SPS: repairing tankers with a steel-elastomer-steel sandwich composite

Feb 11 2021


SPS Technology suggests that tanker operators repair cracks using a structural composite, rather than standard gouging and welding techniques.

SPS Technology suggests to tanker operators that they repair cracks by creating a composite panel in-situ.

 

The company’s patented very stiff and strong composite material SPS comprises two metal plates bonded with elastomer core.

 

When repairing cracks, a thin steel bar is fixed to the steel face plate around the crack, either by welding or adhesive bonding, depending on whether hot work is allowed.

 

The new steel top plate in then welded, bolted or bonded onto this metal bar to create a cavity, and the elastomer is injected into this.

 

To understand the technology, consider that elastomers are used to make car tyres and shoe soles. The material bends easily, but it does not break.

 

In engineering terms we can say that elastomer has low stiffness and high failure strain. Steel, on the other hand, has high stiffness, and high failure strain, apart from its propensity to corrode and get cracked, which elastomers do not have.

 

So by having steel and elastomer layers together, you combine the strengths and benefits of both.   A strong bond is generated between the elastomer and the steel to create one body rather than three layers.

 

This is an alternative method to repairing cracks via the standard “gouging and welding” – gouging out the affected steel, and welding in a section of replacement steel into the hole; a task which requires dry dock and major disruption.

 

SPS Technology and BASF developed the technology and have multiple patents on it.

 

Jonny Lim, director, SPS Technology Asia, says that one major tanker operator first used the method to repair cracks in 2005. It had a crack which was continually re-occurring on one of its vessels, being repaired every time it appeared by gouging and welding techniques.

 

SPS was used to repair the crack, and after the vessel had been in operation for several years,  the crack did not reappear. Now it uses the method as standard for vessels in its fleet.

 

On ships, it can be used to reinstate bottom and side shell plates, decks, tank tops and bulk heads. It has been used on cruise ships, bulk carriers, tankers, ro-ros, general cargo ships, dredgers, barges, LNG carriers, LPG carriers and floating storage units. Over 430 projects have been delivered over the past 20 years in maritime and offshore sectors, the company says.

 

It can also be used to repair corroded steel. Classification societies usually require that steel is repaired or replaced once the corrosion level reaches 20-30 per cent, Mr Lim says.

 

The material has also been used in stadiums, buildings, bridges and the offshore oil and gas industry.

 

The method does not necessarily involve less steel for the main plate – you might replace a 18mm sheet of steel with a 12mm sheet, a 20mm layer of elastomer and a 8mm sheet of steel. But the resulting material will be stronger, so you would not need any additional stiffeners.

 

Further benefits

Another benefit, perhaps more of value to naval vessels but tanker operators may be interested these days, is that the material has strong ballistic strength. It is also providing better blast resistance in comparison with stiffened steel.  

 

Also, it has more vibration dampening properties than steel, and has improved impact resistance, noise dampening, and increased fatigue life, the company says. There are environmental benefits, because SPS is lighter for heavily loaded structures. This means there can be more re-use of the old steel, and less need for additional steel stiffeners.

 

SPS also has in-built thermal insulation properties, perhaps of interest to tanker operators carrying heated cargo.

 

The method has approval of “all major classification societies” for permanent repair and strengthening of existing vessels, SPS says.

 

Cracks on tankers

Cracks on tankers can be very hard to detect visually. It is pretty common that the cracks are identified when cross contamination between tanks occurs , for example oil leaking into ballast water tanks, Mr Lim says.

 

They are usually caused by high stress on a certain part of the structure. This can be due to a combination of factors such as a flaw in the design, corrosion, fatigue, cycle loading, extreme weather conditions, or different cargo loading conditions.

 

The advances in structural analysis software (finite element analysis) can be used to understand the stresses on individual sections of steel, and so better understand why these cracks occur – although this software was not available when many older tankers were built, Mr Lim says.

 



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