Top Riser Research Articles

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.

Casting process of gate valve steel based on ProCAST simulation

The sand casting process of 18CrNiMo7-6 gate valve casting was designed in this paper. The molding and core making methods of castings are sodium silicate CO2 method and self hardening cold box method respectively. The technological parameters of the casting were determined. The middle pouring system and top riser were designed. Using software to draw the assembly and 3D entity of gate valve casting. ProCAST simulation software is used to realize the numerical simulation task of the gate valve casting process. Shrinkage and porosity are the main research objects. It is found that the defects mainly appear in the thick part of the casting and the flange through the simulation experiment feedback parameters. In this paper, from the point of view of reducing and avoiding defects, the casting process is improved and improved. Finally, according to the experimental results, S2 (with chill) is determined as the final scheme.

Influence of Coil Configuration and Operating Conditions on Heat Transfer in Inductively Heated Risers

Electromagnetic (EM) induction heating of open top risers represents an energy-efficient method to preserve mass feeding whilst allowing for the use of a smaller riser and thus higher casting yield. Numerical simulation results for an ill-designed, concentric riser and casting assembly predicting the EM field characteristics and solidification behavior in such systems are presented herein. A vector potential formulation of the EM field was used and solved using a hybrid control volume/integral method, featuring temperature-dependent electrical conductivity. The heat transfer equation with phase change was also solved using the control volume technique. The computed results showed that use of induction heating could shift the hot spot from the casting to the riser. It was also found that use of higher coil frequencies produced a guard heating effect at the periphery of the riser, which was then applied using a new L-shaped coil geometry.

A finite element analysis of axisymmetric flow of air through the top riser of casting using the stream function model

Casting is a manufacturing process for making complex shapes of metal materials. Casting has two stages – filling process requiring a gating system and solidification process requiring a riser. The air in the mould cavity during casting is displaced through the riser by the molten metal in a very rapid manner necessitating the need to understand the air flow behaviour as it exit through the riser. The finite element method and the stream function model were used to analyze the axisymmetric flow of air through the top risers of casting. Results show that the velocity profile at any cross section is parabolic in shape with the maximum velocity at the centre. Comparing results with exact solution shows that the finite element result converged towards the exact solution. http://dx.doi.org/10.4314/njt.v37i3.20

Compressibility of air in casting mould evaluated by finite element method and the stream function model

Casting is a manufacturing process for making complex shapes of metal materials. Casting has two stages – filling process requiring a gating system and solidification process requiring a riser. The air in the mould cavity during casting is displaced through the riser by the molten metal in a very rapid manner necessitating the need to determine its compressibility as it exit through the riser. The finite element method and the stream function model were used to analyze the flow of air through the top risers of casting mould. Results show that the velocity profile at any cross section is parabolic in shape with the maximum velocity at the centre. Comparing the finite element solutions with the exact solutions showed that the solutions converged towards the exact solutions. Further comparing of finite element solutions and experimental results with the local speed of sound (Mach number) showed that the Mach number was greater than one, which established that the air in the mould cavity during casting is compressible as it is displaced by the molten metal. Before now all researches did was to develop empirical equations and optimized molten metal flow in casting mould. This work has gone further to establish the compressibility of airflow in casting mould.Keywords: stream function, top riser, air, finite element

Optimization analysis of the riser top tension force in deepwater drilling: Aiming at the minimum variance of lower flexible joint deflection angle

This paper presents an analysis approach optimizing the top tension force of deepwater drilling riser when the variance of the lower flexible joint deflection angle to the minimum value. The riser is modeled as an Euler-Bernoulli beam subjected to non-uniform axial force and transverse random force. In this paper, the riser transverse vibration behavior is described by transfer matrix method and spectrum analysis, and the distribution inertial force is calculated by the cubic Hermite interpolation polynomial. Finally, the variance of lower flexible joint deflection angle is given, and the minimum variance and the optimal top tension are figured out accordingly. Based on this, the influences of water depth, floating vessel top displacement, riser wall thickness and structure damping coefficient on the optimal top tension are discussed. Analysis results show that, for a given status, there is an optimal top tension making the deflection angle variance be minimum, and the optimal top tension increase with water depth, floating vessel top displacement and riser wall thickness. While the minimum value of the variance increase with water depth and the floating vessel top displacement, and decrease with riser wall thickness. However, the structure damping coefficient has almost no influence on the optimal top tension.

