Riser Design Research Articles

A finite element analysis for unbonded flexible risers under bending loads

As the exploitation of oil and gas resources advances into deeper waters and harsher environments, the design and analysis of the flexible risers has become the research focus in the offshore engineering filed. Due to the complexity of the components and the sliding between the adjacent layers, the bending response of the flexible risers is highly non-linear. This paper presents the finite element analysis of the flexible risers under bending loads. The detailed finite element model of the flexible riser is established in ABAQUS software. This finite element model incorporates all the fine details of the riser to accurately predict its nonlinear structural behavior. Based on the finite element model, the bending moment-curvature relationships of a flexible riser under various axisymmetric loads have been investigated. The results have been compared with the analytical ones obtained from the literature and good agreements have been found. Moreover, the stress of the tendon armors has been studied. The non-linear relationship between the armor tendons' stress and the bending loads has been obtained.

Global design and analysis of deep sea FRP composite risers under combined environmental loads

The use of composite materials in offshore engineering for deep sea oil production riser systems has drawn considerable interest due to the potential weight savings and improvement in durability that can be achieved. The design of composite risers consists of two stages: (1) local design based on critical local load cases (LCs) to obtain the geometric configuration of the riser which will be analysed in the global design stage, and (2) global analysis of the full length composite riser under global loads including top tension force, platform motion, hydrostatic pressure, gravity, buoyancy, wave and current loads to determine and assess critical locations. This study describes the methodology, LCs, analysis procedure and results of the global design of the composite riser based on the geometries of the tubular optimised in the local design stage. The results show that a careful local design of the tubular using inclined reinforcements in addition to axial and hoop reinforcements can offer substantial weight savings and at the same time ensure that the structure is capable of withstanding the global loads applied on it.

Multi-objective non-linear programming problem based on Neutrosophic Optimization Technique and its application in Riser Design Problem

Abstract- Neutrosophic set is a part of neutrosophy which studies the origin, nature, and scope of neutralities, as well as their interactions with different ideational spectra. Neutrosophic set is a powerful general formal framework that has been recently proposed. The paper aims to give computational algorithm to solve a multi-objective non-linear programming problem (MONLPP) using neutrosophic optimization method. The proposed method is for solving MONLPP with single-valued neutrosophic data. We made a comparative study of optimal solution between intuitionistic fuzzy and neutrosophic optimization technique. The developed algorithm has been illustrated by a numerical example. Finally, optimal riser design problem is presented as an application of such technique.

Experiment and Finite Analysis on Resonant Bending Fatigue of Marine Risers

Riser system is a key equipment for offshore oil and gas development. When conducting riser design, fatigue failure mode is the chief one among the many failure modes which should be taken into account. To assess the fatigue performance of riser accurately, it is necessary to conduct fatigue tests. Resonant bending fatigue test is one effective method for fatigue tests of risers. In this paper, the principle of resonant bending fatigue test and test procedures are presented firstly, and then a finite element model using ABAQUS is created to simulate the resonant bending fatigue test, and the results from the finite element model are compared with the experimental results. The good agreements between the FEM results and experimental results verify the accuracy of the finite element model in this paper.

Design Methods for Subsea Equipment for Extreme and Survival Load Conditions

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25352, “Design Method and Load- and Resistance-Factor Design for High-Pressure/High-Temperature Subsea Equipment for Extreme and Survival Load Conditions,” by Parth D. Pathak and Samuel L. Taylor, OneSubsea, prepared for the 2014 Offshore Technology Conference, Houston, 5–8 May. The paper has not been peer reviewed. This paper posits a design method for subsea equipment governed by American Petroleum Institute (API) Specification (spec.) 17 (and particularly 17D). Fatigue-life estimation is also addressed for these conditions. An analysis of a subsea-tree component is reviewed, evaluating its combined load capacity for normal, extreme, and survival conditions. Additionally, the fatigue life-cycle estimation for this component is included. Introduction The API spec. 17D and spec. 6A provisions have been the governing terms for subsea trees and wellheads, providing the design guidelines for normal operating conditions up to 15,000 psi. The recently published API Technical Report (TR) 1PER15K-1 and TR 17TR8, the latter still under development, address the design methodology for normal operating high-pressure/ high-temperature (HP/HT) conditions. But these specifications and technical reports do not provide design guidelines for extreme and survival operating conditions. This paper tries to address design methods for subsea equipment that falls under API spec. 17, with focus on subsea trees and wellheads for such conditions. API Recommended Practice (RP) 17G, which is intended for design of completion and workover risers, addresses these extreme and survival load operating conditions. It provides the equipment’s normal, extreme, and survival operating capacities on the basis of safety factors applied to the ultimate capacity. It also provides allowables based on yield strength with condition factors for extreme and survival conditions. This paper provides a methodology whereby the design factors for the extreme and survival load conditions from API RP 17G are adapted so that they can be applied to API spec. 17D equipment. Because testing the API spec. 17D equipment to its ultimate capacity is not easily feasible, an elastic/plastic analysis method based on American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section VIII, Division 2 or Division 3 (ASME Division 2 or 3) is suggested to determine the ultimate capacity of the equipment.

