In June 1998, the larger ever built monohull HSC came into operations. The craft, built by FINCANTIERI under RINA Class, was equipped with a monitoring system with the purpose of recording the wave induced loads during normal operations. The aim of the campaign was to collect full scale results during operations for at least one year in order to correlate them with the loads assumptions made during the design.
3.2 Vessel characteristics
The vessel is an HSC ro-ro passenger ferry belonging to FINCANTIERI'S MDV 300 series: it is a 146 m (128 between perpendiculars) long monohull able to carry 1800 passengers and more than 460 cars or, alternatively, 30 trucks and 100 cars.
The maximum speed of 42 kn is achieved by a total installed power of 70400 KW and the use of 4 waterjets.
Since June 1998, the vessel operates along the Civitavecchia-Olbia route connecting Italy mainland to Sardinia, by means of two to four journeys per day depending on the season. The route length is about 120 miles and, in favourable weather conditions its duration is slightly in excess of 3 hours, excluding port operations.
The main interest in the full scale data being the wave induced loads, the following quantities have been monitored (see ref. [7] for further details on the monitoring system):
・sea state severity (significant wave height and period) by means of a wave recorder device installed on board
・vertical accelerations at the midship, at the bow and at the stern
・roll and pitch motions at the centre of geometry
・vertical wave bending moment at the midship section
・pressures on the bottom and side hull in the area between 0.5 L to 0.75 L.
3.3 Summary of investigations carried out
The aim of the study was to develop a model for the evaluation of the maximum sea state severity compatible with the assumed design loads. In order to do this, also in this case linear strip theory was selected as the numerical tool. Therefore the investigations carried out covered the following steps
1. comparison between model tests and numerical results wit the aim of verifying the accuracy of the ship hydrodynamic model
2. comparison of numerical results and full scale results in particular for those quantities, e.g. vertical bending moment and local pressures, for which model test results are generally not available
3. use of the model to derive the limiting conditions in terms of sea state severity for different ship wave headings.
3.3 Experimental-numerical comparisons
Model tests results in terms of vertical accelerations at the ship centre of geometry covered more than 30 different combinations of sea state severity, headings and craft ship, see Table 2.
Comparison between model tests (horizontal axis) and calculations (vertical axis) are shown in Figure 5 where deviations from the straight line is a measure of the approximation obtained. It emerges that the accuracy of the numerical results is very good.
As far as full scale results are concerned, the most severe environmental conditions encountered during more than one year operations were considered (cf. Table 3) in the experimental - numerical comparison. Results are shown in Figure 6 for the vertical wave bending moment. In this case the agreement is less good than with respect to model tests, this is due to the fact that in the latter case the environmental conditions, e.g. the shape of wave spectrum, are far better controlled than during full scale.
3.4 Determination of limiting conditions
The design loads considered for the vessel were as follows:
・maximum vertical acceleration at the centre of gravity (rms value) 0.22 g
・maximum vertical acceleration at the craft forward perpendicular (rms value) 0,44 g
・maximum vertical wave bending moment 402 MNm
・maximum impact pressure 180 KPa.
Calculations were carried out in order to assess the variability of each of the above parameters as a function of the sea state severity therefore accounting for significant wave height, wave spectral period, craft-wave heading and craft speed. The limiting wave was then defined as the lowest significant wave height for which the design value of the load was attained.
This procedure is different from the one used for ocean going ships as e.g. adopted in section 3 of this paper. The reason is that in this case the craft operates under controlled environmental conditions and the duration of the voyage is limited to few hours therefore rather than assessing all the environmental conditions likely to occur in the craft 's life, it is sufficient to determine those which have not to be exceeded in order for the loads to be kept within the design envelope: when more severe conditions are forecasted, the craft will not be allowed to operate.
In Figure 7, the resulting limiting wave height and the associated period are displayed as a function of the craft speed. It is observed that a value up to 5 m significant wave height could be safely sustained at full speed (40 kn), such a value is of limited practical interest due to the unavailability of sufficient increased propulsive power needed to keep 40 kn in such high waves.
Of more interest is the limiting value at 25 kn., here the installed power is sufficient up to 6 m significant wave height whereas the structure can sustain up to 7 m.
Finally, the high limiting values obtained for low speed, e.g. 11 m significant wave height for 10 kn., are interesting from a structural safety point of view: in case of a 50% loss of propulsion in severe seas, this means that the craft can safely reach a safe refuge at low speed. This is one of the reasons why this type of craft have normally 4 completely independent propulsion lines.
3.5 Operational considerations
Also in this case, from the operational standpoint, it is very interesting for the ship master to have fairly good ideas of the reduction of loads he can obtain by modifying the craft course or the craft speed.
Based on the calculations carried out, it was possible to derive plots of the reduction of both vertical acceleration achievable (important for the comfort of passengers) and of vertical bending moment (important for structural strength) by means of changes of ship course and / or reduction of ship speed.
