IACS UR S21

IACS UR S21 is the unified requirement for strength of cargo hatch covers, hatch coamings, and closing arrangements on exposed decks, and for the external sea pressures on weathertight structures. It applies to ships other than bulk carriers, ore carriers, and combination carriers, which fall under UR S21A. Design loads are the green-sea pressures on the fore deck, which decrease aft and with height above the waterline, plus the longitudinal force on the forward end of forward covers, taken as a 230 kN/m^2 pressure that can drop to 175 kN/m^2 when a forecastle is fitted. Hatch cover scantlings of stiffened-plate construction are checked for yield, deflection limits, and buckling, with the buckling interface standardized through UR S35.