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Two up-to-date composite riser solutions, specifically, the thermoset and thermoplastic based ones are studied in a global-local framework to examine their technical feasibility for offshore applications. At the local scale, the riser pipe body with multi-layered structure is homogenized through an analytical approach, obtaining equivalent material properties of the riser string model for the global analysis. The metal-composite interface (MCI) end fittings between the riser body and standard connector are examined using finite element (FE) simulations to understand their load-transfer mechanisms for both solutions. At the global scale, an in-house analysis code is performed to study the risers' static and dynamic response in a certain sea condition due to top-tension, current drag and vortex induced vibration (VIV). The analyzed parameters include the resulting riser deflection, axial stress, tension and bending moment distributions. Following that, an integrated analysis utilizing the global results and the local failure envelopes is performed to determine if the maximum stress states of the riser exceed the equivalent structural strength boundary. It is concluded that composite risers exhibit a larger safety margin than traditional steel riser under the same sea condition.

In this paper, a simulation method for predicting global and local hydroelastic response of a ship which couples CFD and FEA is developed and validated. By comparing the prediction with linear/nonlinear strip method, 3D panel method and towing tank test results under various wave conditions in terms of rigid body motions and slamming impact pressure, the effectiveness of the CFD-FEA coupling method is confirmed. The hydroelastic behavior, so-called the whipping moment, evaluated from the CFD-FEA coupling method is further validated by comparing with nonlinear strip method, 3D panel method and tank test results. Finally, the proposed method is applied to a realistic large container ship structure in severe waves. The structural response of the double bottom structure subjected to global and local bending moments is investigated.

This paper addresses collapse behaviour of Very Large Floating Structure (VLFS) under extreme vertical bending moment. The study is performed as part of risk analysis to evaluate the consequence of a rare collapse event. Elastic and plastic deformation of VLFS develops under the extreme vertical bending moment while the deformation interacts with the fluid around VLFS. Therefore, hydro-elastoplasticity analysis need be developed taking account of these effects. In this study, the whole VLFS structure is modeled as two elastic beams with an elasto-plastic hinge embedded at the connection. The structural deformation is formulated by using finite element method (FEM). The hydrodynamic behaviour is modeled by using Rankine source panel method in time domain based on two-dimensional potential theory. The two models are coupled. A series of scaled model tests is performed for validation. Simulation and tank test results for the basic collapse behaviour of VLFS are presented and discussed. The numerical method is applied further to a realistic VLFS design to clarify its collapse behaviour.

As current oil reserves start to deplete, companies are looking to exploit deeper deposits. At these greater depths composite risers, with their high strength-to-weight ratio, reduce the effective tensions and bending moments compared to steel risers. However, there is still limited research into their behaviour, with one key missing element being a comparison with traditional riser designs which accounts for variances in material properties and wave loads. This paper therefore conducts a strength-based reliability analysis of composite catenary risers operating between 1,500 m and 4,000 m. A static global catenary model is combined with Classical Laminate Theory to determine the extreme response and its performance is verified against FEA. This response is evaluated with the Tsai-Wu failure criterion to determine first-ply failure. The effect of laminate moisture absorption on the long-term reliability of submerged composite-based risers is also investigated as it can cause a significant reduction in the strength of composite risers. The reliability analysis is conducted using the Monte Carlo Method, revealing that the composite risers perform well at 4000 m. The degradation in performance from moisture absorption becomes increasingly important at greater depths and needs further investigation for these applications.

Steel stiffened panels are widely used in engineering design and construction. However, numerical modeling and analysis effort for a three-dimensional (3D) stiffened panel may be notable, especially for the ultimate limit state of ship structures. Therefore, a homogenization method is outlined that transforms 3D stiffened panel into an Equivalent Single Layer (ESL) concerning the same mechanical behavior. ESL stiffnesses are obtained with a unit cell analyses based on stiffened panel where periodicity is imposed with boundary conditions based on a first-order shear deformation theory (FSDT). Stiffnesses were determined from the first derivative of a membrane force and bending moment obtained with numerical simulations. The effect of initial imperfection shape was included in the analysis to account for local and global buckling behavior. ESL with non-linear stiffness was implemented in Abaqus UGENS subroutine, allowing incremental evaluation of stiffness. Ultimate strength prediction of a steel grillage model with ESL finite element analysis was in excellent agreement with detailed 3D FEM analysis. The key in this analysis was consideration of non-linear ESL stiffness as linear analysis was unable to detect the point where ultimate strength capacity of the grillage was reached.

