The influence of spacing between two submerged bodies on wave loads of SWATH

Abstract

The determination of the spacing between two submerged bodies is of great significance in preliminary design stage of SWATH. And this paper focus on researching the influence of spacing between two submerged bodies on SWATH wave loads. Wave loads forecast are made for three different loading conditions, which are normal loading, full loading and maximum loading. For each loading condition, no parameter except the spacing of two submerged bodies is changed from the available data of a certain SWATH in practical project to obtain a series of SWATH computation models. By means of 3-D frequency domain, wave load forecast of different computation models is made to obtain the transfer function, long-term and short-term forecast value, then the comparisons between the value of wave load forecast and that of ABS and CCS specifications are done. By comparing the wave load forecast value of different calculation models, the degree and characteristics of impact which the spacing between two submerged bodies has on the wave loads are obtained, thus providing a reference for the preliminary design of SWATH. Introduction With the rapid development of SWATH in the field of shipbuilding, the study of SWATH wave loads has gradually developed and deepened in recent years. Currently, researches about SWATH wave loads are mainly focused on the following: the estimation and empirical formulas of design loads, the dangerous working conditions, the hydrodynamic pressure and motion response characteristics. In preliminary design stage of SWATH ship, the determination of spacing between two submerged bodies is very important, which has seldom been studied in the shipbuilding field. Actually, the underwater part of SWATH mainly consists of two submerged bodies and two pillar bodies, which can provide buoyancy for the whole vessel. Based on source-sink distribution theory, two submerged bodies can be regards as two symmetrically arranged ellipsoids. And the presence of one submerged body will act as an additional wall for the other, while solving the velocity potential. Thus, different spacing between two submerged bodies will surely lead to different wave loads. Therefore, this article focus on researching the influence of spacing between two submerged bodies on SWATH wave loads under three loading conditions. For each loading condition, spacing is the only variable, thus a series computation models with different spacing are obtained. Then COMPASS-WALCS-BASIC software is adopted to do wave loads forecast for those computing models. By comparing the wave load forecast results of different computation models, the influence of spacing between two submerged bodies on SWATH wave loads can be obtained, which can provide a reference for the preliminary design of SWATH ship. Computation parameters To compute the sectional wave loads, 21 transverse sections and a longitudinal section are selected. And all the transverse sections distribute along the captain uniformly, while the longitudinal section locates in longitudinal section of the ship. In regular waves computing, the computed speed is 6kn, and the computed wave interval amounts to 15 °, while 36 frequencies were used (0.2rad/s with steps of 0.05 rad/s up to 2.0 rad/s). Due to space International Conference on Information Sciences, Machinery, Materials and Energy (ICISMME 2015) © 2015. The authors Published by Atlantis Press 1689 limitations, only transfer functions of relevant wave loads of partial frequencies under full loading condition are illustrated in Figs .1~4. Given the results of regular wave calculations, wave loads in irregular waves can be obtained based on relevant mathematical statistics principles. And the wave parameters of short-term forecast are shown in table 1, while NO.34 North Atlantic waves scatter plots recommended by IACS are selected in the long-term forecasts. In the long-term forecasts, the exceedance probability is 10 and the point spread function is 2 2 cos θ π . Table 1. Wave parameters of short-term forecast Scale of state of sea HS(m) TP(s) 4 2.5 6.8 5 4 7.5 6 6 9.6 7 9 11.6 Presentation of the results Due to space limitations, only transfer functions of relevant wave loads of partial frequencies under full loading condition are illustrated in Figs.3~6. And when the vessel is in full loading condition, the short-term wave loads forecast of different calculation models under different scale of state of sea are illustrated in Figs.5~8. Then, comparison between the long-term forecast values of WALCS and specification values of CCS and ABS is shown in Table 2. Fig.1 Transfer function of horizontal split force Fig.2 Transfer function of transverse bending moment Fig.3 Transfer function of sync pitch torsional moment Fig.4 Ttransfer function of horizontal torsional moment