When selecting and installing manhole covers, in addition to complying with relevant conventions, rules, and specifications, strength, purpose, and ease of use should also be considered.
The strength of a manhole cover is mainly reflected in the thickness of the cover plate and the number of fastening bolts. Generally, the thickness of the cover plate should not be less than the thickness of the hull structural steel plate (deck, platform, inner bottom plate, and bulkhead) at the installation location. Since the cover plate is more susceptible to corrosion than the surrounding hull structural steel plate, when the thickness of the hull structural steel plate at the manhole cover installation location is less than 10 mm, the cover plate thickness should be 1 mm greater than the thickness of the hull structural steel plate at that location. If the deck or bulkhead at the manhole cover installation location is determined by strength calculations, the thickness of the manhole cover plate at that location should be able to withstand the same water pressure conditions.
The manhole cover should be selected based on the compartment tightness requirements and the conditions of the installation location, choosing an appropriate structural form. Watertight manhole covers with seawater-resistant (freshwater) rubber gaskets should be used in freshwater tanks, ballast water tanks, and bow and stern tanks.
Oiltight manhole covers with surge-resistant rubber gaskets should be used in fuel oil tanks, lubricating oil tanks, and sludge oil tanks. Type A manhole covers with enclosures should be used on the inner floor of areas prone to water or oil accumulation, such as engine rooms, and on the top of drinking water tanks. When manhole covers are necessary in passageways and living/working compartments that affect personnel movement, recessed manhole covers should be selected. Manhole covers for jet fuel tanks must be made of brass to prevent sparks from impacts. When manhole covers may be exposed to prolonged humid environments, their bolts or studs should be made of stainless steel. Nuts should be made of stainless steel, brass, or galvanized steel.
From the perspective of manhole cover usage, a larger through-hole size is beneficial for personnel access. However, due to factors such as hull structure, mechanical equipment, and pipeline layout, the size of manhole covers is subject to certain limitations. Generally, manhole covers with a through-hole size of 500 mm × 400 mm are more suitable for installation in horizontal locations (decks, platforms, inner bottom plates). Unless there are genuine difficulties in the installation, manhole covers with a through-hole size of 450 mm × 350 mm should be avoided as much as possible. Manholes with a through-hole size of 600 mm × 450 mm should be installed in cabins or other vertical locations, and their long axis should be arranged in the vertical direction. The lower edge of the through-hole should be at least 500 mm above the ground where personnel are standing to facilitate personnel crossing the manhole.