In the present paper, the Local Joint Flexibility (LJF) of tubular T/Y-joints retrofitted with Glass Fiber Reinforced Polymer (GFRP) under IPB moment is studied and discussed. For this aim, a finite element (FE) model was generated and verified with the results of available experimental data and parametric formulas. Afterward, a set of 158 finite element (FE) models was created to evaluate the efficacy of the FRP sheets (number, length, and orientation), the brace inclination angle (θ), and the non-dimensional parameters (β, τ, and γ) on the LJF coefficient (fLJF) and the fLJF ratio of the retrofitted to the associated un-retrofitted joint. In the FE models, the efficacy of the weld profile and the contact between the FRP and the steel members (chord, weld, and brace) was considered. Also, analysis of variance (ANOVA) is used to identify the most dominant parameters which affect the fLJF ratios. Results showed that in the retrofitted joints, the increment of the FRP sheet number results in the notable drop of the fLJF. But, the efficacy of the FRP sheet orientation on the fLJF can be ignored. Despite the considerable efficacy of the FRP sheets on the behavior of the tubular joints, there was not any study on the LJF in the joints retrofitted with FRP. Hence, after an extensive parametric study, the results were used to derive a parametric equation for determining the fLJF of T/Y-joints retrofitted with FRP. Moreover, the derived equation was checked according to the UK DoE acceptance criteria.

It is not possible to measure cross sectional load effects (i.e. bending moments and shear forces) of ships directly. In complex structures, the sectional load effects must be derived from strain measurements. Knowing these load effects is important because they do determine the necessary overall structural strength of the ship, both in ultimate load but also in fatigue loads. Traditionally, load effects in specific prismatic shaped sections are derived using strain gauges placed in the section itself. An alternative approach is to derive the load effects in a cross section using measurements of the global strains across the length of the vessel. When this derivation of load effects is based on the first few global flexural vibration modes of the ship, this method is referred to as the modal method. This paper explains both traditional and modal methods and applies them to a simple barge and a more complex ship structure. Experimental data from full scale trials and model tests are used along with numerical simulations to compare the methods. Although the paper focuses on the sectional load effects, it also shows that the local strains can be calculated through the modal approach. The modal method gives better results and has more advantages compared to the more traditional sectional method in ships with complex configurations and effective house structures.

In this paper, a theoretical calculation method that can be generally applied to responses analysis of irregular bridge span pontoon connected by elastic hinges was proposed based on the elastic foundation beam theory. This method can solve the displacements, rotation angles, bending moments and shear forces of pontoon bridge under vertical static loads. By comparing with the experimental and numerical results in other researches, the correctness and universality of theoretical method are verified. The main response characteristics of the pontoon bridge under vertical static loads are further investigated based on a detailed case study, which contrasts the results of three methods including the theoretical method in this paper, AQWA software and FEM. The results reveal that the rotation stiffness of elastic hinges has a great influence on the responses of the pontoon bridge. The elastic deformation of the pontoon units can be ignored when the rotation stiffness of hinges is small, while it needs to be considered seriously when the rotation stiffness of hinges reaches a certain value. Besides, the flexural rigidity of pontoon bridge also has a sensitive influence on the numerical results. The theoretical method can provide for the optimal design of the pontoon bridge and has significant engineering application prospects.

In this previous study, a consistent theoretical formula was established in single hull structure, taking account of all the structural components affecting the load share of each member, in combination with the combined load effect of direct force from the longitudinal stiffener and shear force on the primary supporting member. What's more, it has been known qualitatively that the bending moment at the root of the web stiffener in double hull structure is less than that in single hull structure. However, the difference between double hull and single hull structures has not been achieved. In this study, the authors develop a theoretical formulation to represent the stresses at the root of the web stiffener due to the load from both the longitudinal stiffener and the shear force on the primary supporting member in the double hull structure. Then, the results calculated by the derived formulae are compared with the results obtained by finite element analysis, and good accuracy of the proposed formula was verified. Finally, the calculated stresses were compared between double hull and single hull structures. On one hand, the share of loading born by the web stiffener is almost comparable between the double hull and single hull structures. On the other hand, the bending moment at the root of the web stiffener is smaller in the double hull structure than in single hull structure, and therefore, the maximum stress is smaller.

