Riser Design Research Articles

FPSO ROLL MOTIONS DRIVEN BY SECOND-ORDER FORCES AND THE IMPACTS ON SLWR RISER DESIGN

Abstract The dynamic behavior of risers is highly influenced by the FPU motions, associated with environmental loads, caused by waves, current and wind. Particularly, as well known, under potential flow modelling, wave loads may be represented through power series expansions, with respect to a small parameter ε. First-order wave forces pulsate in the frequency range of the incident wave spectra, whereas second-order wave forces are related to sum or difference frequencies of the same spectra. The so-called second-order difference frequencies forces may then be responsible by resonating motions of the vessel in that range. Significant low-frequency motions due to second order forces are commonly observed in the horizontal plane, resonating low natural frequencies of moored floating systems characterized by low mooring stiffnesses. Recently, however, this phenomenon was also observed for roll motions in several new FPSOs that will operate in the Pre-Salt of Santos Basin. The phenomenon had already been detected in a deep draft semi-submersible in Campos Basin, but only more recently in FPSOs. The new floating units are characterized by high roll natural periods, above the typical wave period range in the pre-salt region, and the roll response to the second-order difference frequencies forces can be significant. This phenomenon was indeed detected in model tests and confirmed by numerical studies. In fact, this additional FPSO roll behavior occurs in typical periods slightly above 20 s, imposing displacements and accelerations that can directly affect the dynamics of the riser top region. The present paper addresses this phenomenon and its possible impact on steel lazy-wave riser (SLWR) designs.

Integral BLF-Based Adaptive Dynamic Event-Triggered Boundary Control for a Flexible Riser System.

For the flexible riser systems modeled with partial differential equations (PDEs), this article explores the boundary control problem in depth for the first time using a dynamic event-triggered mechanism (DETM). Given the intrinsic time-space coupling characteristic inherent in PDE computations, implementing a state-dependent DETM for PDE-based flexible risers presents a significant challenge. To overcome this difficulty, a novel dynamic event-triggered control method is introduced for flexible riser systems, focusing on optimizing available control inputs. In order to save computational costs from the controller to the actuator, a dynamic event-triggered adaptive boundary controller is designed to effectively reduce boundary position vibrations. Additionally, considering external disturbances, an adaptive bounded compensation term is incorporated to counteract the influence of external disturbances on the system. Addressing boundary position constraints, a new integral barrier Lyapunov function (iBLF) tailored specifically for flexible riser systems is introduced, thereby alleviating conservatism in the controller design of flexible risers modeled by PDEs. At last, the validity of the proposed method is demonstrated through a simulation example.

Configuration design of a steel double lazy wave riser based on metamodel-assisted metaheuristic algorithms

This study presents an approach to the configuration design of a steel double lazy wave riser. Considering the length of each segment and arrangement parameters of buoyancy blocks as design variables, the objectives are to minimize the maximum static stress, horizontal motion scope, and maximum standard deviation of dynamic stress, respectively. A self-developed program was established to calculate the riser dynamics in a fully coupled platform/mooring/riser system. An improved Particle Swarm Optimization (PSO) that can handle both continuous and discrete design variables was proposed to automatically find the optimal solution. Feedforward Neural Networks (FNNs) corresponding were embedded into the optimization algorithm to replace the time-consuming numerical calculation. The results show that the FNN provides high accuracy with an error of no more than 6.34 % and all Determination Coefficients of about 0.95. The static-stress optimization tends to reduce the arch height in the riser; while the dynamic optimizations tend to increase the arch heights. Three optimizations all reach feasible solutions with noticeable objective reductions.

Theoretical and numerical methods for predicting the structural stiffness of unbonded flexible riser for deep-sea mining under axial tension and internal pressure

Composite materials have received great attention in the design of flexible risers for deep-sea mining due to their significant advantages of high strength and lightweight. However, under the influence of various nonlinear factors, the cross-sectional response of composite flexible risers is extremely complex, and the prediction of their structural stiffness is greatly challenged. This paper focuses on a new type of composite flexible riser applied in deep-sea mining. The nonlinearity of materials and detailed geometric features of structures are fully considered. Theoretical and numerical models for predicting structural stiffness under axisymmetric loads are constructed. By iteratively solving the equilibrium equation, the strain and displacement fields of the flexible riser are obtained. Firstly, the accuracy of theoretical and numerical methods is verified based on a 5-layer 8-inch unbonded flexible riser as a case study. Secondly, the influence of fiber volume fraction (FVF) on the structural stiffness and weight of flexible risers is discussed. Finally, the influence of structural design parameters of the reinforcement layer on its axial tensile stiffness and radial stiffness is explored. The research results can provide parameter inputs for the design and dynamic analysis of flexible risers.