Numerical method for subsea wellhead stability analysis in deepwater drilling

A numerical model of under water wellhead stability analysis in deepwater drilling was established using the pile element and nonlinear spring element of ANSYS. Based on the pile foundation theories, assume the constitutive relation of seabed soil obeys ideal elastic–plastic model. The relationships between undrained shear strength and ideal elastic–plastic spring stiffness were given. In the application case, the D–F parameters of nonlinear spring element were calculated by pile foundation theories. Nonlinear spring parameters in different depth were assigned continuously using the APDL language. The results show that when the drilling vessel’s offsets are constant, the wellhead lateral displacement, rotation angle and the conductor bending moment increase with increasing top tension ratio. The higher the wellhead lateral loads, the deeper the maximum moment depth. When the depth reaches 15m under the mudline, the displacement and moment are close to zero. With the drilling vessel’s offset increasing, the wellhead displacement, rotation angle and the conductor body moment increase. Meanwhile, the increasing rate under high tension ratio is much higher than low tension ratio, and the wellhead tends to roll over. When the drilling vessel’s offsets increase, the tension of the top riser should be reduced to avoid wellhead rollover.

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.

Foundry Technique Designing and Optimization of the Cast-Steel Casting

The traditional method has been used to the foundry technique for the cast-steel casting. And the commercial foundry simulation software was used to validate the rationality of the foundry technique. Four side risers and some cylindrical chills were used to decrease the shrinkages which would occur in casting. The results show that the risers could not feed the casting effectively because of the limited feeding distance. A top riser was introduced to eliminate the defects, the simulation results show that still some shrinkages occurred because the heat effect between risers. The side risers were replaced by an annular chill. Finally, the shrinkages would happened in casting were eliminated.

Mass Transfer And Hydrodynamics In A Split-Column Airlift Photobioreactor

Overall gas-liquid mass transfer coefficients of oxygen gas (KLa) at different axiallocations of the riser and downcomer of a split-column Airlift Photobioreactor wereinvestigated using FOXY Fiber Optic oxygen sensor supported by NeoFox viewer software.The effect of bubble dynamics on mass transfer was considered. A 4-point optical probe wasused to measure local interfacial area (a) and other bubble dynamics parameters for both theriser and the downcomer of a split-column airlift photobioreactor. Having both overall masstransfer (KLa) and interfacial area (a) measured at different locations, local oxygen masstransfer coefficient (KL) can be determined for each location along the riser and thedowncomer of the airlift bioreactor. Effects of different operating conditions (0.3 – 2.8 cm/ssuperficial gas velocity) on both mass transfer and bubble dynamics of the riser and thedowncomer have been investigated for air-water system. It was found that both local masstransfer and gas-liquid interfacial area are considerably varying from the bottom to the top ofthe riser and the downcomer of the bioreactor. It was found that local mass transfer coefficientvaries by 100% from the bottom to the top of the riser at low superficial gas velocity (1 cm/s)and by 30% at relatively high velocities (2-2.8 cm/s). The Trend of the variation of the KLa inthe downcomer section of the column is very different from that of the riser, as it was noticedthat insignificant mass transfer takes place at the lower part of the downcomer. The results ofmass transfer and bubble dynamics in the riser and the downcomer of the studied conditionswill be presented and discussed.

Design and analysis of tapered stress joint for Top Tensioned Riser system

Stress Joint (SJ) plays a key role in the Top Tensioned Riser (TTR) system for deep water engineering. A preliminary design method of tapered SJ is proposed in the paper, which could help designers obtain accurate design data. After a further sensitive analysis is carried out, the related parameters choice and control methods are recommended in the engineering practice. By taking the extreme environment conditions into consideration, the effects of bending stress reduction and curve control are analyzed, and the 3-D FE models are established by ABQOUS for numerical evaluation to verify the correctness of design results. At last, dynamic analysis and fatigue analysis, based on actual project, are carried out with designed stress joint. The analysis results prove the feasibility and guidance of this method in the practical engineering applications.

