OTC-25163-MS Subsea Wellhead System Verification Analysis and Validation Testing Jim T. Kaculi, D.Eng., P.E., and Bruce J. Witwer, Dril-Quip Inc.
Copyright 2014, Offshore Technology Conference This paper was prepared for prese presentatio ntation n at the Offshore Technology Technology Conference Conference held in Hous Houston, ton, Texas, USA, 5– 8 May 2014. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.
Abstract As oil and gas drilling and production operations move into much deeper waters and harsher environments, there is ever increasing demand for development of the next generation of tools and technology to meet these challenges. As a critical barrier between the wellbore fluid and the environment, subsea wellhead systems are an important part of the drilling and production equipment and a proper understanding of their behavior under complex loading conditions is crucial. Component and sub-system analysis and limited testing has been performed before but such analysis and testing does not fully evaluate the system as a whole. This is mainly due to the lack of test machines capable of accommodating the size of these systems while generating the sufficiently high loads needed to simulate field conditions. System verification analysis and validation testing is very important to ensure that the load transfer and interactions between the sub-systems and components are properly accounted for during installation and field service of equipment and to provide a true understanding of the wellhead system structural capabilities. Following the recommendations of API-TR-1PER15K-1 with API-TR-1PER15K-1 with regards to system approach evaluation, a complete wellhead system analysis and testing is performed. The system stack includes the wellhead connector, the high and low pressure wellhead housings and the conductor string, and simulates various load combinations. The objective is to verify the system performance envelopes using advanced finite element analysis (FEA) and validate the system using physical testing. Various Var ious loa load d com combin binati ations ons (be (bendi nding, ng, ten tensio sion/co n/compr mpress ession ion,, pre pressu ssure) re) are app applie lied d to gene generat ratee the wellhead system capacity envelopes providing operators a verified wellhead system window of operation. A side by side comparison of testing and analysis results is presented, and proper model calibrations are performed as needed. The physical testing of the wellhead system has provided some invaluable insights to make proper adjustments to the analysis techniques and the inputs required that are not obvious from a theoretical point of view. This effort resulted in a better understanding of the wellhead system and connector interface, and it wi will ll be ve very ry us usef eful ul kn know owle ledg dgee to be ap appl plie ied d fo forr fu futu ture re hi high gh pr pres essu sure re hi high gh te temp mper erat atur uree (H (HPH PHT) T) development work of 20 ksi (or higher) wellhead systems. With an establ established ished and relia reliable ble analysis methodology methodology based on the best indust industry ry practices and proper model mod el cali calibra bratio tion n mat matchi ching ng the tes testt res result ults, s, fut future ure sys system tem ana analys lysis is can pro produce duce rel reliab iable le res result ultss tha thatt
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increase increa se the pro probabi babilit lity y of the suc succes cessfu sfull fir first st tim timee val valida idatio tion n tes testin ting g for var variou iouss wel wellhe lhead ad sys system tem configurations.
