Platform pressure changes force blast wall design review

Schiehallion blast tunnel causes great interest

Mech tools blast walls. [10,873 bytes]

Darlington-based Mech-Tool Engineering has been looking at ways of strengthening blast walls using both conventional and composite materials. This follows completion of the second phase of a joint industry study which has been testing the theory behind predicting the effects of explosions on platforms.

The SCI project, which was launched in 1993, is now in its third phase. But, the result of the second phase, which cost around £4 million, has already indicated that software predictions have under-estimated overpressure buildup caused by the mixture of gas and air. Controlled explosions which took place at the British Gas Research Institute at Spadeam in Cumbria have confirmed bigger changes in air pressure than early computer models predicted when older platforms were first designed.

Mech-Tool has been aware of this problem for some time through a test facility in Cardington that the company designed and built. As a result, it has been upgrading its blast walls to withstand greater pressures - some from 0.4 to 1.6 bar, others from 1.0 to 4.0 bar - depending on their end use.

"We felt we needed to find a cost effective method of increasing the pressure rating," says technical sales manager Simon Bell "so we have entered in to an alliance with Tonen, a Japanese company that supplies carbon fiber composites." Tonen, part-owned by Exxon and Mobil, supplies the composite for seismic reinforcement but as Japan's largest oil processing company, it also uses the technique in its own operations for a variety of blast proofing applications.

In addition to the results of the SCI study affecting the design of new-build platforms, Mech-Tool sees a substantial market for retrofits as they estimate that of the 125 platforms that have been operating on the UKCS for around 15 years, 90 will potentially require upgrading.

Cost benefits

Mech-Tool claims that the carbon fiber route has considerable cost and operational benefits over using conventional steel cladding for blast walls. The material comes in a lightweight roll which does not require special lifting equipment and can be easily fitted into areas with restricted access. No hot work is required to apply the composite material which is formed directly onto the structure offshore.

The process can be completed by a small team so that production is not disrupted. With only a relatively small amount of material required, additional weight on the platform structure is kept to a minimum. In a typical installation, a carbon fiber system upgrade could weigh as little as 30 tons, compared to 84 tons for a conventional steel system.

Carbon fiber cladding is extremely durable and moisture tolerant which makes it ideal for North Sea applications. The cladding is also considerably cheaper than traditional cladding materials, according to Mech-Tool. "The first question I always get asked is how much it is going to cost per sq meter," says Bell. "Obviously, each job is costed individually, but when you take into account the fact that the carbon fiber doesn't require welding, which in turn means that the cost of labor is kept to a minimum and that the platform doesn't have to shut down during the operation, we estimate that the carbon fiber route costs at least 20% less."

Blast protection

Mech-Tool offers newbuild blast protection, in addition to upgrading existing facilities. Its 145-meter pressurized steel escape tunnel with airlock doors for the BP Schiehallion FPSO was delivered to Harland & Wolff's yard in nine sections, avoiding expensive site work. Weighing 150 tons, the jet fire resistant tunnel has a design life of 25 years and is capable of withstanding peak blast overpressures of 500 mbar.

Although blastproof tunnels or escape routes for FPSOs are not yet prescriptive in the North Sea, Mech-Tool believes that there will be a growing demand for this type of blast protection for floaters in the future. "The Schiehallion-type tunnel design has caused considerable interest," says Bell, "and we have recently quoted for both the Shearwater and Elgin/Franklin projects. We are also targeting Norway as a major area of opportunity and have appointed a new agent there as the volume of reasonably large projects looks good."