Adjustable Choke Research Articles

MATHEMATICAL MODEL AND DYNAMICS OF A VARIABLE THROTTLE (CHOKE VALVE) FOR X-MAS TREES

One of the main elements of the subsea production system (SPS) is an adjustable throttle that regulates the supply of the medium (liquid) moved along the wellbore. The drive that controls the position of the shut-off-and-regulating element (SCE) of the throttle should provide increased requirements for reliability and dynamics, based on the requirements for the stability of the SCE, since the occurrence of self-oscillations can be the cause of the «start of disasters». Modern methods for the development and analysis of technical systems are based on the widespread use of mathematical models based on detailed knowledge of the processes and phenomena occurring in the systems under consideration. The use of a mathematical model is of great and urgent importance – it allows not to carry out expensive and costly physical experiments both in terms of time and money. The results obtained in the course of computer simulation can be used to optimize systems, determine operating modes and the causes of possible failures. The article presents the developed mathematical model of the adjustable choke of the X-mas tree. The mathematical model, unlike the existing ones, takes into account the non-stationary nature of the emerging hydrodynamic force on the SCE, as well as the nonlinearity of the hydraulic equipment – the actuator and control distributor. The paper presents the results of a numerical experiment and compares the proposed mathematical model with the results of a numerical experiment. The scientific novelty of the work lies in the creation of an object-oriented mathematical model of an adjustable choke of X-mas trees, taking into account the non-stationary load on the actuator, as well as the non-linearities of the control equipment.

DYNAMICS OF PNEUMATIC DRIVE. LECTURE CYCLE THE EQUATION OF MOTION OF THE WORKING MEDIUM. MULTIPHASE GAS-LIQUID MIXTURES. CAVITATION AND HYDRATES IN THE DRIVE. TRANSMISSION OF INFORMATION VIA A HYDRAULIC COMMUNICATION CHANNEL Continuation

The paper presents the derivation of a mathematical model of the movement of the working environment; the application of the compiled system of equations of motion of the working medium in a cylindrical coordinate system is shown. The validity and possibility of using a rotary rheometer to determine the rheological characteristics of liquids are mathematically proved. An equation is obtained for the moment of resistance during the movement of Newtonian and non-Newtonian fluids in a coaxially cylindrical developed measuring system, taking into account the rheological parameters of the fluid: “k” is measures of fluid consistency and «n» is characteristics of the degree of non-Newtonian behavior of the material. Verification of the obtained results on the example of two liquids is given, and the corresponding conclusions are drawn. The issues of phase transition in gas-liquid mixtures of adjustable chokes are considered. The criteria for the occurrence of cavitation and hydrate formation in liquid and gaseous media are presented. Equations are described that make it possible to determine the condition for a phase transition from a liquid to a gas-liquid state. The paper presents a modern method of protecting the design of an adjustable throttle from the cavitation phenomenon, which consists in the use of throttle cells or throttling stages. The result of using throttle cells is given. An example of the formation of hydrates in a gas-liquid mixture during its flow through an adjustable throttle is considered. The criterion and method for determining the conditions of hydrate formation in the flow channel of an adjustable throttle are given, which consists in constructing an operating curve of an adjustable throttle, an envelope curve and their intersection with the curve of hydrate formation of a gas-liquid mixture to determine the conditions and areas of hydrate formation. The paper gives an example of the use of a hydraulic communication channel for information transfer. The main mathematical dependences of the mathematical model of the hydraulic communication channel, boundary and initial conditions are given. The form of the pressure pulse is obtained, and one of the main methods of encoding information for a hydraulic communication channel is considered. The condition for ensuring the required purity class of the working fluid for hydraulic systems is determined. The scientific novelty of the work includes the relations obtained, which make it possible to study the hydrodynamics of a fluid with variable viscosity, predict and evaluate the values of the shear stress in the fluid during its movement, the shear strain rate, the moment of friction forces, etc., with known rheological characteristics of the fluid. In addition, the scientific novelty of the work is the formulated criteria for the conditions of the phase transition, as well as the provision of practical recommendations to specialists involved in the design and operation of adjustable chokes. This work is based and is implemented as a course of lectures of the course “Dynamics of Pneumatic drive”, is red by the author at the Bauman Moscow State Technical University at the Department of “Hydromechanics, Hydraulic Machines and Hydropneumoautomatics” (E10), as part of the preparation of masters in the specialty 05.04.13 “Hydraulic machines and hydropneumatic drives”.