Introduction The subsea wellhead forms a critical part of the overall drilling and production system as it provides both the structural resistance resistance and press pressure-co ure-containi ntaining ng inter interface face for the dril drilling ling and product production ion equipm equipment. ent. The wellhead system itself is fairly complex as it contains many interfacing sub-systems and components and performs various functions during different phases of drilling and production operations. Some parts of the subsea wellhead system serve as the only barrier between the wellbore fluid and the environment, making it paramount for a robust and safe design. Therefore, a thorough understanding of the system function and behavior under complex loading conditions is important especially for HPHT applications where the equipment is expected to operate under much harsher conditions. System verification analysis followed by validation testing should be performed to ensure the system is fit for service. The wellhead system verification analysis is typically performed using FEA or other analytical tools. Although it is a complex process and requires significant efforts when considering the large size of models required, convergence issues encountered, post-processing of results, and lack of standards that clearly define the analysis methodology and acceptance criteria, it is not as difficult as the validation testing. Validation testing of the wellhead system has been a challenging task due to the lack of test machines capable of producing the high load limits required and of handling the size and weight of the equipment during testing. This task becomes even more complicated during the combined load testing scenarios when considering pressure, bending, tension and compression. However, it is crucial that such testing is done to validate the equipment design and establish confidence in the accuracy of the analysis methodology. Curren Cur rentt ind indust ustry ry code codess req requir uiree tes testin ting g to be per perfor formed med on cer certai tain n wel wellhe lhead ad com compone ponents nts and sub sub-assemblies, but no full system testing is required. ISO-13679 and API API- RP-5C5 RP-5C5 require rigorous testing for premium threaded connections at various assessment levels (CALs). Due to the high loads encountered in deep water applications under HPHT environments, manufacturers are being asked by the industry to perform similar testing for large diameter surface casing and conductor connections to ensure they are fit for service. The casing and conductor connectors, varying in a range of diameters from 20 inch to 42 inch or higher, are an extension of the wellhead system and are located directly below the wellhead which transmits the external loads directly to them. In a worst case discharge, the threaded connectors are buried below the mudline and likely covered by smaller casing c asing strings and a nd are not as critical as the wellhead and wellhead connector when considering that in a case of failure of the wellhead connector oil and gas are exposed expose d direc directly tly to the environment. environment. Because indus industry try codes and some operat operators ors mandate that casing and conductor connectors below the wellhead go through such rigorous testing, it is logical that such testing should be performed on the wellhead system and more specifically the critical wellhead to the wellhead connector interface. In our view such testing is more important on the wellhead system itself, failure of which could have the same or potentially greater catastrophic consequences than failure of the threaded connectors. To meet the industry needs and challenges associated with HPHT environments and development of safe, verified, and validated equipment designs analysis and testing is required. Dril-Quip has designed and manufactured a test machine which is capable of producing static loads of over 20 million foot-pounds bending moment and 13 million pounds axial loads under various internal and external pressures and temperatures. The test machine is a horizontal-type that produces tension, compression, and uniform bending from two end fixtures. It is believed to be the first of its kind in the industry when considering the size of test specimen it can handle, magnitude of the loads it can produce, and the high levels of combined loads that it can accommodate.
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Following the recommendations of API-TR-1PER15K-1 with API-TR-1PER15K-1 with regards to system approach evaluation, a complete wellhead system analysis and testing is performed. The system stack includes the wellhead connector connect or together with the high and low press pressure ure wellhead housings and conduct conductor or string, and simul simulates ates various load combinations including preload, internal pressure, external bending, and axial loads. The objective is to verify the system performance capacity envelopes using advanced FEA, and validate them with physical system testing. The system analysis verification and testing validation is important as it ensures that the load transfer and interactions between the sub-systems and components are properly accounted for during installation and field service of equipment, and provides a true understanding of the wellhead system performance and structural capabilities. The following sections provide detailed description of the wellhead system verification analysis, test fixture, wellhead system test set-up, testing validation, and a comparison of the analysis and test results.
Wellhead System Overview The wellhead system utilized for this effort is comprised of Dril-Quip subsea hardware similar to what is currently installed in markets throughout the world. Modifications to the standard equipment have been made to minimize the site-specific features that may otherwise limit the system capacities. The main modification is the size of the simulated conductor string below the low pressure wellhead housing, where a 41-½ OD 3 wall forging is used to ensure adequate conductor capacity for a worst-case validation of the wellhead system. A partial view of the wellhead system 3-D model used for analysis and testing is shown in Figure in Figure 1. 1. Taking advantage of the inherent symmetry of geometry and loads, a half-symmetry model was used. The 3-D FEA model contained over one million elements. The analysis was completed using ABAQUS/ Standard, Version 6.13 analysis software. Details about modeling techniques, capacity determination, and analysis results for both the wellhead connector and the wellhead housings assemblies are presented.
