Takashi Tsubogo

POTENTIAL OF MICHELL'S INTEGRAL FOR SHIP WAVE RESISTANCE

The Michell’s integral (Michell 1898) for the wave making resistance of a thin ship has not been used widely in practice, since its accuracy is questioned especially for a Froude number range about 0.2 to 0.35 for conventional ships. We examine calculations by Michell’s integral for some ship forms, e.g. a parabolic strut, Wigley hull and so on. As a result, one reason of the disagreement with experiments is revealed. It must be the gradient of hull form in the depth direction. Then a thin ship theory including the hull gradient effect in the depth direction is presented, which improves slightly in low Froude numbers but needs more computing time than Michell’s integral so as to solve a boundary integral equation.

NON-LINEAR WAVE LOADS ACTING ON OBLIQUELY SLOWLY ADVANCING PLATFORM

It is necessary to evaluate wave drift force for ships advancing obliquely. There are some approaches, for instance the strip method, solving the Navier-Stokes equation directly in the fluid domain (CFD), potential theory and so on. In the present study, the non-linear wave loads acting on the ship with constant oblique forward speed is considered based on the potential theory. Consistent perturbation expansion based on two parameters, i.e. the incident wave slope and the ratio of the forward speed compared to the phase velocity of the waves, is performed on a moving frame (body-fixed) coordinate system to simplify the problem. So obtained boundary value problems for each order of potentials is solved by means of the hybrid method. The fluid domain is divided into two regions by an artificial circular cylinder surrounding the body. The potential in the inner region is expressed by an integral over the boundary surface with a Rankin source as its Green function while it is expressed in the eigen function expansion for the outer region. Consequently, the boundary value problems can be solved efficiently. In the present paper, the authors will discuss the effects of the obliquely advancing on the wave drift force in a diffraction wave field up to the order proportional to the advancing speed. An ellipsoid model is used in the calculation and the wave drift force is evaluated for various Froude number.Copyright

RECIPROCAL FORM OF THE WAVE DRIFT FORCE AND MOMENT ACTING ON FLOATING BODY

This paper provides the reciprocal form on wave drift force and moment from the momentum theory. The author in Japan has transformed from the pressure integration on the wetted body surface oscillating in regular waves into the reciprocal form at the near field, then transformed into the form at the far field owing to Green’s second identity, and transformed into Maruo’s and Newman’s formulas. But in this paper the start point is the momentum theory and the goal is the reciprocal form. The obtained reciprocal form at the far field can be transformed into the integration over the wetted floating body surface owing to Green’s second identity.Copyright

A Study on Efficient Methods for Estimating Load Effects and Reliability and Their Application to Preliminary Structural Design of Rectangular-VLFS: 1st Report

This paper is concerned with efficient methods for estimating load effects and reliability and their application to preliminary structural design of box-type VLFS supposed as a floating airport, which is to be put in the west area of the Inland Sea of Japan (“Setonaikai”). In this study, as the first report, structural responses in oblique waves are investigated by using the efficient method based on Weighted Residual Method combined with thin plate and shallow water approximation presented by Tsubogo (2004). Effects of wavelength and wave incident angles on characteristics of the structural response are clarified from results obtained by numerical calculation. These results can be used to evaluate the load effect and the reliability in next report.Copyright

A method for estimating strength and reliability of stiffened plates for pontoon-type VLFS

This paper deals with a simplified method for estimating strength and reliability of stiffened plates for pontoon-type very large floating structures (VLFS), which are supposed floating airport, as a part of studies on reliability based preliminary design systems. Firstly, limit state conditions are shortly presented for the buckling and ultimate collapse strength of stiffened plates under combined compression, shear and lateral pressure in the deck, bulkhead and bottom parts of VLFS, especially, by using a simplified estimation formula. Next, the validity is shown by nonlinear finite element method. Finally, the reliability level is investigated through numerical examples for a 5,000 m-class VLFS under a trial design.

A Method for Structural Design of Pontoon Type VLFS Based on Collapse Behavior and Reliability Analysis

This paper deals with a simplified method for the preliminary design of pontoon-type very large floating structures (VLFS), which are supposed floating airport, based on collapse behavior and reliability analysis in irregular waves. Firstly, a simplified estimation method is presented for the probabilistic load effect model of VLFS under irregular sea-state conditions. Next, limit state conditions are shortly presented for the buckling and ultimate collapse strength of stiffened plates under combined compression, shear and lateral pressure in the deck, bulkhead and bottom parts of VLFS, especially, by using a simplified estimation formula. Then, the validity is shown by non-linear finite element method. Finally, dominant limit state modes of 5,000m-class VLFS under combined loads with bending moment, shear force and lateral pressure are obtained by applying the above methods. Then, the features of the collapse behavior and reliability level are investigated by using above calculation results. Effects of design parameters such as yield stress, plate thickness, stiffener and bulkhead space are also investigated using sensitivity analysis.© 2004 ASME

A Study on Structural Design of Huge Barge Structures Based on Collapsing Behavior and Reliability Analysis

This paper deals with the limit state and reliability analysis of huge barge structures (HBS) which are supposed floating refuse storage and incineration plant based on collapsing behavior analysis in irregular waves as a part of studies on structural reliability-based design methods. First, a limit state and reliability analysis method is shortly presented for the buckling and ultimate collapse strength of deck, bulkhead and bottom panels of HBS. Next, a simplified method is briefly introduced for collapsing behavior and reliability analysis of HBS under extreme sea loads by using a developed system combined with a finite element method and plastic node method using hexahedral element models. Moreover, a simplified estimation method is shortly introduced for the probabilistic load model considering the hydro-elastic response of the structure in irregular waves. Finally, dominant limit state modes of 1,000m-class HBS under combined loads with bending moment, shearing force and lateral pressure are obtained by applying the above methods. Then, the features of the collapsing behavior and reliability level are investigated by using above calculation results. Effects of statistical values such as reduction of thickness due to corrosion, yield stress and design parameters are also investigated using sensitivity analysis.Copyright

A Study on Structural Response Behavior and Strength of Long Submerged Floating Structures With Tension Legs in Waves

In this paper a submerged floating structure with tension legs is dealt with, the hydro-elastic response of which in long waves is investigated as a part of basic studies on marine space utilization structures. The analytical method is applied to the submerged floating structure. Then, the hydro-elastic characteristics of a floating structure model with a submerged plate and tension legs in regular long waves are investigated numerically. Next, in order to demonstrate the validity of theoretical results an experimental study is carried out for a basic model in regular waves. It was found that experimental results for the frequency response amplitude are in good correspondence to theoretical results in the low frequency range. Finally, characteristics of hydro-elastic responses of a 1,000m-class submerged floating structure under trial design are also investigated applying the above method.Copyright

A study on reliability-based design systems of very large floating structures under extreme wave loads

This paper deals with reliability-based design systems of very large floating structures based on the dynamic response and strength analysis considering the hydroelastic wave propagation of the structure under wave-induced loads. Firstly, a limit state and reliability-based sensitivity analysis is shortly presented. Next, a simplified estimation method is introduced for the dynamic response and strength of the structure in regular and irregular waves. Then, the validity of the method is demonstrated through experimental results for a mat-type floating structure model in regular waves. Finally, the features of the dynamic behaviors and reliability-based sensitivity factors are investigated by a numerical example applied to a 5000-m-class very large floating structure under trial design.