Abedrabbo Weatherford Final 2172012

2012 SIMULIA Community Conference 1

Automation of Static and Dynamic FEA Analysis of Bottomhole Assemblies

Nader E. Abedrabbo, Lev Ring & Raju Gandikota Weatherford International

11909 Spencer Road, Houston, TX [email protected]

Abstract: A lower portion of the drillstring used in drilling for oil and gas is called bottomhole assembly (BHA). Drilling forces and vibrations have a great impact not only on the rate of drilling, but also on the survivability of the equipment. Therefore, it is critical to perform detailed analysis on every BHA system to predict critical dynamic behavior and minimize failures. There are several challenges in applying finite element analysis to such problems. The ratio of the length to the diameter is in the order of 105. Beam elements provide a good approximation and result in acceptable combination of accuracy and solution time. However, each BHA model could consist of hundreds of small parts with different section and profile definitions. Contact definition for beam elements and drillstring instabilities pose another complication. An automated method was developed to assist in creating any BHA system from three files: BHA components, well survey and wellbore IDs. Using these, a complete Abaqus input file can be created for each BHA at any location in the well in a matter of seconds. A complete user interface was also created for specifying multiple input parameters, generating input files and then to run and monitor the analysis. The developed GUI allows engineers who are not familiar with FEA and Abaqus to conduct complex static and dynamic analysis. A Standalone post-processor was also developed to automatically extract required data from the ODB file in order to report and view the results in a manner consistent with industry standards.

Keywords: Automation, Bottomhole Assembly, BHA, Bending, Buckling, Drilling, Dynamics, Wellbore, Oil Industry.

1. Introduction

A lower portion of the drillstring used in drilling for oil and gas is called bottomhole assembly (BHA). The BHA consists of a combination of very sophisticated and expensive equipment that is designed to stabilize and direct (navigate) the drill bit to a specific target depth and horizontal deviation (directional drilling) while simultaneously performing real time geophysical measurements (logging-while-drilling). A typical BHA ranged between 500 and 600 feet in length. Figure 1 shows a sample BHA and Figure 2 shows the list of components for the same BHA. Drilling forces and vibrations that occur during drilling operations have great impact not only on the rate of drilling, but also on the survivability of the downhole equipment. Therefore, it is essential to perform detailed analysis on every BHA system in order to predict critical dynamic behavior and minimize premature failures.

2 2012 SIMULIA Community Conference

Figure 1. Schematic of a BHA showing the composition of different parts with

different sections.

Figure 2. Component list of the parts making the sample BHA shown in Figure 1.


There are several challenges in efficiently applying finite element analysis to such models. The ratio of the model length to the typical diameter is in the order of 105 which prohibit using shell and solid elements. Another complication is the presence of multiple and variable areas of contact between drillstring and borehole as well as potential drillstring instabilities (i.e., buckling behavior). Choice of beam elements usually provides a good approximation to the geometry and stress distribution, resulting in acceptable combination of accuracy and solution time. Some modeling challenges are specific to Abaqus [1]. Each BHA model consists of hundreds of small parts with different section and profile definitions. The process of defining contacts in Abaqus using the ITT contact elements (beam-to-beam) is not supported in Abaqus/CAE. Creating a single BHA model using a combination of Abaqus/CAE and manual scripting to prepare the input file could take days to construct. In order to automate and accelerate the analysis, an automated method was developed to assist in creating the BHA systems from three simple input files: BHA component list, well survey and wellbore inner diameter definitions. Using these files, a complete Abaqus input file can be created for each BHA at any location in the well in a matter of seconds. A complete user interface (GUI) was also created for specifying a variety of input parameters, generating Abaqus input files and then running and monitoring the Abaqus analysis. This GUI allows engineers who are not familiar with FEA and Abaqus to conduct complex static and dynamic analysis during pre-job planning and identify root causes of potential field problems. A Standalone post-processor was also developed to extract the required data directly from the ODB file in order to report and view the results in a manner consistent with industry standards.

