2 External Pressures on Exposed Decks

2.1 Application

2.1.1 The external pressures and forces on exposed decks are only to be applied for strength assessment.

2.1.2 The green sea pressures defined in [2.2] for exposed decks are to be considered independently of the pressures due to distributed cargo or other equipment loads and any concentrated forces due to cargo or other unit equipment loads, defined in [2.3.1] and [2.3.2] respectively.

2.2 Green sea loads

2.2.1 Pressure on exposed deck

The external dynamic pressure due to green sea loading, P_D, at any point of an exposed deck, in kN/m², for the static plus dynamic (S+D) design load scenarios is to be derived for each dynamic load case and is to be taken as defined in [2.2.3] to [2.2.4]

The external dynamic pressure due to green sea loading, P_D, at any point of an exposed deck for the static (S) design load scenarios is zero.

2.2.2 If a breakwater is fitted on the exposed deck, no reduction in the green sea pressure is allowed for the area of the exposed deck located aft of the breakwater.

2.2.3 HSM, HSA and FSM load cases

The external pressure, P_D, for HSM, HSA and FSM load cases, at any load point of an exposed deck is to be obtained, in kN/m², from the following formula, see Figure 2 and Figure 3:

P_D = χ P_W

where:

P_W = P_W,D, but not to be taken less than P_D-min.

P_W,D : Pressure, in kN/m², obtained at side of the exposed deck for HSM, HSA and FSM load cases as defined in [1.3].

χ : Coefficient defined in Table 31.

2.2.4 BSR, BSP, OST and OSA load cases

The external pressure, P_D, for BSR, BSP, OST and OSA load cases at any load point of an exposed deck is to be obtained, in kN/m², by linear interpolation between the pressures at the port and starboard deck edges (see also Figure 4, Figure 6, Figure 9 and Figure 10):

P_{D, stb} = χ P_{W, D – stb}

P_{D, pt} = χ P_{W, D – pt}

where:

P_W,D-stb : Pressure obtained at starboard deck edge for BSR, BSP, OST or OSA load cases as defined in [1.3], as appropriate.

P_W,D-pt : Pressure obtained at port deck edge for BSR, BSP, OST and OSA load cases as defined in [1.3], as appropriate.

χ : Coefficient defined in Table 31.

2.2.5 Envelope of dynamic pressures on exposed deck

The envelope of dynamic pressure at any point of an exposed deck, P_D-max, is to be taken as the greatest pressure obtained from any of the load cases determined by [2.2.3] and [2.2.4].

2.3 Load carried on exposed deck

2.3.1 Pressure due to distributed load

If a distributed load is carried on an exposed deck, for example deck cargo or other equipment, the static and dynamic pressures due to this distributed load are to be considered.

The total pressure, P_dl, in kN/m², due to this distributed load for the static (S) design load scenario is to be taken as:

P_dl = P_{dl – s}

The pressure P_dl, in kN/m², due to this distributed load for the static plus dynamic (S+D) design load scenario is to be derived for each dynamic load case and is to be taken as:

P_dl = P_{dl – s} + P_{dl – d}

where:

P_dl-s : Static pressure, in kN/m², due to the distributed load, to be defined by the Designer and, in general, but not less than 10 kN/m².

a_Z : Vertical acceleration, in m/s², at the centre of gravity of the distributed load, for the considered load case, to be obtained according to Ch 4, Sec 3, [3.2.4].

2.3.2 Concentrated force due to unit load

If a unit load, for example deck cargo, is carried on an exposed deck, the static and dynamic forces due to the unit load carried are to be considered.

The force F_U, in kN, due to this concentrated load for the static (S) design load scenarios, is to be taken as:

F_U = F_{U – s}

The force F_U, in kN, due to this concentrated load for the static plus dynamic (S+D) design load scenarios is to be derived for each dynamic load case and is to be taken as:

F_U = F_{U – s} + F_{U – d}

where:

m_U : Mass of the unit load carried, in t.

a_Z : Vertical acceleration, in m/s², at the centre of gravity of the unit load carried for the considered load case, to be obtained according to Ch 4, Sec 3, [3.2.4].