Ventilation and Noise Control Resources

Why you should be using desiccant dehumidifiers instead of air conditioning for offshore lay-ups

Posted by Gary Leseman, P.E. on Nov 19, 2015 4:27:23 PM

Unfortunately, we find ourselves in a downturn situation today that mirrors what we experienced in the mid-1980’s. The big difference, however, is that the offshore drilling rigs* of today are much more expensive and technologically complicated. Since we posted the article Best Practices for Dehumidification of a Mobile Offshore Drilling Rig Lay-up back in September 2014, the response has been overwhelming.

Eldridge has always been a proponent of the useiStock_000044223388Large.jpg of desiccant dehumidifiers on mobile offshore drilling rigs. Back in the 70’s and 80’s, Eldridge designed desiccant dehumidifiers into rig ventilation systems where they were widely used on moored semi-submersible drilling rigs for keeping the lower pontoon machinery spaces dry. Therefore, it was only a natural extension to utilize the desiccant dehumidifier during rig lay-ups back in the 1980’s because the operating personnel were all familiar with this type of simple, easy-to-maintain, and effective equipment.

(*Editor’s Note: Although this article refers to MODUs, the same principles apply to marine service/supply boats.)

Information Gap

As the newer deepwater semis and drillships have come into use, the need for desiccant dehumidification in the pontoons has basically disappeared due to the heat generated by the dynamic positioners. Hence, there is a whole generation of offshore engineers and operators who are not familiar with desiccant dehumidifiers and their benefits for rig preservation.

Many of today’s offshore engineers think air conditioning is the only way to remove moisture from the air. Unfortunately, they are overlooking the tried and true desiccant dehumidifier as the optimum and most efficient solution.

How a Refrigerant Air Conditioner (AC) Works

Moisture removal by a refrigerant AC on a rig lay-up is generally done by a one pass system. That is to say, the AC unit is located on the deck and ambient air passes through the AC unit one time and is then distributed to the machinery space.

A psychometric chart is the easiest way to explain how an AC system works to remove moisture. Let’s take an example of ambient air entering the AC unit at 85°F and 70% RH (128 grains of moisture per pound). The air is first cooled to its dew point temperature of 73.5°F and 100% RH, but no moisture is removed until the air is cooled further to 45°F. The result with the AC unit is the moisture removal is 84 grains of moisture per pound of dry air, but the air is saturated at 100% RH with a 45°F dew point. Once this cold saturated air hits a warm moist machinery space with a RH already at 70% water condensation is going to form on all metal surfaces just like a can of cold beer sweats when it is placed in a warm moist room. This condensation condition can occur at startup and whenever there is infiltration of additional moisture from outside the space into the machinery space such as times when operating personnel go into the space to inspect or service equipment. The result is a rusting rig and equipment—this entirely defeats the preservatory purpose of the system!

For long-term rig stacking, 35%-45% RH is recommended as the optimum relative humidity by ABS in their Guide for Lay-up and for Reactivation of Mobile Offshore Drilling Units.

Most AC systems, however, can only guarantee a 55% RH within the machinery space because further reduction in RH by additional refrigeration will result in the need for an energy-consuming defrost cycle in order to prevent coil freeze up, which can result in restricted air flow and lack of dehumidification.

How a Desiccant Dehumidifier (DH) Works

A desiccant DH removes moisture by continuously drying the air in a repeating cycle. The desiccant DH unit is located within the machinery space and the air from the machinery space is circulated around and through the unit in a self-preserving, multi-pass manner. A solid silicon based desiccant (which is inert and does not deliquesce) is embedded into a wheel comprised of thousands of fluted air passageways. This desiccant coated wheel rotates slowly at 10 revolutions per hour within the DH housing. The desiccant acts like a sponge, picking up moisture from the machinery space air. The desiccant coated wheel rotates through a reactivation section and releases the moisture to a cross flow heated air stream. The moisture laden air off the reactivation section is discharged outside the machinery space. The dry dehumidified air is then distributed through flex ducting to the outer boundaries of the machinery space.

Take a 100,000 cubic foot volume machinery space and use a 600 CFM desiccant DH system with initial entering air at 85°F and 70% RH (same as above AC example). After two complete passes within 6 hours of operation, the moisture removal of the desiccant DH is 106.5 grains of moisture per pound of dry air with a new resulting dew point of 27.5°F. Within 24 hours, the desiccant DH unit shuts off completely. A relative humidity of 35% to 45% RH, as recommended by ABS, can be easily maintained by the desiccant DH system independent of any machinery space or quarters’ internal or external temperature or humidity changes.

The performance ability of a desiccant DH unit far exceeds the performance of the AC system

The power requirement of a 600 CFM desiccant DH unit that can do this is only 17.4 KW. Assuming the overall operational KW power of the DH system is the same as the AC system, the DH system will result in a 80-85% overall power savings since it does not have to run continuously to maintain the desired RH level.

Typically, this means that after the initial start-up 24 hour period the DH system only operates 3-4 hours a day to maintain the desired RH set point. This provides operating cost savings and allows operators to enter and exit the space from time to time for equipment inspection knowing that the desiccant DH system is capable of rapidly reducing the infiltrated humidity back down to the desired RH set point for optimum preservation.

The desiccant DH system is very dependable when moisture levels are very high and in either warm or cold climate conditions. A DH system can maintain the relative humidity in a rig machinery space at the desired 35%-45% RH without any problems. On the other hand, an AC system is only capable of reaching a 55% RH and must constantly run just to play catch up at this less-than-optimal RH level.

By using a desiccant DH system, Eldridge achieves the desired outcome of a well preserved mobile offshore drilling rig during warm or cold stacking and a fast reactivation time when it is time for the rig to go back to work. For more information on this topic, please have a look at our Best Practices article.

Now you know

Desiccant DH systems provide the most efficient and cost effective way to guarantee a 35%-45% RH in the space in order to properly preserve the high tech equipment, machinery spaces, quarters, and electronics on today’s mobile offshore drilling rigs and offshore vessels.

Even the US Naval Reserve Fleet uses desiccant dehumidifier systems for long-term ship storage, allowing the ships to go back into service within only a few days’ notice.

Eldridge has a proven track record of providing rig lay-up plans, equipment, and installation to rig owners using dependable and simple to operate desiccant DH systems. Whenever we have the opportunity to explain the benefits of the desiccant DH system over an AC system, the choice is always clear to our customers.

Give us a call today to discuss your lay-up needs. We are available to:

  1. Determine the stacking plan for your rig
  2. Provide equipment for purchase
  3. Lease equipment for temporary use
  4. Provide installation and start-up services

Preserving the integrity of offshore rigs and equipment is keenly important at this time. Eldridge is the first line of defense when it comes to accomplishing this goal on a global theater. Contact us for a no-obligation consultation and pricing.



Topics: Offshore Ventilation, Dehumidification

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