Abstract Forced harmonic oscillations of seven configurations consisting of horizontal side by side plate elements are performed experimentally and numerically. The configurations are oscillated in vertical direction and represent generalized mudmats of subsea structures. The tests are performed for Keulegan-Carpenter (KC) numbers relevant for force estimation during lifting operations. Hydrodynamic added mass and damping coefficients are presented. The coefficients are found to be amplitude dependent for all configurations tested. The interaction effects between the plates increase with increasing amplitude and decreasing distance between the plates. For oscillation amplitudes small compared with the gap between the plates, the plates behave approximately like individual plates. A study of the relation between the damping force and the added mass force for the tested structures illustrates the importance of applying representative damping coefficients in numerical analysis of marine operations. Numerical results are obtained using a potential flow solver (BEM) and a viscous flow solver (CFD). Low-KC added mass coefficients predicted with the BEM are in accordance with the experiments. There is acceptable agreement between the CFD and the experiments. Best agreement is obtained for small KC numbers. For increasing KC numbers, the differences are, in general, larger. This is possibly due to the CFD being based on two-dimensional laminar flow.

Abstract A two-dimensional numerical analysis on the hydrodynamic force of perforated plates in oscillating flow is presented, and a new semi-analytical force model is proposed. Plates with ten different perforation ratios, τ, from 0.05 to 0.50 are simulated. The Keulegan–Carpenter numbers in the simulations cover a range from 0.002 to 2.2 when made nondimensional with the width of the plates. Resulting hydrodynamic added mass and damping coefficients are presented. All perforated plates with perforation ratios greater than or equal to 10% are found to be damping dominant. The numerical results are obtained using a two-dimensional Navier–Stokes solver (CFD), previously validated against dedicated 2D experiments on perforated plates. Furthermore, we present verification of the code against the analytical solid flat plate results by Graham. The presently obtained hydrodynamic coefficients are compared with the state-of-the-art semi-analytical method for force coefficient calculation of perforated plates by Molin, as well as the recommended practice for estimating hydrodynamic coefficients of perforated structures by DNV GL. Based on the CFD results, a new method for calculating the hydrodynamic force on perforated plates in oscillating flow is presented. The method is based on curve fitting the present CFD results for perforated plates, to the analytical expressions obtained for solid plates by Graham. In addition to its simplicity, a strength of the method is that coefficients for both the added mass and damping are obtained.

Abstract The interaction of small amplitude water waves with a pair of circular-arc-shaped thin porous plates is studied under the assumption of linear theory. The plates are submerged at different depths in deep water and the permeability of the plates varies along the circumference of the plates. Applying Green’s integral theorem to the fundamental potential function and the scattered potential function and using the condition on the porous plates, the problem is reduced to that of solving a set of coupled integral equations for the potential difference functions across the plates. Two kernels of these integral equations are hypersingular and the other two kernels are regular. An expansion-cum-collocation method involving Chebyshev polynomials is employed to obtain the approximate solution of the integral equations. The numerical estimates for the reflection and the transmission coefficients are computed utilizing the approximate solution of the aforesaid integral equations. The numerical results for the reflection and the transmission coefficients are depicted graphically for several values of various parameters. Known results for dual symmetric circular-arc-shaped porous plates are recovered as special case.

Abstract Spar is one of the promising floating structures which support wind turbine at sea. In general, some lower part of spar is used as a ballast tank. It is possible to replace this part with other shapes without water-tightness because this part does not contribute to buoyancy nor stability to support the weight or heeling moment of wind turbine. In the field of oil and gas production, truss spar has been developed with this concept and the lower part consists of heave plates. However, for wind turbine, pitching motion is more important than heaving. The authors changed this part to four vertical damping plates to reduce this motion. The effect of the central gap and holes in the damping plates were investigated because these parameters have effects on flow separation and hydrodynamic forces. The motions in waves of the new spars were compared with a classic spar of the same dimensions and stability. The proposed vertical damping plates, with central gap or holes, work to reduce the pitching motion in wide range of wave frequency. Considering the expected cost reduction and some motion reduction, the promising nature of the proposed spar revealed.