Using Convolutional Neural Networks in Installation Analysis of Lazy-Wave Flexible Risers

Abstract The design phase of offshore installation projects is supported by numerical simulations. These analyses aim to evaluate the mechanical behavior of the equipment involved, such as vessels and flexible pipes, during that operation. Therefore, a common approach is to take the ocean wave loads modeled as deterministic ones (or regular wave approach), which is a simplification that, on the one hand, allows low computational cost, but, on the other one, lacks the representation of the actual behavior of the wave loads, usually better represented by means of an irregular wave modeling. In the way of searching for an irregular wave analysis procedure to be used in the daily design of lazy-wave riser installation analyses, this work proposes an artificial neural network (ANN)-based approach. The proposed model aims to achieve it by training a convolutional neural network (CNN) fed by generated data from short-length finite element-based numerical simulations. This surrogate model can predict quite well the pipe's top tension and approximately the axial tension in the touchdown zone (TDZ) for different configuration stages during the riser's installation operation. Moreover, the proposed model works for different environmental scenarios, which boosts the computational simulation time reduction in this phase of riser design.

Chapter 4. Locating field systems in the southern Peruvian Andes

AbstractIn this paper, we review current understandings of anthropogenic field systems, focusing on trends and variations in the chronology of field construction, use, and in some cases, abandonment, as well as labor organization of agrarian production across the Lake Titicaca Basin. These trends indicate that agricultural intensification increased both during the political centralization of the Tiwanaku state and during periods of political fragmentation. In contrast to prior work on fields in the region, we argue that there was no single cultural, environmental, or historical impetus that ignited the construction of any particular field type. Additionally, we present the results of pedestrian survey of terraces carried out in 2018 in the northern Lake Titicaca Basin. While there were regional commonalities across survey areas in the masonry design of terrace risers, presence of pathways and radiating walls separating vertical tracts of terraces, and a general absence of irrigation, we found deviations from each of these trends in individual terrace complexes. While preliminary evidence indicates that more terraces were built or cultivated during the Late Intermediate period (1100–1450 CE) than in other time periods in the northern Titicaca basin, some terraces were likely built earlier. Our findings point to the multiplicity of strategies that ancient farmers employed in the varied ecological settings of the Lake Titicaca basin under diverse sociopolitical programs. This contrasts with previous research on agrarian field systems, which is mostly single‐sited and tends to emphasize individual strategies over the quiver of agrarian options available to Andean farmers.

Modification of Risers in Buried Pipe Irrigation System to Improve their Performance

This study identified the problems associated with the design, construction, operation, and maintenance of risers in buried pipe irrigation systems and proposed a modified design. The problems of buried pipe irrigation systems were assessed by inspecting three deep tube wells (DTWs) supplying water through buried pipes with twenty-five outlets. One DTW of the Bangladesh Agricultural Development Corporation (BADC) with five outlets in Netrokona, and one with fifteen outlets in Bara Kailati village and one with five outlets in Bali Bazar, both in Netrokona Sadar, were selected. Data were collected from farmers, field staff, and management authorities by employing pretested questionnaires and field investigations. Site visits revealed that the primary problems were the water leakage through outlets and cracks in the riser structure around the outlet. These problems were due to unsuitable outlet design of the buried pipe system. To overcome these problems, the riser design has been modified structurally, that may lead the buried pipe to suitable and durable performance of buried pipe irrigation system and reduce water losses. J Bangladesh Agril Univ 22(2):225-230, 2024

Flow assurance analysis in Front End Engineering Design (FEED) of Subsea Flowline and Riser

Innovative subsea production systems (SPS) are required for deep-water oil and gas exploration in order to transport multiphase fluids—a complex mixture of water, gas, and oil—across long distances under challenging conditions. Significant technological challenges confront these systems, such as flow assurance, which ensures continuous fluid flow through pipelines, flowlines, and wells. Flow assurance issues can significantly impact production and incur costly downtime. This paper emphasizes the critical role of flow assurance analysis during the crucial Front-End Engineering Design (FEED) stage of SPS development. By utilizing industry-standard simulation tools like PIPESIM™, engineers can proactively identify and mitigate potential flow assurance problems that could hinder productivity. These analyses are crucial as FEED decisions significantly impact project cost-effectiveness. To illustrate the practical application of flow assurance principles, the paper presents a case study of a subsea architecture design for a specific manifold system. Considering operator requirements, flow assurance simulations were employed to guide critical decisions regarding material selection and optimal sizing of risers and flowlines. This demonstrates how flow assurance considerations during FEED directly influence cost-effective and optimized SPS design.

