Section 8 Determination of forces for container securing arrangements

8.1.1 The forces acting in the securing system are to be determined for each loading condition and associated set of motions of the ship.

8.1.2 The following forces are to be taken into account:

• Static gravity forces.
• Inertial forces generated by the ship motions in a seaway.
• Wind forces.
• Forces imposed by the securing arrangements.
• Wave impact forces and effects of consequential hull girder whipping.

8.1.3 Forces due to pre-tensioning the securing devices need not, in general, be included in the calculation, provided that they do not exceed 5 kN in any one item. Special consideration will be given to cases where forces obtained from pre-stressing are an integral part of the design of the system.

8.1.4 Although the operation of anti-roll devices or other systems may improve the behaviour of the ship in a seaway, the effect of such devices is not normally to be taken into account to reduce the determination of the forces for container securing arrangements. Where the reliability of such systems can be demonstrated, special consideration of the roll motions will be given. Supporting documentation to demonstrate the effectiveness of an anti-rolling system should identify the redundancy incorporated in the control system and details of actual measurements from the ship over a period of time, including route and season of operation.

8.2.1 The forces acting on each container due to gravity, ship motion accelerations, ship rolling and pitching angles and wind forces and green sea forces are to be calculated as follows.

8.2.2  The equations for ship motion accelerations and other motion parameters are given in Pt 3, Ch 14, 1.7 Symbols and definitions. These are to be used for the calculation of accelerations to derive the forces for the container securing arrangements. Alternatively, the ship motion values may be derived by direct calculation methods using the same principles as those used to derive the Rule equations. The formulae in Pt 3, Ch 14, 1.7 Symbols and definitions are applicable to container ships. Values for other ship types will be specially considered.

8.2.3 The following six Motion Cases (MCs) are to be considered:

Each Motion Case comprises 2 Motion Combination Factor (MCF) sets. Each MCF set represents an Equivalent Design Wave (EDW) that generates response values equivalent to the long-term response values of the critical load components for ship motion forces acting on containers. The Motion Combination Factors are given in Table 14.8.3 Motion Combination Factors (MCFs).

8.2.4 The individual force components for each Motion Case due to gravity, ship motions, wind and green seas acting on a container i are to be determined as follows, see Figure 14.1.1 Diagrammatic representation of symbols and Table 14.9.1 Acceleration force application:

where

8.2.5 The total instantaneous acceleration values acting on container i, including the static gravitational term, are to be taken as:

• Longitudinal acceleration (parallel to deck, positive forward)
  

  a _x = C _xS a _surge + C _xP a _pitch z _i – g sin (C _xG ψ) m/s²

• Transverse acceleration (parallel to deck, positive to port)
  1. For containers on the port side and on the centreline:

     a _y = C _yS a _sway – C _yR a _roll z _i + g sin (C _yG φ) m/s²

  2. For containers on the starboard side:

     a _y = –C _yS a _sway + C _yR a _roll z _i – g sin (C _yG φ) m/s²

  
  
• Vertical acceleration (normal to deck, positive upwards)
  1. For containers on the port side and on the centreline:

     a _z = C _p1 (C _zH a _heave + C _zR a _roll y _i – f _ap C _zP a _pitch x _i ) – g cos (C _xG ψ) cos (C _yG φ) m/s²

  2. For containers on the starboard side

     a _z = C _p1 (C _zH a _heave – C _zR a _roll y _i – f _ap C _zP a _pitch x _i ) – g cos (C _xG ψ) cos (C _yG φ) m/s²

  
  

where the Motion Combination Factors C _xS, C _xP, C _xG, C _yS, C _yR, C _yG, C _zH, C _zR and C _zP for each Motion Case are given in Table 14.8.3 Motion Combination Factors (MCFs).

8.2.6 Wind forces are to be applied to the outer faces of the outermost container stack in accordance with Pt 3, Ch 14, 8.2 Ship motion, wind and green sea forces acting on containers 8.2.4. Wind forces may also need to be applied to inboard stacks in accordance with Pt 3, Ch 14, 8.2 Ship motion, wind and green sea forces acting on containers 8.2.7. The wind forces are to be taken as acting in the athwartships direction. Only positive forces of wind pressure are to be applied; suction forces need not be included. See Figure 14.8.1 Application of wind forces

8.2.8 A container is considered protected from wind in the transverse direction or from green sea loads in the longitudinal or transverse direction if an effective breakwater or similar extends above mid height of the container.

8.2.12 The green sea pressure is given by:

for L >100

for L ≤ 100, P _gs is to be taken as:

where

C _G1, C _G2, C _G3 are defined in Table 14.8.2 Green sea pressure coefficients
Proposals to use other values for green sea forces will be specially considered