Design, Manufacturing and Testing of Filament Wound Composite Risers for Marine and Offshore Applications

The major challenge for producing and manufacturing risers for oil and gas production is to make them light weight so as to reduce the operational cost and improve the overall system requirements to make them an attractive option for marine and offshore industries. In the current research, the composite and metal-composite pipes (1) GFRP only (2) Al-GFRP and (3) PE-GFRP have been manufactured using filament winding machine operated using CADFIL and CADWIND CNC packages. The use of liners (Al and PE) has ensured the fluid tightness and collapse resistance of the pipe system. Small prototype pipes are manufactured based on optimized pipe design parameters for collapse under external pressure and burst under internal pressure using ANSYS 13 FEA analysis software. Experiments are conducted to verify the axial and hoop compressive strengths of the pipes with analytical results. The details of pipe manufacturing process using filament winding machine, simulation procedure to optimize the pipe parameters and validation of the simulation results with experimental results are the focal points of the current work.

An Efficient Analytical Failure Analysis Approach for Multilayered Composite Offshore Production Risers

This paper proposes an efficient analytical failure analysis approach for multilayered composite risers used in the offshore oil industry. This approach is based on a layer-by-layer progressive damage model and a homogenization stress analysis method from which the discontinuous stresses through layers can be efficiently calculated. Different failure theories can be easily integrated into the approach to determine failure initiation in layers with different materials. Progressive failure analysis is based on layer-by-layer material degradation schemes, taking into consideration different failure modes such as yielding, fracture, matrix cracking, fiber broken, etc., in layers with different materials. In this approach, progressive failure information involving failed layers and their failure sequences as well as failure modes can be efficiently predicted for multilayered composite risers under given loading conditions. Failure envelopes of composite risers are generated for either initial failure or ultimate failure in different load spaces, and strengths of composite risers can be predicted under given load ratios. This analytical approach is efficient for failure analysis or strength prediction of composite risers with many layers because stress redistributions in all layers during failure progression can be easily and quickly calculated. A user-friendly interface based on Excel sheets is used to carry out this analytical failure analysis approach. Failure analyses of a 22-layer composite riser under several typical loading conditions are presented to demonstrate the application of the proposed approach. Initial and ultimate failure envelopes of the composite riser are shown in different force spaces. This failure analysis approach provides an efficient way for design of composite risers in the offshore oil and gas industry.

Fatigue analysis of steel catenary risers based on a plasticity model

The most critical issue in the steel catenary riser design is to evaluate the fatigue damage in the touchdown zone accurately. Appropriate modeling of the riser-soil resistance in the touchdown zone can lead to significant cost reduction by optimizing design. This paper presents a plasticity model that can be applied to numerically simulate riser-soil interaction and evaluate dynamic responses and the fatigue damage of a steel catenary riser in the touchdown zone. Utilizing the model, numerous riser-soil elements are attached to the steel catenary riser finite elements, in which each simulates local foundation restraint along the riser touchdown zone. The riser-soil interaction plasticity model accounts for the behavior within an allowable combined loading surface. The model will be represented in this paper, allowing simple numerical implementation. More importantly, it can be incorporated within the structural analysis of a steel catenary riser with the finite element method. The applicability of the model is interpreted theoretically and the results are shown through application to an offshore 8.625″ steel catenary riser example. The fatigue analysis results of the liner elastic riser-soil model are also shown. According to the comparison results of the two models, the fatigue life analysis results of the plasticity framework are reasonable and the horizontal effects of the riser-soil interaction can be included.