Wellhead Connector Assembly The wel wellhe lhead ad con connect nector or is hydr hydraul aulica ically lly act actuat uated ed and inc includ ludes es the Hig High h Pre Pressu ssure re Hou Housin sing, g, Lat Latch ch Segments, Gasket, Lower Body, Outer Body, Connector Body, Cam Ring, and Cover Plate. The connector capacity is usually presented in a chart form similar to the API-TR-6AF2 the API-TR-6AF2 flange flange capacity which gives a relati rel ations onship hip of the int intern ernal al pre pressu ssure re ver versus sus bend bending ing mom moment ent at var variou iouss ten tensio sion/c n/compr ompress ession ion leve levels ls including the makeup forces as shown in Figure in Figure 2. 2. The wellhead connector assembly is shown in Figure in Figure 3. The capacities have been traditionally determined using engineering calculations or 2-D FEA which requires conversion of bending moment to an equivalent tension or compression effect for a simple axisymmetric member or pipe. Equivalent tension or compression conversion as described in API-16R in API-16R and and other standards assumes that the calculated stress at the outer diameter of the pipe wall caused by pure bending is treated the same as the constant cross-section stress caused by pure tension or compression. This is generally a conservative assumption because the peak stress, caused by the bending moment that is being used to determine the equivalent tension or compression force, linearly varies from a peak value at the outer radius of the pipe to zero at the neutral axis, and during the conversion it is assumed that this peak value acts over the whole cross-section of the pipe. The concept of equivalent tension/compression works well and is generally conservativ conser vativee for a strai straight ght round member. However, when this concept is applied to a wellhead connector which whi ch cont contain ainss sev severa erall com comple plex x sha shaped ped com compone ponents nts,, the there re are som somee maj major or conc concern ernss reg regard arding ing the assumptions made for the compression side of bending, the “critical radius” selection, and the assumptions made for non-axisymmetric components. For a wellhead connector, the equivalent tension/compression conversion assumes that the full hub circumference supports the bending load. In the preloaded condition the connector hubs are initially loaded in compression and the stresses due to preload, although not as severe as primary stresses, are
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Figure 1—Wellhead System Model
significant and cannot be ignored. In reality, any external bending load creates additional compressive stresses on the compression side of bending and reduces the preload compressive stress on the tension side of bending. The use of the equivalent tension/compression method assumes that the equivalent compression load from bending is uniformly supported across the full circumference of the hub, which is not realistic and could result in gross error of the connector capacity estimates. This fact is supported with the 3-D FEA results shown in Figure 4, 4, which presents an example of stress distribution of the connector under bending load. As expected, the stress patterns in the tension and compression side of bending are significantly different. When considering complex assemblies such as a wellhead connector the selection of the appropriate “critical radius” for non-uniform component shapes, which is equivalent to the outer radius of a round member, is critical. In the absence of any code or standard defining the critical radius, the selection becomes a matter of user choice and interpretation and can result in a wide range of capacities depending on the critical location selected. Middle of hub contact radius, middle of segment teeth radius, or the outer connector radius are some locations that may be considered as an acceptable critical radius location. For this particular connector case, that results in bending capacity variations of as much as 40% between the different locations. This variation clearly shows how spread out the results could be and how much the connector capacity can be under or over estimated by depending upon the critical radius of choice. The
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Figure 2—Typical Connector Capacity Chart using Equivalent Tension/Compression
Figure 3—Wellhead Connector 3-D Model Mesh
connector capacity chart generated using equivalent tension/compression shown in Figure in Figure 2 assumed 2 assumed that the “critical radius” is at the middle of the latch segment teeth. In a 2-D model, there are simplifications made to non-axisymmetric components of the connector such as the latch segments, holes, slots, etc., by treating them as axisymmetric components with reduced
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Figure 4 —Wellhe —Wellhead ad Conn Connector ector Stress Distr Distributio ibution n Plot with applie applied d Bendin Bending g Momen Momentt
