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Several latching control strategies have been proposed to improve the power performance of wave energy converters (WECs). However, the benefits and challenges vary for different WECs and PTO systems. This paper addresses the power performance of a specific direct mechanical-drive power take-off (DMDPTO) system considering the application of a non-predictive latching control. The WEC is a heaving point absorber (PA) that consists of a cylindrical buoy and a bottom-mounted support structure. A pulley converts the vertical motion of the buoy into rotational motion, and a freewheel system rectifies the rotation direction for the power generation. A speed multiplier amplifies the velocity delivered by the buoy to drive a flywheel, which is rigidly connected to the electrical generator. A time-domain model is developed to simulate the interaction of the wave-buoy-DMDPTO in each time interval calculating hydrodynamic and electro-mechanical forces. An exhaustive optimization strategy is applied to maximize power production, optimizing the DMDPTO parameters. The sensitivity of power production to the wave height and wave period are presented considering the generator’s operational limit. Additionally, the capture width ratio and peak-to-average power ratio are calculated for the controlled PA, and the results are compared to the control-free WEC and linear PTO systems.

Oil-and-gas field development projects are capital-intensive, and optimizing profitability has always been a critical subject for the industry. This paper presents a mixed-integer linear programming (MILP) model to facilitate scenario comparison and selection at the design stage for green offshore oil-field development. The model solves an allocation-scheduling problem with the objective of maximizing the project’s net present value (NPV). Decision variables include the drilling schedule for both production and injection wells, well assignment to FPSO (floating, production, storage and offloading unit), FPSO oil and water production capacity, and water injection capacity. The model represents the waterflood scheme through an injection-production relationship matrix, leading to consistent scheduling of producer-injector pairs. Also, the model allows the assignment of development priorities to blocks or well groups, which corresponds to the “phased-development” concept in real-world application. Methods to estimate production rates are proposed, and a production prediction model is established. Production rates considering scheduling effect are generated by a linear superposition of base production curves from reservoir simulations. Two synthetic reservoirs with properties resembling deepwater offshore Brazil illustrated the performance of the modeling approach. It is observed that in early development years, drilling capacity and oil production capacity are active constraints to the system, while in late development years, active constraints change to water capacity, which propels the “capacity expansion” concept in the late production years, such as subsea water separation. Higher recovery is obtained with expedited production, but NPV does not necessarily increase with increased oil recovery when there is considerable investment on extra FPSOs.

Thermal insulation is widely used in offshore oil production for flow assurance design. Research efforts have concentrated on the thermal and mechanical properties of the insulation material, but few publications have focused on the optimization of the insulation. For certain subsea production systems, several optional insulation materials are available. The distribution of insulation along a subsea system to fulfill thermal requirements is not unique to each insulation material. Manually defined insulation designs often lead to a conservative approach that consumes more material than necessary. To find the most economical design, an optimization method combined with machine learning techniques is presented. A subsea production system using different insulation materials is assessed in the case study and optimization results are discussed. Four different insulation materials are used, and 2000 models are simulated for each material to prepare the training data for the machine learning algorithm. The trained algorithm is able to predict the minimum temperature of the system with an error smaller than 5.5%. Genetic algorithm and particle swarm optimization are used to find the most efficient insulation distribution for each material. The optimized costs related to each insulation material are then compared. The results show that the proposed method is capable of defining material and thickness variations throughout the subsea system with the aim of reducing costs.

The growing increase of offshore wind implementations has been made this modality subject to many resource assessment works. However, the variety of available wind datasets and the difficulty of the validation process over the ocean still leave uncertainty about the most relevant data for offshore wind resource assessment and how much they affect the energy estimation. This study investigates the influence of ERA5, CFSv2, MERRA2 atmospheric reanalyses, and the WRF model on the offshore wind resource technical potential and cost estimation of offshore wind projects. The coastline of the Rio de Janeiro state of Brazil is considered as the focus of the study region. Wind results are validated with in-situ measurements from three meteo-oceanographic buoys by the SIMCosta and PNBOIA programs, an anemometer placed on a floating vessel for production storage and offloading (FPSO), and seven weather stations positioned inland near the coastline from the National Meteorological Institute of Brazil (INMET). Then, energy production and cost estimation are calculated employing the IEA-15 MW wind turbine, and the differences between the models are analyzed. The comparative analysis reveals significant differences in the annual energy production, mainly near to the coastline, impacting the cost estimation of the projects in the planning phase.

