Class Lay-up Recommendations


Before committing to lay up a vessel or rig, the first thing to do is look at the latest class recommendations. They continually change over time as new information comes to light. For example, in 2017, one class surveyor reported

Marine Layup Services - Cold Layup on Europe
Cold Layup on Europe

that over 20 rigs had failed reactivation due to incorrect preservation. Also, some major oil companies are reluctant to accept rigs or offshore support vessels that have been cold stacked for tendering for work. Long-term reliability is a concern. Therefore, it is essential to properly understand what is required to be done and work with someone who can best assist in achieving the best outcome.

Are Dehumidifiers Necessary in Lay-up?

During a recent consultation visit to inspect three rigs for lay-up in Labuan, there was much discussion on the importance of dehumidifiers and whether they are necessary. The information below highlights the three main class recommendations for lay-up and should speak for itself as to whether, in Asia, you can afford not to use dehumidifiers. Some rig & vessel owners tend not to use dehumidifiers. It’s essential to understand the benefits of following correct lay-up procedures. By not controlling humidity levels in tropical regions, corrosion accelerates. As such, dehumidifiers are of great importance for lay-up.

DNV Lay-up Recommendations

Useful links

DNVGL-RP-0290 Lay-up and re-commissioning of ships and mobile offshore units


Accommodation, including the navigation bridge and radio room, should be protected against corrosion and other deterioration by means of a suitable system, e.g., a dehumidified atmosphere with a relative humidity below 60%. Maintaining a dehumidified atmosphere on the navigation bridge is advisable in control rooms and all other rooms containing computers and electronic equipment. The relative humidity should be controlled at regular intervals. Subject to the manufacturer’s recommendations, it may be advisable to keep equipment under constant voltage or put it into service at regular intervals for additional moisture removal and to recondition components.


All provisions under section 9.3 apply, except that the temperature in the engine room and other spaces need not be kept above the ambient temperature but not below 0°C. Areas, equipment and machinery, should be protected by the use of a dehumidified atmosphere with relative humidity below 50%. A safe method for keeping this atmosphere under control with regard to humidity should be established, and arrangements should be made to maintain the relative humidity below the given limit. Cooling water should be drained from machinery, dry lay-up of boilers and steam system to be applied.


Ballast tanks

Ballast tanks in use shall have efficient corrosion protection with coating in good condition throughout. For ballast tanks not in use, a dehumidified atmosphere should be secured, and the tanks sealed off.

Cargo tanks, pump rooms, etc.

A dehumidified atmosphere should be secured for cargo tanks and holds not in use for ballast purposes, pump rooms, cofferdams and pipe tunnels, and the tanks/rooms should be sealed off.

Deck piping

Cargo oil pipes, deck steam pipes, Butterworth lines, heating coils, exhaust, water and air pipes and ballast lines not in use should be cleaned, well-drained, and kept protected by a dehumidified atmosphere.

Deck machinery

Deck machinery which is not needed for instant operation should be protected by a dehumidified atmosphere.

Engine and boiler rooms

The use of a dehumidified atmosphere should protect engine and boiler rooms. Air supply to the engine room for the possible working of combustion engines should be arranged in such a way that the dryness of the atmosphere is not influenced.


Reciprocating machinery

The use of a dehumidified atmosphere should protect the crankcase.

Turbine machinery and reduction gears

The use of a dehumidified atmosphere should protect turbine and gear housings.

Freshwater systems

Cooling water systems on engines not in use should be emptied and dried. Care should be taken to remove all water from cooling spaces, and thorough ventilation with dehumidified air is required.

Starting air system

Starting air receivers should be kept empty, clean, dry and open to the dehumidified atmosphere in the engine room. At least one auxiliary starting air bottle should be fully charged to maintain auxiliary engines ready for start-up. It should be confirmed before lay-up that drains are clear. Air piping should be drained and dried out.

Boilers and steam system


Arrangements should be made to circulate dehumidified air through the boiler and maintain a dehumidified atmosphere.

Water/steam side

A dry lay-up condition should be used. Then, after careful draining, drying should be executed by circulating dehumidified air through the boiler and the steam system.

