Offshore Risers Research Articles

Application of regularization method with sparsity prior in riser vortex-induced vibration reconstruction

In the experimental investigation or in-situ monitoring of the vortex-induced vibration (VIV) of offshore risers, it is often necessary to reconstruct the full-field structural response based on discrete observations. Conventionally, this can be done using the modal decomposition method. However, the method can be invalid with fewer measurements than unknown modes and is unstable when the data contains measurement noises. This study presents a novel sparse modal decomposition method based on the compressed sensing theory and the Compressive Sampling Matching Pursuit (CoSaMP) algorithm. It formulates the reconstruction problem in a regularization framework with a prior related to the sparsity of the modal coordinates, which differs from the classic method that seeks a least-square solution. Verification tests demonstrate that our method outperforms the classic method regarding accuracy and stability, especially when the measurements are noisy and insufficient, highlighting its potential applications in engineering.

Lifetime assessment for riser systems

As marine renewable energy technologies developing, there is a growing need for energy transportation systems. During offshore operations, deep sea risers can be subjected to excessive environmental loadings, causing operational risks. In this study, hydrodynamic loads, caused by in situ sea currents, acting on a riser under real-world sea conditions were modelled and examined, with experimental data being used as a calibration tool. Major safety problems for various offshore energy systems being an accurate assessment of excessive riser external loads, under influence of local sea currents, and hence resulting vortex induced vibrations (VIV).The method outlined in this study may be applied to complex sustainable energy systems, that are exposed to environmental loads, throughout the whole period of their intended service life. Approach advocated in this study offers practical way to estimate failure risks for nonlinear multidimensional dynamic offshore riser systems in an easy yet accurate manner.With regard to defense technology, risers and umbilicals play an important role for modern submarine operations.

Numerical and experimental investigation on tensile fatigue performance of reinforced thermoplastic pipes

With the rapid development of offshore drilling for harvesting oil and gas from the deep sea, the fatigue performance of offshore risers is increasingly demanding. This study investigated the tensile fatigue performance of reinforced thermoplastic pipes (RTPs). The theory of three-dimensional (3D) anisotropic elastic mechanics was combined with a strain potential-energy density function to construct the 3D constitutive relation of RTPs. The stiffness degradation law of RTPs was predicted by introducing a modified 3D Hashin failure criterion and the residual stiffness model. A user-defined material (UMAT) subroutine was developed to evaluate the progressive fatigue failure process of RTPs. A uniaxial tensile fatigue experiment of RTPs was designed to validate the numerical model's correctness. The analysis examined the damage failure modes of RTPs under cyclic loading conditions and revealed the stiffness degradation mechanism. Results suggested that the matrix fatigue damage of the reinforced layer of RTPs is the primary cause of the rapid stiffness degradation, which was observed to be faster with increasing cyclic load stress levels.

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.

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.

Study on the parameters of detached splitter plate for VIV suppression

Cylindrical structures such as offshore risers and offshore platform legs will suffer fatigue damage due to Vortex-induced Vibration (VIV). It is necessary to study the suppression scheme of cylindrical VIV. The rigid splitter plate has the characteristics of simple structure, low maintenance cost, and no external energy. Therefore, it becomes one of the suitable VIV suppression schemes. Based on numerical simulations, the design of a downstream detached splitter plate is investigated at a fixed flow rate. The suppression effects of 3 main parameters, distribution gap, splitter plate width and splitter plate length, are analysed. The study found that there is an obvious effective action interval for the distribution gap. The splitter plate provides better suppression when the gap is within this interval. In addition, the length of the splitter plate is the most significant parameter for the suppression effect. When the length is too small, the vibration response of the cylinder will be further aggravated. As the length continues to increase, so does the overall suppression effect of the splitter plate. Therefore, when the 3 parameters are properly combined and the length is as long as possible within the feasible range, the splitter plate can achieve a significant suppression effect.