During the construction process of the ship's hull, the ultimate strength of the stiffened panels is reduced due to both initial imperfections and residual stresses. In service, these stiffened panels are exposed to damage that causes permanent deformations and localized residual strength, reducing additionally the ultimate strength of damaged panels, which must be considered in the design process to preserve the structural integrity. The paper provides analyses of the effect of damaged stiffened panels on the ultimate strength considering the residual stresses caused by indenting depth and different locations. Experiments were performed using small-scale models representative of a full-scale bottom panels from a cargo compartment at the midship of a typical Suezmax tanker. Experimental tests of the indentation were conducted on the intersection plate-stiffeners, where the force-displacement responses were analyzed. After the indentations, the panels were submitted to uniaxial compression experimental tests, in order to evaluate the loss of ultimate strength compared with the equivalent intact panel. Finite element models were developed by ABAQUS software in three steps sequentially: panel indentation, indenter taking off, and uniaxial compressive loading. Plastic strains and residual stresses caused by the indentation are incorporated in the ultimate strength analysis of the panels. Initial imperfections and maximum denting depth of the panels were measured in the small-scale models for the numerical-experimental correlation. Both indentations and ultimate strength presented a good agreement. A parametric study was performed using the numerical model to determine the residual strength due to the damage and its relationship with both dent depth and location.

A properly arranged subsea production system reduces costs and contributes to production performance due to favorable hydraulic characteristics and flow assurance. Therefore, the layout design of subsea production systems is very important in offshore field development. The design of these systems mainly includes locating the subsea facilities, determining the subsea topology and identifying the pipe route. Each of these three aspects have been studied, for instance, optimization of the pipe network or identification of the optimal single pipe route. However, the combination of these three aspects has not yet been discussed in detail. This paper presents an integrated optimization model for the layout design of a wellhead-manifold-FPSO system, with the aim of obtaining a minimum total pipe length. There are two key details of this model that distinguish it from other models. The first detail is that the seabed topography and obstacles are taken into consideration. The second detail is that all three abovementioned aspects are considered together in the model to determine the optimal number of manifolds, manifold and riser base positions, pipe network topology and pipe routes. The simulated annealing and Dijkstra algorithms are coupled to solve the model by using a newly proposed process. The application of this method is demonstrated by designing the layout of an oil field with 22 wellheads and one FPSO. The results are compared with the situation that neglects the seabed topography, showing a difference in suggested pipe length. In addition, the pipe route effect on both hydraulic and flow assurance is briefly discussed. The model provides a method to link related issues of interest to the layout design, resulting in a practical subsea layout that can be used to more reliably estimate costs, more accurately describe multiphase flow and help in decision-making for flow assurance.

A nonlinear stiffness mechanism installed in a floating point absorber (FPA) in regular waves allowed for studying the influence of the nonlinear behavior on wave energy harvesting. Static analysis of nonlinear stiffness system and time domain numerical simulations based on Cummins’ equation evaluated the effects of power take-off (PTO) damping, system stiffness and geometry dimensions. The results may apply to the evaluation of the balance between energy harvesting performance and practical design limitations. The nonlinear stiffness system improved the efficiency of wave energy harvesting, increasing mean power, by both pushing up the natural period, and broadening resonance, therefore proving more competitiveness.

A methodology for the geometrical optimization of wave energy converters (WEC) based on statistical analysis methods and the hydrodynamics of the system in the frequency domain is presented. The optimization process has been applied on a one-body heaving point absorber for a nearshore region of the Rio de Janeiro coast. The sea characteristics have been described using a five-year wave hindcast and are based on a third generation wind wave model WAVEWATCH III. The optimization procedure is performed based on the resultant wave spectrum and joint probability distribution. The optimization process aims at maximizing both WEC absorbed power and absorption bandwidth when providing a natural period close to the predominant wave periods of the sea site. The optimized geometry of the WEC is determined by running a few simulations in the frequency domain and using the design of experiment (DOE) method. The software ANSYS-AQWA is used for the hydrodynamic diffraction analysis, and the DOE method is applied through the Minitab software to determine the optimized geometry. The two primary advantages of this optimization method are the reduced computational time and the possibility of performing parametric analyses for the WEC geometry.

The exploratory frontier of offshore oil and gas industry comes into deeper waters, with the 3000 m water depth barrier hurdled in the US Gulf of Mexico in 2003. At these water depths, the extremely high external pressures, low temperatures, long distance tie-backs and high environmental loads due to waves, currents, and wind combined brings the employed equipment to its operational limit. This paper presents a literature review on failure events experienced by the industry concerning pipelines, risers, and umbilical cables, describing their causes, consequences, and severity. From the several failure modes reported up to now, it is possible to select the ones that are more frequent and deserves attention from academia and industry. Concerning pipelines, the main failure modes reported are due to mechanical damage, corrosion, construction defect, natural hazards and fatigue. Additionally, a vast review of published researches concerning the pipeline-seabed interaction is presented. With regard to floating risers, approximately 85% of them are of flexible type. Although flexible risers may fail in different ways, collapse due to external pressure is reported as the most frequent failure mode. For umbilical cables, the major failure modes are found to occur under tension or compression, torsion, fatigue, wear and sheaving.