2. BHA Development in Abaqus/CAE

In a drilling procedure, the drilling bit has the largest outer diameter (OD) of all the BHA components. Stabilizers, which have a close-to-gage OD, are added at key locations in the BHA composition in order to provide stability and support for the long BHA. Other components of the BHA (e.g. drill pipes, collars, subs) have varying outer diameters that are usually smaller than the bit and stabilizer ODs. Typically, failures occur due to high bending stresses as well as high vibrations occurring during the drilling procedure. To mitigate these problems, static and dynamic analysis of bottomhole assemblies is frequently conducted before a drilling job is performed to ensure that the drilling forces and vibrations experienced by the BHA components are within acceptable tolerances. If the analysis reveals higher stresses, certain measures can be used to reduce these effects (e.g. installing extra stabilizers, reducing drilling speed). One other factor that affects the BHAs response to static and dynamic forces is the location (i.e. orientation) of the BHA in the well. During drilling, the BHA transitions through different locations based on well planning. As the BHA orientation changes, the static and dynamic forces experienced by the BHA also change. Therefore, it is imperative to perform the required analysis on the BHA in different orientations in the well in order to ensure that a configuration in one orientation that passed the safety limits would not experience higher forces in a different one. Figure 3 shows a side view schematic of a planned well. The dots on the line highlighted by the arrows indicate the positions in the well where static and dynamic analysis must be performed.


Figure 3. Well profile section indicating locations where BHA is to be analyzed.

Due to the ratio of the BHA system length to the typical diameter, beam elements offer the best approach of both accuracy and fast solution time. Beam elements are used to define both the BHA components and the well profile geometry. Contact between the BHA and the well is achieved by using the beam-to-beam contact elements (ITT). In order to define a BHA component in Abaqus and maintain cross sectional accuracy, each component making up the BHA is divided into smaller pipe sections representing the different outer diameter differences. Figure 4 shows a sample BHA part (heavy weight drill pipe) where it has been divided into five separate cylindrical sections, each representing a different OD (typically IDs are similar for a single part). Some of the sections are similar, but they are separated by non-similar parts. These different sections need to be represented individually in Abaqus.

Figure 4. BHA part (heavy weight drill pipe) divided into its different section

representation in Abaqus. Dimensionally equal parts are indicated.

Horizontal Position (ft)

True

Ver

tical

Dep

th (f

t) Positions in well to perform analysis

Section 1

Section 2

Section 3

Section 4

Section 5


The process of creating a beam definition of a BHA system in Abaqus/CAE involves the following:

1. Define the geometry of the whole BHA in Abaqus/CAE Sketcher. In this scheme, the BHA is drawn as a single beam with multiple sections. Each beam section length corresponds to that of a section of the parts making the BHA (see Figure 4). Using this scheme eliminates the need to define elements connecting the different components consequently overhead creation costs are reduced.

2. Define beam profiles using one of the predefined cross section definitions (e.g. tube, circle or the general beam definition method). In the definition, assign the OD and ID of each beam section.

3. Define beam section properties and associate material (e.g. Poissons ratio) to the beam profile.

4. Associate each section of the BHA system to its appropriate beam section definition. A typical BHA could potentially consist of hundreds of subsections for all its parts. For example, one of the BHA systems currently being analyzed has the following definition:

1. Total number of parts: 20 2. Subsections for all parts: 93 + well section definitions 3. Total beam and cross section definitions required to represent the BHA: 186 + well

section definitions After defining the BHA and all its components and manually associating the beam sections to the beam elements, the contact definition between the BHA parts and the well needs to be provided. Since the model is being solved using the Dynamic Implicit approach, the only contact definition available in Abaqus/Implicit is the beam-to-beam contact (e.g. ITT31). These beam definitions, however, are not currently supported in Abaqus/CAE. Manual scripting of the input file must be performed instead. The beam-to-beam contact for beam elements simulates the contact between the OD of each BHA part and the ID/s of the well. Since the BHA is composed of multiple parts with unique ODs, each section of the BHA must have its own beam contact definition. For each contact definition, the interface value, which is the difference between the inner radius of the well and the outer radius of the part, must be supplied in addition to other definitions. Figure 5 shows a sample beam-2-beam contact definition for a single section of the BHA. For the sample BHA described above, a total of 75 beam contact definitions were required to be coded manually. A critical issue that affects the manual definition of beam contacts in Abaqus is the node numbering scheme. Abaqus/CAE tends to assign node numbers to the ends of the beam segments composing the BHA first, then, when all major segments of the line have been assigned, node numbers for the inner regions of the sections are assigned. Abaqus/CAE does not provide a method to renumber nodes in a linear progressive fashion. This numbering scheme further complicates the contact definition for beam elements as shown in Figure 5. The user must keep track and manage differing node numbers increasing the likelihood of confusion and errors. Additionally, this node numbering scheme creates problems when post processing the analysis results as it is difficult to track the OD of each node.


