CUSTOMER: ExxonMobil Mossmoran chemical plant
PROBLEM: Detected on a critical line, operating at pressures of up to 84 bar g (1220psi) and temperatures of some 480C, the small crack had formed between the 16 inch pipe and the weldolet that attached it to a 20 inch header. Although the repair was not termed immediately dangerous by ExxonMobil engineers, repair was clearly essential and, given the operating pressures and temperatures involved, it was initially thought that shutdown was the only option.
The plant at Mossmorran converts the ethane from liquid gas pumped in from offshore, into ethelyene – a fundamental ‘building block’ of the petrochemical industry. Shutdown would not only mean significant costs in terms of lost production but also, to avoid ceasing offshore production, the excess ethane would have to be flared, wasting precious product and raising environmental and community issues.
SOLUTION: Furmanite – already contracted for on-site leak sealing and other core maintenance services at Mossmorran – was called in to help evaluate options for an on-line repair. Furmanite and ExxonMobil jointly developed the concept of using a restraint clamp to relieve the crack opening stress, thus avoiding the requirement for an unscheduled shutdown.
The specialist assembly – comprising a saddle clamp attached to the vertical 20 inch clamp and two gripper clamps linked by pre-tensioned tie-bars fixed to the horizontal 16 inch pipe – is designed to exert a tensile load of approximately 100 tonnes, forcing the pipes together and removing the axial crack opening stress.
The design was fully validated using Furmanite’s in-house Finite Element Analysis.”
To allow installation of the clamp assembly, the defective section needed to be isolated for safety reasons. Prior to clamp installation, a by-pass line was installed to allow this isolation whilst the plant continued in operation. However, while running on by-pass, the plant would be unable to cope with any fluctuations in the incoming gas supply without causing a process trip, or sending significant quantities of gas to flare – an activity that would be unacceptable from an environmental point of view. This made fitting the 2.8 tonne clamp within the minimum possible time a critical aspect.
With the plant switched to by-pass in the early morning there was no time to wait for the pipe surface to cool down. Instead, technicians wore Kevlar gloves to apply the specially designed clamp. The entire assembly was in position and pre-tensioned by mid-afternoon. Steam was then re-introduced to warm the isolated section at a controlled rate to avoid potential deterioration of the crack. Furmanite technicians worked through the night to adjust the pre-tension as the pipe and its clamp were brought back to normal operating pressure and temperature.
The resulting repair is expected to last until 2006 when a full shutdown is planned and the line can be replaced. However – should the noise resulting from the steam leak get worse – an additional leak-sealing clamp has been designed and built in to the restraint clamp. This contingency has been positioned off-set from the crack and can be slid into position in the event of any deterioration.
OUTCOME: A specialist, semi-permanent, on-line clamp has saved an estimated £10 million at the ExxonMobil Mossmorran chemical plant, following discovery of leakage on a tee section of a critical steam line.