Production Risers Research Articles

Analytical Modeling of the Lazy-Wave Hydrogen Production Riser (HPR) with Incorporation of Seabed Interaction in the Touchdown Zone

Analytical Modeling of the Lazy-Wave Hydrogen Production Riser (HPR) with Incorporation of Seabed Interaction in the Touchdown Zone

Emerging Trends in Composite and Advanced Composite Materials Technology for Oil and Gas Industry

Composite and advanced composite materials are new age materials offering multiple benefits such as longer durability, lighter weight, high strength, tailor made properties etc. Lessons have been learnt from successful applications of composite and advanced composite materials and the results indicate that operators, designers and contractors are beginning to take a serious interest in their wide applications in Oil and Gas Industry. The most significant advances in the field of composite materials technology has been the use of glass-reinforced plastics (GRP) for pipelines and piping, structural applications for the top side secondary structures of offshore platforms and onshore installations. The introduction of carbon fibers has opened a new vista for composites applications in oil and gas industry particularly in deepwater operations such as drilling risers, production risers, umbilical, moorings etc. Numerous risers, mooring, pipeline, subsea and topside components made of polyester, titanium, glass/epoxy, and glass/carbon/epoxy materials are in the process of qualification for offshore use. These advanced composite material components have potential to extend capabilities of existing platform concept such as Semi submersibles, TLPs, FPSOs and SPARs. This paper presents the properties and advantages offered by composite and advanced composite materials, their current use and future potential applications in oil and gas industry.

The absolute nodal coordinate formulation in the analysis of offshore floating operations, Part II: Code validation and case study

The absolute nodal coordinate formulation (ANCF) proposed in Part I of this paper series is applied to simulate flexible structures in the offshore operation analysis. The Part II of this paper series aims to validate the in-house code MiNos developed based on ANCF, a module of software KraKen, and employ the code to the static and dynamic analysis in time domain for flexible cables and mooring lines in actual operation scenarios. Three validation cases are conducted to demonstrate the accuracy and robustness of the in-house program for time domain analysis of arbitrary large deformation flexible structures. Furthermore, various floating operations, including crude export, production riser operation, and emergency disconnection of a turret are simulated by MiNos for both static and dynamic analysis, the results of the flexible cables and lines in these cases are compared with commercial software OrcaFlex. The high continuity of ANCF elements ensures high accuracy in the analysis of floating operation which comprises sophisticated vessel motions, wave loads, and soil contact. Besides, the inherent numerical problems of nonlinear dynamic analysis such as non-physics oscillations and peaks can be mitigated by ANCF. Therefore, ANCF could be considered as a promising theory for the dynamic analysis of flexible structure of cables and lines in marine operations.

Wave Effects on Vortex-Induced Vibrations of a Top-Tensioned Riser

Abstract Offshore drilling risers and many production risers are top-tensioned risers (TTRs), connecting the vessel and seabed via joints. External loads such as currents, waves, and vessel motions introduce cyclic loads and motions on riser sections and associated components, such as universal joint, flex joints, and well head at the bottom, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIVs) caused by pure currents, and it is not fully understood. Several model test campaigns on TTR have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves, and vessel motions were modeled, and the riser model responses were measured. In this study, selected cases from such model tests are analyzed, and used to validate a semi-empirical time domain VIV prediction tool—VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of combined wave and VIV prediction is further reduced.

Collision prevention control method for production riser system in deep sea

In the deep-sea oil and gas exploitation process, the adjacent risers under the influence of environmental load and shielding effect will produce large amplitude vibration, which may lead to collision and damage to the riser structure. However, there is a lack of study on active collision-preventing control based on vessel flexible riser array system. This paper aims to better prevent the collision between vessel-risers and increase the motion spacing. Preliminarily, we model the vessel-risers system composed of top production vessel, riser array and environment load based on Hamilton's principle and the improved shielding effect model. In the next part, considering the control spillover problem, we design the collision-preventing boundary controllers directly for distributed parameter system (DPS) without discretization or simplification. And the uniform boundedness of the control system is strictly proved by the Lyapunov direct method. Eventually, we verify the effectiveness and feasibility of the collision-preventing controllers by simulating the system in different ocean currents. At the same time, the parameters of the collision-preventing controllers are independent of those of the riser array system, which ensures the robustness of the system.