Toward Reducing Casting Defects via 3D Risers via 3D Sand-Printing: A Simulation Study

AbstractThe 3D sand printing (3DSP) process is a binder jetting class of additive manufacturing process that can incorporate complex 3D mold designs and consolidate cores with intricate features that were previously inaccessible. Prior studies in 3DSP mold design have been shown to improve pouring and filling conditions for sand casting. However, the opportunity to improve casting quality by exploring 3D riser designs during the solidification stage has not yet been explored. In this research, three novel 3D riser geometries—ellipsoid, spherical, and a fusion riser (combination of cylindrical and ellipsoid riser) were investigated. The results were compared to the benchmark cylindrical risers to assess casting performance (e.g., reduction in shrinkage porosity, increase in solidification time). Computational solidification simulations have been presented to evaluate the characteristics of the novel risers for three different metal alloys- nickel aluminum bronze (NAB), low-carbon steel A216 (WCB), and aluminum alloy (A319) alloy. From the results of this research, spherical risers were found to provide 45% yield improvement of for the three alloys studied. In addition, the riser neck diameter using a spherical riser experienced up to 77% reduction when compared to the recommended dimensions from previous literature. Finally, one of the spherical riser designs provided 18% improvement in terms of riser-pipe safety height over the benchmark design. Findings from this research will help metalcasting industries to optimize their riser designs for complex casting geometries by implementing 3D riser geometries (via 3DSP) into traditional mold making for yield improvement and defect-free castings.

Optimization and empirical studies of riser design in sand casting process using different mould properties

Optimization and empirical studies of riser design in sand casting process using different mould properties

Design of Riser System Based on Target Oilfield and Platform

Abstract: Risers are an important component in the development of offshore oil and gas resources, but they are also one of the weakest components. Therefore, the safety of the design of deep-water riser systems plays a crucial role in the development of offshore oil and gas resources. This article focuses on the selection and initial design of riser systems based on the target oilfield and platform. Taking into account both the transportation capacity of oil and gas pipelines and the strength of casing, multiphase flow theory is used to calculate the pressure drop and transportation capacity of the riser. Based on this, the size of the oil pipe and casing is iteratively calculated. The riser design is based on the specifications and professional software Orcaflex, which is mainly used in the design process, is used for stress analysis of the riser, Taking into account the main scale of the FWPSO platform, reservoir and wellhead data, as well as the marine environment data where the riser is located, the selected oil and casing will be subjected to wall thickness design verification.

The Experimental Study of Dynamic Response of Marine Riser under Coupling Effect of Multiparameter

In order to investigate the dynamic response of a marine riser under the coupling effect of multiparameter, a model experiment of a marine riser was designed and carried out. The main parameters of the model test were divided into riser design parameters and flow field parameters. The riser parameters included the elastic modulus, boundary conditions, and top tension forces, and the flow field parameters were the velocities and wave parameters (wave height and period). The riser materials were aluminum (Al) and polymethyl methacrylate (PMMA), representing the metal riser and fiber-reinforced composite marine riser, respectively, which differ greatly in modulus. Two types of boundary conditions were considered, which were simple supports at both ends (S-S) and simple and fixed supports at each end (S-F). The top tension forces were chosen as 10 N and 30 N, respectively. In terms of the flow velocities, 0.3 m/s and 0.7 m/s were used. For the wave types, the small wave had a period of 1.0 s and a wave height of 5 cm while the large wave had a period of 2.0 s and a wave height of 15 cm. The dynamic response of the riser under 32 different working conditions was studied experimentally, and through the analysis of the experimental data, the effects of various parameters on the dynamic response of the risers were obtained. The results show that the amplitude of the riser was negatively correlated with the elastic modulus, the number of constraints, and the magnitude of the top tension, while it was positively correlated with the flow velocity and wave size. Moreover, the sequences of importance were b3 (flow velocity) > b6 (modulus) > b1 (number of constraints) > b2 (top tension force) > b5 (wave height) > b4 (wave period) for the vibration of the riser in the in-flow direction and b3 (flow velocity) > b1 (number of constraints) > b6 (modulus) > b2 (top tension force) > b5 (wave height) > b4 (wave period) for the vibration of the riser in the cross-flow direction, respectively, according to the multiple linear regression calculation.

