Side Shell Research Articles

Spectral fatigue damage calculation in the side shells of ships, with due account taken of the effect of alternating wet and dry areas

A method for calculating the wave load induced fatigue damage in ship side shells in connections between longitudinals and transverse web frames is presented. The method is based on the Miner–Palmgren fatigue damage formulation and a spectral approach is used to determine the necessary variances of the stress processes. The traditional linear frequency domain analysis is expanded in order to incorporate the non-linear nature of the external water pressure. The pressure process is determined by use of pressure RAOs, and not as previously by relative motion RAOs. Effects of load combinations are included in the method, both combinations of the global vertical and horizontal hull girder bending and combinations of the combined global bending with the local pressure induced load process. The proposed method is used to analyze a double hull tanker, and the results show reasonable agreement with results published by other authors. Effects of load combinations are shown to be significant, and especially it is confirmed that the external pressure is very important to include in the analysis as it accounts for the majority of the calculated damage.

Fatigue Reliability of Maintained Welded Joints in the Side Shell of Tankers

A formulation is presented for the assessment of the reliability of a local ship hull structure regarding fatigue failure. The potential cracks are considered to occur in the side shell, in the connections between longitudinal stiffeners and transverse web frame. The analysis is performed for T-shaped stiffeners. The model accounts for the crack growth process applying linear elastic fracture mechanics. The long-term stress range acting on the elements is defined as a function of the local transverse pressure of internal cargo and outside water, combined with the stresses resulting from the longitudinal bending of the hull. The global hull loading is a combination of horizontal and vertical bending moments. The present paper analyzes the importance of including the transverse pressure loading for the process of the crack growth and also the distribution of the net section area of midship and location of stiffeners. The formulation has been applied to a tanker. The fatigue reliability is predicted by a time variant formulation and the effects of maintenance actions in updating the reliability assessment are shown.

On the Shear Buckling Collapse of Continuous Rectangular Panels

It is important that ships are prevented from fatal failures such as the hull girder collapse under severe sea conditions. On the other hand it is also important to know the reduction of the strength of a ship with lightening caused by the constant demand for lighter ship hulls. Severe shear force acts in the side shell near the bow of the ship during slamming conditions. Thus it becomes important to investigate not only compressive behavior of the deck panels but also shear collapse behavior of the side shell panels to determine the collapse behavior of the ship's hull girder.Simply supported and uniform shear load boundary conditions are suitable to examine the plate behaviour until the shear collapse load. But strength evaluation considering the continuity between side shell panels is needed to analyze the load carrying capacity accurately during the post shear collapse.In this paper it is intended to solve only one panel considering the inplane and rotational continuity between panels as the boundary condition instead of solving many panels to investigate the panel behavior of the post ultimate strength region, such as the load carrying capacity under shear.

Structural Failures of Bulk Carriers

Structural failures of bulk carriers continue at a steady and unacceptable rate, claiming some 150 lives per year. This paper explains why bulk carriers are vulnerable to structural failure either because side shell plating falls out allowing ingress of seawater or the deck cracks open. Inadequate detail design, material without guaranteed fracture toughness and limitations of current inspection programmes are identified as fundamental weaknesses. The paper includes important information from a case study of six oil bulk ore carriers prone to deck cracking. One of the group of six which sank in 1980 was traced 4200 metres down on the floor of the Pacific Ocean in June 1994 and the results of this initial search are reviewed. There is a genuine need to go back to basics. Designing from first principles to determine local stresses is an important first step rather than the current use of prescriptive rules. All subsequent steps in bulk carrier construction and maintenance should be subject to a penetrating review.

A simple formulation for predicting the ultimate strength of ships

The aim of this study is to derive a simple analytical formula for predicting the ultimate collapse strength of a single- and double-hull ship under a vertical bending moment, and also to characterize the accuracy and applicability for earlier approximate formulations. It is known that a ship hull will reach the overall collapse state if both collapse of the compression flange and yielding of the tension flange occur. Side shells in the vicinity of the compression and the tension flanges will often fail also, but the material around the final neutral axis will remain in the elastic state. Based on this observation, a credible distribution of longitudinal stresses around the hull section at the overall collapse state is assumed, and an explicit analytical equation for calculating the hull ultimate strength is obtained. A comparison between the derived formula and existing expressions is made for largescale box girder models, a one-third-scale frigate hull model, and full-scale ship hulls.