Investigating the Topside Choking and Riser Base Gas lifting Methods of Attenuating Severe Slug Formation in Deepwater Production Risers

Severe slugging involves large magnitude fluctuations in pressure and flow rate in a riser due to low gas flow rates and low Gas-liquid ratio (GLR). This work evaluates two mitigation concepts: topside choking and riser base gas lift to determine which technique is more desirable.
 A base case model was created to understand the nature of slugging with an input mass flowrate of 15kg/s, separator operating at 22.5Bar and 370C. After 8hours of simulation, the presence of a severe slug was observed at the production riser. Two models were created from the base case to mitigate the severe slugging. An introduction of topside choke to eliminate or reduce the riser slugging, likewise deployment of Riser base gas lift (RBGL) to achieve the same purpose.
 The result indicated that with the introduction of the topside choke and reducing the opening percentages of the choke from 80% to 5% stepwise, the slugging amplitudes were greatly reduced from 3.3bar to 0.3bar. Though this led to back pressure and a reduction in production. A 50% choke opening was suggested as the optimal choke opening for the simulated case. Results from the introduction of dehydrated gas with a mass flow rate of 1.2kg/s at the riser base indicated slug elimination from the riser and flowline. The results of this simulation were observed at the end of the riser (inlet to the separator) after about 8hours 20mins
 This report identifies that of the two simulated techniques (topside choking and RBGL), RBGL is more efficient in addressing slug issues without impacting field production.

Tailoring the Local Design of Deep Water Composite Risers to Minimise Structural Weight

Following the rising technological advancements on composite marine structures, there is a corresponding surge in the demand for its deployment as ocean engineering applications. The push for exploration activities in deep waters necessitates the need for composite marine structures to reduce structural payload and lessen weights/loads on platform decks. This gain is achieved by its high strength–stiffness modulus and light-in-weight attributes, enabling easier marine/offshore operations. Thus, the development of composite marine risers considers critical composite characteristics to optimize marine risers’ design. Hence, an in-depth study on composite production risers (CPR) is quite pertinent in applying composite materials to deep water applications. Two riser sections of 3 m and 5 m were investigated under a 2030 m water depth environment to minimise structural weight. ANSYS Composites ACP was utilized for the CPR’s finite element model (FEM) under different load conditions. The choice of the material, the fibre orientation, and the lay-up configurations utilised in the modelling technique have been reported. In addition, the behaviour of the composite risers’ layers under four loadings has been investigated under marine conditions. Recommendations were made for the composite tubular structure. Results on stresses and weight savings were obtained from different composite riser configurations. The recommended composite riser design that showed the best performance is AS4/PEEK utilising PEEK liner, however more work is suggested using global design loadings on the CPR.

Local tailored design of deep water composite risers subjected to burst, collapse and tension loads

There is an increasing demand for more sustainable and adaptable technologies in the offshore industry. Thus, composite structures are more frequently used in the industry. Secondly, the shift in offshore exploration from shallow to deep waters has necessitated advancements on more lightweight composite structures to minimise platform loads, ease fluid transport, and service, using conduit pipes like composite production risers (CPR). The properties of composites are harnessed to improve riser weight and strength performance as Composite risers. In this research, the local tailored design of deep water composite risers was subjected to burst, collapse, tension and combined loads. Three design methodologies are considered-conventional, local, and tailored designs for different configurations. The structure is made up of configurations having 17 layers, 18 layers, 19 layers, 20 layers and 21 layers. The numerical technique for the CPR model with 3 m section considered five loadings. The model was developed utilising ANSYS ACP to conduct the numerical stress analysis with parametric studies. From the design, it is recommended to consider liners like PA12 and include environmental loads from global design. This study presents the CPR behaviour. Since CPR structures are response-sensitive and fatigue-sensitive, the global dynamic analysis of full-length composite riser is recommended.