DYNAMICS OF PNEUMATIC DRIVE. LECTURE CYCLE. THE EQUATION OF MOTION OF THE WORKING MEDIUM. MULTIPHASE GAS-LIQUID MIXTURES. CAVITATION AND HYDRATES IN THE DRIVE. TRANSMISSION OF INFORMATION VIA A HYDRAULIC COMMUNICATION CHANNEL

The paper presents the derivation of a mathematical model of the movement of the working environment; the application of the compiled system of equations of motion of the working medium in a cylindrical coordinate system is shown. The validity and possibility of using a rotary rheometer to determine the rheological characteristics of liquids are mathematically proved. An equation is obtained for the moment of resistance during the movement of Newtonian and non-Newtonian fluids in a coaxially cylindrical developed measuring system, taking into account the rheological parameters of the fluid: “k” is measures of fluid consistency and «n» is characteristics of the degree of non-Newtonian behavior of the material. Verification of the obtained results on the example of two liquids is given, and the corresponding conclusions are drawn. The issues of phase transition in gas-liquid mixtures of adjustable chokes are considered. The criteria for the occurrence of cavitation and hydrate formation in liquid and gaseous media are presented. Equations are described that make it possible to determine the condition for a phase transition from a liquid to a gas-liquid state. The paper presents a modern method of protecting the design of an adjustable throttle from the cavitation phenomenon, which consists in the use of throttle cells or throttling stages. The result of using throttle cells is given. An example of the formation of hydrates in a gas-liquid mixture during its flow through an adjustable throttle is considered. The criterion and method for determining the conditions of hydrate formation in the flow channel of an adjustable throttle are given, which consists in constructing an operating curve of an adjustable throttle, an envelope curve and their intersection with the curve of hydrate formation of a gas-liquid mixture to determine the conditions and areas of hydrate formation. The paper gives an example of the use of a hydraulic communication channel for information transfer. The main mathematical dependences of the mathematical model of the hydraulic communication channel, boundary and initial conditions are given. The form of the pressure pulse is obtained, and one of the main methods of encoding information for a hydraulic communication channel is considered. The condition for ensuring the required purity class of the working fluid for hydraulic systems is determined. The scientific novelty of the work includes the relations obtained, which make it possible to study the hydrodynamics of a fluid with variable viscosity, predict and evaluate the values of the shear stress in the fluid during its movement, the shear strain rate, the moment of friction forces, etc., with known rheological characteristics of the fluid. In addition, the scientific novelty of the work is the formulated criteria for the conditions of the phase transition, as well as the provision of practical recommendations to specialists involved in the design and operation of adjustable chokes. This work is based and is implemented as a course of lectures of the course “Dynamics of Pneumatic drive”, is red by the author at the Bauman Moscow State Technical University at the Department of “Hydromechanics, Hydraulic Machines and Hydropneumoautomatics” (E10), as part of the preparation of masters in the specialty 05.04.13 “Hydraulic machines and hydropneumatic drives”.

A Group Air Hydraulic Actuator of Power Stations for Form–Fill–Seal Automatic Packaging Lines

This article examines the aspects of power in-process operations along automatic lines for filling and packaging liquid and half-liquid products. It is shown that individual power station actuators can be replaced with one group air hydraulic actuator without any loss in productivity. Moreover, the dynamics of cutting out ready-made items can be significantly improved. The layout of an air hydraulic cylinder is presented in which an additional introduced high-pressure sealed zone connected to the hydraulic chamber by means of an adjustable choke and a reverse valve allows changing the tool power stroke speed right to creeping.