material properties to account for the material removed and the reduced stiffness. This is acceptable to a certain extent to create proper interactions with other components, but it may not represent a true load distribution and result in less accurate stress levels, especially when evaluating the non-axisymmetric components themselves. Use of equivalent tension/compression and 2-D modeling is useful for sizing purposes during the initial design stage. However, use of 3-D FEA and validation by physical testing is the only reliable method to properly deal with the concerns regarding “compression side of bending”, “critical radius”, nonaxisymmetric geometry and loading, crucial in accurately determining the connector capacity. For the purpose of this study, a 3-D FEA model of the wellhead connector was created. Taking advantage of symmetry, a half 3-D (180°) FEA model was used to analyze the major components of the wellhead drilling connector assembly, as shown in Figure in Figure 3. 3. Over a half-million eight-node linear brick continuum (ABAQUS C3D8) elements were used to model the 3-D assembly. This provided a sufficient mesh refinement in order to accurately capture the stresses and strains throughout the areas of concern in the assembly. Thee ana Th analy lysi siss co cons nsis iste ted d of se sever veral al st step eps. s. In th thee fi firs rstt st step, ep, th thee con conne nect ctor or was pr prel eload oaded ed wi with th th thee appropriate makeup forces. In the subsequent steps, the connector was subjected to a combination of bending moment, tension, and pressure as described in in Table Table 1. 1. Contact elements were included for all interaction inter action surfaces. surfaces. The inter interaction action surfaces were modeled with frict friction ion using ABAQUS surf surface-toace-tosurface interaction (hard contact) formulation. Nonlinear geometry (large displacement) behavior was considered in the analysis. Ambient temperature was assumed for the analysis. Linear elastic behavior was used for the materials. Use of linear elastic analysis for rated capacities is acceptable, however, if thin wall theory cannot be applied or yielding is expected elastic-plastic analysis should be used. The elastic-plastic
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Table 1—Combined 1—Combined Load Cases and Capacities
analysis methodology is described in the ASME Boiler and Pressure Vessel Codes, Section VIII, Division 2 and 3. This paper presents results for only the elastic analysis method, but the same principles apply to the elastic-plastic analysis. Skin or strain gage higher order elements were applied in the model at the areas where strain gages were used. The wellhead connector seal performance was evaluated by monitoring the contact loads/ stresses at the metal sealing gasket, and monitoring connector hub separation. Some partial hub separation on the tension side of bending can be tolerated so long that seal design allows it and a metal-to-metal seal is maintained. In addition to the accuracy of modeling, use of the appropriate allowable stresses and design margins is of equal importance. API specifications 17D and 6A 6A require require that the equipment is designed and rated capacities based on normal working conditions are provided. However, sometimes the operator functional specifications require the manufacturers to verify their wellhead system designs for normal, extreme, and surviv sur vival al loa load d cond conditi itions ons.. Thi Thiss loa load d for format mat is pre presen sented ted in in API-2RD and and API-17G, API-17G, where where the loa loads ds imparted in the wellhead from the drilling and workover risers are categorized as Normal, Extreme and Surviv Sur vival, al, wit with h each cat categor egory y havi having ng dif differ ferent ent des design ign and saf safety ety mar margin gins. s. The all allowab owable le mem membra brane ne (average) (aver age) stresses for normal, extre extreme, me, and survi survival val conditions are typical typically ly limi limited ted to 66.67% 66.67%,, 80%, and 100% of yield, respectively. The membrane plus bending stress allowables are usually 1.5 times higher than membrane stress allowables. Although the rating of the equipment currently is based on working normal conditions, it is practical to provide the capacity of the wellhead system for normal, extreme, and survival conditions as it gives the operators higher confidence and better understanding of equipment capacity limits. This study provides the connector capacity based on 3D FEA for normal, extreme, and survival load condit con dition ions. s. For a giv given en pre pressu ssure re and ten tensio sion/c n/comp ompres ressio sion n loa load, d, the bend bending ing mom moment ent was inc increa reased sed incrementally until the allowable stresses for each load case/category are reached. The bending capacities 1. determined from the connector analysis from various load combinations are summarized in in Table 1. Representative load cases determined from this analysis are used in the testing of the wellhead system. The connector capacity chart determined with 3-D FEA is shown in Figure in Figure 5. 5.
