Trelleborg in January 2017 indicated slight ozone cracking had started to appear in the outer surface of some fenders. While this was in keeping with the expected aging effects of these high performance fenders, the inspection report recommended two of the SCK cell fenders be sent for compression testing to verify its performance and integrity.

Factory testing in Singapore showed that the performance of the fenders was still within the initial manufacturing tolerance and the minor ozone cracking had no negative impact on the fenders’ operational capability. Franco Cheng of Hong Kong International Terminals says, “Even with 12 years of continuous use, the SCK cell fenders have maintained their high performance and this has had a positive effect on berthing uptime at CT8. It certainly shows our decision in 2004 to opt for a high quality fender with a proven track record was the right one. More than a decade later and we are still delighted to be partnering with Trelleborg. Their unrivalled industry expertise and first-rate aftercare means we will continue to work with them for years to come.”

A high-quality compound is one where carbon black is broken down and distributed uniformly throughout the rubber matrix. Ideally, it should be broken down into nanoparticles before distribution. However, it requires an extremely advanced mixing machine to break it down to an optimal level.

The percentage of carbon black dispersion in the final compound controls the quality of the end product. Poor dispersion can lead to damaging effects, such as reduced product life, poor performance, appearance, processing characteristics and even product uniformity.

To achieve a high and uniform carbon black dispersion, operators must ensure close control over the mixing process, and the machinery used in this process is critical.

There are a number of machinery related parameters that affect the carbon black dispersion of the final mix. These include ram pressure, rotor speed and design, fill factor, coolant temperature, mixing sequence, time and the number of passes through the mixer.

Trelleborg’s ‘Mixing it Up’ whitepaper describes the essential parameters to ensure a high and uniform distribution of carbon black within the rubber matrix, and the machinery options available to manufacturers to achieve this.

Measuring success

Hardness is the current industry practice for judging the energy absorption capacity and reaction force of a rubber fender. However, this method of measuring a fender’s performance is too simplistic and largely inaccurate.

The current market perception is that softer fenders will have a lower energy absorption capacity and harder fenders will have higher energy absorption ability, but it is easy to increase the hardness of a rubber fender by using non-reinforcing white fillers and recycled rubber.

Modulus (stiffness) is the slope of the stress / strain graph during tensile strength measurement of a cured rubber sample. Having a higher modulus of a rubber compound indicates a higher energy absorption capacity of a fender. Therefore, as a more robust alternative, Trelleborg suggests that the industry starts to measure modulus, rather than hardness of a rubber compound to corelate with fender performance.

Superior compound (no recycled rubber) Inferior compound (high percentage of recycled rubber)
Internal Mixer 1 Internal Mixer 2
Internal Mixer 1 Internal Mixer 2
77 74
77 76
Compound Modulus (MPa)
14.4 13.3
8.7 8.4

An example of this can be seen in the table above: a superior compound with no recycled rubber, and an inferior one with a high percentage of recycled rubber mixed in two different internal mixers. Although the hardness values of the compounds are similar, there is a greater difference in the modulus values. Evidently, measuring only hardness will provide a false impression of fender performance.

It’s important that the industry works towards a deeper understanding of the impact of the manufacturing process to ensure that mixing quality does not impact product performance.

Port owners, operators, contractors and consultants need comprehensive specifications and testing methods covering ingredient selection, mixing procedure and production process to stipulate the performance of finished products.

For more information on how the mixing process can be measured, in order to guarantee compliance to specifications and assure performance, watch the ‘Assuring Fender Performance’ webinar to hear from Trelleborg’s materials expert, Mishra Kumar.

Molecular orientation

Rubber is a long chain polymer, and its strength and performance depends on the alignment or orientation of the long chain molecules in its final state. Clearly, molecular orientation in the rubber element of a fender system is critical to its characteristics. Any given rubber sample will have a unique strength, dependent on the direction in which the molecules are oriented.

These differences in strength and, subsequently, performance can be substantial. The orientation of rubber molecules determines the tensile strength, modulus and other physical properties of rubber compounds.

The building process

The building process of fenders determine the orientation of the rubber molecules in the final product. It is for a given fender is dependent on the type and size of the fenders, and the application they will be used in. For example, large cone fenders are produced through a wrapping process, whereas small cone fenders are produced by extruding the rubber compound into the mold. Tug boat fenders are produced using an extrusion process, whereas cylindrical fenders are produced through the wrapping process.

Optimizing manufacturing

The orientation of rubber chains is determined by the manufacturing process. Most fenders are produced either by extrusion into a mold, by filling up molds with rubber blanks, or wrapping of rubber sheet on a mandrel.

The orientation of the molecules inside the mold differ between each of the three processes, significantly impacting the properties of the rubber compound and, subsequently, the performance of the final fenders. Filling up the mold with extruded block or wrapping process provides better performance than direct extrusion into the mold. It’s essential that due consideration is given to the process used.

Curing and performance

When the building process is complete, curing is the next step in fender manufacturing, and equally important to the quality of the final product. This is the step that takes soft, commercially unusable rubber fenders and converts them into hard and useful products.

Chemical curing is a cross linking process in which individual long molecules of rubber are converted into a three dimensional network of interconnected chains, through chemical cross links.

The most important ingredients in the curing process are sulfur and accelerators. Accelerators are used to increase the rate of reaction between the rubber molecules and the sulfur. Sulfur is the element that makes the links between rubber chains.

