Best Practices for Dehumidification of a Mobile Offshore Drilling Rig Lay-Up

Posted by Gary Leseman on September 29th, 2014
Lately, we have been reading and hearing reports from mobile offshore drilling rig owners that a number of MODUs are going to be stacked—unfortunately, this is a cycle that we have been through before. From past experience, we have learned that laying up rigs with proper dehumidification equipment installed helps to preserve the integrity of the below deck equipment and the quarters. When it comes time for the rig to be reactivated, it turns to the right faster and in better condition than a rig that is cold stacked at dockside or in the middle of a bay.

Iron begins to significantly corrode (rust) at 60% RH and above (Garverick, Corrosion in the Petrochemical Industry). According to the American Bureau of Shipping, for the best results in preserving below deck equipment and machinery spaces, they recommend “complete dehumidification at 35% to 45% RH…to prevent sweating or humidity corrosion damage”. Holding a relative humidity of 40% to 75% will preserve the integrity of seals and gaskets. Given all these factors, we usually recommend around 45% RH for rig layups. (Check out this great resource for more information.)

There are two basic types of Dehumidifiers (DH) available:

  • Desiccant Wheel DH Units
  • Refrigerant DH Units

Desiccant Wheel DH Units are used by the US Naval Reserve Fleet and are the “gold standard” for lay-ups because they are not adversely affected by low ambient air temperatures like Refrigerant DH Units.

Top view of helicopter pick up passenger on the offshore oil rig.



The Desiccant Wheel DH Unit uses a rotating, regenerative, open fluted matrix wheel that is impregnated with a hygroscopic, non-toxic element. A humidistat monitors the relative humidity in the space and turns the DH Unit on or off to achieve and maintain the desired humidity set point. The humidistat can be built into some DH Units at the return air inlet, which is where the air is going to be the most humid.

Dry air is heavier than humid air, so the dry air will displace the humid air and push the humid air back to the DH Unit, which is installed in the dehumidified space. Dry air will also seek out places where there is more humid air. So hard-to-reach areas, crawl spaces, tanks, and voids that are allowed to be common to the area being dehumidified will also be dried out and properly preserved.


Normally, we recommend one dehumidifier for all the below deck machinery spaces and one dehumidifier for the quarters—the same approach applies to both spaces. Dehumidifying the machinery spaces below deck is obvious. However, it is also a good idea to dehumidify the living quarters if the rig is not going to be inhabited during lay-up. Dehumidifying the quarters prevents mold and mildew from growing in the enclosed quarters—all the bedding stays nice and dry and the overhead ducting and structure does not rust.

There are also a number of semi-submersibles with DH Units in the caissons and pontoons, which should be used for preservation of these areas in the event of a lay-up.

To determine the proper size of the dehumidifier, we would calculate the gross volume of the area to be dried and then figure that the DH Unit will turn that gross volume over every 4 to 6 hours on a stacked rig. This usually turns out to be a small DH Unit such as a 600 CFM or 1200 CFM size unit.

With a Desiccant Wheel DH Unit, we extend flex duct that handles the dry supply air to the farthest point away from the DH location. Then, the return air circulates back to the DH Unit through all the open doors. The reactivation air intake and exhaust needs to be ducted from outside the dehumidified space to prevent ambient, humid air leaks. Low cost, insulated flexible duct can be used.

You can also use very small DH Units (60 CFM) to dehumidify isolated areas such as crane cabs and drillers/dog houses. Usually small Refrigerant DH Units are used in these areas and only a drain line needs to be run outside of the area.

Typically, rig hands and rig electricians are more than capable of installing a dehumidification system on a rig layup, which eliminates the costly need to hire an outside contractor for this work


No one likes to think about rig lay-ups. That is not what this energy industry is all about. We want full throttle forward. Not reverse. Not stop. But a rig that is properly preserved and dehumidified will survive being temporarily out of action and will be able to return to operation faster than one that is not.

Should the need arise to stack a rig, contact Eldridge and we will help you with dehumidifier application, sizing, and flex duct/fitting routing layout as a value added service.

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