Cause & Consequence
- Release of a harmful substance
Other Cause & ConsequenceFailure to check clamp sufficiently
- Control of work
Description of Process
An incident occured on an offshore production platform where a 1" diameter Vector 'Techlock' pipework clamp connector catastrophically failed causing a gas release. The failure was caused by poor heat treatment during the manufacturing process, which led to failure by cracking as shown:
Further enquiry revealed similar failures on other small-sized (1", 1.5" and 2" diameter) clamp connector segments.
Description of Incident
The primary cause of failure of the clamp was due to hydrogen cracking. Evidence from this clamp and others checked later indicates that high hardness, with values in excess of 48-50HRC (Rockwell hardness value), being the underlying cause. High hardness increases the material's susceptibility to cracking in general from reduced ductility.
Investigations revealed that prior to 2010 the clamps, which require quench and temper heat treatment, were not subject to 100% hardness testing. Hence the quality controls were not sufficient to detect components that had received improper heat treatment and lacked ductility.
Clamp connector segments are produced by a forging process in a number of foundries in the UK and are manufactured from AISI 4140 alloy steel with a recommended Rockwell hardness value of 22HRC.
To date all defective clamps found were produced by one manufacturer (George Dykes) and supplied to Vector Technology Group (Techlok) up until 2010. These were limited to 2" diameter clamps and below. It has subsequently been established that the similar 'G' clamp supplied by Destec Engineering, was also manufactured by George Dykes. A Destec 'G' clamp is also known to have failed in service in 2014.
The confirmed numbers of failed clamps is low and they appear to be random and not batch related, therefore it is not possible to identify them through heat numbers or heat treatment records.
Good Practice Guidance
Duty Holders should identify if they have in use a 2" or below Techlok or Destec 'G' clamp connectors supplied before 2010, or have the potential to be used (e.g. spares kept in stores) on any of their installations and to verify their fitness for service.
Verification could be established from supplier records whereby the material properties of the clamps are fully certified, or by appropriate NDT inspection and hardness tests.
Any found to exceed acceptable hardness values must be taken out of service. Note: clamp segments may also have been supplied by other vendors along with their main products (e.g. valve assemblies) and duty holders should ensure that these are subject to the same checks.
The evidence suggests that segments manufactured after 2010 will not require inspection and hardness checks.
There are a number of suppliers of this type of clamp. Whilst distinguishing the Techlok or Destec versions from the others is possible through raised stamp marks on the hub, this is likely to become increasingly more difficult after a period in service. In such circumstances to ensure fitness for service, it will be likely that all small diameter clamp connectors will require hardness checking.
Hub Clamp Suppliers (in bold known to be affected):
Vector (Techlok), Destec (G Clamp), Galperti (G Clamp), Oceaneering (Grayloc)
UK Manufacturers/Foundaries (in bold known to be affected):
George Dykes, Caparo, Kimber Drop Forgings