Review of Composite Marine Risers for Deep-Water Applications: Design, Development and Mechanics

In recent times, the utilisation of marine composites in tubular structures has grown in popularity. These applications include composite risers and related SURF (subsea umbilicals, risers and flowlines) units. The composite industry has evolved in the development of advanced composites, such as thermoplastic composite pipes (TCP) and hybrid composite structures. However, there are gaps in the understanding of its performance in composite risers, hence the need for this review on the design, hydrodynamics and mechanics of composite risers. The review covers both the structure of the composite production riser (CPR) and its end-fittings for offshore marine applications. It also reviews the mechanical behaviour of composite risers, their microstructure and strength/stress profiles. In principle, designers now have a greater grasp of composite materials. It was concluded that composites differ from standard materials such as steel. Basically, composites have weight savings and a comparative stiffness-to-strength ratio, which are advantageous in marine composites. Also, the offshore sector has grown in response to newer innovations in composite structures such as composite risers, thereby providing new cost-effective techniques. This comprehensive review shows the necessity of optimising existing designs of composite risers. Conclusions drawn portray issues facing composite riser research. Recommendations were made to encourage composite riser developments, including elaboration of necessary standards and specifications.

Study on mechanical properties of natural gas hydrate production riser considering hydrate phase transition and marine environmental loads

A dynamic analysis model for natural gas hydrate (NGH) production riser was established considering the internal phase transition of hydrate multiphase flow and the external marine environmental loads. The model was numerically discretized by the finite element method (FEM) and solved by Newmark β method. The correctness of the analysis model was verified by an experiment and field testing data. The results indicate that the NGH begins to rapidly decompose in the middle of the riser, causing the sudden pressure drop in the riser. The internal NGH decomposition will significantly increase the axial tension and Von Mises stress of the riser. The Von Mises stress reaches maximum at the top end of the riser subjected to the distribution of effective axial tension. The lateral displacement and bending moment of the riser increase with the increase of flow rate and mixed fluid density in the middle and lower sections, and the stress decreases with the increase of flow rate and mixed fluid density. The increase of hydrate content and seawater temperature will decrease the lateral displacement and bending moment and the upper part stress of the riser, but it will significantly increase the stress in the lower part.

On the development of a nonlinear time-domain numerical method for describing vortex-induced vibration and wake interference of two cylinders using experimental results

A nonlinear mathematical model is developed in the time domain to simulate the behaviour of two identical flexibly mounted cylinders in tandem while undergoing vortex-induced vibration (VIV). Subsequently, the model is validated and modified against experimental results. Placing an array of bluff bodies in proximity frequently happens in different engineering fields. Chimney stacks, power transmission lines and oil production risers are few engineering structures that may be impacted by VIV. The coinciding of the vibration frequency with the structure natural frequency could have destructive consequences. The main objective of this study is to provide a symplectic and reliable model capable of capturing the wake interference phenomenon. This study shows the influence of the leading cylinder on the trailing body and attempts to capture the change in added mass and damping coefficients due to the upstream wake. The model is using two coupled equations to simulate the structural response and hydrodynamic force in each of cross-flow and stream-wise directions. Thus, four equations describe the fluid–structure interaction of each cylinder. A Duffing equation describes the structural motion, and the van der Pol wake oscillator defines the hydrodynamic force. The system of equations is solved analytically. Two modification terms are added to the excitation side of the Duffing equation to adjust the hydrodynamic force and incorporate the effect of upstream wake on the trailing cylinder. Both terms are functions of upstream shedding frequency (Strouhal number). Additionally, the added mass modification coefficient is a function of structural acceleration and the damping modification coefficient is a function of velocity. The modification coefficients values are determined by curve fitting to the difference between upstream and downstream wake forces, obtained from experiments. The damping modification coefficient is determined by optimizing the model against the same set of experiments. Values of the coefficients at seven different spacings are used to define a universal function of spacing for each modification coefficient so that they can be obtained for any given distance between two cylinders. The model is capable of capturing lock-in range and maximum amplitude.

