Full-scale measurements

Full-scale measurements are an important method for investigating slamming behaviour. Generally, in these experiments the motions of the vessel are measured using a gyro and/or accelerometers; strain gauges can be used to measure local or global structural stresses. The wave environment is usually measured using either an onboard radar system or a deployed wave buoy.

Several full-scale monohull vessel slamming experiments are reported in the literature some of which are outlined in Kapsenberg’s [8] review paper on slamming. The probability of slamming, propeller emergence occurrence rates, structural stresses, Vertical Bending Moments (VBM) and slam pressure measurement were among the parameters analysed in these trials. For example, a relationship between the impact velocity and slam peak pressures was established by Ochi et al. [15]. One problem with full-scale trials has been the high uncertainties in measuring the incident waves. Methods of parallel measurements of the seaways such as using buoys and onboard radars are mostly recommended [5, 8]. The complicated instrumentation and finding the right slamming weather conditions in normal ship routes are reported as other big challenges in full-scale experiments.

Full-scale measurements of wetdeck slamming on a 30 m catamaran were undertaken by Haugen and Faltinsen [16]; accelerations, ship motions, wave conditions and hull strains were measured. The importance of the relative normal impact velocity between the wetdeck and the water surface, wetdeck angle, pitch angle and forward speed was identified. Wetdeck slamming events were characterised by large vertical accelerations with substantial vibrations in the hull girder. It was found that wetdeck slamming may occur even “when the vessel operates in sea states well below the operational limits given by the DNV rules”. The vibration following a slam is called whipping and can reduce the fatigue life due to the increased cyclical loadings [3].

Full-scale experiments during the delivery voyage on an 86 m Austal high-speed catamaran were reported by Steinman, Fach and Menon in 1999 [17], with motions, wave heights and some hull strains being measured. The measured hull strain responses in slamming have shown an initial forced

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impact followed by a strong backlash (excursion from the mean stress level in the opposite direction) then fluctuations with global mode structural response. Significant differences were observed between the design loads required by the classification societies and the full-scale measurements.

Full-scale tests were conducted by Roberts et.al [11] on an 81m and a 96m INCAT Wave-Piercing Catamaran (WPC) and the slam induced loads were found to be significantly larger than the global wave loads. The results of full-scale trials of an 81m INCAT wave-piercing catamaran were used by Yakimoff [18] in conjunction with a Finite Element (FE) model to investigate the fatigue life of these vessels. Whipping due to the slamming and its subsequent load strain peaks were identified to be the cause of 66% of fatigue damage in the vessel.

Thomas et al. [19] carried out full-scale strain and motion measurements on a 96m INCAT WPC to identify slam events. Slam events were characterised by structural loading, relative vertical velocity, heading angle and frequency of occurrence. The stresses in the hull structure due to slamming, were shown to be up to seven times greater than the underlying wave loads. It was also shown that the VBM from a severe asymmetric slam load could exceed the design maximum VBM from DNV rules for such vessels [2]. The difficulty with the analysis of these full-scale trial results has been the definition of the slam loads from the response of the vessels structure. The hull form is complex and it is not easy to use a straightforward formula for estimation of slam loads. Therefore, a FE model of the ship was developed to use the strain results of the full-scale tests for identifying slam loads. For this purpose, wave profiles were proposed for estimating global wave loads. An iterative process to determine a distribution for slam loads from full-scale strains in the FE model was adopted [2]. Similar measurements were conducted by Thomas et.al [6] on a 86 m INCAT wave piercing catamaran (Hull 042, Figure 1.3).

Amin et al. [14, 20, 21] completed quasi static FE analysis on a 98m INCAT hull strains in which he used a “reverse engineering” technique to estimate the slam loads. These loads then were applied to a FE model of the vessel and reasonable strain correlations were achieved. This process of deriving slam loads is difficult and can have large uncertainties due to the large range of possible load distributions.

Full-scale trials were conducted on Sea Fighter SFS-1 by Naval Surface Warfare Centre in 2007 [22] to characterise wetdeck slamming. A complex set of wave measuring systems were employed onboard including TSK radars (Doppler Effect radar correcting for ship motions), sonars and video cameras along with motion and acceleration sensors. The experiments were successful in identifying and synchronising encountered wave profiles, motions and acceleration responses in four slam cases. The results indicated that prior to all slam cases the demihull bows were out of the water. Also two consecutive high waves were necessary to cause a severe wetdeck slam. No hull strain or pressure measurements were reported in this work.

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From all these full-scale sea trials and measurements, several practical and analytical issues have been identified:

- Complicated instrumentation and measurement processes are involved in acquiring data during sea trials, which makes these experiments expensive and time consuming.

- It is difficult to find the right seaway conditions for slamming to occur. The environmental factors cannot be controlled.

- The measurement of the encountered waves is difficult and has high uncertainties.

- It is difficult to collect pressure data on the hull as the owners/operators are reluctant to drill holes in the vessel’s hull.

- The key difficulty is to relate the structural response of the vessel to slam loads. A complicated process using a Finite Element model can be used to obtain a load case in a “reverse engineering” process.

- It is not possible to easily investigate influence of hull form on slam behaviour.

These issues have led researchers to explore other methods to investigate slamming. Methods such as model tests or analytical/numerical calculations have been used for both load definition and response prediction.