Abstract In the present study, the hydrodynamic performance of a novel and efficient concept of a floating Oscillating Water Column device has been investigated. The new concept consists of two chambers that are placed in the upstream (fore chamber) and in the downstream (rear chamber) with respect to the incident wave direction. The rear chamber acts mainly similar to a Backward Bent Duct Buoy system, while the design of the fore chamber follows conventional types of Oscillating Water Column systems with the harbour plates (bottom plate as well as side plates) elongated outside of the fore chamber. The primary efficiency of the devised concept has been investigated in the frequency domain. In this context, to solve the corresponding diffraction and radiation problems due to the influence of the air pressure inside the chambers as well as motions of the body, an in-house code has been developed in 2D using the Boundary Element Method based on linear wave theory. The obtained numerical results show that the introduced concept has advanced hydrodynamic efficiency in a broad range of waves.

Abstract Application of steel plates with highly ductile is beneficial for ship owners, in order to improve structural redundancy and prevent environmental burden in accidental condition such as collision or grounding. Highly ductile steel, in which breaking strain is improved by 20% in comparison with conventional steels has developed, by optimizing microstructure. Excellent ductility was achieved selecting ferrite-perlite dual phase structure of ferrite phase which is advantageous for stable actual production. Morphology of ferrite phase control which consists of optimization of volume fraction and strengthening of ferrite phase itself by adding Si plays an important role to enhance ductility. The highly ductile steel of YP315 grade had acquired Class approval, and for ships to which high ductile steels are applied, ClassNK would assign the notation “Hull Protection by Highly Ductile Steel” (HP-HDS). MITSUBISHI SHIPBUILDING has evaluated the effect of highly ductile steel by numerical simulation for LPG (Liquefied Petroleum Gas) Carrier owned by Astomos Energy Corporation and Iino Kaiun Kaisha, Ltd. It has been confirmed that highly ductile steel can improve the structural redundancy by 20% in accidental condition by being applying to side structure.

Abstract Fatigue crack growth is a major challenge to the structural integrity of steel structures. In technical practice, surface cracks are of great importance since cracks in components and structures often exhibit this geometry. Fiber-reinforced polymer (FRP) strengthening technology is a reliable technique to repair cracks in steel structures. Yet the investigation on FRP repairing surface cracks in steel structures is lacking. What’s more, the crack growth might cause crack-induced debonding at the interface of FRP reinforcement, generating negative effects to the reinforcement effectiveness. Unfortunately, there are limited studies in the open literature for this issue. In this paper, we conduct the investigation on surface crack growth in steel plates reinforced with Carbon Fiber-reinforced polymer (CFRP) under tensile load. Three-dimensional finite element models are built to predict the stress intensity factors of the surface cracks. The crack-induced debonding is considered in the finite element analysis by introducing the cohesive zone model and a bond failure criterion. In accordance with Paris law, surface crack growth rate of different models are predicted. The influential parameters of crack-induced debonding are analyzed by means of parametric studies. The results indicate that CFRP reinforcement could significantly decrease the surface crack growth rate, while the crack-induced debonding might generate negative effect on CFRP reinforcement. In addition, the crack-induced debonding is affected by not only the interfacial properties, but also the reinforcement scheme, such as thickness of the adhesive layer, CFRP layer number and its elastic modulus, and the depth of surface cracks.

Abstract The challenges of performing full-thickness fracture toughness tests on steel plates of 100mm thickness and greater means that the use of sub-size specimens is desirable. In this work, 100mm thick parent plate of S690 high strength steel was characterised using SENB fracture toughness specimens with thickness of 12mm, 25mm, 50mm and 100mm. Sub-size specimens were extracted at two different locations through the plate thickness; mid-wall and quarter wall. Sufficient specimens were tested to apply the Master Curve method in ASTM E1921 to predict the behaviour of 100mm thick material from each set of sub-size specimens. The through-thickness microstructural variation in these heavy-wall steel plates meant that significantly different predictions of full-thickness fracture toughness were obtained from the two sampling locations. However, when sampled from the mid-wall location, sub-size specimens down to 25mm thick were able to conservatively predict full-thickness fracture toughness using Master Curve methods.