As one of the renewable and clean energy sources, wave energy has high density and all-day availability characteristics. However, the development of wave energy converters has been hampered, mainly because of the low efficiency induced high levelized energy costs. The present study introduces an improved bistable mechanism composed of three linear springs, aiming to enhance the performance of a point absorber employed as a wave energy converter. The improved bistable mechanism features an asymmetric configuration of three linear stiffness springs: one horizontal and two obliques on a vertical plane. Numerical simulations based on Cummins equations in the time domain describe the point absorber dynamic behavior in regular and irregular waves. The improved bistable wave energy converter featured a wider low equivalent stiffness range than linear and conventional bistable counterparts, improving its ability to frequency shifting. Its lower potential barrier greatly improves the capture width ratio and frequency bandwidth at low excitations. Such characteristics facilitate the improved bistable wave energy converter to achieve higher efficiency than its linear and conventional bistable counterparts at low frequencies, and enhance its robustness against power-take-off damping detuning and sea state changes. These features altogether make the improved bistable mechanism an efficient alternative to explore the benefits of the bistable dynamics applied to wave energy converters.

This paper provides a theoretical method for calculating the compression limits of a subsea connector's sealing structure to prevent natural gas leakages. The amount of compression deformation is related to the compressive load between contact surfaces. Because of the different compressive loads on the gasket under preloading and operation conditions, the amount of compression will change. Therefore, the load transmission relations between subsea connector components are first analyzed under different working conditions, and the relationship between the sealing contact load and the locking force is obtained. Secondly, a contact model between the lenticular gasket ring and hubs is established on the basis of Hertz contact theory. An analytical equation for the amount of compression is deduced. Considering the demands for sealing and strength, compression limit equations are put forward. In order to apply the compression limits equations, an optimization model is built for a locking mechanism, and the structure design parameters of one certain subsea connector are optimized. This study provides a theoretical method and guidance for the structural design of subsea connectors with a metallic lenticular gasket.

Sandwich pipe (SP) combining high-strength performance and thermal insulation has been considered an effective solution for oil and gas transportation in ultra-deepwater. Strain hardening cementitious composite (SHCC) is well known for its capacity to withstand both tensile load and external hydrostatic pressure. The sandwich pipe considered in the research is constituted of concentric steel pipes with SHCC annular layer. In the present research work, the SHCC was manufactured, and full scale sandwich pipes were assembled. Intact and damaged specimens were submitted to controlled external pressure in a hyperbaric chamber to obtain the collapse and propagation pressures, respectively. Modeling and simulation of the buckle propagation of the SPs were correlated with the experimental results. The results show that sandwich pipe with SHCC core has an excellent structural strength under high external pressure in both intact and damaged conditions. Moreover, the results also show that the interaction between the annular and the inner/outer pipes provides a significant contribution to the buckling resistance under propagation pressure.

Sandwich pipe (SP), composed of two concentrically steel tubes with a strain-hardening cementitious composite (SHCC) filling core, is a new concept designed for oil and gas transportation in deep waters. Bending is a primary loading condition for the SP during the installation process. There is currently a lack of research on SPs' bending capacity, considering the actual interlayer behavior. The extreme adhesion conditions of fully bounded and unbounded layers have been commonly adopted in numerical simulations. This paper aims to study the actual interlayer behavior through push-out and self-stress tests and proposes an SP numerical model with a bond-slip layer, based on the finite element method. The results from experimental tests for bending on a rigid surface to simulate the reeling installation method are correlated with the results from the finite element model, presenting good agreement. A parametric study is then performed to investigate the influence of geometric parameters and the interlayer bond behavior on the ultimate bending strength.

This paper focuses on the concept of a subsea satellite well system in deep water oil field, and a mixed-integer nonlinear programming model (MINLP) is proposed to help design the layout to minimize the payback period. The proposed model has two key aspects. First, the objective function, payback period, reflects the link between the total cost and production. And second, the Floating Production Storage and Offloading units (FPSOs), subsea wells, flowlines, and subsea obstacles are integrated for the modeling, presenting the interaction of the components from underground to the sea level. A decomposition strategy based on gradient descent algorithm is proposed to solve the model. The case studies indicate the model's feasibility. Besides, two different applications of the model are presented, one is to optimize the layout under given horizontal displacements and the other is the cost comparison between drilling horizontal well and vertical well, indicating the model's flexibility.

Over the last decade, a dramatic increase in the size of commercial wind turbines is noticeable. The optimal structural design and reliable fatigue-life prediction of these large-scale structures depend on the accurate modeling of the turbulent flow around the rotors. The present paper aims at extending the knowledge of the turbulent flow characteristics around horizontal axis wind turbines and assessing the performance of several URANS turbulence models, principally Reynolds stress turbulence (RST) models, in predicting the wind turbine aerodynamics under stall condition. The simulations are performed using the NREL phase VI wind turbine over a wide range of wind speeds. Three RST models and the linear and quadratic variants of the k − ω SST turbulence model are examined. The results demonstrate that the RST models generally provide more reliable predictions. An evaluation of the Boussinesq hypothesis questions the applicability of the turbulence models employing this hypothesis to the wind turbine aerodynamic problems. Finally, the elliptic blending RST model, which is one of the latest formulations of the RST models, appears to provide the best trade-off between accuracy and computational cost.