Modelling and simulation of A segregates in steel castings using thermal criterion function Part II – Optimisation of real industrial cast part

The present paper is the second of two papers in which a methodology on how to exploit thermal criteria for modelling and optimising, i.e. minimising, the formation of A segregates in steel castings is presented. The first part of the paper dealt with incorporating one specific thermal criterion into a transient three-dimensional thermal fluid model inside a commercial simulation software package and validating this against experimental data obtained from a manufacturing foundry for a cast forging ram. In the second part of the present paper, the found criterion is then taken one step further by being applied to shape optimise the original casting layout of the forging ram where unknown optimal shapes and sizes of the top riser and chills are sought. This was carried out using genetic algorithms for establishing a better solidification pattern and thermal conditions inside the ram, which would eliminate the likelihood of centreline porosity and A segregates.

Modelling and simulation of A segregates in steel castings using thermal criterion function Part I – Background and validation

The present paper is the first out of two papers in which thermal criteria for modelling and optimising, i.e. minimising, the formation of A segregates are investigated. One specific thermal criterion has been incorporated into a transient three-dimensional thermal fluid model inside a commercial simulation software package. It is then used for predicting A segregates inside a large steel casting, i.e. a forging ram. In part I, experimental data obtained from a foundry serve to validate the given criterion and to evaluate the critical value for A segregate initiation for one alloy composition on the forging ram. In part II, the criterion forms the basis of shape optimisation of the original casting layout for the ram. More specifically, unknown optimal shapes and sizes of the top riser and chills are sought by means of autonomous optimisation to establish a better solidification pattern, which would eliminate the likelihood of centreline porosity and A segregates.

Application of View Cast Software in Foundry Technique Designing

Taking large scale steel casting for example; View Cast software was introduced to the designing process of the large scale steel casting technique. Solidification results show that the annular pop gate is beneficial to the casting. The side risers are used but it cannot eliminate the shrinkages because of the limited feeding distance according to the simulation results. So a top riser replaced it and some chills are used in the necessary regions. The results show that the new feeding system can help to progressive solidification and the shrinkages are eliminated. The temperature distribution is very clearly and the solidified part of casting is transparent which improved the visible effect and made it easy to read. People can estimate the shrinkage position easily.

Albacora Leste Field - Subsea Production- System Development

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18044, "Albacora Leste Field - Subsea Production System Development," by C.E.C.V. Longo, S.J.A. Neto, and M.T.R. de Paula, Petrobras; F.M. Lopes Jr., Technip; and C.A.F. Godinho, SPE, Prysmian, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. The Albacora Leste field-development plan comprises 30 horizontal wells tied back to a floating production, storage, and offloading (FPSO) platform moored by use of an innovative spread mooring system (SMS). All risers are flexible and installed in a free-hanging configuration. Environmental conditions and FPSO motion were factors in flexible-riser design. All flexible lines were anchored by means of torpedo piles. The full-length paper describes field development, including planning, design, qualification, manufacturing, and installation activities. Introduction The Albacora Leste field is in the deep north area of the Campos basin approximately 120 km off the coast of Brazil. Water depths in this field range from 800 to 2000 m, and oil densities range from 17 to 21°API. The field development comprises 30 horizontal wells (16 production wells and 14 injection wells, three of which are in a piggyback configuration) tied back to a host FPSO, P-50 by flexible risers and flowlines. Bending-stiffener and top riser accessories development and qualification represented an important and challenging success factor for the project. Three test benches were designed and assembled to perform the long-term fatigue tests for the gas-export flexible riser and to validate the new concept of the stiffener hanging systems. The FPSO will be moored in 1230 m of water by use of an innovative differential-compliance anchoring system. The oil, after treatment and storage on the FPSO, is pumped to a shuttle tanker. The produced gas then is cleaned and compressed, and a fraction of the total volume is used for power generation, gas lift, and internal consumption; the remaining volume is exported to shore by way of Platform PGP-1 in the Garoupa field, Campos basin. During the design phase, continuous gas lift and electrical-submersible-pump (ESP) artificial-lift methods were selected as potential candidates for the wells. However, at the time of the design phase, an ESP that could handle the high oil flow rates expected was not available, so the decision was made to use continuous gas lift as the artificial-lift method for the production wells. The approved gas lift design strategy eliminated pressure-operated valves and adopted a very simple and reliable well completion that included a single gas lift orifice valve. A high-discharge-pressure gas compressor was installed topside to guarantee continuous gas injection through the orifice valve at normal well-operation conditions and kickoff.