Development of a Procedure for Risk-Based Qualification of Additively Manufactured Components: Adopting to Oil and Gas Industrial Applications

Recent advances in additive manufacturing (AM) technology provide the potential for on-demand and rapid production of spare parts during urgent repair times. Recently, big oil and gas companies have shown early progress in using additive technology in manufacturing specific heat exchangers, downhole cleanout tool nozzles, offshore risers, gas turbine nozzles, and subsea chemical stick injection tools. Despite the mentioned progress, the current adoption level of additive technology for the offshore oil and gas industry is very limited. Non-destructive and destructive evaluation methods of additively manufactured metallic components have been studied extensively. However, the technique selection procedure and scope of the required test methods have not been studied sufficiently. This paper discusses various elements related to the qualification of additively manufactured components for application in the oil and gas industry. A risk-based qualification method for identifying the scopes of required non-destructive and destructive tests and the resulting qualification procedure for additively manufactured spare components in offshore oil and gas applications is suggested.

Variation in Vortex-Induced Vibration Phenomenon Due to Surface Roughness on Low- and High-Mass-Ratio Circular Cylinders: A Numerical Study

Fluid–structure interaction has been widely studied in the last few decades due to its wide range of applications in engineering fields. This phenomenon plays an important design role, for example, in offshore risers, high slender buildings, chimney stacks and heat exchangers. The vortex shedding generated from a bluff body can induce high-amplitude oscillations, known as vortex-induced vibrations (VIVs). This study presents a numerical analysis to investigate the impact of surface roughness on VIV in the crossflow direction of a circular cylinder. The study also investigates the impact of surface roughness with variation in mass ratio from 2.4 to 11 at a high Reynolds number (Re) = 104 using Reynolds-averaged Navier–Stokes (RANS) equations. The study concludes that roughness on a cylinder results in a reduction in amplitude response. Furthermore, the lock-in region is narrower compared to that of a smooth cylinder, irrespective of the mass ratio. However, it is observed that the impact of surface roughness is more significant in high-mass-ratio cylinders where the lock-in region is more squeezed and shifted toward lower reduced velocities. Furthermore, the vortex mode beyond reduced velocities Ur = 5.84 and 7.52 was observed to be 2S for high and low mass ratios, respectively.

Analysis of flow-induced vibration characteristics for flexible riser attached with non-rotating water-drop fairing

One of the measures of suppressing vortex-induced vibration (VIV) for the offshore riser is to install a streamlined fairing device. However, the galloping phenomenon may occur for a non-rotating water-drop fairing, which has a serious influence on the riser operation. An improved meshless discrete vortex method (DVM) in a two-dimensional framework is employed to calculate the fluid forces for flow past an arbitrary geometry. Meanwhile, by coupling the mass-damping-stiffness kinematic equation, VIV responses of a circular cylinder with one degree-of-freedom (DOF) under a wide range of reduced velocities can be determined. Then, systematic simulations are carried out to investigate the effects of physical parameters, including free-stream velocity and trailing edge angle, on the galloping characteristics of fairing structures. At last, a quasi-three-dimensional DVM-finite element method (FEM) coupled strategy is employed to calculate the vibration responses of a flexible riser attached with the water-drop fairing. The results indicate that the non-rotating water-drop fairing can bring about intense galloping instability behaviour. The predicted vibration amplitudes, frequencies and fluid forces for different fairings as well as the circular cylinder are compared. Furthermore, a comparison between flow-induced vibration (FIV) characteristics of the bare riser and flexible risers fitted with various fairing devices is conducted.

Development of a prototype autonomous inspection robot for offshore riser cables

The motion and large environmental loads experienced by riser cables connected to floating offshore wind turbines put them at higher risk of failure than grounded sections of cable. We propose an Autonomous Riser Inspection System (ARIS) to facilitate regular inspection of riser cables for early fault detection and gathering engineering data to improve future cable design. Novel robotic methods for automatic attachment, traversal and inspection of cables are described. We develop the sensing and intelligent processing needed to (i) enable autonomous traversal and position estimation, and (ii) gather information about the state and condition of riser cables. Information from an array of cameras and electric actuator current monitoring allows autonomous navigation around obstacles and identifying the end of the inspection. A visual processing algorithm identifies damage to the surface of the cable and sensing methods are developed to measures the cable catenary, where methods to estimate the robot’s position along the cable are developed to contextualise and efficiently re-locate observations. Technology concepts are verified through a combination of dry, lab-based experiments using a full-scale prototype and simulations. The results provide proof of concept that requirements for untethered, autonomous riser cable inspection can be met by the proposed system.