In this work, the influence of initial geometric imperfection modes on the ultimate strength of a ship’s hull is studied, with a focus on the buckling behavior of stiffened panels that initiates the structural hull failure. A numerical model of a cargo compartment at the midship of a Suezmax tanker is developed by using the finite element method and by considering both geometric and material nonlinearities. Analyses are conducted under hogging conditions to evaluate the double bottom stiffened panels experiencing axial compression by hull girder bending. Different imperfection modes on the bottom and inner bottom plates are considered in the model. Two cases are studied. In the first one, a half-wave imperfection mode is employed in the longitudinal and transversal directions. In the second case, the imperfection mode coincides with the main natural buckling mode of the plate between the stiffeners. Experimental tests were performed using small-scale models that are representative of the bottom panels, and the results are correlated with those from numerical simulations to define a proper mesh refinement to reproduce the buckling phenomenon. The ultimate strength of the ship hull, in full-scale, employed the same mesh refinement for the bottom panels, assuming the two proposed initial imperfections. The results from the ultimate strength are compared with the reference values for the design bending moment recommended by a classification society.

T-joints are one of the most common welded joints used in the construction of offshore structures, including ships and platforms. In the present study, a sequentially coupled thermo-mechanical finite element model that considers temperature-dependent material properties, high temperature effects and a moving volumetric heat source was used to investigate the effect of welding sequence on the residual stresses and distortions in T-joint welds. The parameters of Goldak's double ellipsoidal heat source model were predicted using a neural network. The numerical models were successfully validated by the experimental tests. The results show that the welding sequences have significant effects on the residual stresses and distortions, both in the magnitude and distribution mode. The optimization of the welding sequences should be investigated numerically or experimentally before the construction welded structure.

The concept of sandwich pipes for subsea applications has been studied independently of an available connection to joint the pipe segments. Therefore, it is necessary to propose and study the design of a pipe segment connection to build a continuous sandwich pipeline. The reported research work suggests using threaded connections of integral type with square teeth and metal-to-metal sealing. The threaded connector is analyzed by a two-dimensional numerical model using nonlinear finite elements to obtain both contact stress and maximum tension under make-up torque, external pressure, and axial loads. The make-up torque is applied through radial interference for different tightening torques. The results indicate that the threaded connector is a feasible solution for the sandwich pipe joint, especially for the watertight requirement under combined loads.

The interest in predicting the force required for a subsea blowout preventer (BOP) to cut and isolate the drilling pipe has been growing since the Macondo accident. In previous research, numerical simulation using the nonlinear finite element method combined with the blowout preventer (BOP) shear test has been a common approach. As various fracture criteria exist, the selection and application of a suitable fracture criterion are of great interest to achieve a successful ram shearing simulation. In this paper, the CrashFEM criterion, previously employed in crashworthiness simulation, is first adopted in BOP ram shearing simulation to capture the onset of damage to the pipe. The Johnson-Cook (J-C) and Modified Mohr-Coulomb (MMC) criteria are utilized to carry out a comparative study. The TRIP 690 experimental data are used to derive the fracture parameters. These parameters enable the building of numerical models based on boundary and loading conditions for the BOP shear test. Based on the numerical model, a mesh sensitivity study is carried out. The simulation results using refined meshes show that the shearing forces predicted by the CrashFEM and MMC criteria are generally consistent with the experimental data. The results from the three criteria indicate differences originating from the shape of the fracture limit curve, the damage initiation concerning the equivalent plastic strain accumulation, and the damage distribution in the shearing area. These findings are explained in detail in the paper. A comprehensive application of the CrashFEM criteria for BOP shearing simulation is presented and recommendations are provided about the selection of suitable fracture criteria.

This paper focuses on the study of the influence of indentation parameters on the ultimate strength and behavior of the damaged panels for two types of stiffeners, through an experimental-numerical correlation and a parametric study. Experiments were performed using stiffened panels in small-scale models, with stiffener type T and flat-bar, representing typical full-scale bottom and deck panels of a ship, respectively. A series of indentation tests have been carried out in the plate-stiffener intersection of the small-scale models, where the indentation force and displacement pattern are analyzed for each type of stiffener. After indentation, the panel model is submitted to an uniaxial compression test, in order to measure the loss of ultimate strength of the damaged panel in relation to the intact one. Numerical simulations of the ultimate strength behavior for the small-scale panels are performed by a finite element program for both indentation and panel collapse. The results from the numerical-experimental correlation study presented a good agreement. Therefore, a parametric study is performed to analyze the indentation parameters, such as, the dent depth, indenter size, indentation location and the stiffener column slenderness. The research work contributes to a better understanding of the damaged panel behavior in order to define more robust safe design bases, as well as, to support decision on repair procedures.