Figure 5. Beam-to-beam contact definition for a single section of the BHA.

Another significant issue exists with using the manual method to define beam elements for a BHA in Abaqus/CAE. As described earlier, the BHA must be analyzed at different location in the well. As shown in Figure 3, not all positions in the well conform to a straight line. Analysis locations in the well may include BHA systems that are partially or even completely curved. Creating a curved beam of the BHA components while accounting for the exact lengths of each section or part is nearly impossible to do manually. One solution to this problem is to start each analysis with the BHA in a vertical position, dynamically insert the BHA to the required position, stop the analysis, and finally perform a static analysis in this position. Using this approach, however, is both time consuming, and creates more work load. In the BHA example described previously and illustrated in the last point of Figure 3, the analysis took two days to dynamically insert the BHA to the required position. This solution time is prohibitive especially when the analysis must be conducted for several locations and for multiple BHA configurations. Assuming all the previous manual definition limiting issues were resolved, constructing the input file for Abaqus using a combination of Abaqus/CAE and manual editing would require at least two days for each BHA system and for each test position within those systems (for positions shown in Figure 3). In order to build, test, and analyze multiple configurations rapidly and effectively, an automated method is required to speed up the FEA analysis.

3. Motivation

At Weatherford International and in the oil industry in general, bottomhole assembly analysis is currently conducted using proprietary third party programs. These programs, however, suffer from several limitations (as reported by our users and personal testing):

1. Majority of the programs only give limited output results regarding the behavior of the BHA. For example, the deformed shape of the BHA in one plane, usually the lateral

****************************************************************** ITT31 - SLIDE LINE INTERACTIONS FOR BHA AND WELL *** ****************************************************************ELEMENT, TYPE=ITT31, ELSET=TUBE_SL_1 100001, 1 *ELGEN, ELSET=TUBE_SL_1 100001, 6, 1, 1 *SLIDE LINE, ELSET=TUBE_SL_1, TYPE=LINEAR, GENERATE, SMOOTH=0.5 791, 796, 1 *INTERFACE, ELSET=TUBE_SL_1, NAME=ISL_1 0.00521, *FRICTION 0.1, *SURFACE BEHAVIOR, PRESSURE-OVERCLOSURE=EXPONENTIAL 0.001,7200000.


axes, is only supplied. In order to accurately predict helical and sinusoidal buckling, however, viewing the out-of-plane deformed shape of the BHA is a necessity.

2. The ability to extract the analysis results in text or CSV file formats is limited. 3. One of the major problems with these programs is that they limit the length (or divisions)

of the BHA being analyzed. For example, one of the programs currently in use relies on a student version of ANSYS as a solver. Due to this fact, the user is limited to a small number of nodes that can be used to describe the BHA and the well (500 node limit). Because of this limitation, the users are forced to reduce both the length and complexity of the BHA system being analyzed by limiting the number of varying OD and ID sections. The more complex the system being studied, the shorter the length of the BHA must be for the analysis to run. The reduction in BHA complexity limits the amount of useful information available to accurately analyze BHA systems.

4. Some of these programs suffer from major bugs and crash incidents. One of the programs currently in use suffers from a crash rate of 50%.

5. As mentioned earlier, every BHA must be analyzed at different locations in the well as shown in Figure 3. However, most the programs currently in use do not use the real well trajectory to position the BHA for the analysis. Instead, they rely on a very short description of the well. In this description, the BHA is either in the vertical, horizontal or completely curved (i.e., arc) position. This description, although useful, does not accurately describe the actual well in use, especially for conditions where the BHA is in a non-uniform well.

4. Development Requirements

With these shortcomings of current BHA analysis programs in mind, future developed procedures for analyzing BHAs using Abaqus FEA should satisfy the following criteria:

1. FEA (and Abaqus) Knowledge: Users with limited to no knowledge of the FEA process or Abaqus should be able to use the program with ease.

2. Usability: The BHA and all its complex components must be created with ease and with minimum user effort. Also, no limitations on BHA lengths being analyzed should be imposed.