Dynamic Analysis of Semi-Submersible Production Platform Under the Failure of Mooring Lines

This paper quantitatively studies the transient dynamic response of a semi-submersible production platform with the loss of one or several positioning mooring lines. A semi-submersible platform, production risers, and positioning mooring lines are all included in the numerical simulation. Increased motion of the semi-submersible platform, tension variation of the remaining mooring lines/risers and the risk of mooring line or riser clashing are all investigated through fully coupled time-domain analysis. Combined environmental loads are selected from irregular waves and the steady current varying from very rough to extreme sea conditions. Three dimension radiation/diffraction theories and Morison’s equation are applied to calculate first-order wave force and second-order mean drift force of floating semi-submersible platform. Nonlinear time-domain finite element methods are employed to analyze the behavior of mooring lines and risers. Results show that the failure of mooring lines seriously reduce the platform’s stability performance. The tension of the rest lines is also increased accordingly. Remaining lines which are closer to the failed lines will have larger tension increase to compensate. Line-Line distance provides practical information for the risk of clashing investigation.

How Real-Time Evaluation of Slugging Severity Can Help Maximize Production

In recent years, Aker BP has explored and developed a number of digital improvements to optimize production. The underlying business drivers are meant to improve efficiency, increase production and reserves, decrease costs, and reduce the carbon footprint from operations. The example described in this article has innovative elements of digitalization and automation of workflows which provide a new approach for better handling of slugging in subsea developments with long tiebacks. The new solution has a potential for optimizing production and limiting the amount of flaring. Flow Instabilities in the Vilje Field The Aker BP-operated Vilje field in the Norwegian Continental Shelf has occasionally experienced production-flow instabilities in the production pipelines and risers due to slugging. The company worked with Turbulent Flux to develop a software solution, the FLUX Stability Adviser, to continuously and precisely monitor production in real time. Field Description and Outline of Problem The Vilje field is a subsea development with three horizontal producers tied back to the Alvheim floating production, storage, and offloading (FPSO) facility through a production line longer than 20 km. The inlet separator at Alvheim is shared with other third-party developments. After an initial period of dry-oil production at the field, which started production in 2008, the oil rate has gradually decreased as a result of increased water cuts (WC) over time. Gas lift has been used to sustain production. The occasional slugging at the inlet separator had been controlled by increasing the backpressure to the production line and/or shutting in one of the producers. However, this practice affects production potential and may lead to production losses or deferral. Explanation of the Solution The two companies developed a Stability Adviser application to advise operators of the settings to optimize production in real time while lowering the risks related to slugging at any point in time. The developed solution runs on a cloud infrastructure with an interactive web-user interface. The main user interface contains a 3D heat map which is based on output from a series of pre-run transient simulations and a statistical analysis of the associated slugging severity. The model quality of the pre-run simulations was benchmarked with field data, and a similarity index was introduced to evaluate the degree of matching. Transient multiphase flow simulations represent the state of the art in the prediction and analysis of slugging behavior in multiphase transport pipelines (both terrain-induced and hydrodynamic slugging). These simulators capture and follow slugs as they are forming along the pipeline. That said, these simulations are time consuming, which limits their usefulness in a real-time operational context.