A comprehensive study on the collapse pressure and post-buckling behaviour of a sandwich pipe (SP) with a core of strain-hardening cementitious composite (SHCC) was carried out in Part 1 of two companion papers. The results in the Part 1 paper show that an SP with an SHCC core has a different collapse mechanism from an SP with a polypropylene core. Because of its weak inter-layer adhesion and a relatively hard core, the collapse pressure and the characteristic response of an SP with an SHCC core are more influenced by its strongest layer than by the summed strength of all its layers. Since this behaviour has never been reported before, current prediction equations for the collapse pressure of SP systems cannot capture the special behaviour of an SP with an SHCC core. Therefore, utilising the available prediction equations for an SP with an SHCC core may lead to unreliable estimates. This Part 2 paper is dedicated to addressing the challenge by proposing a suitable prediction equation. Based on the extensive simulation results carried out by the numerical model verified by experiments in the Part 1 paper, supervised machine learning techniques were applied to support the regression of different equation forms, which come from three sources: (a) equation forms proposed by previous researchers, (b) equation forms found by the automatic machine learning software EUREQA, and (c) equation forms proposed by us. Further, the performances of the equation forms in predicting accurate results for the collapse pressure were compared. Based on the comparative performances and accuracy, an equation was recommended for the design of SPs under external pressure.

Combining both thermal insulation and structural strength in the core, sandwich pipes (SP) are a promising alternative for conventional single wall pipes in deep-water applications. The ultimate strength of SPs under external pressure has always been a major concern in the design process. An insightful understanding of the collapse pressure and post-buckling behavior of SPs is critical not only for safe application, but also for optimized design. This paper and its companion paper present a comprehensive study on the collapse pressure of SPs with strain-hardening cementitious composite (SHCC). Part 1 presents experiments investigating the collapse under external pressure of SPs composed of two steel tubes and the strain-hardening cementitious composite (SHCC) core. The experimental results were employed in correlation studies with the finite element model using the ABAQUS software; good agreement was reached. The proposed numerical model was used in a parametric study for simulating collapse in 6000 different practical configurations. The influence of the geometric parameters and material properties of the SP on its ultimate structural strength under external pressure and post-buckling behavior was systematically analyzed. The results revealed some interesting phenomena not previously reported. For example, assuming frictionless inter-layer conditions, the collapse pressure of an SP with an SHCC core decreases with increasing steel layer thickness. The reasons behind these phenomena were thoroughly discussed and the special behavior of an SP with an SHCC core subjected to external pressure is reported. The results developed in this paper were utilized in Part 2: A suitable prediction equation, where different equation forms are evaluated and one prediction equation is recommended for the design of an SP with an SHCC core.

This study investigates the welding residual stress and distortion in T-joint welds under various mechanical boundary conditions. An experimentally calibrated and sequentially coupled thermal and mechanical 3D finite element (FE) model is developed, and Goldak's double ellipsoidal heat source model is implemented into the numerical model. The results show that the transverse residual stress, vertical displacement, angular distortion and transverse shrinkage depend significantly on the mechanical boundary conditions, and the influence on the longitudinal residual stress is not significant. Applying the clamped condition along the edges during the welding process and releasing it after the T-joint cools down to ambient temperature can significantly reduce the welding-induced geometric imperfections.

The pipeline end manifold (PLEM) is an important subsea facility, which can greatly reduce the cost and risk of the development scenarios of deepwater oil and gas fields by declining the number of export pipelines and risers. However, the employment of PLEMs is a multidiscipline task involving substantial financial and technical factors. Due to various uncertainties of influencing factors, the evaluation process may take several months or years by the engineers with rich project experience. Thus, how to develop quantified reference tools using mathematical models to assist engineers in efficiently making their crucial decisions is essential. In this paper, the optimization of the layout scenarios of cluster manifolds with PLEMs is discussed, where a proposed mathematical model and its dedicated algorithm are illustrated. The optimal solution at the lowest cost can be obtained through in-house routine in MATLAB, including the optimal layout scenario, the number and locations of PLEMs, and the connection relations. Besides, the numerical simulations are performed to demonstrate the validity of the proposed mathematical model and its algorithm. The results show that this optimization layout problem in engineering can be described accurately by the presented mathematical model and the convergence rate of the given algorithm is efficient.

