How to preserve subsea equipment

Last summer, we were asked how to preserve subsea equipment to be stored on a quay with the North Sea as its closest neighbor. The equipment must be on standby and capable of being put into operation immediately if something happens to the active equipment on the seabed in the North Sea. This places high demands on preservation when it is to stand out in the weather and wind on the west coast of Norway. The main challenge is often removing ions and salts, which traditional cleaning agents are not specialized to handle.

Together with the engineers at Zerust, we came up with an 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 specifically developed to remove

salts, ions, and chlorides from surfaces. By removing ions, corrosion is stopped. This also achieves an optimal foundation for extended protection using, for example, VCI wax coatings or wrapping.

WBA-10 also ensures that rinsed surfaces do not quickly deteriorate due to flash rust.

On painted surfaces at lower pressure, WBA-10 acts as a detergent that removes salts particularly well.

Surfaces are only air-dried after treatment

1.2 Preservation Termination points

  Connection points, flanges, couplings, lifting eyes, and other items that need to maintain a mechanically undamaged surface are coated with Zerust Axxacoat 90, a self-healing wax coating with inhibitors that provides protection for 1 year in outdoor storage, and much longer with simple measures.

The coating is slightly brownish, making it possible to see where it has been applied. It can be applied by spray, brush, or dipping. Experience shows that Zerust Axxacoat 90 must be thinned 50/50 with preservation oil, e.g., Coatway, to provide good results when applying by spray.

1.3 Preservation Outdoor surfaces

Painted surfaces stored outdoors are also protected with Zerust Axxacoat 90. Whether it should be thinned with preservation oil depends on the project. If the wax coating is to be removed before the equipment is used, it can be an advantage to thin it out to make it easier to remove. Axxacoat 90 is relatively thick (about like syrup), and to get an even coverage with a spray, it should be thinned with Coatway. For application by brush or dipping, however, it is best as it is. 

The mixing ratio is adapted by the project itself, based on their own experience and equipment.

1.4 Preservation Seawater hydraulics

Seawater hydraulics are flushed with fresh water to remove salts and early marine growth. The systems are then drained and filled with fresh water supplemented with 5% FVS during the storage period.

FVS is an inhibitor that contains a mixture of contact inhibitor and

vapor phase inhibitor (VCI). This achieves optimal protection of wet surfaces and air pockets inside the pipe system.

FVS functions excellently in water/glycol mixtures.

Any remaining chlorides and salts after flushing will be dissolved in the FVS mixture.

1.5 Preservation Dry cavities and tanks

Tanks 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 does not vent out.

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 large surface area relative to the volume, as in a heat exchanger, the amount of powder is increased. 

2.0 Preservation Long-term storage

If the equipment is to stand for a long time, it will in many cases be sensible to pack the equipment in plastic after preservation.

Before packing: Zerust VCI Capsules are installed in any junction boxes and

marked with the installation date using stickers; gaskets are greased with vaseline. Then, the equipment is packed in shrink film that is shrunk tight. When equipment is stored in a closed atmosphere, condensation will form as a result of temperature differences; it is therefore important that the preceding preservation has been performed correctly. Also ensure that the condensation has the possibility to drain out. Desiccant can work against moisture if the packaging is tight and made of material with low moisture permeability.