Forensics

Fibreglass Failures

Introduction

This new water tank collapsed the first time it was filled. An FSM investigation showed that the failure initiated at a man-hole and that the construction did not conform to the requirements of the British Standard to which it had been ordered.

Fibreglass, sometimes called glass reinforced plastic, is an unusual material. It is a composite, consisting of glass fibres set in a solid plastic such as polyester or epoxy. Fibreglass is distinctly different to the lightweight glass fibre mats used for insulation.

Well made, it has a remarkable strength to weight ratio and excellent corrosion resistance. One of its applications is in the construction of tanks to contain water or process fluids. It is this application that has given rise to the majority of the failures that we have been asked to investigate.

Characteristics of Fibreglass Failures

Following the collapse of a 120,000 litre tank at a chemical factory in Indonesia, an insurance claim was lodged on the basis that the tank had exploded. An investigation by FSM on behalf of insurers revealed this was not the case. The tank remains were laid out (above) and the failure origin located. It was determined that the tank had been cooled externally by water sprays and this had resulted in accelerated corrosion of the bolts holding the tank panels together (left).

Fibreglass is not very forgiving to poor design, faulty construction or to misuse. In addition, once a fracture begins it tends to go all the way, whereas the same article made of carbon steel might give signs of impending trouble before the failure itself.

Another unusual feature of fibreglass failures is that a significant proportion of failures originate not from the fibreglass itself, but from the deterioration of steel fasteners or lugs incorporated into the construction.

The presence of the fibreglass does not accelerate the steel corrosion. The problem is more that the owners treat the tank as 100% fibreglass, ignoring the small steel portion. It is almost as if it is assumed that the steel will somehow inherit corrosion resistance from the fibreglass.

The Need for Inspection

Cracks in a fibreglass bracket after only one year in service.

For at least 50% of the failures we have examined, there were clear signs of impending problems. Sometimes these signs were apparent externally, while with others the tank would have had to be emptied for the deterioration to be apparent. The lesson to be learnt is that a good proportion of failures could be prevented by regular inspection.

It can also be concluded that deterioration can begin soon after manufacture. Experience has shown the incorrectness of the belief that there is an initial time interval during which fibreglass tanks do not need to be inspected.

 
This chemical tank at a palm oil refinery in Johor Bahru collapsed after 10 years of service.  Investigation by FSM revealed that the failure initiated at a nozzle where the fibreglass had degraded with time as a result of the combined effects of vibration and localised corrosive environment.