Sandwich pipes (SP) with steel tubes and a lightweight flexible core are under consideration as a potential solution for ultra-deepwater submarine pipelines, combining high structural resistance with thermal insulation. Besides polymeric materials, strain hardening cementitious composites (SHCC) reinforced with polyvinyl alcohol (PVA) fibers, a micromechanically designed material with high tensile ductility, can be an option for the annular material. The purpose of this work was to investigate experimentally and numerically the collapse behavior of SP filled with SHCC under external hydrostatic pressure. The preparation procedure for SHCC and the fabrication process for SP were presented in detail. The full-scale laboratorial tests of SP were performed using a hyperbaric chamber to analyze the collapse pressure subjected to external pressure. The mechanical behavior of SHCC was simulated using a concrete damaged plasticity (CDP) model provided by the commercial finite element package ABAQUS, whose parameters were estimated by tension and compression tests. In addition, a systematic parametric study was performed to analyze the effects of ovality, thickness and outer/inner radius ratio on the collapse pressure of SP with SHCC core.

Mechanical restoration of a point absorber (PA) wave energy converter (WEC) featured a nonlinear stiffness system built by conventional mechanical compression springs (NSMech). Numerical simulations conducted here considered irregular waves in a specific sea site. Results showed considerable improvement for a given suitable spring configuration (length and stiffness parameters). However, practical implementation of NSMech imposed limiting constraints on the geometrical and physical characteristics of the mechanical compression springs. Such constraints limited very much the feasible region of NSMech configurations, restricting significantly applications for wave energy converter. One alternative approach using pneumatic cylinder springs proved to overcome such a limitation aggregating obvious advantages with fewer elements and bringing more enhancement to the WEC performance.

Subsea pipe route selection is very complex which involves several issues due to the complicated sea environment. A properly selected subsea pipe route could help reduce costs and meet engineering requirements. While simultaneously considering the related requirements, this task becomes a complicated optimization problem. This paper proposes a nonlinear constrained optimization model for selecting pipe route with minimum length that considers seabed topography, obstacles and pipe curvature requirements. Besides, the process of determining the intersection between the pipe route and obstacles is presented. The Particle Swarm Optimization based Augmented Lagrangian multiplier method followed by dynamic programming is applied to solve the proposed model. Both flat and undulating seabed with obstacles were considered for the case studies, and the proposed model was compared with Dijkstra algorithm, indicating the feasibility, stability and reliability of the proposed model and solution method. This work provides an initial understanding of pipe route optimization, and other engineering requirements related to safety and cost can be easily added to the proposed model, making the proposed model flexible to the needs of users for different application scenarios.

Sandwich pipes (SP) can be an effective solution for the ultra-deepwater submarine pipeline, combining high structural resistance with thermal insulation capability. Besides polymer, steel fiber reinforced concrete (SFRC) can be an another choice for the annular material, based on the characteristics of high fracture toughness and good adhesion with metal. The purpose of this work was to investigate numerically the ultimate strength of SP filled with SFRC under external pressure and longitudinal bending. The mechanical behaviour of SFRC was simulated using a Concrete Damaged Plasticity (CDP) model whose parameters were estimated by uniaxial tension, compression and four-point bending tests. The applicability of the parameters obtained was verified by simulating the compression and four-point bending tests, where the results showed good correlation between measured and predicted numerical values. Pressure–curvature ultimate strength for SP with perfect adhesion and no adhesion interface condition was obtained. Besides, a parametric study was performed to investigate the effect of the thickness of each layer on the pressure–curvature collapse envelope of SP. It was found that the adhesion between layers and the lateral confinement effect on SFRC play a dominant role in the ultimate strength behaviour of SP, which lead to the non-monotonicity of the collapse envelope.

This paper presents an experimental study of the behavior of the residual welding stresses for butt-jointed steel plates. Two different welding processes were used to joint the plates, one with a single electrode and the other with double electrodes. The stresses were monitored over two weeks after the removal of the welding constraints. The measurements were performed at the deposited metal, the heat-affected zone, the base metal close to the weld joint and along the plate using the X-ray diffraction method and magnetic method based on the magnetostriction effect. The experimental results showed the continuous process of welding stress relaxation over a relatively short length of time with a significant difference in the final welding stress distribution from just after the removal of the welding restraints, both for the single-electrode and the double-electrode welding. The observed stress redistribution trend was characterized by a reduction in and a uniformity of the values of the maximum shear stresses. Microstructure analysis showed the absence of microcracks at the deposited metal, the heat-affected zone and the base metal close to the weld joint at the welded specimens. Thus, the possibility that the observed welding stress relaxation was the result of material failure is ruled out.