Research on bypass pigging in offshore riser system to mitigate severe slugging

During pigging operations in offshore pipelines, severe slugging can block the pig or cause sudden velocity changes. Therefore, this paper proposed a method involving bypass pigs to mitigate the influence of severe slugging on the pigging process. The impact of different inlet mass flow rates and pig bypass fractions on the characteristic parameters, such as the pressure in the riser bottom, pig velocity, and liquid-phase distribution patterns, were analyzed by combining OLGA and CFD simulations. Furthermore, the accuracy of the simulations was verified by the comparisons of OLGA and CFD, as well as the transient indoor experiments. The results show that the bypass pig significantly reduces pressure fluctuations in the riser bottom and pig velocity in severe slugging conditions, compared with the conventional pig. Consequently, the bypass pig dissipates downstream liquid slugs, reducing the mass flow rate and liquid holdup in the riser outlet. In addition, an increase in bypass fraction reduces the pressure fluctuations in the riser bottom and pig velocity, increases the ability to dissipate the downstream liquid slug, and decreases the mass flow rate and liquid holdup in the riser outlet. Increasing the inlet mass flow rate improves the ability to dissipate the downstream liquid slug. However, the inlet mass flow rate has a lower mitigation effect on severe slugging than the bypass fraction.

Vortex-induced vibration of risers with staggered buoyancy modules of small aspect ratio

Vortex-induced vibration (VIV) is one of the critical design considerations for the offshore riser and pipeline system. Significant research effort has been devoted to better understanding and predicting riser VIV. Compared to the bare uniform riser configuration, the attached staggered buoyancy modules of a small aspect ratio will extensively alter riser hydrodynamic characters, and hence the riser dynamic response. In the current study, experiments were performed on forced inline (IL)-crossflow (CF) combined vibration of a rigid cylinder with staggered modules (module to bare cylinder diameter and length ratio of 2.5 and 1.0.) as well as self-induced vibration of two flexible models with 100% and 50% buoyancy module coverage ratios. From the rigid model experiment, positive regions for lift coefficient in the phase of velocity (Clv) for both riser and buoyancy module induced vibration are observed. The result indicates potential power-in excitation from both riser and buoyancy modules, which is later confirmed by the bi-frequency and bi-modal vibration from the flexible cylinder experiment. Furthermore, several other phenomena have been reported, including low reduced frequency for buoyancy module induced motion, the phase between CF and IL trajectory, and buoyancy module VIV suppression ability, emphasizing the connection between the rigid and flexible cylinder experiment.

Fatigue damage assessment of offshore riser subjected to vortex-induced vibrations by SHEAR7

Marine risers are a critical component in transporting the produced hydrocarbons from the seabed to the topside facility and vice versa. These risers are subjected to VIV when encountered with ocean currents, leading to the accumulation of fatigue damage, which reduces the risers' service life. Estimation of fatigue damage is an important aspect to assure reliable and operable conditions. SHEAR7 is used in this analysis to estimate the fatigue damage of the Top Tensioned Riser (TTR). SHEAR7 is widely used semi-empirical software in the offshore industry to estimate the fatigue damage induced by VIV. Recently, SHEAR7 has been revised to account for the higher harmonic corrections based on experimental and field data. The key pertaining to the revision of adding higher harmonics can contribute 50% higher strain along the length of the riser at some locations (Jhingran and Vandiver, 2007). VIV fatigue damage analysis was conducted using SHEAR7 v4.7b and v4.9b to investigate the effect of the higher harmonics considered in the later version. The TTR model used in this analysis was modelled in OrcaFlex and validated with available references in the literature. TTR with different model scenarios was developed, and comparisons were made between the predicated responses from SHEAR7 v4.b and v4.9b for the riser. It was observed that higher harmonic correction factors have a great impact on the performance of the riser. Finally, sensitivity analyses were carried out to check the effect of individual parameters and higher harmonic correction factors on predicted responses between the two versions of SHEAR7. • The fatigue damage of the top tensioned riser subjected to vortex-induced vibrations was analyzed. • The effect of higher harmonic frequencies on the fatigue performance of the riser was numerically investigated. • The difference between the riser responses for with and without higher harmonic frequencies was statistically evaluated. • The parametric study was carried out to estimate the effect of individual design parameters on the riser performance.

