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Because of the long lead-in when ordering equipment for offshore installations together with the time it takes for fabrication of the structure there becomes a need for long term storage of the equipment at the offshore construction yard. Leaving new or used equipment that hasn't had preservation maintenance standing in a warehouse, stores, compound or can be detrimental to the equipment. This is true whether it is electrical, mechanical or HVAC. Offshore installations guidance is needed for both the design and construction of warehouses to ensure that such costly equipment and machinery is not ruined.
Temperature and relative humidity in the storage location must be controlled. If humidity is too high, condensation can enter the machinery which is fatal for cloth & card filters, electric motors, control panels and switchgear. Too low relative humidity can make the air cool, again not great for electrical equipment or operatives.
Controlled relative humidity along with correct temperature is the answer. The optimum combination is 45% Relative humidity and 20°C. I have used these values in the North of Scotland summer and winter and never lost any equipment due to warehouse atmosphere. These values can be checked by using a combined temperature and humidity indicators.
Humidity Indicator Cards should be placed at specific locations throughout the warehouse to enable an immediate visual confirmation of humidity (these cards are designed to turn from blue (safe atmosphere) to pink (warning of damp atmosphere).
Large pieces of equipment which cannot be stored inside. Pedestal cranes and HVAC AIR handling units should have their motor and control panels anti-condensation heaters energized. For HVAC units, any filters which cannot be removed for storage in warehouse should have desiccant bags inserted in alongside them to prevent dampening of paper/card. These desiccant bags are cloth bags filled with silica-gel granules which can be supplied with indicator cards attached.
For the protection of small unventilated electrical isolation boxes against corrosion of contacts, Vapor Phased Corrosion Inhibitors should be used. These come in foam adhesive backed strips and can easily be cut to size. I have left these intact for a year.
Now we shall look at the different types of equipment in this article;
- Steel Vessels.
- Stainless Steel Vessels
In future articles;
- Mechanical equipment (Pumps, Gensets, cranes and lifeboats)
- Electrical equipment (Switch gear, transformers, batteries, HV motors and terminal boxes)
- HVAC (fans, dampers)
The importance of protecting vessel internally against corrosion cannot be overstressed. In the petrol/chemical/offshore fields, the majority of vessels need to be spotless inside, as any rust can lead to pump or instrument failure because the liquid is stored in the vessel and circulated by the pump through orifice plates and other instrumental measuring devices and small-bore/metering valves.
Vessels are notoriously hard to clean internally, all of which is done manually. The vessel is cleaned by hand using steel wool discs, followed by brushing up and removing all of the debris. Not a nice task, I have been there! It is well worth the effort to keep the vessel in pristine condition, through storage and installation ready to hand over to commissioning.
If, at initial receipt and inspection, the vessel is found to be rusty inside, the vendor should be contacted immediately to make arrangements for him to clean it, as per spec.
Once the vessel has passed inspection, desiccant bags should be tied together and hung clear of the vessel internal shell. They must not be allowed to come in contact with the steel internal surface; this can cause a rust spot as the silica-gel absorbs any moisture. If it is not possible to suspend the bags then a suitable barrier should be placed between the vessel and the bags (a rubber mat for instance).
Red warning tags pronouncing ‘Desiccant Bags Inside’ should be firmly wired to the outside of the vessel; preferably about the man-way door area. The number of bags inserted along with the date should be clearly marked on the tag.
Note: The man-way door gasket should be replaced by a temporary card gasket, and secured by four bolts. The remaining bolts and original gasket should be bagged and tagged and stored in warehouse. Monthly internal inspections should be carried out to ensure no deterioration due to corrosion at which time the desiccant bags should be replaced (the old bags can be regenerated by leaving them in the heated warehouse). The outside of the vessel should also be inspected to ensure integrity of the paint work and any bright metal e.g. man-way bolting, hinges etc, have remained unblemished.
Stainless Steel Vessels
No internal preservation is required in stainless vessels. However, all nozzles should be spaded and tails painted red. The spades should be made from stainless steel, not carbon steel, to prevent galvanic corrosion.
An internal inspection every three months is sufficient, but monthly external inspections for damage should be maintained. No rust-proofing greasing is required, except for lubrication of the man-way hinges, where silicon-based grease should be applied.
Note: On installation, there should be tufnal or similar insulation blocks placed between the vessel stainless vessel saddles and steel supporting steelwork, preventing galvanic corrosion.
- Authors Experience