Steam system

The whole steam system should be preserved using dehumidified air. An arrangement combined with the boiler should be established.

Electrical installation


Local arrangements with dehumidified atmosphere should be established for components such as switchboards, starter boxes, instrumentation units etc. Furthermore, regular control of relative humidity on components and regular change/reactivation of possible drying agents should be carried out.

Instrumentation and automation control room

The control room in the engine room and other rooms containing sensitive electronic equipment should be protected by the use of a dehumidified atmosphere

Lloyds Register

Useful Links

Lay-up Guide. (

Accommodation areas

Personnel living on board should be accommodated in one area to allow all other areas to be dehumidified or at least provided with heaters to reduce humidity to an acceptable level. If the ship’s galley is being used by lay-up staff, the galley exhaust fans, and grease trap should be regularly inspected and cleaned. The humidity level in accommodation areas should be reduced and maintained at 45-55% relative humidity (RH) by dehumidifiers for all unoccupied areas. This is particularly important for spaces such as the radio room, Navigation Bridge and other spaces housing electrical machinery or electronic control equipment. Ships’ linen and napery should be stored in one dry compartment with mattresses stowed on their edge to assist free air circulation. All provision room, cabin and cabinet doors should be secured in the open position. Water services in unoccupied areas should be shut off and drained, and sanitary fittings and toilet bowls should be sealed.


General Machinery

The temperature in machinery spaces should be maintained above 0°C (32°F). Dehumidifying equipment should be installed to protect machinery spaces from atmospheric corrosion by maintaining relative humidity within the range of 30-50% RH. Power should be available for continuous operation of the dehumidifiers and the occasional turning of machinery. To achieve humidity control of the machinery space, funnel openings, grills, ventilator openings, doors, etc., should be closed and sealed. Access to the space should only be restricted to two openings, and it is advantageous if these are double airlock doors.

Diesel Machinery

Main engine crankcases should be supplied with dehumidified air suitably vented at the opposite end of the engine. Reference should be made to the engine maker’s recommendation for laying up. Water cooling systems for the main engine should be completely drained, washed with fresh water and left open to the atmosphere. Air starting valves should be dismantled and lubricated. Fuel valves should be removed, overhauled and stored outside the engine. All bright work should be protected with a smear of grease or oil. Engines, including all shafting, should be turned weekly to circulate oil (e.g. one complete turn plus one-quarter turn). Cylinder lubricants should be operated by hand before turning. Ideally, diesel generators should be maintained in operational condition and operated (rather than turned) once weekly for about two hours.

Steam plant

For lay-ups over three months, boilers should be drained and stored with the drums and header doors open to ensure maximum air circulation.

Water systems

All SW and FW systems and pumps not in use should be washed with fresh water, drained and left open to the atmosphere when a dehumidifier is used. (Otherwise, they should be left full of suitably treated clean water.) Any pumps that are unavailable for power should be turned weekly by hand. Dehumidifiers should be suitably equipped with vent trunking to forcibly ventilate heat exchangers, condensers and steam piping after removing appropriate inspection covers, crank doors, non-return valves, etc.

Electrical, electronic and software systems

Electronic and software systems

A large amount of computer processing equipment is of particular concern when laying up modern ships. Preventative measures must be taken: Equipment containing printed circuit boards should be kept dry and moisture-free, and excessive temperatures should be avoided. The electric supply systems in modern ships consist of electrotechnical components, equipment and systems which could be susceptible to deterioration if the environmental conditions are inappropriate. These systems may require special protection and specialist testing during reactivation. Software back-ups should be made available for reactivation should computers fail to boot up or restart on their own. This may involve keeping all programs and databases (for planned maintenance, etc.) duplicated ashore.

Electrical equipment and machinery

Heating or dehumidification techniques should be employed to prevent condensation within electrotechnical systems where degradation could occur if the environmental conditions are inappropriate. These systems include:

  1. a) main and emergency generators and switchboards
  2. b) all motors and starters associated with propulsion machinery, pumping duties, steering gear, cargo handling, deck machinery and domestic services
  3. c) converters, harmonic filters and transformers
  4. d) all radio and navigational equipment, and
  5. e) all engine, boiler and wheelhouse control consoles.