A new method for strake configuration design of Steel Catenary Risers

A number of studies of vortex-induced vibration (VIV) having important effect on the fatigue performance of riser have performed analytically as well as experimentally in the last decade. However, there are few research results on the configuration design of VIV suppression devices such as fairings, strakes, and etc. Therefore, a systematic design procedure of strake configuration (i.e., location and length) using the modal approach for steel catenary riser (SCR) is proposed in this study. The proposed method considers the dominant mode for VIV suppression through modal analysis of bare riser to find appropriate location and various design parameters are introduced to calculate appropriate length of VIV suppression devices. From the numerical example of SCR, it is shown that the required strake length from the optimal location for VIV suppression of SCR is much shorter than that of the conventional method. This is achieved by staggered strake installation at the effective location. Furthermore, no additional calculations or engineering cost are required for the proposed method, because the proposed method follows exact same analysis procedure of the conventional one except using the modal information of bare SCR obtained from initial analysis of bare SCR.

Design and hydrodynamic loading analysis of production riser for the Arctic

Production risers are usually designed to have an operating life of about 15-25 years. Based on the lumped mass method, rigid and flexible risers in varied configurations have been applied to a global analysis in order to evaluate the static, dynamic and fatigue behaviour of the riser suitable for operation in the Arctic. Analyses were performed to assess the feasibility of the risers by considering their stress performance and fatigue life under the applied environmental loads of Shtokman field in Eastern Barents Sea. The results from the global analysis indicate that rigid risers perform poorly in 'shallow' water with von Mises stress higher than allowable stress as indicated by Det Norske Veritas (DNV) and America Petroleum Institute (API). However, this research paper also suggests that flexible risers are not very sensitive to hydrodynamic loading as indicated by the reduced von Mises stress and improved mechanical performance of the risers.

Re-examination of natural frequencies of marine risers by variational iteration method

Marine risers are important components operating in offshore oil and gas industry. The vortex-induced vibration design of marine risers requires accurate knowledge of natural frequencies and mode shapes. Free vibration of marine risers are re-examined in this paper by means of variational iteration method, which is relatively new technique capable of dealing with eigenvalue problems rather efficiently. Solutions from the variational iteration method are compared to approximate solutions previously proposed in literatures via a numerical example. Furthermore, validation of the technique is demonstrated by comparing experimentally measured natural frequencies of model marine riser with the predicted ones.

Coupled fluid–structure simulations for evaluating a performance of full-scale deepwater composite riser

A global–local analysis methodology based on fluid–structure coupling is used to investigate the mechanical responses of both composite and steel risers. Since the design of the riser system can be a daunting task, involving hundreds of load cases for global analysis, semi-empirical fluid load models are considered for the reduced order computations of full-scale riser models. The structural performance of composite risers under real sea current conditions is investigated systematically and discussed with regard to the practical concerns in full-scale settings. The failure envelops of internal liners are found to be within that of the composite layers, which reveals that the liner is the weakest link for composite riser design. Results show that the composite risers can be more prone to vortex-induced vibration (VIV) due to their lower structural frequencies. In the present study, the composite riser yields 25.5% higher RMS strains than the steel riser. Placement of buoyancy modules along the riser may be critical for the design against VIV, and our results show that the modules are not recommended at the top region of the riser, especially if a top-sheared current is expected. Instead, it is preferable to implement them at the bottom-half portion of the riser and as a continuously buoyed region rather than short discrete buoys separated with gap spaces. Composite risers with different metallic liners are studied, and the titanium liner riser is found to be favourable over the steel and aluminum liner risers.

