Cost of Ownership for Drill String Components Subject to Extreme Wear Environments: A Case Study

Abstract Total cost of ownership is an often-overlooked factor when evaluating drilling tool procurement decisions. Initially attractive, low cost investment options that consider only upfront purchase price may backfire when considering long-term ownership costs. Factoring in repair costs and expected component life cycles in a broader financial analysis can, and in the case of many directional drilling components, will affect the total cost of ownership significantly. This paper presents a total cost of ownership case study for a generic drill string stabilizer, a widely used component of directional drilling tool assemblies that, as the name suggests, adds stability to the advancing tool underground but are subject to extreme wear conditions. The hypothetical analysis of the long-term costs presented here discusses the available options for wear protection technologies and the total cost of ownership. The relative analysis in this work is meant to present a typical scenario faced by procurement and operations supervisors and the effect of their decisions on the total cost through the life cycle of these parts. This evaluation supports that laser cladding repair of initially laser clad drill string stabilizers is the most financially prudent decision considering long term ownership of these parts encompassing manufacturing costs, inspection periods, expected service life, and typical repair costs.

Introduction This paper presents a critical analysis for the cost of ownership of a drill string stabilizer, a single component of a multicomponent downhole tool assembly, used in modern drilling applications. Drill string stabilizers are subjected to extreme in-service conditions, and therefore require special protective coatings to ensure performance and survivability in some of the most severe environments in the planet. The impact of the technical decisions of coating type and deposition technology can have a significant impact on the run time hours for new components, the expected repair cycles, and overall service-lifetime of a drill string stabilizer, all of which affect ownership costs. The consequences of these decision and their effect on total cost of ownership cannot be fully understood until the entire life cycle of the component is analyzed. In this analysis, a hypothetical case for a generic string stabilizer is broken down into the five most common scenarios for coating material selection and corresponding coating application technology for initial manufacturing steps and repair cycles. The total cost of ownership is considered for a 1000-hour service life, which is desirable for a drill string stabilizer. A relative cost analysis between the technologies is presented considering all relevant factors that must be considered by procurement and operations staff for the entire lifetime of the component. The appendix includes relevant background material on drill string stabilizers and their role in down-hole tool assemblies. Additional information regarding the base materials, coating materials, and welding application technologies is provided in the appendix to support life-cycle decisions in the body of the paper.

Cost of Ownership Analysis The cost of ownership for a generic drill string stabilizer can be broken down into three relevant cost categories: initial manufacturing, repair, and replacement costs. The drill string stabilizer itself is manufactured from high strength, non-magnetic stainless steel. The base material is very expensive, and hardfacing is applied to maximize the total service-life and to reduce operational cost of early replacement. Wear protection coatings applied to the surface of the stabilizer that are necessary for successful in-service operation can be repaired in some cases multiple times, which carries substantial financial benefits. The strategy is to employ wear protection systems (material and application process) that can maximize the lifetime of the stabilizer based on application costs and potential for multiple repair cycles. Two materials are primarily used for wear protection for drill string stabilizer applications: tungsten carbide bricks with a nickel-based matrix to hold them in place (hereafter referred to as tungsten carbide bricks) and spherical nickel tungsten carbide in a primarily nickel matrix (tungsten carbide powders). Three different welding processes are used to deposit these materials: laser cladding (LC), plasma transferred arc welding (PTAW), and oxy-fuel brazing (BR). Additional technical information on the base materials, wear protection materials, and welding processes is included in the appendix. The five most industrially relevant process-material combinations are as follows:

1. (LC-LC): Laser cladding with tungsten carbide powders initial manufacture with laser clad tungsten carbide powder repair cycles

2. (PTAW-PTAW): Plasma transfer arc welding with tungsten carbide powders initial manufacture with plasma transfer arc welding tungsten carbide powder repair cycles

3. (BR-BR): Oxy-fuel brazing tungsten carbide bricks initial manufacture with oxy-fuel brazing tungsten carbide brick repair cycles

4. (PTAW-LC): Plasma transfer arc welding with tungsten carbide powders initial manufacture with laser clad tungsten carbide powder repair cycles

5. (BR-LC): Oxy-fuel brazing tungsten carbide bricks initial manufacture with laser clad tungsten carbide powder repair cycles

The following analysis outlines the total cost of ownership for each of the above listed process-material combinations. It is possible to create high quality coatings as manufactured for all five combinations, but there are significant differences in cost, repair service intervals, and expected component life that are fundamentally linked to the materials and processes used to deposit them.

1. LC-LC Lifecycle Summary: Laser cladding of nickel tungsten carbide powders is a proven high-quality overlay coating process that can be done efficiently and repeatedly using automation in the form of CNC or robotic welding. The deposition rate is the slowest of all the weld overlay processes considered here (5-10lbs/hr), which adds cost to the initial manufacture and repair processes compared to PTAW. The heat input of laser cladding is significantly less: approximately 10 times less than PTAW

comparing typical Apollo-Clad process parameters to published values for PTAW from the University of Alberta for the same material [1]. The laser clad coating can be refurbished at regular intervals with minimal additional heat during the welding process to the benefit of the coating and base material integrity. It is typical for drill string stabilizers to be regularly repaired at approximately 250 operating hour intervals and achieve at least the desired 1000-hour service life of the part.

