Performance Verification of Marine Fenders
Marine rubber fenders play a critical role in the operation of ports. They enable vessels that weigh several thousand tons to berth against vital infrastructure without damaging the wharf or the vessel. Fenders are crucial as they turn the kinetic energy of a berthing vessel into known reactions when they absorb its energy.
Prior to the publication of “Guidelines for the design of Fender Systems” [PIANC 2002], there was a lack of standardization in the design, specifications, and testing of fender systems.
This paper will reference this publication and its role in ensuring proper testing procedures for fenders. For a fender to be designed and procured properly, consultants must perform each of the following steps:
- Determine the expected normal berthing energy of the vessel. Then, apply an appropriate safety factor to establish the energy requirement for an abnormal berthing
- Select an appropriate fender including all correction factors that affect the nominal performance of the fender
- Verify from testing that the fenders produced actually meet the project performance requirements specified
Failure to complete all three steps can result in serious consequences.
It’s critical that specifiers ensure fenders installed at their facility are actually able to absorb the prescribed energy. The current industry practice of verifying the performance of rubber fenders is carried out by suppliers themselves. This practice is readily accepted because suppliers own equipment capable of testing such large items.
The conflict of interest between the supplier and the port is obvious when considering the high cost of manufacturing some of the world’s largest rubber parts.
This paper discusses potential ways that the purchaser of marine fenders can independently verify the fenders and whether they meet project specifications.
Performance verification testing
Rubber fenders are almost always manufactured to order as there are too many models, sizes, and grades to keep in stock. Performance verification testing (or a Factory Acceptance Test), is a test performed on the actual fenders produced for a project.
To ensure fenders are produced correctly and in accordance with project specifications, 10% of the order quantity is usually tested. These tests differ from the scale model testing performed to establish catalogue rated performance values, RPD, or for determining the various correction factors which are described in PIANC 2002, Appendix A, sections one to five.
How to perform verification test
Performance verification testing is usually performed in a large press or test frame with either load cells or pressure transducers. These are installed in the hydraulic circuit of the press to measure the load and a displacement transducer to measure the deflection. The sheer size of even just a mid-sized rubber fender must be taken into account. Aside from the large specimen size, the testing of rubber fenders requires a larger deflection capability than most frames can produce.
Around the world, there are only a limited number of publicly available test frames that are capable of testing rubber fenders. Therefore, performance verification testing is almost always performed at the manufacturer’s facility. There are obvious reasons that should concern the customer when the manufacturer elects to test their fenders at their own factory.
The problems with industry practices
There are serious concerns with the way fender performance verification testing is currently performed. Some of these involve the authenticity of the reported performance, as very little thought is given to the need for independent certification of the reported results.
This is because, the fender being tested can easily be specifically selected for the test rather than randomly selected. Manufacturers can build test fenders that will pass the tests, while building the rest of the fenders with substandard materials.
Testing results can also be manipulated for commercial reasons. It is much cheaper to build low quality fenders that do not meet the performance requirements and manipulate the test results instead of building every single fender to meet the requirements.
An undisclosed truth about witnessed testing
The common practice in the industry is to rely on factory testing that is witnessed by either a third party or by the consultant. However, there are several reasons why this is inadequate.
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. It is easy for the manufacturer to manipulate the recorded data in the computer without the witness’ knowledge.
Many project specifications require a third party inspection agency to witness the test. Unfortunately, these are not doing anything more than simply witnessing a test. They do not provide oversight on how the test data was acquired or if the report they are asked to endorse is even from the test they witnessed. The inspection agencies are not in any way guaranteeing the validity of the data they are endorsing. If the data being validated and presented to the customer for acceptance cannot be guaranteed then what useful purpose does the test serve?
Independent construction materials testing is a common practice in the construction business. It should be standard practice in the verification of fender performance, especially given its critical effect on safety.
Independently verifying fender performance during the performance verification test is not an easy task, but it is imperative if the specified performance is to be guaranteed.
Independent verification testing is possible with any one of the following methods:
- At an independent structural laboratory
- At the manufacturer’s factory using their test frame but with independently recorded performance data
Each of the two methods has advantages and disadvantages.
The user shouldn’t rely on simple witness testing to determine fender performance. Real-time data should be shown on an external display and the results printed in real-time, so the witness has direct access to the data.
The paper will go on to discuss how the industry could move to true independent testing and the pros and cons of the methods presented.