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Figure 5—Wellhead Connector Capacity Chart
The limiting capacity location of the connector is dependent upon the combined load case applied. For example, for bending loads with no internal pressure and no external tension/compression, the limit is typically on the hub on the compression side of bending, or on the latching segment teeth interface on the tension side of bending. For cases with high internal pressure and external tension/compression, the limitation is usually on the teeth on the tension side of bending. Therefore, it is important for all load combinations to be considered in the analysis. Depending on the bore size, the internal pressure end-load effects could result in millions of pounds of add addit itio iona nall ax axia iall fo forc rce, e, an and d ha have ve si sign gnif ific ican antt im impac pactt on th thee cap capac acit ity y of th thee con connec necto torr an and d se seal al performance. Therefore, it is crucial c rucial that the capacity charts clearly indicate the assumptions made for the pressure end-load effects to ensure that the information is not misinterpreted, and an “apples-to-apples” comparison of various connector capacities can be done.
Wellhead High and Low Pressure Housings Assembly A typical subsea wellhead system assembly is shown in Figure in Figure 6 and 6 and is comprised of many components and the number and size of the components varies depending on the specific applications used. The main components compone nts that are present in every system are the high press pressure ure housing, low press pressure ure housing, landing ring, bending reaction ring, rigid lock-down split ring, and the actuator sleeve, and are the main focus of this work. It is a challenging task to determine the capacity of a wellhead system, because it is affected by several factors that could change depending on the site-specific application. Some of the parameters that have a direct impact on the capacity of the wellhead system are the system preload, the casing program and weights, soil properties, cement levels, and external loads transmitted to the top of the wellhead. A preloaded or rigidly-locked-down wellhead provides great benefits to the fatigue performance of the system, and the preload magnitude affects the load sharing between the conductor and casing strings. The magnitude of casing weight loads affects the system preload and the remaining capacity of
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Figure 6—Wellhead System Global FEA Model
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the components in the load path. The soil properties properties and cement levels could significantl significantly y impact the load distribution between the components and the fatigue performance. The external loads at the top of the wellhead are typically determined via riser global analysis and account for the environmental loads, currents, waves, VIV, vessel motions, etc., and combined with the casing weights have a significant impact on the wellhead system preload, load sharing, and load distribution. The 2-D modeling and equivalent tension/compression for a wellhead system have similar limitations as described for the wellhead connector. To properly account for all the factors described above, a 3-D model of the wellhead system is required. Taking advantage of symmetry, a half 3-D model is sufficient to accurately capture the global response and component load transfer of the wellhead system by applying the act actual ual bend bending ing mom moment ent,, ten tensio sion/c n/compr ompress ession ion,, she shear, ar, and pre pressu ssure. re. The 3-D mod model el giv gives es a tru truee representation of the amount of load transfer and load sharing among the components of the wellhead system. The tension and compression side of bending, component interactions and separations, nonuniform loading of load shoulders and hubs, stress concentrations in geometric discontinuities (slots, splits, holes, etc.), sealing performance and other factors which cannot be fully captured in the 2-D model, are evaluated with higher accuracy in the 3-D model. The analysis model should include all the major components of the system and should be carefully sequenced and preloaded in a method analogous to real-world field installation. System preload, global external and internal loads, and casing loads should be applied to the model. The model should extend about 200 feet below the mudline, a depth considered sufficient for the wellhead system to be free of any boundary effects. It should be recognized that this is a very challenging and time consuming task considering the large size of the analysis model, large number of components and their interactions, cements, soils, convergence issues encountered, and post processing of the results. A typical example of the 3-D model of the wellhead system is shown in Figure in Figure 6. 6. This particular analysis model consisted of the High Pressure Housing (HPH), Rigid Lockdown Actuator Sleeve, Rigid Lockdown Split Ring, Landing Ring, Bending Reaction Ring, Low Pressure Housing (LPH), Supplemental Adapter, 38 Conductor String, 28 Positive Stop Hanger, 28 Casing String, 14 Casing Hanger, Casing Hanger Load Ring, Casing Hanger Load Ring Actuator Sleeve, 14 Casing String, 22 Casing String, Lock Down Sleeve, Lockdown Sleeve Retainer Ring, Lockdown Sleeve Lock Ring, Lockdown Sleeve Actuator Ring, and Cement and Soils Soils.. All components, components, includ including ing cements, are modeled with solid elements. Soil properties are modeled as non-linear springs using the appropriate soil resistance data (P-y curves). The sequence of installation and load steps required for proper modeling of the wellhead system are presented in detail the Appendix A. For the scope of this work, some simplifications were made to the wellhead system by focusing on the main components of the wellhead and excluding the cements and soils from testing. These modifications were driven by practical reasons relating to validation testing. However, creating a 3-D model of the “as-tested “as-t ested”” condit condition ion of equipm equipment ent (Figure (Figure 7) 7) is sufficient to validate the analysis methodology and provide confidence in the results and accurately models the critical wellhead connector to wellhead interface, which were the main goals of this work. A plot that shows the pattern of stress distribution of the wellhead housings under a bending load is shown in Figure in Figure 8. 8.