SIMULATION OF WEAR OF THE CHRISTMAS TREE CHOKE ELEMENTS

The effectiveness of the use of machinery and equipment in the oil and gas industry is impossible without solving the problems associated with improving the wear resistance of their components and parts.
 One of the types of equipment that is subject to intensive wear is the elements of the gushing fittings (valves, chokes, etc.). During operation, the throttle locking elements (tip and nozzle) are subject to the destructive influence of the working environment, as a result of which hydro and gas-dynamic wear of the tip and nozzle occurs. As a result, it becomes impossible to control the flow of a well.
 The aim of the work is to use simulation modeling to study the wear process of the elements of the choke of the Christmas tree fittings to determine the areas that are the most worn, taking into account the size of abrasive particles in the gas stream.
 According to the results of simulation, it was found that the size and place of maximum wear of the throttle elements is different, depending on the size of the grains of sand. The amount of wear on the throttle tip is 15 mm/year, and most of all, it is not the tip that wears, but the nozzle (46 mm / year). It was also found that the amount of wear of the surfaces located in front of the nozzle, compared with the wear of the tip and the nozzle, is much less, and is 3 mm/year.
 Based on the above simulation algorithm and the obtained results, the model of an adjustable choke can later serve as a basis for its subsequent research, namely: optimizing the tip cone to reduce the amount of wear of the tip itself; studies of the effect of speed and pressure drop on the amount of wear of the throttle elements; design optimization.

Reduction of Crude Oil Production Cost in Nigerian Indigenous Company using Activity Based Costing

The aim of this study is to investigate the impact of activity based costing in reducing crude oil production cost in Nigerian indigenous oil and gas company. This research work identified strategies to effectively reduce the cost of crude oil production by adopting a cost reduction tool for crude oil production and to establish a good crude oil flow to the surface for production. Activity based costing was the cost reduction tool used for this work. The tool helps to differentiate between value added costing and non-value added costing. Non-value added costs must be reduced or eliminated during production so as to maximise profit. Data was collected from an indigenous oil service company. The collated data were tabulated and graphs were plotted with the aid of Microsoft excel. The analysis revealed a total sum of ? 416,978,977 was wrongly spent for a duration of three years on crude oil production due to non-value added costing. The activities are: poor transportation of crude oil, that is, use of mobile tanker for haulage instead of laying 4 inches coated pipes for a distance of 5km and contracting the treatment of produced water to a contractor instead of setting up a water treatment plant. Also, using a diesel engine generator for electric power supply while gas was available as a fuel gas for natural gas consuming generator was a non-value added activity. Lastly, inadequate oil well flowing practice by flowing the well through an adjustable choke for a long period of time instead of using a fixed choke. This is a huge loss for indigenous oil producing fields operated by an indigenous oil service company in Nigeria. The loss was due to inability of the producers/field location owners to set up few equipment to meet up with complete operation standard.

Reduction of Crude Oil Production Cost in Nigerian Indigenous Company using Activity Based Costing

The aim of this study is to investigate the impact of activity based costing in reducing crude oil production cost in Nigerian indigenous oil and gas company. This research work identified strategies to effectively reduce the cost of crude oil production by adopting a cost reduction tool for crude oil production and to establish a good crude oil flow to the surface for production. Activity based costing was the cost reduction tool used for this work. The tool helps to differentiate between value added costing and non-value added costing. Non-value added costs must be reduced or eliminated during production so as to maximise profit. Data was collected from an indigenous oil service company. The collated data were tabulated and graphs were plotted with the aid of Microsoft excel. The analysis revealed a total sum of ₦ 416,978,977 was wrongly spent for a duration of three years on crude oil production due to non-value added costing. The activities are: poor transportation of crude oil, that is, use of mobile tanker for haulage instead of laying 4 inches coated pipes for a distance of 5km and contracting the treatment of produced water to a contractor instead of setting up a water treatment plant. Also, using a diesel engine generator for electric power supply while gas was available as a fuel gas for natural gas consuming generator was a non-value added activity. Lastly, inadequate oil well flowing practice by flowing the well through an adjustable choke for a long period of time instead of using a fixed choke. This is a huge loss for indigenous oil producing fields operated by an indigenous oil service company in Nigeria. The loss was due to inability of the producers/field location owners to set up few equipment to meet up with complete operation standard.