An efficient probabilistic framework for the long-term fatigue assessment of large diameter steel risers

Using recent advances in data analytics techniques and tools, this paper proposes a novel data-centric paradigm for riser fatigue analysis in which practitioners can efficiently make use of large hindcast and measured metocean datasets directly in the design process. In the conventional fatigue design method for offshore risers systems, in order to reduce the computational cost practitioners tend to condense large hindcast metocean datasets into small wave datasets, so-called wave scatter diagrams, to represent the long-term variation in significant wave height, Hs, and peak period, Tp, at a site of interest. These methods tend to predict rather conservative riser loading conditions and hence fatigue lives due to the information lost in the condensing method, which can limit the application of steel catenary and lazy wave risers (SCRs and SLWRs) with larger diameters (outside diameter, OD > 18") whose design is governed by fatigue. This paper proposes enhanced frameworks for more accurate prediction of the fatigue life of SCRs and SLWRs located in swell dominant regions by enabling the direct use of large measured and hindcast metocean datasets in the fatigue design process of risers. Firstly, we introduce a novel detailed framework that uses an “ANN-based technique” with a “representative (P50) year” concept to enable large metocean datasets to be directly used for accurate estimation of riser fatigue life that is suitable for detailed design stages. Secondly, we propose an alternative innovative framework based on the use of Monte Carlo methods for more efficient long-term fatigue assessment of risers. Finally, we demonstrate the usefulness of the proposed frameworks by calculating the riser fatigue lives and comparing them against those obtained via conventional wave-condensing strategies. For the case studied here, the results show that using the proposed framework can significantly increase the estimated riser fatigue life over the values estimated using conventional wave condensing methods.

Numerical modelling of offshore risers conveying slug flow under the ALE–ANCF framework

A nonuniform distribution of internal flow along the offshore riser can lead to transient in-plane excitation. To predict the global riser response due to slug flow by nonlinear time domain analyses, a finite element method is developed in this study by combining the absolute nodal coordinate formula (ANCF) and arbitrary Lagrange-Eulerian (ALE) description. This proposed method has the capacity to effectively simulate both the nonuniform gravity distribution of slug flow and geometric nonlinearity of the riser. The nodal position and slope vectors described in the global coordinate system avoid the complex coordinate transformation of the elements while the internal fluid is effectively integrated into the framework. After the accuracy of the proposed program is verified, a kind of steep wave riser (SWR) with internal flow is taken as an example to discuss the effect of slug flow. The results show that slug flow can result in large local deformations and lead to variable pattern changes in the bending moment.

Experimental investigation on vortex-induced vibration of variable diameter riser with uniform flow

Diameter design of the riser is a very important link in offshore riser engineering, which will affect a series of parameter changes of the riser. In this paper, the riser is a variable-diameter riser with different cross-sections spliced by riser sections with diameters of 18, 25, and 30. In the wave-flow combined flume, dynamic response parameters such as strain-time history curve and root-mean-square of dimensionless strain were obtained by changing the outflow velocity and diameter of the riser, the effect of variable diameter on vortex-induced vibration of the riser was studied in detail. The results show that the vibration amplitude of the variable diameter riser varies with the axial position of the riser, but the frequency is dominated by the excitation of the thickest section riser; the variable diameter riser is always dominated by the first-order frequency in the outflow velocity range, and the relationship between the in-line and the cross-flow dominant frequency is always 2 times. With the increase of the flow velocity, the root mean square value of strain increases steadily.