The final properties of a rubber compound depend on the cross link density and number of sulfur molecules in a link, otherwise described as the type of cross link. The cross link density is defined by the number of crossings between the rubber chains. Both cross link type and density depend on:

  • Sulfur dosage
  • Accelerator type
  • Sulfur and accelerator ratio
  • Curing time and temperature

Generally, a low sulfur to accelerator ratio leads to cross links with one or two sulfur molecules present. Products from such a curing system exhibit better heat stability, suitable for products used in hot climates.

On the other hand, products produced using a higher sulfur to accelerator ratio will have higher tensile strength, tear strength, and fatigue resistance due to more sulfur molecules in a link. This system provides higher longevity of products at medium to low environmental temperatures.

Depending on the sulfur to accelerator ratio, cure systems for NR or SBR can be classified as:

  • Conventional cure (CV)
  • Efficient vulcanization cure (EV)
  • Semi efficient vulcanization cure (SEV)

The Sulfur Analyzer

An instrument called a sulfur analyzer is available to help specifiers substantiate the amount of sulfur in a rubber product, using just a small sample from the end product.

Vulcanization System Conventional (CV) Efficient (EV) Semi Efficient (Semi EV)
Sulfur Dosage, phr 2.0 - 3.5 0.4 - 0.8 1.0 - 1.7
Accelerator Dosage, phr 0.4 - 1.2 2.0 - 5.0 1.2 - 2.4
Accelerator/Sulfur Ratio 0.1 - 0.6 2.5 - 12.0 0.7 - 2.5

A series of cone fenders were tested after curing at various curing temperatures. It was observed from the following graphs that the curing temperature and time have an impact on the load deflection graph. Therefore, it is important to make sure the manufacturers are able to vulcanize fenders, after they undergo the correct building process. Ensuring proper curing process will reduce the variation in the properties of the final products.

Curing Study of Cone fenders

Curing Study Diagram

Performance verification testing

Performance verification testing, often referred to as a fender acceptance test, is a test performed on actual fenders produced for a project. Rubber fenders are almost always manufactured to order as there are too many models, sizes, and grades to stock. To ensure fenders are produced correctly and according to specification, 10% are usually tested as per PIANC’s recommendation. These tests differ from the scale model testing performed to establish catalog rated performance, RPD, or for determining the various correction factors.

Conducting performance verification

Performance verification testing is usually performed in a large press or test frame with either load cells or pressure transducers measuring load, and a displacement transducer measuring deflection.

There are a limited number of publicly available test frames around the world capable of testing rubber fenders, which is why performance verification testing is almost always performed at the manufacturer’s facility.

Trelleborg’s whitepaper, ‘A User Guide to Performance Verification of Marine Fenders’, provides deeper information on how correction factors apply. It also explains why the application of safety factor is not necessarily enough to ensure safe berthing, and discusses the impact of improper energy absorption capabilities.

There are a number of machinery related parameters that affect the carbon black dispersion of the final mix. These include ram pressure, rotor speed and design, fill factor, coolant temperature, mixing sequence, time and the number of passes through the mixer.

Trelleborg’s ‘Mixing it Up’ whitepaper describes the essential parameters to ensure a high and uniform distribution of carbon black within the rubber matrix, and the machinery options available to manufacturers to achieve this.

Why verification testing cannot be left to the manufacturer

Testing results are routinely manipulated for competitive reasons. It is much less expensive to build a low quality fender that does not meet performance requirements, and manipulate test results, rather than building it to the requirements. If a manufacturer decides to do this, a company that spends the money to meet the requirements will find it difficult to compete.

The truth about witnessed testing

Common industry practices rely on factory testing, witnessed either by a third party or a consultant. There are several reasons why this is inadequate. The primary reason being that there is no easy way for a witness to verify that the results being put forth for approval are actually from the test witnessed. Since performance verification testing of rubber fenders is a specialized topic, the witness rarely has an understanding of how the tests should be run or how the data acquisition system functions.

Most specifications simply require a third party inspection agency to witness the test. It should be absolutely clear that they are indeed doing that, and only that, witnessing a test. There is no requirement for them to investigate how the test data was acquired or if the report they are asked to approve is from the test they witnessed.

When testing is performed at the factory, the fender being tested can even be specifically selected for the test, rather than randomly selected. Manufacturers can build special test fenders that will pass the test and build the remaining production run with substandard materials. To combat this, the inspector should select fenders from the production run after all fenders are manufactured.

Verifying verification

Independently verifying fender performance during the performance verification test is not easy, but it is critical if the intended performance of the fender is to be guaranteed.

Independent verification testing is possible with one of two methods:

  • Testing at an independent structural laboratory
  • Testing at the manufacturer’s factory using their test frame but independently recorded performance data

Each of the two methods has its advantages and disadvantages, which are detailed in ‘A user guide to the performance verification of marine fenders’.

Either way, it is time for the fender industry to accept that performance verification testing needs to be conducted independently, or at least outside of the manufacturer’s control. There is simply too much of an incentive for the manufacturer to just “make it pass” when such large contracts at high dollar amounts are at stake.

Given the lack of suitable test presses worldwide, the long term goal for the industry should be for manufacturers to offer independent testing at their own facilities, but with results guaranteed to be independent. This will require the industry to adopt standards and methods that are easy to implement, cost effective, and easy to understand by independent inspectors and consultants.

Trelleborg proposes that the industry develop a load monitoring system that provides independently verifiable load readings that are outside the control of the manufacturer. If this monitoring system is standardized and then adopted by all manufacturers then inspectors will only have to be trained on one system.

Eventually, the industry should develop a working group to write a specification on how third party inspection companies can verify performance, not just witness it. This will take time, of course.