Numerical simulations of 2-DOF vortex-induced vibration of a circular cylinder in two and three dimensions: A comparison study

Finding a reliable and efficient numerical method is of great importance for the safety design of a offshore riser whose model can be simplified as vortex-induced vibration (VIV) of an elastically-mounted circular cylinder. In the current study, two-degree-of-freedom (2-DOF) VIV responses of a circular cylinder with a small mass ratio m*=2.6 and small mass damping parameter (m*+1)ζ=0.013 is numerically investigated by two- and three-dimensional method. The simulations by using the URANS (Unsteady Reynolds Averaged Navier-Stokes) in combination with the SST (Shear Stress Transport) k−ω turbulence model are performed at subcritical Reynolds numbers (Re ranges from 1×103 to 1.5×104). From the overall results, both the 2D and 3D simulations can obtain relatively accurate statistical results including VIV response amplitudes and frequency values. The main differences between the 2D and 3D simulations lie on the three-dimensional effects that exist in the supper upper branch and the flow transition condition. However, the 2D numerical simulations can save hundreds of times of the computational resources compared with a 3D simulation, hence is more suitable for engineering VIV prediction under such conditions. The comparison of simulation and experimental results in this study provides research support for the selection of appropriate simulation methods (2D or 3D) for 2-DOF VIV of an offshore riser in research and engineering.

Cross-sectional mechanical characteristics and sensitivity analysis of unbonded flexible risers under axial loads

Unbonded flexible risers exhibit complex structures. Different structural layers can withstand axial loads and exhibit different degrees of coupled deformation. Based on the different material properties and structural forms of each layer of an unbonded flexible riser, the structural layers are divided into three types: cylindrical, steel helical, and polymer helical layers. This study establishes a theoretical model of flexible risers under axial loads based on the law of conservation of energy and the geometry of deformation, and deduces theoretical expressions for the axial load and axial stiffness of flexible risers. MATLAB was used to compile calculation programs to calculate the cross-sectional mechanical properties of flexible risers under axial tensile and compressive loads and to compare the calculation results with the experimental results and the results of other researchers to verify the reliability of the theoretical derivation and calculation programs. By further calculating the cross-sectional force distribution of each structural layer of the flexible risers under axial tensile loads, it is clarified that the tensile armor layer is the main component that can withstand axial tensile loads. A sensitivity analysis of the helix angle and number of helical strips of the tensile armor layer on the tensile properties of flexible risers was conducted; the results show that the helix angle had a more obvious influence on the tensile properties of flexible risers. The results of this study can provide a reference for the structural design and optimization of flexible risers.

Evaluasi Efisensi Riser Untuk Bentuk Riser Samping yang Berbeda Menggunakan Metode Simulasi Casting

As the solidification reaches the hot spot area, no molten metal remains and shrinkage is formed. To anticipate the shrinkage, a riser is added to the casting system. An optimal riser design would produce free shrinkage components. One of the factors that affect riser efficiency is the riser shape. This study aims to find the most efficient side riser shape by using simulation software. The riser shape of tubes, tubes with a half sphere on top, hemispheres, conical tubes, tubes with an oval cross-section, and cubes are used in this experiment, with the volume of all risers kept constant. The most efficient shape of the riser is the tube. The tube riser produces a larger modulus. The tube riser generated directional solidification. The same pattern can be seen in the niyama criteria and solidification temperature, where the tube riser has a more continuous pattern compared to other riser shapes.

Casting process design of high pressure and heat resistant steam chamber casting

This paper is about the casting process of high pressure and heat resistant steam chamber casting. The castings are made of ZG15Cr2Mo1. The internal and external quality requirements of the casting are demanding. There are many grade 0 and grade 1 defect areas in castings, which requires that these positions should not have shrinkage cavity and porosity and other defects.This casting scheme includes the design scheme of modeling, modeling material selection, coating selection, parting surface and casting system establishment, process parameters establishment, sand core design, casting riser design, cold iron design and so on. By using AnyCasting and MAGMA to simulate and predict the feasibility of the casting process scheme, a more appropriate casting process design scheme was finally determined to ensure the density of the casting metal at the flaw detection site, and eliminate the defects of shrinkage porosity and porosity of the casting, so as to ensure the requirements of leakage of the casting in practice, and make the casting meet the defect rating. At the same time, the production rate of the scheme is relatively high, which can reduce the production cost.