Fatigue damage in the side shells of ships

More than 40% of the registered fatigue cracks in ship structures are observed to occur in the side shell, more specifically in the connections of longitudinals to transverse web frames. The fatigue damage is caused partly by vertical and horizontal wave-induced hull bending and partly by outside water pressure on the side shell. Due to the non-linear nature of the outside water pressure the fatigue damage of the combined stress cannot be solved via a traditional frequency domain analysis. This paper solves the combination problem by application of a realistic wave model. The proposed model is used to analyze a segregated ballast tanker, and the results are compared to previously registered fatigue cracks. The analysis shows that it is very important to include the water pressure in the fatigue analysis, as this accounts for the majority of the expected fatigue damage. The analysis is performed for both I-shaped and L-shaped stiffeners.

Strength of Double Hull Afra-Max Tanker

FEM analysis is carried out to clarify the strength of double hull Afra-Max tanker which has neither center line longitudinal tight bulkhead nor swash bulkhead. And following results are obtained. The depth of double bottom is most important item on the design viewpoint of double hull Afra-Max tanker, and the deformations at bottom and side shell of double hull Afra-Max tanker are larger than those of ordinary single hull tanker, The bottom girder and side stringer are effective to reduce the deformation of double bottom and side shell, especially to reduce the relative deformation between the bulkhead and the adjacent transverse web. And the side stringer already attached to double hull tanker is very useful to reduce the stress in transverse web. The shearing stress is dominant at the point where large von Mises equivalent stress is calculated in transverse web, which can be reduced by using thicker plate.

On the Characteristics and Long-term Prediction Procedure of Wave-induced Pressure Flutuation on a VLCC Hull

There has been much discussion on the characteristics of wave-induced pressure fluctuation on the side shells of VLCCs since a considerable number of fatigue cracks have been found on side- longitudinals of second generation VLCCs. The main theme of the discussion is whether the characteristics of pressure fluctuation are predictable even for small values of wave-length to ship-length ratio such as 0.1-0.3 and how peak-to-trough value can be obtained for wave-induced pressure fluctuation on the side shell close to the load- water-line. Tank experiments on a scale model ship were carried out to answer these questions, and it was shown that the measured results agree with the results calculated by the strip method after a simple correction on non-linearity effect caused by the crossing of the free-surface. Results of long-term prediction with the same correction also show good agreement with the results of the full-scale measurements.

A Study on Structural Reliability of Large Bulk Carrier Based on Collapse Mode Analysis

A new system for plate structures using spatial membrane elements was developed to offer the expansion of the applicablilty of the structural system reliability presented by the authors in previous studies. In this paper, the features of the structural reliability of a large bulk carrier (150,000 DWT) based on collapse mode analysis are investigated by applying the newly developed system. From the analytical results for the spatial plate structures idealized for modelling the midship part and fore/aft L/4 parts of the ship hull structure under towing conditions, the main conclusions are: (1) New formulas of the compressive strength are derived for the longitudinal and transverse frame system panels under in- plane compressive and shearing stresses and lateral pressure, and the effects of the shearing stress and lateral pressure on collapse modes and the system reliability are investigated. (2) Effects of the shearing stress on the dominant collapse modes including damages of side shell elements and their occurrence probabilities in the fore/aft L/4 parts of the ship hull structure are more remarkable than in the midship part. (3) It is also pointed out that results for deteriorated structures considering the notional corrosion damage referred to the class NK report are in comparatively good agreement to the features of structural damage due to corrosion at fore/aft parts of aged bulk carriers.

On modelling of thin-walled girders and accuracy of vibration analysis performed by the finite element technique related to ship structures

An analytical solution of a pontoon flexural vibration and vibration of its substructures, i.e. membrane side plating and shell deck-bottom plating, is presented, and is used to evaluate the reliability of the finite element method and show the importance of rational modelling. The use of one-, two- and three-dimensional models, and the application of direct, substructure, superelement and modal synthesis techniques, for ship structures, is discussed.

Longitudinal Strength of The Fore Body of Ships Suffering from Slamming

It has been reported that not a few ships were fatally damaged at their fore bodies due to heavy slamming among rough seas.A series of model tests has been conducted to study the mechanism of failure and to evaluate the ultimate longitudinal strength of the fore bodies of hulls by applying forces simulating impact load due to slamming. Three types of structural models of fore bodies were tested they were designed to be similar to a 50, 000 DWT bulk carrier, a 120, 000 DWT ore carrier and a 1, 400 TEU containership with scale ratios from one sixth to one eighth. In all cases, their side shells were firstly observed to yield due to heavy shear stress. Then the yield zone spread throughout the side shells and resulted in a progressive rise of the longitudinal stresses in the deck plating, which might eventually cause buckling. In the case of bulk carriers and ore carriers with U-form sections, the collapse was brought about immediately after buckling occurred in a whole section of the deck structure. In the case of containerships with V-form sections, the spread of yielding throughout the side shells led to the collapse.This paper also presents an analytical method to predict the ultimate strength of the fore bodies of hulls subjected not only to bending moment but also to heavy shearing force. Compared with the experiments, the calculation method is found to be effective to estimate the final stage at which a section of the hull collapses along the whole circumference and it does not carry any more load. The yielding throughout the side shell plays a fatal role because it gives rapid increase to the deck stresses as well as brings about the loss of shear rigidity.