Feeding and risering of high-alloy steel castings

A more accurate, less conservative set of feeding distance (FD) and riser sizing rules is developed for high-alloy steel castings produced from alloy grades CF-8M, CA-15, HH, HK, and HP. These rules are designed to produce radiographically sound castings at 2 pct sensitivity. By comparing results between plate casting trials and the corresponding simulations of those trials, a relationship is shown to exist between a local thermal parameter known as the Niyama criterion and ASTM shrinkage X-ray level. This relationship is then used in an extensive set of casting simulations to numerically determine FDs for a wide range of casting conditions. It is shown that the FD rule developed in an analogous earlier study for carbon and low-alloy (C&LA) steels can also be used for these high-alloy grades, provided that the FD is modified by a multiplier that accounts for the high-alloy steel grade. In addition, it is shown that multipliers for superheat, sand mold materials, and the use of chills developed in the earlier work are also valid with these high-alloy steel grades. In comparison with previously published high-alloy FD rules, the present rules are shown to provide longer FDs (and hence higher casting yields) in most casting situations. This study also investigates riser sizing rules. It is determined that for open top risers, the previously published C&LA riser sizing rule is also valid for high-alloy steels. This rule is less conservative than existing high-alloy riser sizing rules, specifying smaller risers that produce higher casting yields. In addition, for vented blind top risers, it is shown that the previously published rules are also overly conservative.

Development of new feeding-distance rules using casting simulation: Part II. The new rules

Based on a correlation between the Niyama criterion and radiographic casting soundness developed in Part I of this work, a new set of riser feeding-distance rules is developed for low-alloy steel castings. These rules are designed to produce radiographically sound castings at 2 pct sensitivity. Rules are provided for the riser-zone length, end-zone length, end-effect feeding distance and lateral feeding distance for top risers, and feeding distance for side risers. In addition, the relationships between the end-zone length, riser-zone length, and the various feeding distances are discussed. Multipliers are given to apply these rules with end chills and drag chills, and multipliers are also provided to tailor these rules to different steel alloy compositions, sand mold materials, and pouring superheats. In comparison with previously published rules, the present rules are shown to provide longer feeding distances in most casting situations.