An integrated methodology for the design of mooring systems and risers

Traditionally, the design of mooring lines and risers of floating production systems (FPS) has been performed separately, by different teams, employing uncoupled analysis tools that do not consider the nonlinear interaction between the platform hull and the mooring lines and risers. Design processes have been focused on fulfilling the design criteria of the respective component (mooring/riser) alone, with few or no consideration to the other component, and little interaction between the design teams. Nowadays the importance of employing analysis tools based on coupled formulations is widely recognized, and analysis strategies have been proposed to consider feedback between mooring lines and risers within their respective design processes.In this context, this work details a proposal of one single and fully integrated design methodology for mooring systems and risers for deep-water FPS. In this methodology, the design stages of both risers and mooring lines are incorporated in a single spiral, allowing the full interaction of different teams; mooring design implicitly considers the riser integrity, and vice-versa, leading to gains in efficiency and cost reduction.Different analysis strategies are employed, taking advantage of uncoupled and coupled numerical models. The models generated at the initial/intermediate design stages can be reused in subsequent stages: simpler models are used in the initial stages, and more refined models are gradually introduced, to reach an ideal balance between computational cost and accuracy of results. In the advanced stages, the exchange of information between mooring/riser also allows the definition of criteria for the selection of governing/critical loading cases to be revised and verified in detail. This leads to the reduction of the original loading case matrix, allowing a feasible use of time-consuming fully coupled analysis.Results of a case study illustrating the application of some of the main processes of the methodology are included.

Artificial neural network development for stress analysis of steel catenary risers: Sensitivity study and approximation of static stress range

Fatigue design of steel catenary risers (SCRs) is an important challenge especially in the touchdown zone (TDZ). Numerous parameters affect the fatigue damage in the TDZ, including those pertaining to riser motions, riser characteristics and soil properties. So far, only a few sensitivity studies have been published with limited applications, considering small ranges, investigating only a selection of input parameters or failing to examine the interactions between input parameters.This paper aims to test the robustness of previous research and extend the ranges of the input parameters for SCR systems under static loading, by means of numerical simulations. An approximation of the critical stress range in the TDZ defined by the authors previously was refined to assist the sensitivity studies. A large database was created using an automation subroutine coded in Python programming language that links the marine analysis software OrcaFlex and the optimisation software modeFRONTIER. Design of experiment techniques were used for post-processing and quantify the relative effects of the various dimensionless groups and their interactions. An approximation using a series of artificial neural networks is presented; it successfully approximates over 99% of the cases of the database with an accuracy of ±5%.

Dynamic analysis of coupling between floating top-end heave and riser's vortex-induced vibration by using finite element simulations

The dynamic coupling between floating top and submarine riser becomes more remarkable owing to larger fluctuation amplitude of floating platform in deeper water, compared to fixed platform in shallow water. In this study, the impacts of top-end vertical motion (heave) on riser's vortex-induced vibration (VIV) are explored by means of finite element simulations. A coupled hydrodynamic force approach, regarding vortex-induced lift force along with fluid drag force, is developed, which takes into account of the interaction between instantaneous structure motion and fluid dynamics. Then the dynamic responses of the integrate system including both floating top-end and a riser undergoing VIV are examined based on our numerical simulations. The influences of platform heave, in terms of heave frequencies and tension ratios, on riser's VIV are presented. Our numerical results show that the dynamic response displacement of riser becomes several times larger than the displacement for the case without top-end motion. The impact of top heave on riser's VIV gets larger as the modal order number drops. Moreover, an interesting phenomenon, called the mode transition, is observed particularly at lower vibration frequencies due to the natural dynamic characteristics of the slender riser. We suggest that, in practices of riser design, a combined excitation needs to be considered for the accurate dynamic analysis of slender marine structures subjected to a top-end motion and VIV.

VIV response of a flexible cylinder with varied coverage by buoyancy elements and helical strakes

Many significant engineering challenges have emerged as the petroleum industry has moved their field development and production activities into increasingly deeper water depths. The design of deepwater marine risers presents the combined challenges to minimize top tensioning requirements, mitigate any flow-induced vibrations, and if possible to increase the expected fatigue life of these slender structural members. As part of the design process to achieve these goals external buoyancy modules and strakes have been employed. To gain insight into the complex multi-mode response behavior a recent experimental study was performed and the analysis of selected data sets is presented. In the experiments a horizontal cylinder with a length to diameter ratio of 263 was fitted with a variety of strake and buoyancy element configurations. The models were towed at uniform speeds ranging from 0.4 to 2.0 m/s and fiber optic strain gages were used to measure both in-line and cross-flow strain response. The resulting time series information was processed utilizing the method of time domain decomposition formulated for strain data input and the introduction of modal assurance criterion to resolve the modal strain information that included frequency, mode shape, and critical damping ratio information. The pre-tensioned cylinder without appendages was used as a base case and the results were basically consistent with expectations. In the case of 0.8 m/s low-tension test, multiple closely spaced non-overlapping peaks were observed in both in-line and cross-flow directions and were identified as being of the same mode with mode shapes distorted away from purely sinusoidal behavior. The test data for the 100% coverage by helical stakes demonstrated the effectiveness of that suppression device over the range of current velocities investigated. The most interesting case was that of a staggered combination of helical strakes and buoyancy element whose total for each type of coverage was equal. This effective asymmetric VIV suppression approach is presented and discussed in detail.