Trailer valve study with adjustable choke function based on the prior volume

With the continuous development of China’s market economy, the demand for commercial vehicles is increasing, and the loading capacity of traditional tractors in the transportation sector cannot meet the demand. As the problem of carrying capacity has been solved by the advent of trailers, the reliability and environmental protection of the products are more and more concerned and studied by the automobile developers. This study describes the structural design principle of the trailer brake control system and analyzes the key component – trailer braking valve through the connections and output line with hand brake valve and foot brake valve and the product performance and environmental protection.

First Three-Zone Intelligent Completion in Brazilian Presalt: Challenges and Lessons

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28136, “First Three-Zone Intelligent Completion in the Brazilian Presalt: Design Considerations, Challenges, and Lessons Learned,” by Filipe Del Vecchio, Potiani Maciel, Francisco Salom, José Chagas, and Antonio Ortiz, Baker Hughes, a GE Company, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission. Since the first intelligent completion was installed 20 years ago, the systems have become increasingly complex in order to reach productivity and optimization goals, allowing real-time independent monitoring and management of each zone in the well. Accompanying the search for greater system efficiency is the goal of reliability assurance. This paper presents the complexity and challenges of planning and installing the first intelligent completion in a subsea well with three producing zones in the Brazilian presalt. Introduction Below water depths greater than 7,200 ft, below salt layers up to 6,500 ft thick, and at true vertical depths (TVDs) of approximately 18,000 ft lie the presalt fields. The distance from shore (as much as 300 km), high carbon dioxide and hydrogen sulfide concentrations, high temperatures (sometimes greater than 100°C), and the existence of multiple intervals with different porosities all add to the challenges of producing from the presalt successfully. Through the installation of permanent downhole gauges (PDGs), intelligent completion allows real-time monitoring of downhole parameters such as pressure and temperature, providing a better understanding of what is happening at any given time. Hydraulically actuated flow-control valves give the operators the ability to access each interval with the touch of a button, greatly improving reaction time and reducing the need for costly interventions. Furthermore, valves with adjustable choke settings increase ultimate recovery from the reservoir by fine tuning the production/injection of each individual zone. Chemicals can be injected to ensure productivity and avoid potential problems to production such as scale and asphaltene buildup. Equipment Design and Configuration Flat-pack (FP) and control-line protectors (clamps) were designed to ensure maximum flexibility, allowing for wells to be completed with one, two, or three zones without the need for additional equipment. The splice subs also were designed with flexibility in mind. The lower the splice sub is positioned, the fewer the slots that are necessary. However, by supplying all of the subs with the same number of slots, the same slot sizes, and the same slot positioning, the equipment could be used in any type of well layout and inventory management was greatly improved. The design of the pressure and temperature gauges and their carriers was also optimized. Instead of the PDG dictating whether the data are acquired from the tubing or the annulus, the mandrels do that. This was made possible by having two different configurations for the area where the gauge is mounted to the carrier. The tubing mandrel has a through hole allowing fluid from the inside of the string to contact the gauge’s inlet port, while the annulus mandrel has a blank interface with cross-shaped slots through which annulus fluids enter the sensor’s inlet.