Abstract The exploration of oil & gas fields towards more demanding environments require the continuous development of line pipe steels capable to withstand severe working conditions (high pressure, high temperature, sour environment, large deformations) maintaining good weldability in terms of hardness control in the Heat Affected Zone (HAZ). Therefore, the response to welding thermal cycles needs to be taken into account since the first steps of the product development. This paper describes a methodology to ascertain the weldability of new line pipe steels. A matrix of welding trials, based on different welding technologies, was designed; the manufactured welds were assessed in terms of hardness and microstructure using standard and non-standard approach. Gas Tungsten Arc Welding (GTAW) cold wire technique with anthropomorphic robot was selected as principal process, being extremely robust, accurate and repeatable and able to reproduce with good accuracy the thermal field of different arc welding operations. Furthermore, GTAW process permits to manage separately the so called “Arc Energy” (Volt*Ampere) and wire feeding: this additional degree of freedom gives the possibility to produce different combinations of Heat Inputs and Diluting Ratios. Single and multi-pass bead on plates (WT∼40mm) with no pre-heating were used to promote high restrained configurations and high residual stresses to reproduce the worst welding condition. Furthermore bead on plate have been performed by Nd:YAG laser to generate fastest cooling rates and narrowest welding pools. The investigation activity was performed according to the API RP 2Z for what concerns the characterization of the main Heat Affected Zone both for single and multi-pass. For the individuation and investigation of the HAZ sub-zones a thermal model has been selected, validated by dedicated instrumented welding activity and used to predict the thermal cycle in selected points/zones of the HAZ (e.g. CGHAZ, FGHAZ...). In depth metallographic/hardness analysis was performed on the welded samples using non-standard methods: low, medium and high magnification optical microscope analysis to generate a complete morphological map, suitable hardness paths with low and standard loads along the main direction of heat propagation in the bead and “tint etching” to better individuate the phase fractions of the different sub-zones of the HAZ. Finally, proprietary and commercial predictive tools, using different input parameters (e.g. heat input, cooling rate, composition...) were used to calculate hardness and phase fractions under the experimental operative conditions. Results were compared to those obtained by the experimental activity.

Abstract The present paper aims at obtaining insights on the strength behaviors of ship hull plates under uniaxial cyclic loads. The residual ultimate strength of ship hull plates under uniaxial cyclic load is studied by numerical analysis using nonlinear finite element method. A series of tests are also conducted for comparison and verify the reasonability of numerical results. The accumulated plastic damage and crack propagation from the low cycle fatigue damage during cyclic load are both considered in present study. From the numerical results and tests some significant conclusions are drawn to the residual ultimate strength of plates under uniaxial cyclic loads.

Abstract In recent times, drag-based vertical-axis wind turbine rotors have gained increasing interests in offshore applications because of their performance potential and reliability. Their advantages like simplicity, easier manufacture and lower maintenance cost have attracted the researcher’s attention toward improving their design further. However, this type of rotor is still suffering from lower efficiency than the lift-based Darrius and the horizontal-axis wind turbine rotors. A recently developed elliptical-bladed Savonius rotor has shown its potential to harvest wind energy more efficiently. However, the geometric parameters of this rotor such as aspect ratio, overlap ratio, number of blades, shaft and end plates, the aerodynamic parameters such as Reynolds number, lift and drag coefficients are needed to be optimized for further improvement of its performance. In the present investigation, the wind tunnel tests have been conducted to analyze the effect of shaft and end-plates of a newly developed elliptical-bladed vertical-axis Savonius wind turbine rotor. Experiments have been conducted over a range of tip speed ratios to find the torque and power coefficients of a two-bladed rotor system for two individual cases viz., the rotor with a shaft and the rotor with end-plates. In order to have a direct comparison, the experimental data are also obtained for the same rotor without the shaft and without the end-plates. The wind tunnel tests have demonstrated an improvement of power coefficient by 26.31% for the rotor with the end plates.