Flexible pipes are key equipment for offshore oil and gas production systems, conveying fluids between the platform and subsea wells. The structural arrangement of unbonded flexible pipes is quite complex, encompassing several layers with polymeric, metallic and textile materials. Different topologies and a large amount of intricate nonlinear contact interactions between and within their components, especially because of the relative stick-slip mechanism during bending, makes numerical analysis challenging. This paper presents an alternative three-dimensional nonlinear finite element model that describes the response of flexible pipes subjected to combined axisymmetric and bending loads. To simulate the response of a flexible pipe under axial tension or compression combined with uniform curvature, an equivalent thermal loading is employed on the external sheath, which is modelled as an orthotropic thermal expansion material with temperature-independent mechanical properties. To assess the feasibility of the proposed model, the bending moment versus curvature of the finite element solution is compared with experimental results obtained in literature and good agreements are found between them. Detailed finite element results such as contact pressures, armour wire slip displacements and friction, normal and transverse bending stresses are also shown and compared with available analytical models.

In this study, we propose a numerical procedure to analyze floating plate structures with multiple hinge connections in regular waves and investigate the maximum bending moment and deflection in the plate structures. The directly coupled equations of motion for the hydroelastic analysis are discretized by the boundary element method for fluid and the finite element method for plates. The hinge connection is modeled by releasing the rotational degrees of freedom of the plate finite elements, in which a complete condensation procedure is used considering structural mass and stiffness and fluid-structure interaction terms. Wave tank experiments have been performed to verify the numerical results. The modeling capability of the proposed numerical procedure is demonstrated through floating plate problems with 1- and 2-directional multiple hinge connections. The numerical analyses show the effect of the number of hinge connections used on the maximum bending moment and deflection of the floating plate structures according to the aspect ratio, bending stiffness and incident wavelength.

Pressurized girth-welded steel pipe is an important element in offshore and inter-field pipelines. This paper presents finite element (FE) analyses to clarify the effects of internal pressure on the residual stresses in girth-welded steel pipes. FE simulation of the girth welding process was first carried out to obtain the weld-induced residual stresses employing a sequentially coupled three-dimensional (3-D) thermo-mechanical FE formulation. Then, 3-D elastic–plastic FE analyses incorporating the residual stresses and plastic strains obtained from the preceding FE simulation were performed to investigate the residual stress behavior in the girth-welded pipes under internal pressure. The simulated results showed that when internal pressure is applied to girth-welded steel pipes, secondary bending moment is generated at the girth weld attributed to the circumferential shrinkage during the welding process, thus significantly affecting the axial and hoop residual stress evolution in the course of the mechanical loading.

This is the first of two companion papers dealing with nonlinear finite element modelling and analysis of the ultimate strength of a bulk carrier hull girder under alternate hold loading (AHL) condition. The purpose is to contribute to establishing rational ultimate longitudinal strength criteria for the hull girder under combined loading. The focus is on the hogging condition. An important issue is the significant double bottom bending in empty holds in AHL due to combined global hull girder bending moment and local loads. The local loads may substantially reduce the strength of the hull girder. Different AHL conditions, i.e. fully loaded cargo and (partially) heavy cargo are considered. A critical review of external and internal design pressures for different AHL conditions is accomplished using both CSR-BC rules and DNV rules. A methodology for nonlinear finite element modelling of hold tanks of a bulk carrier under AHL is presented by use of ABAQUS. A mesh convergence study is carried out in order to find the appropriate mesh for the model. The implication of using different design pressures on the hull girder strength is assessed. The FE results can be used as a basis for establishing simplified methods applicable to practical design of ship hulls under combined loadings. This issue is discussed in the companion paper.

The paper deals with grounding and stranding of ships. An important issue is the influence of size and shape of the sea floor during grounding. From a range of finite element simulations, the resistance to penetration of the ship bottom is found, for stranding at various locations and different sea bottom topologies. A description of the state of the art material and failure methods is given, and the theory is applied in finite element analyses. Furthermore, a mesh convergence study is carried out in order to find the appropriate mesh for the grounding simulations. Integrated local/global grounding analyses are performed. Stranded ships may be subjected to tidal changes, which may result in further bottom damage. The loss in water level yields a re-distribution of hydrostatic forces due to grounding actions. In this process large hull bending moments are formed, which may influence the resistance to penetration and the damage modes significantly.

The use of a pipe subject to bending moment with an equivalent plate subject to tension has been tried by a few researchers to avoid the complexity usually involved with experimental crack growth investigations of pipes with initial surface flaws. This approach also minimizes the use of more sophisticated monitoring instruments, thereby offering significant cost savings. This equivalency has been done for both experimental and finite element investigations. This paper studies the validity of this approach and evaluates the ranges of the crack depth ratio and elliptical crack diameter ratio for which this approach would be admissible. A series of finite element analysis was carried out to both verify the values of the stress intensity factors reported in the literature, and verify the results of the interpolation function used in the computational simulation in this research. Based on the computational simulations and demonstrating that the crack front follows a semi-circular shape during its growth, a dimensionless relationship between the stress intensity factor of a pipe under bending moment and that of a plate under pure tension has been introduced. A series of experimental investigation was performed to verify the validity of the proposed computational simulation. The results show the rationality and admissibility of this approach when considering the fatigue crack growth of pipes under bending.