Electro-Pneumatic Control System with Hydraulically Positioning Actuator Motor

A compressibility of the actuating fluid of a pneumatic drive (compressed air) leads to significant landing of the pneumatic cylinder piston at the time of stop and hold of the load, a constant component of which can fluctuate significantly for the holding period. There are a lot of factors, which have a significant impact on the landing value of piston. Those are: an initial position of the piston at its stop, which determines the volume of the an active area of the piston, a value of the constant load component at the time of stop and its variation for the holding period, a transfer coefficient of the position component of the load, an active area of the pneumatic cylinder piston, as well as reduction in atmospheric pressure, which can significantly affect the operation of the control systems of small aircrafts flying at high altitudes. To reduce the landing value of piston due to changing value of the constant load component for its holding period, it is proposed to use a hydraulic positioner, which comprises a hydraulic cylinder the rod of which is rigidly connected to the rod of the pneumatic cylinder through the traverse, a cross-feed valve of the hydro-cylinder cavities with discrete electro-magnetic control, and adjustable chokes. A programmable logic controller provides the hydraulic positioner control. At the moment the piston stops and the load is held the cross-feed valve overlaps the hydro-cylinder cavities thereby locking the pneumatic cylinder piston and preventing its landing. With available pneumatic cylinder-controlled signal the cross-feed valve connects the piston and rod cavities of the positioner hydro-cylinder, the pneumatic cylinder piston is released and becomes capable of moving. A numerical estimate of landing of the pneumatic cylinder piston and its positioning quality is of essential interest. For this purpose, a technique to calculate the landing of piston has been developed taking into consideration that different factors, which determine operation conditions of the pneumatic cylinder, have the impact on it. Experimental investigation of piston landing was conducted using the pneumatic cylinder with piston and rod diameters of 32 and 16 mm, respectively, at operating pressure of 0.8 MPa. With a vertical arrangement of the pneumatic cylinder axis, the control weights of a dead-weight pressure-gauge tester created a constant component of the load and its variation, and an indicator with resolution of 0.05 mm estimated a landing value. The article shows the calculated data of piston landing, which are in good compliance with the experimental results. To substantially reduce the piston landing for the holding period of the load varying in value and sign was designed a hydraulic positioner of pneumatic cylinder piston, and its operation quality is studied on the physical layout of electro-pneumatic control system operating in the discrete and tracking modes. For control system service, work programs of the programmable logic controller integrated in the control circuit of the hydraulic positioner in discrete mode, and, as a digital discriminator, in tracking mode are developed. In conclusion as a result of the research activities the article estimates the impact of factors on the quality of the load position at the time of its hold and notes a high efficiency of the hydraulic positioner of pneumatic cylinder piston both in discrete mode and in tracking one.

Technology Focus: Well Control (January 2010)

Technology Focus It has been stated many times that all the easy wells have been drilled. It would stand to reason then that only problem wells remain. Even though this last statement is not entirely correct, our industry is facing increasingly costly incidences of pressure-related nonproductive time. Problems include narrow pore-/fracture-pressure windows, wellbore stability, depleted formations, formation damage, and excessive casing strings, among others. The industry has responded by developing managed-pressure-drilling (MPD) technologies. The goal of MPD is precise management of the pressure in the entire annulus. In most instances, a rotating control head is used, diverting the mud returns through an adjustable choke. This closed system allows precise imposition of surface backpressure on the annulus and enables precise measurement of the mud-return rate. The use a rotating control device and choke in this way is very similar to a constant-bottomhole-pressure method of well control. In September 2009, the SPE Drilling & Completions Applied Technology Workshop on Well Control was held in Rio de Janeiro, during which several companies involved in MPD made presentations on how their MPD technologies have enhanced well control. The closed system, which precisely measures surface pressures and mud-return rates, enables the detection of kicks (sometimes with less than 1 bbl of influx). Then, the mud-return rate is slowed to the prekick rate long enough to stop the influx and the well is killed, all without shutting in the well. In some cases, when a very small influx is detected, it is not even necessary to stop drilling. The papers selected to be highlighted in this issue all detail unconventional well-control methods that have been used recently. On the basis of these papers and presentations at the Applied Technology Workshop, it appears that well control with MPD will quickly become “conventional.” Well Control additional reading available at OnePetro: www.onepetro.org IPTC 11970 • “Killing a Gas Well: Successful Implementation of Innovative Approaches in a Middle-Eastern Carbonate Field—A Field Case” by S. Salehi, University of Calgary, et al. SPE 121045 • “Prevention of Vertical Gas Flow in a Collapsed Well Using Silicate/Polymer/Urea Method” by Istvan Lakatos, SPE, Reseach Institute of Applied Earth Sciences, University of Miskolc, and Geoengineering Research Group, Hungarian Academy of Science, Miskolc, Hungary, et al.

An Experimental Study on the Influence of Vortex Generators on the Shock-Induced Boundary Layer Separation at M=1.4

The results of an investigation into the effects that sub-boundary layer vortex generators (SBVGs) have on reducing normal shock-induced turbulent boundary layer separation are presented. The freestream Mach number and Reynolds number were M=1.45 and R=15.9×106/m, respectively. Total pressure profiles, static pressure distributions, surface total pressure (Preston pressure) distributions, oil flow visualization, and Schlieren photographs were used in the result analysis. The effects of SBVG height and the location upstream of the shock were investigated. A novel tetrahedron shape SBVG with different lengths (30 mm and 60 mm) was used for these experiments. The effect of streamwise location of the longer SBVG on the interaction was also investigated. The location of the shock wave was controlled by an adjustable choke mechanism located downstream of the working section. The results show that an increase in the distance for the longer SBVG from 17.4δR to 25.5δR did not remove the separation entirely, but the shorter SBVG provided higher total pressure distribution within the boundary layer in the recovery region. This also provided a healthier boundary layer profile downstream of the interaction region with lower displacement thickness and shape factor.

