How to preserve subsea equipment
Last summer we were asked how to preserve subsea equipment that will be stored on a quay with the North Sea as its nearest neighbour. The equipment must be ready and must be able to be put into operation immediately if something happens to the active equipment on the seabed in the North Sea. High demands are then placed on preservation when it has to be left outside in the wind and weather on the Westland coast. The main challenge is often to remove ions and salts, which traditional cleaning agents are not specialized to manage.
Together with the engineers at Zerust, we came up with a manageable three-step process: Washing, surface protection and long-term storage.
1.1 Preservation All surfaces upon receipt
Equipment is high-pressure washed upon receipt with 1.5-2% Zerust Axxavis WBA-10 added to the water.
WBA-10 (water blast additive) is an inhibitor specially developed to remove
salts, ions, chlorides from surfaces. By removing ions, corrosion is stopped. This also provides an optimal basis for extended protection by, for example, VCI wax coatings or wrapping.
WBA-10 also ensures that flushed surfaces are not quickly deteriorated by flash rust.
On painted surfaces at lower pressure, WBA-10 works as a detergent that removes salts particularly well.
The surfaces are air-dried only after treatment
1.2 Preservation Termination points
The coating is slightly brownish, which makes it possible to see where it has been applied. It can be applied by spray, brush or by dipping. Based on experience , Zerust Axxacoat 90 must be thinned 50/50 with preservation oil, e.g. Coatway to give a good result when applied by spray
1.3 Preservation Outdoor surfaces
Painted surfaces that are stored outdoors are also protected with Zerust Axxacoat 90 . Whether it is thinned with preservative oil or not depends on the project. If the wax coating is to be removed before the equipment is to be used, it may be advantageous to thin it out to make it easier to remove. Axxacoat 90 is relatively thick (roughly like syrup) and in order to get an even coating by spraying it should be thinned with Coatway. Application with a brush or dipping, on the other hand, is best as it is.
The mixing ratio is adapted by the project itself, based on its own experience and equipment.
1.4 Preservation Seawater hydraulics
Seawater hydraulics are flushed with fresh water to remove salts and feed to marine growing. The systems are drained and then filled up with fresh water with 5% FVS added during the storage period
FVS is an inhibitor that contains a mixture of contact inhibitor and
vapor phase inhibitor (VCI). Optimum protection of wet surfaces and air pockets inside the pipe system is thus achieved.
FVS works excellently in water/glycol mixtures.
Any residual chlorides and salts after flushing will dissolve in the FVS mixture
1.5 Preservation Dry cavities and tanks
Tanks that are stored dry can be fogged with dry FVS powder for up to 5 years of storage. Openings must be sealed so that the VCI powder is not vented.
FVS is an inhibitor that contains a mixture of contact inhibitor and vapor phase inhibitor (VCI). Used dry, 300 grams of FVS can protect 1m3 of cavity. The powder is distributed evenly in a powder cloud using compressed air.
The VCI vapor will penetrate and protect areas that cannot be reached mechanically.
If there is a lot of surface in relation to the volume, as in a tube exchanger, the amount of powder is increased.
2.0 Preservation Long-term storage
If the equipment is to be left for a long time, it will in many cases make sense to wrap the equipment in plastic after preservation.
Before packing: Zerust VCI Capsules are installed in any connection boxes and
are marked with the installation date using stickers , gaskets are smeared with Vaseline . The equipment is then wrapped in shrink film which is crimped tightly. When equipment is packed in a closed atmosphere, condensation will form as a result of the temperature differences, it is therefore important that the impending preservation is carried out correctly. Also ensure that the condensation has the opportunity to drain out. Desiccant can work against moisture if the packaging is tight and made of material that has little moisture penetration.