Test Fixture Design To meet the challenges of applying loads on the scale of subsea wellhead system capacities in such a manner that presented the worst-case scenario, the fixture was designed to apply uniform bending across the entire specimen length. This is accomplished with loaded push/pull rods flanking a specimen in a parallel axis. Attention was given to the sizing of the cylinders and push/pull rods to ensure that adequate tension/compression loads could be transmitted. The size and weight of what was deemed necessary to accomplish these tasks ultimately led to a horizontal fixture. Placed in the horizontal position, center of gravity-related stability concerns were virtually eliminated. However, making use of the fixture in the
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Figure 7—Wellhead 7—Wellhead Housi Housings ngs Model Mesh
Figuree 8 —Wellh Figur —Wellhead ead Housings Stress Distribution Distribution Plot with Applied Bending Moment
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Figure 9 —Horiz —Horizonta ontall Test Fixture witho without ut Specime Specimen n Insta Installed lled
horizontal position was not without its challenges. The apparatus makes use of a fixed end where the cylinders are securely mounted. The opposite end is free to move as a function of applied load, specimen stiffness, and rolling resistance between it and the surface it rests on, among other friction forces. This articulati artic ulating ng end is suppor supported ted on specia speciall floor flooring ing material and height height-adjus -adjustable table roller devices to ensur ensuree as smooth and low friction movement as possible. Much of the load path within the fixture is preloaded to minimi min imize ze cyc cyclic lical al str stress ess ran ranges ges,, and the theref refore ore pro provid videe the fix fixtur turee a lon longer ger len length gth of tim timee bet between ween inspections, and overall increased lifespan. The test fixture without and with test specimen installed is shown in Figure in Figure 9 and Figure Figure 10, 10, respectively. Compression within the subsea system can be a limiting factor, most notably due to bending. Casing weights within or below the high pressure wellhead housing may increase the amount of compression within the system. To capture the effects of the casing weights in and/or below the high pressure housing, a hydraulic ram was incorporated into the fixture design to simulate 6 million pounds of casing weight within the wellhead. This ram also incorporates a spherical rod end to reduce added stiffness that would otherwise benefit the system’s bending capacity, thereby providing a more conservative (i.e. increased) load transfer between the high pressure and low pressure housings. The 6 million pound value was targeted as a reasonably conservative amount of weight from casing strings within the wellhead housing, pipe and supplemental casing strings below the wellhead housing. Due to time limitations, this feature was not used in this test, but will be done in future testing.
Test Fixture Capacities Designed to output the harshest static conditions of the offshore environment, the fixture utilizes twin 6.5 million pounds double acting cylinders. With equal piston areas, the total cylinder output in both tension and compression is 13 million pounds. The total frame tension is a function of the cylinder output and specimen size with internal specimen pressure. Spacing of the cylinders equates to uniform bending capacities beyond 20 million foot-pounds. Auxiliary systems include the hardware required to apply nitrogen or hydraulic pressure to the test specimen, both of which are sized for 20 ksi applications. Pressure is applied via hydraulic intensifying
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Figure 10 —Horiz —Horizonta ontall Test Fixture with Specim Specimen en Insta Installed lled
and gas bo and boos osti ting ng pu pump mps. s. Pr Prov ovis isio ions ns we were re ma made de duri du ring ng th thee fi fixt xtur uree des desig ign n to be abl ablee to per perfo form rm equipment testing under various thermal conditions, when necessary.