An investigation has been carried out based on multi-domain boundary element method to analyze the mitigation of wave-induced hydrodynamic loads on a pair of floating rectangular bridges by retrofitting the structures with external porous plates. The study is based on the assumptions of small amplitude water wave theory in finite water depth with the characteristics of wave-body interactions remain unaltered along the bridge. Wave past porous structure is modelled using Darcy’s law. Various hydrodynamic characteristics are studied by analyzing the wave forces acting on the floating bridges and the retrofitted porous structures for different wave and structural parameters. With the introduction of a retrofit, the horizontal force on the bridge reduces irrespective of wave and structural parameters, whilst vertical force increases under certain conditions. Moreover, when the distance between the bridges is an integer multiple of half of the wavelength of the incident waves, both the bridges experience optima in horizontal and vertical wave forces, with both these forces being 180° out of phase. The present study is expected to be useful in the design of efficient bridge structures which will reduce wave-induced hydrodynamics loads on the structure and thus enhance the service life of floating bridges.

A coupled SPH-FEM model is established to study the hydroelastic interactions between waves and submerged horizontal plates. The fluid motion is governed by the continuity equation and Navier-Stokes (NS) equations, which are solved based on the smoothed particle hydrodynamics (SPH). The solid boundary condition is implemented with the dynamic boundary particles (DBPs), and the multi-layer DBPs technique is used to describe the thin plate. As for the structural deformation, it is governed by the structural dynamic equation and solved using the finite element method (FEM). Four-node square elements are used for mesh generation and the Newmark scheme is employed for time integration. To couple these two models, a novel attachment algorithm for the fluid-solid interface is proposed, and the Dirichlet–Neumann boundary condition is enforced in the weak coupling regime. The accuracy and robustness of the coupling model are checked through a numerical test of a hydrostatic water column on an elastic plate. Then the model is used to compare the hydroelastic performances of submerged horizontal plates with different rigidities. An equivalent conversion method is introduced to avoid using different spatial resolutions and excessive computational resources in the comparison. On this base, some valuable conclusions that have not been previously reported are drawn.

Welding is one of the most used joining methods in the ship industry. However, residual stresses are induced in the welded joints due to the rapid heating and cooling leading to inhomogenously distributed dimensional changes and non-uniform plastic and thermal strains. A number of factors, such as welding speed, boundary conditions, weld geometry, weld thickness, welding current/voltage, number of weld passes, pre-/post-heating etc, influence the residual stress distribution. The main aim of this work is to estimate the residual stresses in welded joints through finite element analysis and to investigate the effects of boundary conditions, welding speed and plate thickness on through the thickness/surface distributions of residual stresses. The welding process is simulated using 3D Finite element model in ABAQUS FE software in two steps: 1. Transient thermal analysis and 2. Quasi-static thermo-elasto-plastic analysis. The normal residual stresses along and across the weld in the weld tow region are found to be significant with nonlinear distribution. The residual stresses increase with the increase in the thickness of the plates being welded. The nature of the normal residual stress along the weld is found to be tensile-compressive-tensile and the nature of normal residual stress across the weld is found to be tensile along the thickness direction.

Simplified two-dimensional models, representing components of complex subsea structures, are experimentally investigated. Individual as well as combinations of components in different configurations are tested, in order to study the effect of hydrodynamic interaction. The components include porous plates and cylindrical pipes with circular cross-section. Hydrodynamic added mass and damping coefficients, relevant for force estimation during lifting operations, are presented. The coefficients are obtained based on forced oscillation tests for a large range of Keulegan–Carpenter (KC) numbers and forcing periods, and compared to numerical source panel results for the low KC limit, as well as recommendations given by DNV GL, where relevant. Coefficients for all configurations are found to be highly amplitude dependent. Significant interaction effects are found for the assembled structures, causing either reduced or increased total added mass and damping coefficients compared to the super-position of the coefficients for individual members.