Sand Exclusion and Management in the Okwori Subsea Oil Field, Nigeria

Summary Several downhole-sand-control failures in Okwori subsea oil producers triggered a significant overhaul of the downhole-sand-control method and a review of topside sand management. Because the eventuality of further downhole-sand-control failures in existing producers could not be ignored, a surface-sand-handling and -management facility was designed. For the future development producers, the downhole-sand-control method was re-engineered to increase its reliability. The initial multizone, fully selective sand-control-completion system, with expanded sand screens inside the casing, was abandoned. A revised openhole sand control was designed with similar completion selectivity. The new completion system implied a change in drilling practices, in reservoir drill-in fluid, and in filter-cake-cleanup techniques that required simultaneous engineering. These revisions were validated and implemented only a few months after encountering the first major sand-control failure. The Okwori floating production, storage, and offloading (FPSO) vessel was not expected to deal with sand production. Temporary sand traps were installed to minimize production downtime and to determine flow regimes that minimized sand production. Severe-service adjustable chokes were introduced to avoid valve erosion and stabilize production rates. Finally, a long-term topside-sand-management solution was designed and recommended for implementation.

Adjustable Downhole Choke Technology Facilitates Intelligent-Completion Push

If Susquehanna Financial Group’s estimates are correct, the intelligent-wells sector is about to take off, with the market doubling this year vs. last, to U.S. $200 million, and potentially growing to $600 million by 2010. Susquehanna is basing its estimates on two trends: a shift to longer-term, multiwell contracts and greater demand for intelligent completions from independent oil and gas producers. Another factor that may also be influential in the projected growth of this market is the evolution of adjustable downhole choke technology. For example, since the first installation of Baker Oil Tools’ HCM-A variable downhole choke in December 2003, the remotely controlled, multiposition hydraulic choke has been the fastest-growing segment of the company’s intelligent-completions business. Adjustable chokes offer advantages in flow control and reliability to solve a wider range of completion and production problems than standard open/close flow-control technology. The technology has been shown to improve the economics of marginal fields and could help to explain the Susquehanna report’s conclusion that, although they have been late coming to the party, independents are now viewed as repeat customers with increased demand for intelligent completions. Choke Anatomy The HCM-A combines the basic chassis from Baker Oil Tools’ HCM surface-controlled sliding sleeves with incremental-choking technology. A balanced hydraulic piston opens and closes the choke. A tungsten carbide choke assembly and J-mechanism selectively adjust the choke without the need for complex electronics downhole. The choke is infinitely variable, with the understanding that six intermediate choke settings between fully open and fully closed must be selected by machining a profile before tool installation. Nodal analysis and reservoir studies are used to determine the optimum choke settings to achieve completion objectives. These may include any or all of the following: maximized oil production, reduced water cut, improved oil blend quality, sand control, avoidance of crossflow and consequent production losses, compliance with regulatory production restrictions, and production above the bubblepoint to avoid gas dropout in the reservoir. Input parameters for the nodal analysis include completion configuration, fluid properties, productivity indexes, wellhead pressures, formation pressures, and artificial-lift design. An advantage of the design is that the choke settings can be selected and changed in the field without the need to ship the tool back to the plant for rebuild. With demonstrated reliability and the additional functionality of a variable choke, intelligent-well systems can provide value by solving a wide range of completion and production problems, including addressing crossflow risks in commingled production, distributing water injection among multiple zones, and matching productivity or injectivity to optimize electrical-submersible-pump (ESP) performance. Dump-Flood Application In one case study, an operator in Nigeria elected to use an adjustable-choke-enabled intelligent well to initiate a waterflood without the need for a costly and long-lead-time surface infra-structure system. In this case, the pressure in the producing interval has been significantly depleted. However, significant additional oil volumes remain. To recover the remaining incremental reserves, the operator opted to initiate a waterflood to increase the reservoir pressure and sweep additional volumes of oil into the producing wells. The drawback to this option was the time and cost associated with implementing the surface infrastructure to inject upwards of 100,000 bbl of water.