The Spar Platform: A Design That Transformed Deepwater Development

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30709, “The Spar Platform: Transforming Deepwater Development,” by Anil Kumar Sablok and Tim Otis Weaver, TechnipFMC/Genesis, and John Edwin Halkyard, Deep Reach Technology, prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The spar is the only successful dry-tree solution for deepwater production that can operate successfully in the deepest fields and the most severe environments. Its deep draft results in natural periods outside the range of waves, which has led to its wide acceptance for different field scenarios. The complete paper is an extensive review of the evolution of spar designs, focusing on the progression of work that ultimately led to the application of a transformative concept to the oil industry. Introduction The spar can support a drilling rig as well as top-tensioned production risers in water depths thousands of feet greater than the water depth limit for a tension-leg platform (TLP). It is especially well equipped to support steel catenary risers (SCRs) using the pull-tube option, which allows the SCR to serve as a continuous welded steel containment for hydrocarbons from the seafloor to the topsides and protects the riser from vortex-induced vibration in the fastest part of the current profile. Broadly speaking, there are three configurations of spars: classic, truss, and cell, with the common feature being that the center of buoyancy is higher than center of gravity. Table 1 of the complete paper lists all oil and gas spar production platforms that have been installed at the time of writing, in chronological order of installation. The complete paper devotes several pages to the spar’s initial development, including the crucial role of Edward Horton, the inventor and designer behind the spar production and storage concept and the TLP, and some of his colleagues. Years of development and navigation of various design challenges culminated in the installation of the Neptune spar in 1996 on time and budget. After the installation and success of Neptune, several other classic-design spars were implemented. The riser system on the Neptune spar had two unique features: buoyancy cans provided the tension, and the riser passed through a point of high bending and potential wear at the keel. The keel joint was a straightforward design; a sleeve around the main riser pipe pro-vided wear protection and distributed the bending in the riser to two endpoints rather than at a single contact point.

Strength check for production riser during dry tree unit coil tubing operations under different environmental conditions

In this paper, we introduce the modelling, loads, result evaluation method and boundary for production riser strength analysis. We investigated the different result evaluation method and different boundary condition based on a real project. The strength checks for the riser coil tubing stack up system in different environmental conditions was performed by SACS Software, calculate the most critical members UC and the maximum loads at critical flange locations of the riser during the whole dry tree unit coil tubing operations. According to the corresponding specification, the analysis results show that all the values of UC less than 1 and the project meet design requirements.

Performance change of a semi-submersible production platform system with broken mooring line or riser

Under high sea states, positioning mooring lines or production risers could be led to failure. Such an accident will change the watching circle of the platform or even cause production to shut down. The present paper is aimed at investigating the transient/unsteady response of a semi-submersible platform, tension variation of the remaining lines, and evaluate the risk of risers clashing after mooring lines or risers failure. The fully coupled time-domain analysis is set up for the whole system which including a semi-submersible platform, 12 mooring lines, and 22 production risers. One or several mooring lines and risers are intentionally disconnected from the fairleads at a certain time. Combined environmental loads are selected from irregular waves and the steady current varying from very rough to phenomenal sea conditions. Hydrodynamic response of the semi-submersible with regarding first-order and second-order wave forces is calculated with the combination of three-dimension radiation/diffraction theories and Morison's equation. Nonlinear time-domain finite element methods are employed to analyze the behavior of mooring lines and risers. Results indicated that the platform has a large horizontal offset with the lost positioning forces. This will lead to the successive failure of the neighboring objects. The tension of the rest lines is also increased accordingly. Remaining lines that are closer to the failed lines will have larger tension increase to compensate. A practical methodology for investigating the risk of risers collision is also provided in this paper.