3. Flexibility: The developed methods must have the ability to apply different boundary conditions based on the analysis type and create different test scenarios quickly (e.g. static analysis at different locations in the well).

4. Accuracy: The FEA analysis must be able to run to completion with minimal code crashes and produce accurate results.

5. Post-Processing: The user should have the ability to extract the desired results from the analysis ODB file and plot them in a fashion close to other programs for easy comparison.


5. Developed Solution

Based on the requirements stated above, a standalone program (GUI) was developed for the creation and analysis of bottomhole assemblies which uses Abaqus FEA as a solver engine. The developed program with its standalone GUI was developed using the Python programing language [2]. Also, a post-processor was developed where the required output results are extracted directly from the Abaqus ODB results file using a Python script. The extracted data is then manipulated and the post-processor is used to display the analysis results in the required format. After generating the required input files for Abaqus, the developed program then calls the Abaqus FEA solver to perform the required analysis from within the GUI. Monitoring of the progress of the analysis is also provided. In the developed program, a complete input file for Abaqus can be generated for any BHA system at any position in the well (e.g. horizontal, vertical, curved or in-between) in less than two seconds, compared to more than two days using a combination of Abaqus/CAE and manual editing (if feasible). Running the analysis to solve the BHA problem (Implicit Dynamic step followed by a Static step) takes between 10 to 45 minutes, depending on the location of the BHA in the well.

5.1 Procedures

The developed procedure for generating the required BHA input files for Abaqus in any location in the well and also for post-processing of the results is as follows:

1. General Settings: In this window (as shown in Figure 6), the user specifies all the necessary information and boundary conditions for the analysis:

a. Solution directory; b. Project name which is used in naming of the (*.inp) files for easy identification. c. Analysis type:

i. Static Analysis: This is used to perform the static analysis of the BHA. The analysis is achieved using a multiple step procedure combining an Implicit/Dynamic step followed by an Implicit/Static step.

ii. Buckle Analysis: This is used to generate imperfections to be included in the Static Analysis step in order to improve buckling predictions.

iii. Frequency Analysis: This is used to extract the eigenvalues and modes of the model. Contact between BHA parts and the wellbore are enforced using BC definitions.

d. Boundary conditions: The user can specify all the necessary boundary conditions for the analysis. For example:

i. Fix top of BHA: The drill bit is usually fixed to the centerline of the wellbore. The user can also choose to fix the top of the BHA to the centerline, in which case only lateral movement will be allowed.


ii. Test MD: As shown in Figure 3, this is the position in the well where the analysis is to be performed.

iii. WOB: This is the weight on bit that is to be applied as an axial force to the top of the BHA.

iv. Other boundary conditions

Figure 6. General setting window of the developed GUI.

2. Well Survey: In this window, the user supplies the well survey information in the spherical coordinate system: Measured Depth, Inclination and Azimuth. In order for the well survey data to be used in the finite element method, the spherical coordinate data must be converted into the Cartesian coordinate system. This is achieved internally in the program using the Minimum Curvature Method [3]. After conversion, several plots are provided of the well survey for better visualization (e.g. 3D, side view, planar view). Figure 7 shows a 3D plot of a sample well.


Figure 7. 3D plot a sample survey data after being converted into real coordinates.

3. Well Intervals: In this window, the user supplies the well inner diameters. Some wellbores can have multiple inner diameters; therefore, the inner diameter of the well is supplied as a function of measured depth. A plot is also provided to the user for better visualization of the well inner diameters as shown in Figure 8.

Figure 8. Well intervals input showing inner diameter as a function of depth.


4. BHA Breakdown: In this window the user defines the composition of each component of the BHA. Each BHA part is divided into its subsections based on outer diameter as described in Figure 4. For some parts (e.g. drill pipe, heavy weight drill pipes), a built-in database has been developed such that the user only needs to specify the part name and nominal outer diameter to identify the part. Two plots are also supplied for the user in order to review the composition of the BHA: 2D sectional view and 3D view. Figure 9 shows the BHA breakdown of a sample BHA system. A plot of the cross section of the parts is also shown.

Figure 9. Breakdown of BHA components into subsections.

5. Generate Abaqus Input Files: After the user supplies all the necessary information about the well, BHA components, required analysis type and boundary conditions, the Abaqus input files can be generated. As seen in Figure 10, generation of the input files for the system illustrated in Figure 9 based was accomplished in 1.25 seconds.