Subsea Adjustable Choke Valves: A Focus on a Critical Component

Summary With emphasis on deepwater wells and marginal offshore fields growing, the search for reliable subsea production systems has become a high priority. A reliable subsea adjustable choke is essential to the realization of such a system, and recent advances are producing the degree of reliability required. Technological developments have been primarily in (1) trim material (including polycrystalline diamond), (2) trim configuration, (3) computer programs for trim sizing, (4) component materials, and (5) diver/remote-operated-vehicle (ROV) interfaces. These five facets are overviewed and progress to date is reported. A 15- to 20-year service life for adjustable subsea chokes is now a reality. Another factor vital to efficient use of these technological developments is to involve the choke manufacturer and ROV/diver personnel in initial system conceptualization. In this manner, maximum benefit can be derived from the latest technology. Major areas of development still required and under way are listed, and the paper closes with a tabulation of successful subsea choke installations in recent years.

A Feasibility Study on the Use of Subsea Chokes in Well Control Operations on Floating Drilling Vessels

Summary The use of a subsea remote-operated adjustable choke between the blowout preventer (BOP) stack and the marine riser was considered as a means for eliminating the need for long subsea flowlines. Well behavior was predicted through computer simulations and showed/that existing problems could be reduced but not eliminated entirely. Introduction During the past 20 years, offshore drilling in water depths up to 350 ft (107 m) has become routine. Deepwater exploratory drilling from floating vessels in water depths as great as 5,000 ft (1524 m) has been achieved in the past few years. Currently drilling vessels are being constructed that are capable of drilling water depths of more than 6,000 ft (1829 m). As drilling extends to such great water depths, the blowout control problem becomes much more difficult. Well control equipment and procedures were developed first for drilling operations on land. With only minor changes, these well control techniques have been applied to bottom-supported drilling rigs such as submersibles, jackups, and rigs operating on offshore platforms. More extensive changes in BOP equipment and procedures were required for floating vessels, which are used almost exclusively for deepwater operations in water depths of more than 350 ft (107 m). The most significant change was the location of the BOP stack at the seafloor rather than at the surface. This paper considers the need for development of additional subsea well control equipment as the search for petroleum is extended to greater water depths. The special well control problems for floating drilling vessels stem primarily from the reduced fracture resistance of the marine sediments and the need for long vertical subsea chokelines between the subsea BOP stack and the drilling vessel. The shallow marine sediments often are unconsolidated and undercompacted, having a relatively low bulk density up to several thousand feet below the mudline. The fracture problem is aggravated because the mud column extends far above the mudline to the flowline above sea level, and mud density generally exceeds seawater density. Thus, when a kick occurs during deepwater drilling operations, formation fracture usually will occur at a lower equivalent mud density than for a similar situation on land. For the example shown in Fig. 1, the fracture gradient computed at a casing seat 3,000 ft (914 m) below the mud line in 3,000 ft (914 m) of water is only 10.6 lbm/gal (1.27 kg/m3). The use of long vertical subsea chokelines in deep water between the subsea BOP stack and the drilling vessel has two disadvantages. The increased chokeline length can result in unacceptably high circulating frictional pressure losses in the chokelines. This can be extremely important because current well control procedures are based on the assumption that frictional pressure losses held against the well annulus are small and can be applied as a convenient safety margin when circulation of the kick is begun. In addition, since the subsea chokelines extend vertically over a considerable length, the density of the fluid in the chokelines contributes directly to the hydrostatic pressure in the well. When the top of a slug of low-density kick fluid reaches the subsea BOP stack, the hydrostatic pressure of the drilling fluid in the chokeline is lost quickly during the rapid elongation of the slug as it exits the large casing and proceeds upward through the small-diameter chokeline. This too is extremely important because current well control procedures are based on the assumption that significant well pressure changes will occur more slowly than the unsteady-state readjustment time of the surface drillpipe pressure, on which choke manipulation is based.