TLP Rigid Riser: A Case Study

Summary This case summarizes tension-leg platform (TLP) rigid riser designconsiderations and presents results of TLP riser analysis for aproduction/injection riser in the North production/injection riser in the North Sea. Analysis methods, design criteria, and design optimization are addressed. The riser is designed for 300-m water depth in compliance with Norwegianregulations. Emphasis is placed on application of the regulations andquantitative comparison of alternative methods for analysis of fatigue andextremes. Introduction This paper summarizes TLP rigid riser design considerations and presentsresults of the TLP riser analysis. TLP functions include drilling, production/injection, and export through rigid risers. TLP design is mostsensitive to the many production/injection risers. Such parameters as riserspacing, top tension, stroke, and pontoon clearance strongly influence globaldimensions and deck spacing/layout. Parameters used in this case study aresummarized in Table 1. Fig. 1 is a schematic of the TLP riser system. Thedesign is based on Norwegian Petroleum Directorate regulations and guidelines. Criteria for local design are summarized in Ref. 3. Analysis Methods Both frequency domain and time do analysis methods were used. Fatigueanalysis is primarily based on frequency domain and extremes on time domain. The riser is a nonlinear dynamic system, and the inaccuracy in using frequencydomain (assuming constant top tension and using stochastic linearization) isdiscussed below. The approach adopted for extreme responses is regular waveanalysis. This method was used to overcome uncertainty in statisticalextrapolation from an irregular time simulation and, for efficiency, to enablemany design iterations. Dynamic analysis was carried out without introducingload factors. Load factors are applied to responses when carrying out codechecks and when preparing design values for component specifications (e.g., stroke results). Boundary conditions include TLP motion, tensionercharacteristics (see Stroke section) and template/subsea wellhead interface(see Components and Interfaces). The riser analysis included TLP offsets in therange corresponding to TLP extreme motions. First-order response is includeddirectly as a transfer function in the riser analysis, and all other TLP motioncomponents are taken into account by defining a range of riser mean positionsupon which the first-order response is superimposed. TLP setdown is included ateach timestep in the time domain analysis. Sensitivity and Optimization As usual for deterministic design, an extensive sensitivity study must becarried out. The following were investigated:wave periods;currentvelocity;riser periods;current velocity;riser location andgeometric phase;hydrodynamic parameters, variation with depth, Reynoldsparameters, variation with depth, Reynolds number, Keulegan-Carpenter's number, and roughness;annulus and tubing fluids;top tension and tensionercharacteristics;TLP offset;extent of marine growth; andTLP huginfluence on wave kinematics. Quantitative sensitivity results vary widely forthe various responses along the riser, and the resulting priority is influencedby those responses that govern the design. The rigid risers tend to be inconflict with the overall TLP optimization on such parameters as top tensionand riser spacing. parameters as top tension and riser spacing. Optimization, therefore, is an iterative process carried out on the basis of the sensitivityresults for both the risers and the TLP. Handling and operational weatherlimitations are most sensitive to riser spacing and wellbay layout. Theserequire early detailed consideration because they are important to TLPefficiency in service. An alternative analytical approach to sensitivityanalysis and optimization with probabilistic methods could be used whereprobabilistic methods could be used where quantitative sensitivity results aregenerated directly in one analysis. This method has been applied successfullyto TLP tethers. Extreme Stresses Allowable tensile stresses in the riser wall were checked against values in Ref. 2. The Von Mises (combined) stresses were solved with a minimum wallthickness, taking into account corrosion/wear allowance and wall-thicknesstolerances. Ref. 9 is used for sum calculations. Compressive stresses also werechecked for load where axial stresses were low. Local buckling checks did notlead to additional design requirements. Extreme results vary considerably withanalysis method.

GRAVITY- AND SOLIDIFICATION-SHRINKAGE-INDUCED LIQUID FLOW IN A HORIZONTALLY SOLIDIFIED ALLOY INGOT

A basic-variable, explicit finite difference scheme is used to solve the unsteady and strongly pressure-coupled velocity problems of liquid flows induced both by thermosolutal buoyancies and solidification shrinkage in a binary alloy solidification process. A sample calculation, performed on an IBM personal computer, for a horizontally solidified Al-4.5%Cu alloy in a high-H/L ratio cavity with a top riser shows that the shrinkage established pressure gradient in the mushy region can be several hundred, even several thousand, times larger than that in the bulk liquid region.

Frequency domain analysis of marine risers with time dependent tension

A model of a marine riser system which is appropriate for the study of its dynamic behavior in deep water conditions is developed. This model is used to estimate the steady-state riser response to harmonic excitation. In this regard the finite element method yields a linear discrete multidegree-of-freedom structural model. The resulting stiffness matrix is time dependent due to fluctuations in the riser top tension. Further, Morison's equation is used for estimating the hydrodynamic load on the riser. Due to the nonlinearity of the drag term appearing in this equation, the equation of motion of the riser is nonlinear. An approximate analysis procedure based on the concepts of equivalent linearization and time averaging leads to efficient determination of the riser maximum stress. A continuous beam model under a constant tension equal to the average of the top and bottom riser tensions is used to provide an alterate estimate of its natural frequencies. Numerical results from a variety of parametric studies are reported. The computational features of the proposed method make it quite appropriate for implementation on personal computers which can be used in the preliminary design stage of marine risers.