Effect of welding on microstructure and mechanical response of X100Q bainitic steel through nanoindentation, tensile, cyclic plasticity and fatigue characterisation

This paper presents an experimental characterisation of fatigue at welded connections for the next-generation high-strength low-alloy offshore riser steel, X100Q. An instrumented girth weld is conducted with a parallel programme of physical-thermal simulation (Gleeble) to develop heat affected zone (HAZ) test specimens. X100Q is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. Significant differences are demonstrated between the parent material and simulated HAZ in terms of hardness, monotonic strength and cyclic plasticity response, which can be related to the observed microstructural transformations: the refined grain and bainitic block size in the fine-grained HAZ are shown to give a harder and stronger response than parent material, whereas the coarsened bainitic lath structure in the intercritical HAZ gives a softer and weaker response. The simulated HAZ materials exhibit superior fatigue performance to the parent material and weld metal. A significant reduction in life is shown for cross-weld specimens, indicating susceptibility to failure due to HAZ softening for matched or over-matched X100Q welds.

Suppression of Vortex-Induced Vibration by Fairings on Marine Risers

Vortex-induced vibration is quite common during the operation of offshore risers or umbilical cables, commonly leading to serious damage to risers and reduced service life. Vortex-induced vibration of the offshore risers could be effectively suppressed by fairing devices. In this paper, a newly developed vortex-induced vibration fairing and large eddy simulation model of the FLUENT software were used for numerical analysis, experimental research and stimulating vortex-induced vibration at 0.1–2 m s –1 . The data of the numerical model with fairing was compared and analyzed to study the vortex shedding frequency at different Reynolds numbers and changes in drag and lift coefficients. The displacement state of 12 in risers with and without fairing was experimentally tested using a five degree-of-freedom balance. The vortex-induced vibration effect of the fairing was tested at different velocities. The result shows the drag reduction effect of the fairing is more obvious when the flow velocity is 0.4–1.2 m s –1 and the maximum drag reduction reaches 55.6% when the flow velocity is 0.6 m s –1 . Additionally, the drag reduction effect was obvious when the flow velocity was greater than 1.3 m s –1 and less than 0.3. The result indicates that the developed 12 in fairing, with good potential in engineering applications, has good vortex-induced vibration-suppression effects.

Active SHM for composite pipes using piezoelectric sensors

Composite materials, in addition to the high specific mechanical properties, have properties enabling their applications in high-pressure, high-temperature (HPHT) and corrosive environment as occurs in deep water. Their use for manufacturing pipelines and offshore risers can provide relevant performance advantages over steel such as lower weight, improved fatigue capacity, corrosion resistance and higher strain limits. However, composite materials are more complex to use in design than metallic materials due to their anisotropic properties and lack of accurate failure prediction models. Thus, a continuous in-situ and in real-time Structural Health Monitoring (SHM) of composite components would be necessary and useful to promote their use in a wider range of operational conditions. In this work, an FRP pipe sample was instrumented with Piezoelectric Wafer Active Sensors (PWAS) used either as passive receivers or as transmitters of guided waves for active health monitoring in pitch-catch configuration. The propagation properties of guided waves in glass fibres reinforced composites were studied by developing MATLAB scripts, running FE simulation and experiments. The numerical and experimental signals were post-processed in MATLAB by Short Time Fourier Transform (STFT) in order to evaluate their frequency and time-frequency content. Furthermore, the guided waves were used to detect artificial defects imposed on the structure identifying their location. Several testcases were studied to find out the limitations and the most suitable conditions of using guided waves for defects monitoring in a composite pipe. The work proposes effective methods for pipe structural health evaluation by non-destructive techniques and ultrasonic guided waves.

Failure analysis of thermoplastic composite pipe (TCP) under combined pressure, tension and thermal gradient for an offshore riser application

Useful characteristics of fibre-reinforced thermoplastics, including high specific strengths and moduli and excellent corrosion resistance, make them ideal candidates for offshore riser pipe applications. When in operation, risers are subjected to combined mechanical and thermal loading. In the present paper, a 3D finite element (FE) model is used to analyse stress state in a section of thermoplastic composite pipe (TCP), consisting of a fibre-reinforced thermoplastic laminate fully bonded between inner and outer thermoplastic liners, under conditions illustrative of a deepwater riser application. The effects of varying combinations of pressure and thermal differentials on the distribution of stress-based failure coefficient are examined. Failure responses under different axial tensions at low/high pressures and temperatures are assessed for configurations with different laminate ply stacking sequences. Temperature-dependent material properties are considered in the thermomechanical analysis.