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Mike Watson
Tube Tech International Ltd.
ESSEX, UK -- When a platform's heavy plant fails, it means highly involved and often hazardous operations to disconnect it, move it off line, and then transfer it to a supply ship to be taken ashore for repair.
Plant-like Waste Heat Recovery Units (WHRUs) and shell-and-tube heat exchangers are comprised of coiled tubes that can become restricted with carbon deposits. Standard industry approaches to solving the blockage problem are:
None of these options are completely satisfactory. Bypassing the unit causes inefficient burning of fuel and higher platform operating costs. Cutting the U-bends is time-consuming, with no guarantee there will be access to reattach the bends. When the exchanger is returned to duty, it under-performs because deposits remain in the U- bends, so the exchanger shortly has to be cleaned again. Replacing the complete unit is the most expensive option due to the cost of the replacement unit and platform production loss from lengthy downtime.
A WHRU on a North Sea platform became restricted and had to be shutdown. The unit already had been disconnected and transported ashore when the operator decided to have Tube Tech try to clean it before ordering a $500,000 replacement. It can take up to a year for a new replacement unit to be installed because of a six-month manufacturing lead-time plus WHRUs can only be taken off line once a year when the platform's gas turbines are down for maintenance.
Tube Tech established a three-stage plan. Phase one was locating the position and length of blockages within the unit; phase two was a systematic unblocking; and, phase three was a thorough cleaning. This did not involve removing any U-bends.
Because of the size of the WHRU, two 14.5-metric ton (14.9-ton) cranes were used to change the position of the unit during cleaning. The WHRU was de-scaled, unblocked in two weeks, and restored to peak efficiency.
The operator saved £150,000 ($297,350) in possible manufacturing costs and subsequent rig production losses through on-site cleaning. After considering cost implications of a failure in one of two other WHRUs they owned, the operator contracted Tech Tube to clean both units on-site as a precaution.
A serpentine shell-and-tube exchanger on a Persian Gulf platform developed a similar fouling problem. Tube Tech decided on a seawater-fed jetting system they had devised. It features low-flow, high-pressure water delivered through unblocking and de-scaling nozzles, supplemented with a specially-developed U-bend nozzle which can negotiate close-radius bends. Even in the limited confines of the platform, a comprehensive clean was completed on time and on-budget.
Now that the concept of on-site cleaning has proven successful, the industry no longer has to accept the high cost and inconvenience of lengthy plant downtime.
5/9/2007
