Fabrication and Installation of Offshore HVAC Systems

The ductwork is fabricated in individual spools from the fabrication drawings, usually from 4mm mild steel sheet, 3mm stainless steel sheet being used for ductwork exposed to the elements, reducing to between 1mm to 1.5mm for internal round section stainless steel ducts.

As the majority of offshore ducting is manufactured from mild steel sheet, we shall examine the fabrication and installation of mild steel ducting below:

After the steel sheet has been cut to size using a guillotine, it is bent to the required shape using a bending machine, and the open edges welded. The flanges are cut from flat bar, holes punched and then welded into the rectangular shape and dimensions of the duct it is to be welded to.

The flange is then welded to the duct paying attention to keeping the edges parallel to the duct edges.

Any stiffeners are then welded onto the duct spool along with earth bosses, and once completed the flange faces are given a run over with a grinder to remove any weld splatter.

The duct is then given a unique spool No. from the fabrication drawing; we used to have this number hard stamped onto a plate, and the plate welded onto the duct.

Once sufficient spools have been fabricated to make up a couple of truck-loads, the ducts are sent offsite to be hot-dipped-galvanized. On return to the construction yard, the spools are checked for galvanizing quality and stored ready for installation.

Any spools containing a right angle bend will have to be fitted with turning vanes. These are made from stainless steel and prevent internal erosion of the plate by diverting the air round the corner into the next section.

Now, stainless steel and galvanized mild steel will cause galvanic corrosion on contact so the turning vane is insulated from the plate using a thin piece of material such as Tufnol (polyurethane laminated material). We used Whale Brand Tufnol for this purpose.

Stainless studs are used to secure the turning vanes, with flat Tufnol washers against the galvanized duct plate. A sketch is shown below;

Reference Web: standard - specifications for Offshore HVAC fabrication and installation

Equipment is as follows:

• Air Handling Units.

These are large rectangular steel modules containing a plenum, heating, and air conditioning equipment along with suction/discharge air filters. Some of the more modern ones also house the main centrifugal fans.

• Centrifugal Fans

These are the main system fans and can be housed in a dedicated HVAC fan room or in the air handling unit. Either way, the suction should be from under the deck, away from prevailing wind and any hazardous drains or wind-blown spray.

• Fire Dampers

Fire dampers are manufactured in stainless steel, and designed with a pneumatically open-fail shut operation. These shut down the airflow on a gas escape or fire situation.

• Manual Dampers

These are manufactured from stainless steel or galvanized steel plate, being used to balance the system.

• Attenuators

These are really silencers which cut the noise of the air flowing through the ducting.

• In-line Fans

In-line fans are used in segregated systems such as the laundry, galley, battery room, and switch-rooms. Fans are usually supplied as two units; one operating and the other on standby.

• Grills, Diffusers and Louvers

Grills are fitted at the outside duct suctions preventing any ingress of large pieces of rubbish being sucked into the system.

Diffusers and louvers are fitted to internal suction and discharge ends of the ductwork.

• Filters

The different HVAC systems have suction filters between the fan intake grills and the fan, along with discharge filters between the fan discharge and the louvers or diffusers.

• Hanging Spring Supports

These are adjustable to increase/reduce tension and are used to support duct runs, along with the structural duct supports, whilst reducing vibration.

• Main Ductwork Supports

These are fabricated from mild steel channel, being shot-blasted and painted to a high spec. and welded to the structure. The ductwork can be laid directly onto the supports or on top of a rubber anti-vibration strip glued onto the main supports. An insulation material is placed between the duct and main supports on A/C systems to prevent condensation.

Reference Web: ishrae - overview of offshore HVAC systems

As we seen from the introduction and previous sections, the installation of the ducts is ongoing as the platform construction progresses, especially as the areas become less accessible with the installation of other systems such as piping and cable rack.

Galvanic corrosion takes place between stainless and galvanized surfaces, so any stainless equipment will have to be insulated against the mild steel galvanized ducts. This is carried out using "top hat kits" (I have sketched these methods in the Image Section at the end of the article). These consist of Tufnol tubes with an integral flat top in the shape of a washer, and a separate Tufnol washer. Gaskets between the flanges are made from a hard 6mm insulating rubber with the bolt-holes punched slightly larger than the flange bolt-holes.

The tubes are fitted through the adjoining flange holes, with the top hat washer in contact with the flange flat plate. The bolts are fitted through the tubes, and a flat washer and nut fittedas shown below;

Reference Web: standard - specifications for Offshore HVAC fabrication and installation

Note:

The next procedure is the pressure testing of the HVAC system, followed by the commissioning of the system, leaving as little as possible for the off-shore hook-up team to complete.

I will cover these procedures in a subsequent article on Pressure Testing and Commissioning of Offshore HVAC Systems.