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Abstract

The current industry practice to verify the performance of a rubber fender lies with manufacturers. The reason this is so readily accepted is that generally only the manufacturers own equipment capable of testing such large and specialist items. The conflict of interest between the manufacturer and Customer is obvious, especially when considering the high cost of these goods.

Introduction

For a fender to be procured properly, the customer (or his design consultant) must perform each of the following steps;

  • Select an appropriate fender inclusive of all correction factors that affect the nominal performance of the fender.
  • Verify from testing that the fenders produced for the project actually meet the performance requirements specified.
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Performance Verification Testing

Performance verification testing, (Factory Acceptance Test- FAT), is a test performed on the actual fenders produced for a project. Rubber fenders are almost always manufactured to order as they are large and there are too many models, sizes, etc to hold a stock of fenders. To ensure the fenders are produced to the particular specification of the intended project, some quanty are tested, usually 10%.

How to Perform the Verification Test

Performance verification testing is usually performed in a large press with either load cells or pressure transducers, measuring the load and deflection. From which, the energy absorption can be determined.

Besides the large specimen size, testing of rubber fenders requires a large deflection/stroke capability of the test press; there are a limited number of publicly available test presses around the world capable of testing rubber fenders. For this reason, performance verification testing is almost always performed at the manufacturer's facility.

Consequences of Out of Specification Fenders

There are two primary concerns if the fenders being installed do not meet the performance specified:

Energy absorption below specified value

When the fender is incapable of absorbing the specified energy, the kinetic energy of the berthing ship will not be fully absorbed by the fender. Thus, the fender would become fully compressed and the residual kinetic energy of the ship will be transformed into a hard impact on the quay/ jetty which could lead to costly damage to the vessel or quay.

Reaction above specified value

When the fender being installed has a maximum reaction force larger than specified, the quay will see an unacceptably high reaction force which could destabilise or damage the structure. This has very serious consequences for load sensitive structures such as monopiles or timber pile quays.

Problems with Current Industry Testing Practices

There are several serious concerns with the way fender performance verification testing is currently performed. These concerns mainly relate to the authenticity of the reported performance.

Due to the high cost of marine fenders, there is a large commercial incentive for manufacturers to cheat the performance verification tests. It is more cost efficient to build a low quality fender that does not meet performance requirements and just manipulate the tests than to build the fenders to the specified requirements

Why Verification Testing Cannot Be Left to the Manufacturer

When fender performance is left to be verified by the manufacturer only there are multiple ways in which the manufacturers can adapt the test fender or the test itself to show that an out-of-spec fender is acceptable:

  • The manufacturer can select his preferred (best in production run) fenders and present the test results of those fenders only (i.e. not a random sample of the production run).
  • The manufacturer can build special test fenders that will pass the test and build the remaining production run with substandard inexpensive materials.
  • The temperature of the fender body can be raised or lowered prior to the test (which affects the fender performance) to ensure the fender passes the test
  • The manufacturer can alter the speed of compression of the test rig and as rubber is a viscoelastic material, this will affect the test results.
  • The manufacturer can complete numerous compression cycles to 'work' the fenders prior to the test which would again affect the test results.
  • The manufacturer can re-calibrate their test rig to record different reaction values to ensure the fender passes the performance test.
  • The manufacturer can manipulate the raw test data (for example when converting the voltage recorded from the load cells into a force) to show that the fenders have passed.
  • The manufacturer can present test data which is completely unrelated to test performed.
Current Practice with Witnessed Testing

Common practice in the industry has relied on manufacturer self testing with witnessing by either a 3rd party or by the consultant. There are several reasons why this is inadequate but the primary reason being there is no easy way for a witness to verify the results independently of what the manufacturer is reporting. Modern data acquisition methods rely on computers to interpret the data and produce a report. The witness rarely has any understanding of how the data acquisition system functions. There is little difficulty in the manufacturer adjusting the recorded data inside the computer without the witness' knowledge.

The 3rd party inspection agencies are not in any way guaranteeing the validity of the data they are stamping.

How to Test independently with confidence

Independently verifying fender performance during the performance verification test 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 with independent equipment.
Independent Laboratory

The advantages and disadvantages of independent laboratory testing include:

  • The laboratory is a 3rd party testing laboratory that has no incentive to manipulate the results.
  • Laboratories are often nationally accredited.
  • However, these laboratories are far and few between around the world, which results in fenders having to be shipped from the manufacturer's factory to some distant test laboratory before then being shipped to the customers location. This additional shipping would have obvious additional costs and programme implications for the customer.
  • The independent laboratories are not purpose built for fender testing and therefore rarely have an ideal fender test set-up. This can sometimes lead to the manufacturer contesting the results of the independent laboratory.
  • The independent laboratory is often in another country compared to the fender manufacturer and therefore may adhere to different test regulations and hold different test accreditations.
  • The independent test laboratories rarely have sufficient space and facilities to complete the fender temperature stabilisation required prior to the performance test.
Manufacturer's Facility

The advantages of independent verification using the manufacturer's test rig include:

  • Manufacturers are accustomed to the specific needs of fender testing and they are already setup to easily test fenders in accordance with the relevant standards at their factories.
  • Convenient in that the fenders are can be manufactured and tested in one location, meaning there is no additional logistics to consider.
  • The independent test equipment can be inserted into the manufacturer's frame with ease.
  • The independent test operative can randomly select the fenders to be tested from the full production run.
  • The independent test operative can ensure that the fenders are the correct temperature, the fenders are compressed at the correct speed and that the fenders are not worked prior to the verification test.
  • The independent test operative can check the dimensions and the weight of the fenders to ensure they have be manufactured with the correct materials and mould size.
  • The independent test equipment can be calibrated in the same country as the fender manufacturer's factory to rule out any disagreements related to test calibration or national accreditations.
  • The independent test equipment is not reliant on the manufacturer's raw data and therefore there is no opportunity for the raw data to be manipulated.
  • Other inspections of the fenders such as build quality and dimensions can occur during one inspection visit.

Summary and conclusions

The marine industry needs to accept that independent verification testing needs to be done without any influence or manipulation by the manufacture. There is simply too large an incentive for the manufacturer to modify the test fender or the test itself when such large monetary amounts are at stake.

Given the importance of fenders in protecting the Quay and vessel, it is difficult to understand how little attention some port operators/ owners give to the procurement of fenders.

No matter how well the designer understands the fender specification process it will hardly matter if the performance verification testing is not independently verified with independent equipment and qualified independent test operatives.

references

PIANC 2002, Guidelines for the Design of Fender Systems: 2002, Report of Working Group 33 - MARCON, PIANC, Brussels, Belgium.