Hybrid Ultradeepwater-Riser Configuration Saves Costs, Pushes Depths

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29389, “Single Independent Riser: A Cost-Efficient Ultradeepwater Riser,” by François Lirola, Eric Revault, and Jean-François Lunven, Saipem, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission. The single independent riser (SIR) is a hybrid riser configuration optimized for ultradeepwater field development. The SIR is composed of a flexible jumper and a rigid part vertically tensioned by means of distributed or continuous buoyancy. This configuration features improved dynamic behavior under fatigue and extreme environmental conditions for the rigid riser section thanks to its compliant shape. The design is versatile and can be staggered easily to comply with design constraints or congested layouts. Introduction The SIR is adapted to both turret-moored and spread-moored vessels as well as most other types. The configuration is also suitable for a wide spectrum of environments, from the mild conditions of west Africa to those in the Gulf of Mexico or the North Sea. The configuration can be designed for production risers, whether single-pipe or pipe-in-pipe, and injection and export risers. The strength of this configuration lies in its compliant, fatigue-resistant behavior. Its steep-wave configuration facilitates the mitigation of potential interference; the relative position of the sag and hog of each neighboring riser can be adjusted while reducing the load at the hang-off location. Overview The SIR features a steep-wave shape. The upper part is a flexible jumper, and the lower part is a rigid, steel-pipe string maintained in a near-vertical position either by means of continuous buoyancy or distributed buoyancy modules. These methods are used instead of a conventional air can in order to provide the up-thrust necessary to keep the riser vertical. The use of continuous or distributed buoyancy implies that each section compensates for its own weight, meaning that the concept is applicable to any water depth for which buoyancy materials are qualified. The transition between the rigid and the flexible parts is achieved with a sub-sea connector. An overview of the SIR configuration is given in Fig. 1. The SIR features the following technical advantages: The design relies on field-proven technology such as rotolatches to connect the rigid section to its anchor or standard shallow-water flexible-jumper technology to perform the transition between the rigid section and the floater. Though the SIR can be deployed in any water depth, the cost of the flexible jumper limits the suitability of the SIR for water depths below 1000 m, which should be considered the minimum applicable water depth. The SIR is compatible with any installation method. The fabrication process of the SIR can be reversed fully, meaning the system can be retrieved for relocation or maintenance purposes. The SIR features a small dynamic response, thus leading to less-stringent fatigue and welding requirements.

Study on the static behavior of installing a deep-water drilling riser on a production platform

Riser interference caused by wake effects between adjacent risers severely threatens the safety during riser installation process on a Dry-Tree Tension Leg Platform (TLP) system. Due to the ignorance of interference effect, the static deflection of the drilling riser during installation is highly over-estimated in previous work. To obtain a static behavior of a drilling riser during installation, this paper built a dual-riser model where two risers are arranged in tandem and considered as Bernoulli beam subjected to steady ocean current. The modified Huse model is applied to study the interference between object risers. Then, the structural equation is solved by Finite-Difference-Method (FDM). Case studies show that an upstream production riser can largely reduce the deflection of a downstream hung-off drilling riser, hence increasing the probability of the collision of two risers. The influence of water depth, riser wall thickness, Blowout Preventer (BOP) weight, etc. are also studied. Finally, the current condition based weather windows with respect to wind and tidal current velocity are obtained corresponding to different operation conditions. Results show that installing a drilling riser with lower BOP weight in shallow water is most feasible for multi-riser TLP systems. Moreover, drilling riser installation operability can be improved by increasing the top tension of the production riser.

Numerical Examination on the Effect of Internal Fluid Presssure on the Hydrodynamic Response of a Marine Riser

Marine risers are long slender structures which links the floating vessel on the sea surface and its manifold on the sea bottom. It acts as a transportation means for the hydrocarbon resources underneath the sea bed. A riser mainly undergoes hydrodynamic loading which leads to Vortex induced vibrations (VIV) or Flow induced vibrations. These are motions induced on bodies interacting with an external fluid flow producing periodic irregularities on the flow which leads to fatigue damage of offshore oil exploration and production risers. Therefore, suppressing of VIV by providing helical strakes, fairings etc. is necessary in order to reduce the fatigue damage of risers due to hydrodynamic loading. The present paper deals with the numerical study on the response of a marine riserdue to the effect of internalfluid pressure. The initial work is carried out in ANSYS ICEM CFD software. The CFD solution after analysis is obtained from ANSYS FLUENT. The hydrodynamic effects like lift and drag forces along with motion responses is obtained.

Static Analysis of Suspended Pipes

Abstract We analyze a suspended pipe with the application to offshore pipelaying and suspended production risers. Basic equations from the planar theory of elastic rods are reduced to a second-order ordinary differential equation with special end conditions that is solved numerically. Dimensionless results are given for the maximum curvature in the sag portion of the pipe and the slope angle at the upper end of the pipe in the absence of hydrodynamic forces. Both a rigid and a compliant seafloor are treated. We find that tension is highly effective in limiting curvature. A equation is found for the applied tension at the upper end of the pipe. Our results are intended for preliminary design studies that can be followed by more sophisticated analyses.