Yongwon Lee

Wave loads and flexible fluid-structure interactions: current developments and future directions

The function of a Classification Society includes the setting of standards for the design, construction and maintenance of ship hulls to ensure adequate safety throughout their service life. Fundamental to this is the determination of the design loads to support the prescriptive Rule requirements and for application in direct calculations. The current design philosophy for the prediction of motions and wave-induced loads is driven by first-principles calculation procedures based on well-proven applications such as ship motion prediction programs. In recent years, the software and computer technology available to predict design loads has improved dramatically. With the stepwise increase in ship size and complexity it is necessary to utilise the latest technologies to assess the design loads on new ship designs. This paper discusses some of the recent experiences of Lloyds Register with regard to the current state of the art in the assessment of design loads and structural responses by reviewing recent work on the effects of flexible fluid-structure interaction for hull girder and also for sloshing applications. The paper also discusses the Lloyds Register strategic research programme on hydrodynamics, involving the use of state-of-the-art technologies for the solution of ship dynamic response problems.

Global wave loads on a damaged ship

A computational tool was applied based on a two-dimensional linear method to predict the hydrodynamic loads for damaged ships. Experimental tests on a ship model have also been carried out to predict the hydrodynamic loads in various design conditions. The results of the theoretical method and experimental tests are compared to validate the theoretical method. The extreme wave-induced loads have been calculated by short-term prediction. For the loads in intact condition, the prediction with a duration of 20 years at sea state 5 is used, while for loads in damaged conditions, the prediction with 96 hours of exposure time at sea state 3 is used. The maximum values of the most probable extreme amplitudes of dynamic wave-induced loads in damaged conditions are much less than those in intact condition because of the reduced time. An opening could change the distribution of not only still-water bending moment but also wave-induced bending moment. It is observed that although some cross-sections are not structurally damaged, the total loads acting on these cross-sections after damage may be dramatically increased compared with the original design load in intact condition.

Time Domain Analysis of Springing and Whipping Responses Acting on a Large Container Ship

As part of WILS II (Wave Induced Loads on Ships) Joint Industry Project organised by MOERI (Maritime and Ocean Engineering Research Institute, Korea), Lloyd’s Register has undertaken time domain springing and whipping analyses for a 10,000 TEU class container ship using computational tools developed in the Co-operative Research Ships (CRS) JIP [1]. For idealising the ship and handling the flexible modes of the structure, a boundary element method and a finite element method are employed for coupling fluid and structure domain problems respectively. The hydrodynamic module takes into account nonlinear effects of Froude-Krylov and restoring forces. This Fluid Structure Interaction (FSI) model is also coupled with slamming loads to predict wave loads due to whipping effects. Vibration modes and natural frequencies of the ship hull girder are calculated by idealising the ship structure as a Timoshenko beam. The results from springing and whipping analyses are compared with the results from linear and nonlinear time domain calculations for the rigid body. The results from the computational analyses in regular waves have been correlated with those from model tests undertaken by MOERI. Further the global effects of springing and whipping acting on large container ships are summarised and discussed.Copyright

Numerical Investigation on Wave Load Characteristics of a High Speed Trimaran in Oblique Waves

Since the wave load characteristics of trimaran ships cannot be estimated from experiences with conventional mono-hull vessel and Catamaran, investigations on these hydrodynamic aspects of trimarans are urgently needed to exploit their potential. In this study, the global loads of a trimaran in oblique waves are investigated by means of the 3D translating-pulsating source distribution model. The wave loads including the shear forces, bending moments and torsional moments at different transverse sections of the trimaran ship hull and longitudinal section of connected deck are predicted at various wave heading and frequencies. The characteristics of longitudinal and transverse sectional wave load responses of the trimaran ship will be dicussed.