Section 2 Theoretical analysis

2.1.1 The determination of dynamic loads is to be based on theoretical analysis of ship load and motions and a long term prediction method.

2.1.2 Hydrodynamic analysis is to be undertaken to determine the long term values of the main load and motion parameters. The ship load and motion analysis will provide the phase relationship between each load component to be applied to the FE model.

2.1.3 The necessary level of complexity of the hydrodynamic software and its validation procedure is to be agreed upon with LR.

2.2.1 It is likely that only one sailing condition will need to be evaluated for the Trimaran. However, if other sailing conditions exist where there is a large variation of draft, trim, VCG, GM or distribution of cargo or deadweight, then additional conditions may need to be evaluated.

2.2.2 It is suggested that the following range of data is used in the program:

• Ship speeds – For calculation of long term values in high sea states, analysis is to be based on a manoeuvring speed of 5 knots. For ships operating at higher speeds in high sea states, speeds should be selected that are representative of this operational range, i.e. 75 per cent and 100 per cent of full service speed. However, speed selection should consider the operational limitations of the vessel in the selected sea state.
• Ship headings – The motion and load responses should be determined for at least every 30 degree from head to stern seas.
• Wave frequencies – The range and interval of wave frequencies should cover the complete spectrum of wave energy. The required frequency range is 0,25 radians to 1,5 radians in 0,05 increments.

2.3.1 The long term response values are to be evaluated for the following load and motion parameters:

1. Vertical wave bending moment (Hog and Sag);

2. Horizontal wave bending moment;

3. Longitudinal torsional moment;

4. Transverse torsional moment;

5. Splitting moment in cross-deck structure (Hog and Sag);

6. Vertical shear force in hull;

7. Splitting shear force in cross-deck structure;

8. Maximum roll angle.

2.3.2 Response Amplitude Operators (RAO) are to be generated in regular waves for all load and motion parameters specified in Vol 4, Pt 1, Ch 3, 2.3 Load and motion parameters 2.3.1 for each heading/ship speed combination.

2.3.3 A suitable wave energy spectrum is to be applied to the RAOs of each load and motion parameter to calculate the short term response.

2.3.4 In analysing the response data, the short crested option is to be used with a Cos² spreading function.

2.3.5 The long term responses for each load and motion parameter are to be derived using the appropriate wave scatter diagram and target design values.

2.3.6 Information about appropriate wave energy spectra and wave scatter diagrams may be obtained from IACS Resolution 34. The sea area considered to produce the worst operating environment is the North Atlantic. Acceptable wave spectra for the North Atlantic sea area include the Pierson- Moskovitz or Bretschneider spectra.

2.3.7 It is to be assumed that the ship has equal probability of encountering waves from all directions. Weighted headings based on the operational profile may only be used if the results are more conservative than the assumption of equal probability of all headings.

2.3.8 When evaluating the long term response, the operating life is generally to be taken as 20 years, which is assumed to correspond to 10⁸ wave encounters or a long term probability level of 10^–8.

2.4.1 The following supporting information is to be submitted, as applicable:

1. Reference to the technical program used and an outline of the load calculation procedure adopted.

2. Input data.

3. A summary of analysis parameters including environmental conditions, speeds and headings.

4. Details of weight distributions.

5. A comprehensive summary of calculation results. Sample calculations are to be submitted where appropriate.

6. Verification and validation of the target design values.

2.4.2 Documentation of adequate validation may be required for software used for derivations of loads.