Table 2—Test Fixture Calibration Load Program
Control System The control system of the fixture was designed with the intent intent to mee meet, t, as a min minimu imum, m, the validati validation on requirements of of ISO 13679/ 13679/API-RP-5C5 API-RP-5C5.. Because these industry documents require loading close to the specimen yield strength, both accuracy and precision were needed to reach and maintain target test values. The brain which provides this accuracy and precision of the control system lies in a multi-axis position control CPU. This CPU, under the guidance of custom-tuned proportional-integral-derivative (PID) controller algorithms, algorithms, utili utilizes zes feedbac feedback k from strain gauges, magnetostrictive linear-position sensors, senso rs, thermo thermocouples couples and press pressure ure trans transducers ducers.. User input to the fixture is recipe-based, whereby a data file with loads, bends, pressures, positions and timing events is “fed” into the custom control software.
Test Fixture Calibration Accuracy of the control system of the fixture was quantified with 36 OD 2 wall API 5L X-80 pipe with documented geometry and material properties. With two planes of strain gauges installed at 90° apart
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Figure 11—Calibration Experiment Setup
(qty. 4), the pipe was loaded to the limits of the wellhead system validation test. These combined loads are presented in Table in Table 2. 2. Because the princi pal stress axes of the specimen were known, 90° tee rosette gauges were used exclusively for their ability to measure biaxial strain. The two planes, labeled east and west array, are shown in Figure in Figure 11. 11. API 5C5/ 5C5/ISO 13679 requires 13679 requires an annual calibration with a national standards body-traceable load cell ce ll at th thee re requ quir ired ed cap capac acit itie ies, s, i. i.e. e. th thee sc scop opee of loads used for testing. For a large number of subsea systems, sub-systems and even many components, Figure 12—Calibratio 12—Calibration n Specim Specimen en in Test Fixture load cells of this size/magnitude are not commercially available. Without the availability of a load celll tra cel traceab ceable le to a nat nation ional al sta standar ndards ds body body,, duediligence was employed to ensure that the test frame was properly calibrated. Comparisons of commanded loads and test frame output ensure that the control system is designed to transmit accurate loads into the test specimen, and live monitoring of strain gauge data throughout testing ensures that these loads are being transmitted for all load cases. It is recommended that these requirements be used for all systems lacking the ability to calibrate with such a load cell. For the calibration testing, API 5L pipe was used in place of the load cell. The test fixture calibration specimen is shown in in Figure Figure 12. 12. Becausee the control system monitors Becaus monitors exact averaged strains between the 2 gauge arrays of the pipe and adjusts cylinder output to reach the desired strains, within a single microstrain before starting the hold period, the output of the cylinders c ylinders based on pressure and cylinder area a rea can be reviewed to ensure adequate correlation to the target values. The comparison of the control system output, based on real-time strain gauge readings, and calculated system output, based on real-time cylinder pressures and areas, can be seen in Figure in Figure 13. 13. Given the fact that all calculated outputs are larger or start larger than the control system outputs, disparities in these values can be attributed to friction factors. Friction plays a role in the linkages of the
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Figure 13—Comparison of Control System and Calculated Outputs
Figure 14 —Comp —Compariso arison n of Measu Measured red and Theore Theoretical tical Cylinder Relative Position
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Figure 15—HPH & LPH Housing Test Preload
fixture and a major role in the articulating beam, especially in bending, where cylinder loads to induce a bending moment are relatively small compared to the cylinder loads required to generate tension and compression at the levels of this test. During bending, the relatively small cylinder loads are increased until static friction has been overcome in the articulating beam. Once it has been overcome, a certain amount amo unt of the bending bending ove oversh rshoot oot is captured captured wit within hin the sys system tem and the app applie lied d bend bending ing is red reduce uced d accordingly. accord ingly. Another review review can be made on the diff difference erence in cylinder movement for the variou variouss bending moments applied during the test. As shown in Figure in Figure 14, 14, the measured relative cylinder movement (i.e. the total difference of position in a single axis between the two loading cylinders of the fixture) is compared to the theoretical cylinder movement at each bending step, based on specimen geometry and material properties only. This assessment is not made for a direct comparison of values, because the theoretical values do not account for fixture stiffness, mechanical slack within the system or hysteresis, but merely to highlight proper trends in the cylinder movement and application of relative loads between cylinders. The results of this calibration testing are in line with the ability of the control system to output loads to reach defined strain values. Error in the calibration of this system can be found in the inherent use of strain gauges, material testing and physical measurement devices.