The present paper aims at studying the behavior of stiffened panels from a column segment of a new generation of semi-submersible platforms up to the peak compressive load and in the post-buckling condition. Previous studies have demonstrated a strong influence of the mode and magnitude of initial geometric imperfections, as well as boundary conditions, on the structure's axial load capacity. Numerical–experimental correlation study for small-scale models was performed to define the proper numerical model to be used in more complex numerical simulations of the failure behavior of full-scale column structures. The stiffened panels were assessed to identify the buckling onset in a specific plate and its interaction with longitudinal and transversal stiffeners during the progressive column failure. Measurements of the geometric imperfection distribution of full-scale stiffened panels were collected during construction to better understand the buckling mechanism. Initial geometric imperfections were measured by means of laser-based equipment including a portable measuring system that uses laser technology with sub-millimeter accuracy.

Oscillating bodies represent an important class of wave energy converters (WECs). In these systems, the steps of converting wave energy into electricity usually employ a floating body that is either connected to a hydraulic system and a turbine (or motor) coupled to an electric generator or connected directly to a linear electric generator. The improvement of the efficiency of the extracted energy from these systems is closely related to different control strategies. Currently, the reactive control and the phase control by latching are the most analyzed control strategies used for this purpose. This paper proposes a strategy based on phase control concepts for a hyperbaric wave energy converter—a WEC with an oscillating body linked to a hyperbaric chamber through a lever arm. A proportional–proportional integral (P–PI) cascade controller is adopted to regulate the position of a sliding mass, which is used to change the inertia of the floating body. The analysis of the body motion in regular waves is also presented. Simulation results that consider both the regular and irregular waves illustrate the performance of the proposed control scheme.

It is well known that no criterion about seismic design for risers is available, and relevant research has not been reported. A comprehensive study of riser dynamics during earthquakes is performed in this paper. A dynamic model for seismic analysis of risers is developed in accordance with the working environment of the risers and the influence of inertia force of the pipelines. The dynamic equations for the developed model are derived and resolved on the basis of the energy theory of beams. Numerical simulation for an engineering project in the Bohai Oil Field, China shows that the fundamental frequency of the riser plays the major role in the seismic responses, and for platforms in shallow water in Bohai Bay, the risers demonstrate a much lower stress response due to prominent differences between the riser frequency and the earthquake wave frequency. The presented model and its corresponding method for seismic analysis are practical and important for riser design resistant to earthquake waves.

The Brazilian North and Northeast regions have possibilities on harnessing tidal power, where the greatest tidal amplitudes are to be found. In the 1970s, a barrage constructed in the Bacanga estuary, a civil construction project for reducing the distance between the city of Sao Luis do Maranhao and the port of Itaqui, was planned to be the first Brazilian tidal power plant. The favorable conditions associated with the considerable tidal heights, which can be as much as 6.5 m, made this an ideal place for building a pilot plant. Although, a scheme had been proposed to harness tidal energy in a conventional way with the use of the reservoir, the tidal plant in its original form was not implemented due economic and technical restrictions originated from the turbo generators chosen at that time. This paper presents an alternative concept for tidal power plant when the reservoir presents limitations caused by urban occupation, silting up and deterioration of the original structures. These limitations have been verified following the construction of the Bacanga barrage which led to an alternative proposal for the future installation of a pilot tidal power plant. The power generation possible to be harnessed was simulated through a model for each hour from the water head data, outflow, levels, areas and volumes of the reservoir and estuary. The development of the conceptual project, with the utilization of low-head turbines, a floating platform for the turbines and, particularly, taking into consideration those restrictions to be found at the present time, it can be concluded that Bacanga tidal power plant of is technically viable.

This paper aims at assessing the reliability of pipelines with corrosion defects subjected to internal pressure using the first-order reliability method (FORM). The limit-state function is defined based on the results of a series of small-scale experiments and three-dimensional non-linear finite element analysis of the burst pressure of intact and corroded pipelines. A sensitivity analysis is performed for different levels of corrosion damage to identify the influence of the various parameters in the probability of burst collapse of corroded and intact pipes. The Monte Carlo simulation method is used to assess the uncertainty of the estimates of the burst pressure of corroded pipelines. The results of the reliability, sensitivity and uncertainty analysis are compared with results obtained from codes currently used in practice.

Sandwich pipes composed of two steel layers separated by a polypropylene annular can be used for deepwater oil and gas transportation, combining high structural resistance with thermal insulation in order to prevent paraffin and hydrates formation. In this work, sandwich pipes with typical inner diameters of those employed in the offshore production are analyzed numerically regarding to the influence of the inter-layer adhesion between steel and polymer on the ultimate strength under external pressure and longitudinal bending. The effect of the reeling method of installation is also simulated. The maximum shear stress of the metal–polymer interface is obtained by experimental tests. Numerical simulation of the adhesion is modeled by contact surfaces adopting a maximum shear stress value to allow the relative displacement between the layers. It was observed that the ultimate strength of the sandwich pipe is strongly dependent on the shear stress acting at the interface.

