Section 4 Target design values

4.1.1 In general, the target design value is to be taken as the extreme value in the most onerous conditions as specified in Vol 4, Pt 1, Ch 3, 3.4 Model test conditions 3.4.1. This value may be used instead of the Rule based value in the structural analysis.

4.1.2 The design value is to be derived from the extreme values for each load and motion parameter, see Vol 4, Pt 1, Ch 3, 2.3 Load and motion parameters 2.3.1.

4.1.3 The design values obtained through theoretical analysis may need to be adjusted or calibrated to take account of scale issues, non-linearities, etc. This comparison or validation data is to include results from model tests, full scale tests or ship motion analyses of similar size or shape ships. All calibration factors and methods are to be agreed with LR prior to further work being undertaken.

4.1.4 The design values obtained from the model tests are to be determined at an appropriate probability level as agreed by LR based on the statistic fitting for each load and motion parameter as stated in Vol 4, Pt 1, Ch 3, 3.5 Parameters to be measured 3.5.2.

4.2.1 Having established the magnitude of the target design values, it is then necessary to determine an appropriate wave condition which will result in the target design value and then may be applied to the FE model. This wave condition is referred to as the characteristic wave and is to be derived as follows.

4.2.2 RAOs for all relevant speeds and headings are to be reviewed and the RAO, for a particular speed/heading combination, with the maximum response for the specified load or motion parameter is identified.

4.2.3 The peak of this selected RAO curve then provides the encounter frequency, ω_e, and the peak amplitude, a, of the required characteristic wave.

4.2.4 The wave height, h, required to obtain the specified load or motion parameter is given by:

4.2.5 The characteristic regular wave is given by the following equation:

The corresponding maximised motion response is

4.2.6 The time instant at which the maximum response occurs is when the cosine response is maximum or minimum as appropriate, hence,

4.2.7 The instantaneous responses of all other load and motion parameters are to be derived at the time instant given by Vol 4, Pt 1, Ch 3, 4.2 Equivalent design wave 4.2.6.

4.2.8 The summation of all dynamic pressures and inertial loads acting on the ship at this time instant, as calculated in the ship motion program, are to be checked to ensure reasonable balance is obtained in all global degrees of freedom.

4.3.1 The load cases to be applied and assessed are as follows:

1. Maximum vertical wave bending moment in hog;

2. Maximum vertical wave bending moment in sag;

3. Maximum horizontal wave bending moment;

4. Maximum longitudinal torsional moment;

5. Maximum transverse torsional moment;

6. Maximum sagging splitting moment in cross-deck structure;

7. Maximum hogging splitting moment in cross-deck structure;

8. Maximum wave shear force in hull;

9. Maximum splitting shear force in cross-deck structure;

10. Maximum roll angle.

4.3.2 For cases (a) through (j), the still water loads are to be applied in addition to the pressure and inertial loads. The still water loads are included in case (k).

4.3.3 From the evaluation of the RAOs, there is now a set of characteristic cosine waves for each load parameter with a phase relationship which is unique for each maximised parameter. An appropriate program may be used to convert this information into pressure and inertial loads for application to the structural model.

4.3.4 The pressure and inertial loads are to be applied to the finite element model. All inertial loads are to be applied to both deadweight and lightweight items in the FE model. These loads are dynamic loads. The still water loads, as described in Vol 4, Pt 1, Ch 2 Structural Strength Analysis and Verification, will also need to be applied to the structural model. It is recommended that this is done as a separate load case and the two load cases are added together using the principle of superposition.

4.3.5 The loads applied to the structural model are also to be checked for reasonable balance in all degrees of freedom.

4.3.6 Alternatively, the loads may be applied to the FE model using sinusoidal analysis techniques where the RAOs are considered to be represented by complex numbers.