Wellhead System and Connector Test Setup The 18-¾ high pressure wellhead housing, was installed and preloaded to the low pressure housing as it would be in the field condition. The preloading of these housings calculated from test measurements is shown in Figure in Figure 15. 15. This calculation is based on uniform geometry of the two housings at the strain gage locati loc ations ons.. The com compar pariso ison n of mea measur sured ed str strains ains ver versus sus anal analysi ysiss str strain ainss are pre presen sented ted in in Table 3. 3. The
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Table 3—Analysis and Test Results Sample Comparison - Rated without PEL
reduction in preload over the installation time period is inherent to the wellhead preload and installation device. Two sub-systems, the pressure boundary plug and hydraulic ram for simulated casing weight, were installed within the high pressure housing in order to facilitate a more accurate representation of subsea loads and pressures. Pressure has a critical influence on a subsea system. To quantify this influence, a pressure boundary plug was installed to facilitate 15 ksi or higher pressure applications within the connector and wellhead bodies. The boundary plug was designed with a floating head and spherical rod ends to virtu virtually ally eliminate eliminate any stiffness stiffness it may have otherwise provided provided the system during the applic application ation of external loads and/or internal pressure. The wellhead connector was designed for drilling environments with high bending and tension loads. The annular piston/cam ring provides the preload needed to prevent hub-face separation. With the use of linear displacement transducers and strain gauges located on the connector locking segments, the preload between the connector and wellhead, as well as the hub-face separation, separ ation, were quanti quantified fied throughout testing. Strain gauges were installed throughout throughout the wellh wellhead ead system. syst em. Their locations are detail detailed ed Figure 1. 1. The strain data were collected throughout testing and compared to the analysis strains at respective locations.
Analysis and Test Results Comparison The 3-D FEA verification was fully completed for the wellhead system that includes the connector and HPH and LPH assemblies for normal, extreme, and survival conditions at various load combinations as presented in in Table Table 1. 1. This resulted in a total of 72 analysis case runs, which required significant efforts
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OTC-25163-MS
Table 4—Analysis and Test Results Sample Comparison - Extreme without PEL
when considering running time, convergence issue, model calibration, and data post processing. Due to the time constraints, testing validation was done for 36 load combinations, which is considered a sufficient number of cases to validate the analys analysis is methodology methodology and shows that the wellh wellhead ead system confirms to the specified design limits. For the test cases involving pressure, no signs of leakage or loss of pressure were observed throughout the testing. This confirmed that the seals performed as intended. A comparison of analysis and test strain results was performed. Samples of this data comparison are presented in in Table Table 3 through Table 5. 5. The results indicate that there is a good correlation of analysis and test data with a match within 1% and with a maximum variation of 11%. These variations are attributed to several factors assumed to be perfect in the analysis, but in reality are different. Some of the factors includ inc ludee the str strain ain gage err error or and ori orient entati ation/ on/ali alignm gnment ent,, com compone ponent nt tol tolera erance ncess and out out-of -of-ro -round undnes nesss effects, positioning of the non-axisymmetric with respect to the plane of bending, and material properties. The hub contact which is important for the seal performance, was monitored during testing using linear displacement transducers, and showed very good correlation with the analysis results. The wellhead system test specimen was disassembled after testing was completed. An inspection of the major components was performed and there were no signs of yielding or damage of the wellhead system components. This is another indication that confirms the validity of the analysis methodology used. A picture of the disassembled wellhead system is shown in in Figure Figure 16. 16.