Bottom Slab Strength of PC-Barge while Alighting on Sea Bed

The strength of bottom slab of PC-barge under the operation of alighting on the sea bed was studied experimentally and theoretically. Large cone penetration tests were performed on three model slabs. The penetration-resistance curves were obtained. The initial destruction mode was the punching shear, and then the concrete crashed. Before crashing out of the concrete, the punching shear strength of JACE's formula was in good agreement with the resistance. After crashing out of the concrete, the resistance was generated from tensile forces of reinforcements and prestressing strands.The bottom slab strength against the local contact forces of the foundation soils, the side shell strength against the local ice pressure loads, and the strength against the ship collision were discussed. The local contact forces of the foundation soils were too small to cause damage to the bottom slab. However, the local ice pressure loads were very huge, so that it was almost impossible to design the side shell free from damage. The steel-concrete sandwich wall would be the better structural element against the local ice pressure loads than the PC-slab dealt with in this paper. The strength against the ship collision was evaluated with the energy absorption capability. The absorbed energy of the PC models were much more than that of the steel models.

In Case of a Bulk Carrier

Shear lag in the cross section of a bulk carrier will be discussed in this paper. The hull girder of a bulk carrier consists of multi-cellular box girder. As to the single cell box girder, the approximate methods of calculation were proposed by Kuranishi M. and Miyashita M. and by Reissner E., independently.To apply these methods to the hull girder of a bulk carrier, the former is preferable to the latter because of the flexibility in the mathematical formulation of its warping function.At first, authors developed the former method to the multi-cellular box girder. And then, after some numerical calculations on the bulk carrier of 60 thousands Dead Weight tons, in the condition of loading alternatively in every other hold, the following results were obtained : (1) In girthwise, the warping and the warping normal stress due to shear lag become greater in the bilge part and in the double bottom plates near the ship centre line than other part, mainly. As to the warping. shear flow, it acts so as to decrease the maximum value of shear flow and to make smooth the 'distribution pattern in the side shell.(2) In spanwise, the warping and the warping normal stress and shear flow due to shear lag are much affected by the magnitude and distributing pattern of the shearing force acting on the hull girder.

A Simplified Method to Analyse the Strength of Double Hulled Structures in Collision (3rd Report)

The effects of various design parameters on the strength of double hulled structures in collision were examined. At first, the scantlings and the strength characteristics of double hulls were investigated, which included double sides and double bottoms. Static destruction tests were carried out using a series of double hulled models which covered a wide strength distribution range. The validity of the simplified method to analyse the strength of double hulled structures in collision which was presented in the previous paper was confirmed by comparing the calculated results and the experimental results. Finally, choosing a double hulled ship of 100, 000 tons displacement as a standard type ship, parametric calculations were done using this method, where the side shell thickness, the transverse web thickness and the double hull breadth were taken as design parameters. It was found that the most effective method to increase the energy absorption capacity is to increase the transverse web thickness.

Fracture Characteristics and Reliability Analysis of Side Shell to Annular Fillet Weld Joint in Flat Bottom Tank

In order to study the fracture behavior at the side shell to annular fillet weld toe in the flat bottom oil storage tank, the burst test of model tanks has been carried out by hydraulic internal pressure. It is clarified from the results that the initiation of the ductile crack is the critical event in the fracture of the tanks, and the ductile fracture toughness of materials and the geometry of the weld toe have the significant effect on the fracture.So far, L-shape bend test has been performed and the effect of the geometry of the weld toe and mechanical properties was formulated. In the present study, the fracture criterion of fillet weld toe in the tank is derived by modifying that formula based on the crack initiation characteristics.Finally, the reliability analysis has been carried out by means of Monte-Carlo simulation. The probability distribution for the weld toe geometries is assumed to be normal and Weibull function based on the data measured in the real tanks. The ductile fracture toughness δi is evaluated using two and three parameters Weibull distribution functions. The reliability of tanks are disscussed from these results.

