More independent laboratories are needed to verify fender performance and quality for tug owners
Fenders play a vital role in harbour operations, preventing damage to both tugs and the assisted ships. It is therefore crucial the quality of fenders is verified through independent testing. However, fender manufacturers can self-test in their own factories and with their own choice of witness.
Performance verification analysis is usually performed in a large press or test frame with either load cells or pressure transducers. Outside manufacturers’ facilities, these test frames are extremely rare, meaning performance testing usually occurs in manufacturers’ factories. Results are not always fully objective, says Trelleborg president for marine and infrastructure operations Richard Hepworth.
“Effective and reliable fender systems are mission-critical equipment,” he explains to Tug Technology & Business. “Testing procedures need to keep pace to ensure fender performance standards are maintained.” He says new manufacturing factories have opened and different materials were developed, leading to questionable performance and quality.
Mr Hepworth questions whether a test certificate is enough to guarantee performance. “The fender industry involves big contracts and vast sums of money. There is too much at stake to allow manufacturers to serve as their own regulators,” he warns.
“Our industry must move towards true independent testing, rather than just witnessed testing”
“Our industry must move towards true independent testing, rather than just witnessed testing.” If independent laboratories are unavailable, manufacturers could have testing at their own facilities but with independently recorded results.
“This will remove any uncertainty from the results and enable end users to have confidence that the lifecycle and performance of fenders meets specification, and that fenders are therefore fit for purpose,” says Mr Hepworth.
He thinks a deeper understanding of the testing procedures and their impact on fender performance will help tug owners and other stakeholders make the right decisions. “This will ensure designers, operators and owners of port infrastructure invest in equipment that guarantees quality and reliable performance over the long-term,” Mr Hepworth says.
However, there are flaws in manufacturer fender testing. For example, there are claims that manufacturers select specific fenders for testing, instead of taking random samples. Or fenders could be built to pass performance tests. “But, when it comes to creating the products that actually go to market, some manufacturers may use low-cost, substandard materials in production,” Mr Hepworth claims.
If there are not enough external testing laboratories, the industry should introduce independent witnessing to verify the authenticity of results.
Berthing angle, velocity and temperature
Performance verification should include testing the velocity factor (VF). While it is impossible to replicate the actual berthing velocity of vessels in testing, which is up to 500 mm/sec, it is possible to conduct rigorous testing to simulate berthing conditions.
The type of raw rubber used in compound formulation significantly influences VF, says Mr Hepworth. VF is highly dependent on the blend and ratio of natural rubber (NR) and synthetic based rubber (SBR) used in the overall rubber compound formulation.
“Fender manufacturers should always provide commentary in relation to the application of VF in their rubber compounds and fender designs,” he says.
Testing VF enables fender suppliers and manufacturers to improve standards across the industry. “Robust chemical and physical material testing is also required to ensure that reclaimed rubber and non-reinforcing fillers are not used in fender production,” Mr Hepworth says. These substitutions can negatively impact the fender’s ability to absorb the designated berthing energy of a vessel and therefore protect berthing vessels and port infrastructure.
Temperature factor (TF) is also important in understanding changes to reaction force and energy absorption of fenders in operating conditions. The stiffness (modulus) of the rubber compound changes dramatically with temperature which, in turn, impacts how the fender performs in situ.
“Ideally, rubber elements for fender systems should be tested on a case-by-case basis in accordance with the temperatures they will be subjected to in the field,” explains Mr Hepworth.
TF is highly sensitive to the type of rubber used – NR or SBR, or a blend of the two, and inclusion of recycled rubber. TF varies with fender type and manufacturer, meaning test results are unique to each individual fender.
Fender performance will also vary depending on the angle of contact that results from a vessel’s berthing approach. A fender system should be designed to have enough energy absorption capacity to accommodate the appropriate berthing patterns.
Capacity of a fender system designed to accommodate parallel berthing will differ significantly from one with an acute angle of approach. “Understanding how well a fender is able to absorb energy at different angles is critical to its performance at the quayside,” says Mr Hepworth.
To meet active standard ASTM F2192 and PIANC 2002 guidelines requirements, fender rated performance data (RPD) must include testing at a zero-degree angle of approach. “By zeroing the angle factor, this should allow for easier comparison of fender performance data using other testing parameters (temperature, velocity, deflection),” says Mr Hepworth.
Manufacturers should also provide adjustment factor information for contact angles at 3°, 5°, 8°, 10°, 15° and 20° for each fender type. “This makes it possible to determine if there is a reduction in energy absorption at larger berthing angles,” says Mr Hepworth.
RPD needs to be indicated if chain restraints are factored in, as this will impact results data. Similarly, manufacturers should show angle factor testing data in relation to both transverse and longitudinal angular berthing to illustrate performance under different berthing conditions.
Designers select tug fender types and shapes
Cylindrical and double loop soft fenders were fixed to Z-Tech escort tug Mark E Kuebler
Naval architects consider the vessel's operational requirements when selecting the fender shape and type. Chartwell Marine managing director Andy Page explains to Tug Technology & Business which type of fender is used in workboat designs.
Initially D-shaped fenders were incorporated in vessels designed to operate in offshore renewables projects. “We also looked at W-profile fenders with forward facing grooves for more grip,” he says. A solid strip of rubber block, around 600 mm deep, was effective, but heavy.
“We looked at foam-filled fenders with a polyurethane rubber skin,” says Mr Page. These have less weight, but “They tend to split after a period of time,” he explains. “Then the foam gets impregnated with water.”
Robert Allan included cylindrical fenders of 915-mm outer diameter and 457-mm inner diameter on Z-Tech tug designs for a series of 10 tugs Gulf Island Shipyards is building for Bay Houston Towing and Suderman & Young. These ship-handling fenders are on the bow at the main deck level with double loop soft fenders 405-mm thick between the main deck and the knuckle at the bow and along the sheer lines of main deck. On the stern there will be cylindrical fendering of 405-mm outer diameter and 203-mm inner diameter on each of these 10 tugs.
The first two of these Z-Tech 7500 design escort tugs, Mark E Kuebler for Bay Houston Towing and Ted C Litton for Suderman & Young, were delivered in Q1 2019.
The main types of tug fenders are:
- Cylindrical tug fenders
- D-shaped fenders
- Block fenders
- M-shaped fenders
- W-shaped fenders