Analysis of Vortex-Induced Vibration of Riser using Spalart-Almaras Model

Vortex-induced vibration is natural phenomena where an object is exposed to moving fluid caused vibration of the object. Vortex-induced vibration occurred due to vortex shedding behind the object. One of the offshore structures that experience this vortex-induced vibration is riser. The riser experience vortex-induced vibration due to vortex shedding caused by external load which is sea current. The effect of this vortex shedding to the riser is fatigue damage. Vortex-induced vibration of riser becomes the main concern in oil and gas industry since there will be a lots of money to be invested for the installation and maintenance of the riser. The previous studies of this vortex-induced vibration have been conducted by experimental method and Computational Fluid Dynamics (CFD) method in order to predict the vortex shedding behaviour behind the riser body for the determination of way to improve the riser design. This thesis represented the analysis of vortex induced vibration of rigid riser in two-dimensional. The analysis is conducted using Computational Fluid Dynamic (CFD) simulations at Reynolds number at 40, 200, 1000, and 1500. The simulations were performed using Spalart-Allmaras turbulent model to solve the transport equation of turbulent viscosity. The simulations results at Reynolds number 40 and 200 is compared with the other studies for the validation of the simulation, then further simulations were conducted at Reynolds number of 1000 and 1500. The coefficient of lift and drag were obtained from the simulations. The comparison of lift and drag coefficient between the simulation results in this study and experiment results from the other studies showed good agreement. Besides that, the in-line vibration and cross-flow vibration at different Reynolds number were also investigated. The drag coefficient obtained from the simulation results remain unchanged as the Reynolds number increased from 200 to 1500. The lift coefficient obtained from the simulations increased as the Reynolds number increased from 40 to 1500.

Influence of riser design on macrosegregation in static castings

The flow between a casting and riser and the consequent macrosegregation development in an aluminium alloy is examined. The riser is designed using Campbell’s rules and solidification is simulated. The influence of cooling conditions and riser shape on macrosegregation patterns is studied. The relative importance of buoyancy and shrinkage induced flows is also simulated. For castings with a fixed riser shape, the average composition in the cast part increases due to the exchange of interdendritic fluid between the riser and the part. This exchange is smaller at higher solidification rates and when the bottom of the mould is chilled. Although the buoyancy induced flow does move some interdendritic liquid from the riser to the casting, shrinkage is more significant in increasing its average composition. A wider riser provides a barrier to flow, slowing the introduction of heavier interdendritic liquid from the riser into the casting, keeping it closer to the nominal composition.

Wave Forces on Linear Arrays of Rigid Vertical Circular Cylinders in Regular Waves

The present investigation aims to experimentally determine the variation of forces and force coefficients acting on circular cylinders, which are arranged in a linear array along the direction of the waves. Most commonly used structural and non-structural elements in the construction of offshore platforms are circular cylindrical members. In many cases, these members are found in very close neighbourhood of each other, thus modifying the surrounding flow and wave forces acting on them. Model tests were conducted in the wave tank on a maximum of four cylinders of the same diameter. A reasonable scale factor was chosen considering the pertinent factors such as water depth, wave generating capability and accuracy of measurements. The cylinders were installed inside the wave tank as vertical cantilevers fixed at the top. Wave forces acting on the cylinders were measured using special wave force sensors exclusively designed and fabricated for the present project, while the wave profiles were recorded using wave probes installed in the wave basin. The results confirmed the presence of a force shielding effect on the trailing cylinders by the leading cylinders with few exceptions. The findings also substantiated the significant modification of the forces on cylinders when they are present in a linear array. A common practice adopted for the design of offshore platforms was identified with a possibility of underestimating the wave forces acting on the cylindrical elements. In many cases, the experimentally computed hydrodynamic force coefficients were found to be lower than the standard values adopted by various design codes. These findings portray the significance of the present work in achieving economy in the design of jacket platforms and risers.

Enhanced multi-layer fatigue-analysis approach for unbonded flexible risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.