It is concluded that an inventory of these systems is to be prepared by the shipowner before lay-up, as each vessel has different requirements. Where required, anti-condensation heaters should be provided for those systems located outside the heated or dehumidified areas. Electrical equipment on deck should be covered and sealed, with a suitable method to ensure that any moisture within the sealed equipment is absorbed, such as using a desiccant material. Heaters should be distributed throughout the machinery spaces and those spaces that contain the electrical and control/software-based systems to maintain reasonable temperatures (above 0°C) and prevent condensation.


Useful links

Guide for Lay-Up and Reactivation of Mobile Offshore Drilling Units (

Guide for Lay-up and Reactivation of Offshore Support Vessels 2021 (

Accommodation spaces, the navigation bridge, and other control rooms should be sealed and controlled by dehumidification. Complete dehumidification at  45% to 55% RH is typically required to prevent sweating or equipment damage.

Machinery Spaces and Machinery

The machinery and related engine room equipment should be protected against accelerated or localised corrosion, seizing and freezing. This usually involves stabilised or preservative lubricants, prevention of acid concentrations, and regularly scheduled rotation or movement of machinery parts to shift contact surfaces. In addition, there should be a means or source of power for lighting and turning over machinery. Continuous heating for the machinery spaces, including the steering gear room, to maintain a temperature a few degrees above atmospheric; alternatively, complete dehumidification at 35% to 45% RH is usually required to prevent sweating or humidity corrosion damage.

Equipment and engine manufacturer lay-up instructions should be referenced and adhered to.

All rotating machinery in the spaces should be turned over several revolutions and stopped at a new position at least once a month. Where fitted, pressure lubrication systems should be actuated and cylinder lubricators manually operated before the machinery turns over.

The steering gear should be operated and moved full travel at the same intervals. Deck machinery should be turned over at least quarterly. Monthly engine turning may cause the removal of preservative oil from some surfaces. Therefore, the surfaces should be re-covered with preservative oil after the scheduled turning operations. Main and auxiliary engines which are not in operation shall be turned once a month with the main lube oil pump (main engine) and pre-lube oil pump (auxiliary engines) running to ensure oil coverage of bearing journals. Cooling water circulation should also be considered.

Enclosed engine compartments should be dehumidified to avoid corrosion.

Lube oil in engines and turbines should be thoroughly centrifuged and water separated or dehumidified before shutting down. After this, the oil should be analysed periodically to confirm stability and the absence of harmful acidity. Any lube oil reservoir or sump vents to the exposed atmosphere should be closed off and opened only to a dry space. Where lube oil tanks are contiguous with the hull plating, accumulation of condensation should be provided for.

Electrical Installation

The electrical system should be protected against insulation deterioration, primarily from moisture absorption or water ingress, and the rotating elements should be protected against corrosion damage in the bearings.

Electronics are better protected with constant power applied in a warm and dry environment. Sealed and dehumidified ships preserve the equipment longer. Desiccants should be placed inside all electrical units to avoid the risk of condensation. Desiccants should be changed at the manufacturer’s recommended intervals. Open cable ways should be sealed to reduce desiccant degradation and other component damage.

Before securing, electrical motors and generators should be thoroughly cleaned of carbon or other hygroscopic foreign matter, heat dried to obtain good insulation resistance readings, and the bearings lubricated with a stable grease or oil. Any carbon brushes should be lifted to prevent spot corrosion on the commutator or slip rings. Insulation readings should be taken and recorded at least monthly after that, and where found abnormal, immediately corrected by heating, drying or cleaning.

All electrical apparatus should be maintained internally a few degrees above atmospheric through built-in heaters, if fitted, or by other means such as strip heaters or heat lamps. Alternatively, the humidity control may be maintained by opening the unit to the effects of a dehumidifier or by sealing the unit with desiccant. This latter approach would require the replacement of the desiccant and re-sealing at least monthly or based on a moisture indicator.

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