The purpose of this paper is to propose a methodology to assess the time-variant ultimate strength of ship hull girder under the degradations of corrosion and fatigue. The effects of fatigue cracks on the tensile and compressive residual ultimate strength of stiffened panels and unstiffened plates are analyzed by an FE method. Based on FE analysis results, some empirical formulae are provided for effective calculation of the compressive or tensile ultimate strength of cracked or intact unstiffened plates or stiffened panels. A non-linear corrosion model is used to determine the corrosion rate of plates, webs and flanges, respectively. The effects of inspection and repair are taken into account. A minimum net thickness rule is used to determine repair policies. A procedure is proposed to determine the maximum allowable corrosion thicknesses of different parts of the hull cross section. The procedure developed is illustrated by application to a tanker. For a given set of inspection and repair criteria, the ultimate bending moment and reliability as a function of ship age is predicted.

Welded tubular connections exhibit significant stress concentrations at the weld vicinity, which may result in fatigue failure. Stress concentration factors (SCFs) in DT-joint are computed, based on an extensive finite element study. The present paper develops a methodology for the calculation of the maximum local stress, referred to as “hot-spot stress”, in a multi-planar DT-joint, with particular emphasis on the effects of bending moments on the braces and the chord. Special attention is focussed on the location where critical stress concentration occurs, as well as on the so-called “carry-over phenomenon” and its implications on the hot-spot stress value. Simplified design equations for fatigue design are proposed to determine SCF values due to bending in order to improve predictions with respect to existing design tools.

In order to reduce the large loads that are developed in connecting semi-submersible platforms on a 5000 ft long mobile offshore base (MOB), a compliant connector concept was developed. This concept employs the cable compliant technology developed by NASA-Goddard and is based upon the use of 3-D Through-the-Thickness® braid reinforced elastomeric tubes as the compliant element in the connector. By selectively placing and orienting the composite tubes, the connector can be designed to provide stiffness in degrees of freedom critical to maintaining runway smoothness (such as relative heave or roll) while permitting larger displacements in other degrees of freedom (such as relative pitch or surge). Two different connectors were designed: a four-degree-of-freedom (4 DOF) connector for the deck attachment and a 6 DOF connector for connecting at the pontoon level. The work presented includes material characterization testing, including some fatigue studies, and constitutive model development to fully understand the material behavior. The constitutive model for the material included both the material nonlinearity of the urethane elastomer matrix and geometric nonlinearities to include the fiber reorientation during loading. Tests were performed on a subscale connector design validate the analytical technique used to predict the connector performance. Six different tests configurations were performed with different loading conditions and connector configurations, and good correlation was found between the experimental and analytical results. The performance of full-scale connectors were determined through analysis and incorporated into a finite element model of a scaled MOB design. The connector loads of the compliant connector are compared with the rigid or hinge-type connector for two different hull configurations and various sea conditions and headings and is shown to reduce the loads over the conventional design.

Experiments for the ship motions and sea loads were carried out on a segmented model of a container ship in ballast condition. Comparisons between the measurements and the theoretical results were carried out for the vertical motions and bending moments. For the evaluation of the primary stresses it is assumed that the total vertical bending moment induced by waves is divided into one component obtained by the linear theory and another one is due to the slamming loads. Several formulations for the determination of the slamming loads are compared with experimental results. The vibratory response of the model is calculated by modelling the hull with rotational springs and rigid links. Linear finite elements with a consistent mass formulation are adopted for the structural model and the response is obtained by modal superimposing and direct integration methods.

At the initial design stage, for rapid evaluation of strength of ship structures, finite element analysis using beam elements is carried out as a rule. In beam modeling of ship structures, brackets are usually represented by rigid elements to simplify the analysis. The extent of rigid ends, which is called the span point, can be determined from the three kinds of view points, i.e., bending, shearing and axial deformation. In this paper, a novel beam element is developed. The developed novel beam element, referred to as the rigid-ended beam element, can consider the effect of three kinds of span point within one element, which was impossible in modeling with the ordinary beam element. Calculated results agree with the exact solution for a cantilever beam and also with results obtained from finite element analysis using membrane elements. Structural analysis using the rigid-ended beam element is revealed to have good computing efficiency due to the elements not needing to correspond to the brackets.