6. Run Abaqus Analysis: When the input files have been generated based on the supplied data, the user can run the Abaqus analysis from within the GUI. The user has the ability also to select the number of CPUs to use for the Analysis. The state and progress of the analysis can be monitored using the Event Log window and also through the utilization of progress bars. Figure 11 shows an example of a case being solved using Abaqus solver and the solution monitored within the GUI.


Figure 10. Generation of Abaqus input files based on supplied data. Building complete Abaqus input files for the model was accomplished in 1.25 sec.

Figure 11. Running of Abaqus analysis and monitoring of the analysis is done from within the GUI.


7. Post-Processing: After the model is analyzed using Abaqus solver, post-processing of

the data in a format that is easy to understand by the users is necessary. Due to the fact that the ratio of the model length to the typical diameter is in the order of 105, viewing of a model that is 500ft long in Abaqus/CAE does not produce useful viewable information. Figure 12 shows the result from a sample BHA analysis using Abaqus/CAE. The beam profile rendering was activated with a magnification scale factor of 10. Figure 12 clearly illustrates that even with the magnification, the BHA deformations are not easy to visualize in the current format.

Figure 12. BHA analysis result of the sample BHA in Figure 1 as viewed in Abaqus/CAE. The beam profile is magnified by a scale factor of 10. Furthermore, the extraction of several required variables in the Abaqus/CAE post-processor, while possible, is time-consuming. In order to extract the results of the analysis and view them in a usable format, a standalone post-processor was developed. All required analysis results are extracted from the ODB file using a specially written Python script file. When the analysis is completed, the user then calls the analysis extraction module where the ODB file is supplied. The data from the ODB file is extracted and saved to a special file. The extracted analysis results is then viewed in the post-processor. In order to view the long BHA in a useful format, the post-processor automatically manipulates the BHA from its original curved or vertical position to a straight, horizontal


orientation. This allows the deformations in the BHA to be viewed in a much clearer format than shown in Figure 12. Two views are shown for the BHA: front cross-section view and top cross section view. In this format, sinusoidal and helical buckling in the BHA can be easily visualized. Also, the ability to project the von Mises stresses in contour format can be projected on the cross section as shown in Figure 13. The post-processor displays a list of data values that can be viewed alongside the deformed BHA shape. The user can also extract the name of each part in the list by mouse clicking on the plots for easy identification of parts. Figure 13 shows a sample result for the BHA as illustrated in Figure 9. Only the first 250 feet of the BHA is shown.

Figure 12. BHA analysis result of the sample BHA in Figure 9 as viewed in Abaqus/CAE. The beam profile is magnified by a scale factor of 10. For the frequency analysis capability, the post-processor shows the natural modes of the solved system in multiple formats. One of the more useful views is the 3D plot, which has the ability to apply the beam profile rendering as well as a scale factor for the deformation intensity of the modes. Figure 13 shows a sample natural mode (mode #14) for the previous system with a deformation scale factor of 30. The post-processor also offers the ability to export all plot data to CSV files if required by the user.


Figure 13. Natural modes of the sample BHA shown in Figure 9 for mode #14.

6. Conclusion

A standalone user interface was developed that allows an engineer, even one with little or no knowledge of Abaqus or FEA, to perform detailed FEA analysis of a bottomhole assembly. By supplying the program with required input data, the GUI builds complete input files for a BHA, including nodes, elements, contacts and step definitions. The program further allows the user to run the analysis using Abaqus as a solver. When the analysis is completed, the program automatically extracts all the required results from the ODB file. Finally, a standalone post-processor was also developed to view the analysis results in a usable format. The developed user interface, analysis procedure, and post-processor described in this paper will result in increased speed, efficiency, and accuracy in analyzing bottomhole assemblies with efficiency.

7. References

[1] Abaqus is a product of Dassault Systmes Simulia Corp., Providence, RI, USA. [2] www.python.org [3] http://www.drillingformulas.com/minimum-curvature-method/

Automation of Static and Dynamic FEA Analysis of Bottomhole Assemblies1. Introduction2. BHA Development in Abaqus/CAE3. Motivation4. Development Requirements5. Developed Solution5.1 Procedures

6. Conclusion7. References

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Abedrabbo Weatherford Final 2172012