A Simplified Method to Analyse the Strength of Double Hulled Structures in Collision

The strength of ship structures in collision is an important matter in terms of saving life, preventing pollution and from an economic point of view, especially in the case of hazardous cargo carriers, oil tankers and so on. So far, several analysis methods or formulae have been presented to estimate the collision strength of such ships. However, there are few papers in which the accuracy of the methods have been verified.In this paper, the collision strength of a typical double hulled structure was examined. The types of collisions were classified into five groups from a geometric view point between a colliding bow and a hull. Two kinds of experiment (which were considered to be more critical than others among those five groups), were carried out using large scale models. The first was an experiment in which a stem collided against the ship's side and in the other a bulbous bow collided against it. In both cases, the bow models were assumed to be rigid.A simplified method to analyse the damage of double hulled structures in collision was developed on the basis of the results from the experiments. As far as this method, is concerned, a double hulled structure is considered to consist of three main structural members, the side shell, trans web frames and side stringers. The side shell is modelled into plastic membrane plates, and trans webs and side stringers are assumed to support the side shell rigidly and buckle when critical load is imposed.The method was formulated in matrix form, where triangular membrane elements and buckling elements are used to model a double hulled structure.The accuracy of the analysis method was then examined by comparing the calculated values with the experimental ones. From these, a strong correlation emerged.

Development of New Steel and Welding Procedure for Hull Structure of Ice Breaking Vessels

Recently, it has been very active to develop various types of marine structure and ice breaking vessels in order to exploit energy resources in ice fields. These steel structures are to be operated in severe environments of low temperature in the arctic area. Expecially, ice breaking vessels are required to navigate smoothly with breaking ice floe, thus the side shell and the deck structure should be fabricated of heavy plating with a good notch toughness.To obtain excellent notch toughness and mechanical properties, the authors introduced the new steel manufacturing process, namely CLC process and have successfully developed 50 kgf/mm2 class high tensile strength steel of heavy thickness.Then, experiments associated with fabrication, welding procedure and joint performance were carried out for this newly developed heavy section steel to study its applicability to hull structures of ice breaking vessel. It was consequently made clear that this steel could be welded by conventional welding procedure and showed a satisfactory performance which could assure the safety to be required for ice breaking vessel.

Analysis of Structural Damages of the Fore Body of a Container Ship due to Slamming

A container ship suffered serious structural damage in her fore body due to slamming in heavy seas in the North Pacific Ocean in January, 1977. She was in the fully laden condition and voyaged to Kobe from Oakland, California, U. S. A. A long brittle crack propagated in the fore and aft direction in shell plating of the flare part on the starbord side. The inner structures, such as longitudinals, web frames and side stringers, on the side shell of the same side were collapsed, and deck and shell plating on the port side was buckled. Furthermore, No. 2 Hatch was distorted in a parallelepiped. In the present paper, the damages are investigated from the view point of ship hydroelasticity with the aid of fracture mechanics. The results obtained suggest the importance of ship handling as well as structural design for preventing serious damages in large container ships.

On the Vibration of Shell Structure

A great deal of fatigue damage due to severe vibration arises at after ship body, because there are some exciting sources. In order to avoid the resonanse between the ship structure and the exciting forces, it is necessary to estimate exactly the natural frequency of ship structure. The side shell structure of after ship body which recieves directly the propeller surface force is not the flat plate as a general rule, therefore it is not easy to calculate the natural vibration of side shell structure.In this paper, a method to calculate the natural vibration of rectangular and trapezoid shallow shell was presented. And the results by present method were compared to the results by the finite element method. The natural vibration of shallow shell can be fairly good estimated by using the present method, but the magnitude of maximum initial deflection which this method can be applied to is about 20% to the side length. It was also clarified that the plate thickness-maximum initial deflection ratio is one of the parameter which decides the natural frequency of the shallow cylindrical shell. According to the calculated results for the trapezoid shell, in the case that the upper side-lower side ratio c/a is about 0.91.0, the natural frequency is almost equal to that of rectangular shell.

A new concept of a hull form to improve wave response characteristics for marine construction barges

The object of the new hull form is to provide a single hull which possesses long natural periods of roll and heave and has substantially reduced motion response amplitudes in very high sea states. Model tests and preliminary estimates indicated that the new hull form can be designed for roll and heave motions nearly equivalent to those of much larger semisubmersible units. All existing conventional marine construction barges have rectangular cross section hull. The new hull form consists of a system of upper side tanks and lower side tanks added onto a rectangular cross section hull. The upper tanks and lower tanks form longitudinal troughs on the port and starboard sides. Structural grillage of any open type is to connect the upper and lower tanks at the side of the vessel. Figure 1 indicates a profile and a typical transverse section of the new hull form. The new hull comprises the concept of reduced water plane area which is turn results in low transverse metacentric height and low tons per in. immersion. The novel features of combining low GM T and low TPI with extremely heavy damping and added mass of the entrained water characteristics result in very long natural periods of roll and heave and considerably small rolling and heaving amplitudes in high sea states. The open side shell plating on the side of the vessel functions to dissipate wave energy at the side of the vessel which would have otherwise been transmitted to the vessel and caused the vessel to respond. This paper presents the conceptual foundation and outline of the new hull form. Model test results are presented and implemented. Also presented is the design philosophy.