The loss of metal in a pipeline due to corrosion usually results in localized pits with various depths and irregular shapes on its external and internal surfaces. The effect of external corrosion defects was studied via a series of small-scale experiments and through a nonlinear numerical model based on the finite element method. After calibration was conducted, based on the experimental results, the model was used to determine the burst pressure as a function of material and geometric parameters of different pipes and defects. This paper briefly summarizes these results, which are subsequently used to develop a simple procedure for estimating the burst pressure of corroded pipes.

This paper presents the results of an extensive sensitivity analysis carried out by the Committee III.1 “ Ultimate Strength ” of ISSC’2003 in the framework of a benchmark on the ultimate strength of aluminium stiffened panels. Previously, different benchmarks were presented by ISSC committees on ultimate strength. The goal has typically been to give guidance to the designer on how to predict the ultimate strength and to indicate what level of accuracy would be expected. This time, the target of this benchmark is to present reliable finite element models to study the behaviour of axially compressed stiffened aluminium panels (including extruded profiles). Main objectives are to compare codes/models and to perform quantitative sensitivity analysis of the ultimate strength of a welded aluminium panel on various parameters (typically the heat-affected zone). Two phases were planned. In Phase A , all members analysed the same structure with a defined set of parameters and using different codes. It was expected that all the codes/models predict the same results. In Phase B , to boost the scope of the analysis, the different members (using their own model) performed FE analyses for a range of variation of different parameters (sensitivity analysis).

A theoretical explicit formulation for numerical simulation of the buckle propagation in deepwater pipelines is proposed. It is based on thin shell theory incorporating large rotations and material elastic–plastic behavior for infinitesimal strains. The equilibrium equations are solved numerically through a computer program using the finite difference method associated to the dynamic relaxation technique. The pipe post-buckling behavior is determined by the arc-length method used in convolute regions of the load–displacement curve. The numerical results are correlated with experimental data from small scale laboratory tests.

Theoretical and experimental research on the ultimate strength of rigid pipelines for deepwater applications conducted at the Laboratory for Submarine Technology—COPPE has been reviewed. Small scale laboratory tests included intact pipes under external pressure combined with longitudinal bending as well as damaged pipes under external pressure. Results obtained from specialist computer programmes and analytical formulations have been correlated with experimental results in order to propose ultimate strength equations to be used in the design procedures. Aspects related to the pipeline installation by reeling method are considered.

Two different buckle arrestor configurations were tested experimentally in order to evaluate the feasibility of using them for deepwater pipelines. Long steel alloy pipes with diameter-to-thickness ratios of 16 and 23 were provided with buckle arrestors and tested under quasi-static conditions in a hyperbaric chamber. Empirical formulae were obtained from the experimental results taking into account both pipe and buckle arrestor geometries as well as material properties.

Existing limit state formulations for tendon and steel riser bodies used in tension leg platforms have been reviewed. These include API Bull. 5C3 (for casings), API RP 2A LRFD, BSI Sub-sea Pipeline, Danish Pipeline Code, DnV Submarine Pipeline Standard and NPD Offshore Structures Regulations, as well as formulations proposed by Johns and McConnell (Battelle), Jensen and Pedersen (Technical University of Denmark), Loh (Exxon), Murphey and Langner (Shell), Tamano et al. (Nippon Steel) and Winter et al. (TNO). Ultimate strength formulations for tension, bending and pressure acting singly or in pairs of loads are compared. There are significant differences in the strength formulae for pressure load, mainly due to the treatment of initial ovality; and for bending capacity depending upon whether plastic or first yield bending capacity is utilized. Some formulae for interaction between pressure and bending, as well as pressure and tension, imply little mode interaction, while others, specially for low diameter-to-thickness ratio, indicate strong interaction, i.e. almost a linear relationship as in all formulae for tension combined with bending.

This paper is concerned with ultimate strength formulations for long circular tubes under combined pressure, tension and longitudinal bending. The study is motivated primarily by the design needs of tendon and riser bodies used in the offshore industry, but it also relevant for offshore pipelines. The tubulars considered have a diameter-to-thickness ratio between 15 and 40 and a yield stress primarily in the range 300–600 MPa. The stress-strain characteristics are typically like those of X-52 and X-77. The goal has been to develop a simple, explicit strength formulation that can be used in reliability analysis as well as load and resistance factor design (LRFD) checks of riser and tendon bodies. This goal is achieved by a systematic comparison of existing formulations for tubes subjected to pressure, tension and longitudinal bending, with relevant experimental data as well as numerical analyses. The study resulted in a new formulation for the interaction between the three loads.

A finite difference computer program considering shell large deflections, elastoplastic material behaviour and initial geometric imperfections has been implemented to evaluate the collapse loads of pipelines under external pressure and longitudinal bending. Experimental tests on tubes with typical diameter-to-thickness ratio of deepwater offshore pipelines have been carried out. Correlation studies between numerical and experimental results were performed to calibrate the proposed numerical formulation. Actual steel pipelines were analyzed and the results compred with predictions from design codes.