This paper presents the experimental tests of a 16th-scale model of the Spar-buoy oscillating water column (OWC) carried out at a large scale wave flume. The model is slack-moored to the flume floor by two mooring lines. The turbine effect is replicated using calibrated orifice plates. The device six degree-of-freedom motion, inner free surface displacement and air chamber pressure are measured. The influence of wave height, turbine damping and mass distribution on the system dynamics is analysed for regular waves. An in-house developed numerical time-domain model is used to simulate the motion and power absorption under the same wave conditions as the physical model tests. The formulation considers linear hydrodynamic forces, viscous drag effects and drift forces. The floater is assumed to oscillate in six degrees of freedom and the OWC can move vertically in the tube. The mooring system is simulated using a quasi-static model. The comparison between experiments and numerical simulations shows a good agreement for wave periods outside the zone where parametric resonance in roll and pitch occurs.

There is an increasing interest in the lightweight design of ship and offshore structures, more specifically, choosing aluminum alloys or other lightweight high-performance materials to build structure components and ship equipments. Due to its better mechanical properties and easy assembly nature, extruded aluminum alloy stiffened plates are widely used in hull structures. When the load on the hull reaches a certain level during sailing, partial or overall instability of stiffened plate makes significant contribution in an event of collapse of the hull structure. It is very necessary to investigate the ultimate strength of aluminum alloy stiffened plate to ensure the ultimate bearing capacity of large aluminum alloy hull structure. Most of studies of the ultimate strength of stiffened plates deal with stiffened plates with T–shaped stiffeners. Stiffeners of other shapes have seldom been explored. In this research, the ultimate strength of six different cross–section aluminum alloy stiffened plates and one steel stiffened plate was studied based on the non–linear finite element analysis (FEA). Taking into account stiffness, weight and other issues, the new cross–section aluminum stiffener has finally been concluded for replacing the original steel stiffener in upper deck of a warship.

Ships and offshore structures are often exposed to various types of repeated impact loads, such as wave slamming, floating ice impacts and ship collisions which will cause large deformation or even fracture. With imperfections due to the process of construction or damage caused by accidents, the load carrying capacity of structures will decrease. This paper investigates the load carrying capacity of aluminum alloy plate with an initial crack under repeated impact loads by means of experiments and numerical simulations. In the experiments, the prepared specimens with crack and without crack are impacted repeatedly up to plate perforation by releasing a hemispherical-headed cylindrical hammer. Numerical simulations are carried out with ABAQUS/Explicit software. The numerical models are built according to the actual experimental conditions. Comparison of the numerical predictions with the experimental results shows reasonable agreement. It is found that aluminum alloy plates under repeated impacts are sensitive to initial cracks. The fracture mode and plastic deformation of aluminum alloy plates can also be affected.

For evaluating reliable load carrying capacity of actual hull girder, experiments for similar scale model are necessary. The object of the present paper is to figure out a reliable FE analysis method in the similar scale model experiment regarding hull girder ultimate strength. The compared results between the true hull girder of a typical ultra large container ship (ULCS) and the scale experimental model created by the similarity criterion proposed in this paper assessed by finite element method (FEM) under longitudinal bending moment considering the effects of initial deflections are obtained guaranteeing the similarity in both elastic and inelastic range during the progressive collapse of plates, stiffened panels and hull girder. Finally, a series of elastic-plastic large deflection analyses is conducted to ensure the failure mode of hull girder is consistent with the actual ship.

A series of experiments were carried out with a flat plate towed normal to the flow in quiescent fluid. The focus was given to the analysis of the drag force seen by the plate as a function of its aspect ratio and hydraulic diameter. The effect of towing the plate near the water free surface was also investigated thoroughly. Plates of aspect ratio ranging from 0.25 to 4 were towed in a still water tank at different Reynolds numbers in the range from 15000 to 60000. Submergence depth was measured from the upper edge to the free surface and varied from zero to the centre of the tank. Forces on the plates were measured using a submersible bending beam load cell and the carriage motion was monitored by a rotary potentiometer. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this effect being more dominant in lower aspect ratio plates. The abrupt rise in the drag is due to the interaction of the upper edge of the plate with the free surface resulting in a large shrinkage of the recirculation zone. The non-unit low aspect ratio plates also showed another drag peak around 50% depth, especially at lower speeds. Overall, the trends were Reynolds number independent, except when the aspect ratios was in the range from 0.75 to 1.33 and the plate was near the free surface.