Dynamic drift-off warning limits for dynamically positioned MODU with Deepwater Artificial Seabed (DAS) system coupling

A new Deepwater Artificial Seabed (DAS) drilling system, which enables the shallow-water rated drilling equipment to operate in deep and ultra-deep water, has been developed for dynamically positioned Mobile Offshore Drilling Unit (MODU), with a focus on key issues of well access and riser design. Nevertheless, the MODU may drift-off due to the critical failures of the dynamic positioning (DP) system and thus the well integrity will be gravely threatened. This study is committed to establishing the quantitative criteria of drift-off warning limits, which consist of the yellow limit, red limit and physical limit, for dynamically positioned MODU with the coupled DAS drilling system. To achieve this aim, a three-phase methodology is proposed on the basis of a fully coupled model of the MODU-DAS drilling system. Furthermore, a hybrid method integrating the orthogonal experimental design, back propagation neural network, and Garson's algorithm is developed to systematically investigate the importance of the correlative influencing factors on the drift-off warning limits in consideration of both the efficiency and accuracy of the numerical calculations. The validity of the proposed methodology is demonstrated by a case study. The results indicate that the dynamic drift-off warning limits are able to assist in decision-making of safe DP operations on the MODU and the operating depth of the artificial seabed is the most critical factor affecting the envelope of drift-off warning limits. These findings and recommendations could improve the safety of DP drilling operations.

The Effect of Ice Floe on the Strength, Stability, and Fatigue of Hybrid Flexible Risers in the Arctic Sea

Flexible risers have proven to be a popular choice for deepwater exploration due to their ability to withstand functional and environmental stress while maintaining system integrity. In the challenging arctic conditions, lightweight hybrid composite flexible risers are likely to be employed to mitigate the increase in effective tension. This study investigates the strength and stability performance of production hybrid composite flexible risers with composite pressure armour in the harsh environmental conditions of the Arctic seas. At a water depth of 340 m, the flexible riser was analysed in various global configurations to evaluate the static, dynamic, and lamina-scale performance of its carbon fibre-reinforced thermoplastic polymer composite layer. The drifting ice in the region generated additional load on the riser system, and the effects of this ice on the riser design and its dynamic and lamina-level performances were also analysed. The results indicate that the current riser design incorporating the composite layer is insufficient to ensure system integrity without mitigating the effects of ice loading. The carbon fibre direction in each lamina must be optimised for excess axial stress emanating from the combined action of hoop, axial, and bending stresses. Finally, recommendations on how to improve the life of the lightweight hybrid composite riser in arctic conditions are provided.

Investigation of trench effect on fatigue response of steel catenary risers using an effective stress analysis

The design of Steel Catenary Risers (SCR) heavily relies on the interaction between the riser and seabed in the touchdown zone (TDZ). To accurately estimate the SCR-seabed stiffness, the soil behaviour must be assessed in two main phases: (1) during cyclic SCR motions, which generate excess pore pressure, leading to soil softening and remoulding in the TDZ, and (2) during inactivity periods throughout the SCR’s lifespan, causing dissipation of excess pore pressure associated with the consolidation state. The latter was neglected by the existing non-linear hysteretic soil models, which are based on the total stress approach. In the current study, a global riser analysis was conducted and the consolidation effect was incorporated using an effective stress framework. Long-term soil stiffness was determined by capturing both the remoulding and consolidation effects, which are respectively associated with the damage accumulation during the SCR cyclic motions and soil strength recovery during the intervening pause period. The constructed model was then combined with a new methodology named Hybrid Trench Model to investigate how the consolidation effect contributes to the fatigue performance in different trench depths. Stochastic fatigue analysis showed that the consolidation effect might increase the damage by around 10%-40% over the long-term assessment.

Fatigue reliability assessment of offshore catenary risers conveying internal slug flow

AbstractThis paper proposes a fatigue reliability assessment method of offshore steel catenary risers (SCRs) caused by internal slug flow. Offshore risers subject to slug flow are modeled first by employing a finite element method scheme. Subsequently, a combination of Kriging metamodel and Monte Carlo simulation (MCS) is established to assess the fatigue reliability of SCRs conveying internal slug flow. An explanatory example of a 16‐inch SCR is presented to validate the performance of the proposed method and the results indicate that the failure rates of the SCR are significantly affected by the frequency of slug flow. Moreover, the accuracy and feasibility of the present method are validated by a comprehensive comparison with the existing experiments and numerical results. The present method contributes to the design and maintenance of offshore risers with internal slug flow.