2. PTAW-PTAW Lifecycle Summary Plasma transfer arc welding has been commonly used for depositing tungsten carbide powders for oil and gas applications. From a deposition perspective it can deposit coatings at rates up to 25lb/hr, which is in the realm of two and a half to five times that of laser cladding processes. Though significantly faster, PTAW is normally only quoted at slightly lower initial purchase cost than comparable laser clad coatings gaining a more meaningful competitive advantage on larger coating area much larger than required by a drill string stabilizer. The sacrifice to using PTAW comes in terms of the heat of the process, which is dramatically higher than laser cladding. The quality of PTAW hardfacing can vary among suppliers but it is common that due to the excessive heat input of the process, that cracking will occur in service requiring premature replacement. For the non-magnetic stainless-steel drill string stabilizer base materials considered here, the heat input limits the life-cycle to a single repair phase at the standard 250-hour schedule before cumulative effects of the heat on the base material necessitate part replacement. Typical life expectancy is approximately 500 hours. Significantly short part life cycles have been observed due to cracking associated with PTAW on high strength, non-magnetic stainless steels.

3. BR-BR Lifecycle Summary Brazed tungsten carbide bricks represent the highest initial cost of manufacture of all processes in this analysis. The cost is not completely driven by cost of materials, but rather by manufacturing time, which requires both a skilled operator to apply the materials manually and significant flame heat to the entire component for successful application. The preheating times, multistep nature of the application of a variety of materials, and manual application add approximately 25% additional cost to manufacturing the drill string stabilizer. The performance of the bricks can be superior compared to tungsten carbide powder overlay systems and not require a repair cycle after the first 250 hours of run time. At 500 hours, the stabilizer will need repair, and, with a reapplication of bricks and the associated heat of the process on the base material, it is expected to last half as long as a newly manufactured stabilizer. At 750 hours, complete replacement of the drill string stabilizer should be anticipated.

4. PTAW-LC Lifecycle Summary

The PTAW initial manufacture remains the same as previously described in the PTAW-PTAW lifecycle summary. The original damaged PTAW coating needs to be stripped off to apply laser clad tungsten carbide, which brings slightly higher coating removal costs compared to removing a laser clad coating at the first repair cycle of 250 hours. The regular 250-hour repair cycle will continue, but optimistically after two repair cycles the drill string stabilizer will need to be replaced because the laser clad cannot undo thermal damage to the base material caused by the initial higher heat input of the PTAW process.

5. Br-LC Lifecycle Summary The 500-hour anticipated first repair cycle for tungsten carbide bricks remains the unchanged as described in the BR-BR summary at the same initial manufacturing costs. Similar to the PTAW-LC scenario, there is slightly increased material removal costs to repair the drill string stabilizer with laser cladding. The anticipated number of repair cycles is increased from one in the BR-BR scenario to two; however, due to the heating effects of the original brick application the laser clad repair lifecycles limiting its overall performance compared to the LC-LC anticipated minimum three repair cycles. Figure 1 visually presents the cost of ownership for drill string stabilizer during the 1000-hour time period, which outlines when ownership costs are incurred and the relative amount. The figure is split into comparing each option against the lowest cost laser clad with laser clad repair All values have been normalized to the highest cost of ownership option, which is tungsten carbide bricks with tungsten carbide brick repair. Figure 2 present the information in Figure 1 categorically rather than chronologically, which helps to break down contributions to total cost of ownership of purchase, repair, and replacement decisions.

Figure 1: Cost of ownership of a drill string stabilizer considering initial manufacturing and repair cycle materials and deposition technologies during a 1000-hour service life. Data normalized to the most expensive option, bricks manufacture with bricks repair and compared to the low-cost option, laser clad manufacture with laser clad repair. Figure 1(a): PTAW manufacture with PTAW repair. 1(b): Bricks manufacture with brick repair. 1(c): PTAW manufacture with laser clad repair. 1(d): Bricks manufacture with laser clad repair.

Figure 2: Categorical cost breakdown for drill string stabilizer ownership for initial manufacturing, repair, and replacement costs.

With a typical target service life of 1000 hours, the cheapest ownership option is the new laser clad drill string stabilizer with laser clad repair every 250 hours. Process-material combinations that result in premature part failure prior to the 1000-hour service life are costlier long-term evidenced by the three of the five most costly ownership options requiring part replacement. With only relatively small differences (within 10%) in repair and initial manufacturing costs, risk associated with premature replacement is a key factor that if overlooked can double the total cost of ownership as shown in both Figure 1(b) and Figure 2. It is important to understand that laser cladding, though superior in this example and in many drilling applications, may not always represent the lowest ownership costs. Many of the underlying assumptions in this analysis regarding formation type, operating conditions, and quality of the coating application materials and process may impact the results significantly. For example, in more difficult (harder) formations, tungsten carbide bricks have a reputation for small but meaningful performance boosts that can justify higher initial manufacturing costs. However as shown in Figure 1(d), brick repairs should be with laser cladding coatings to maximize service life and minimize cost of ownership.

The analysis presented here is meant to outline that between the available technologies and materials used today for protecting drill string stabilizers, laser cladding should be seriously considered as it can represent the lowest total cost of ownership. Though hypothetical in nature, the data for this example is based on the experience of Apollo-Clad Laser Cladding who offers laser clad and tungsten carbide brick surfacing solutions and competes against PTAW technologies in these and other similar drilling component applications. Apollo-Clad has over 10 years’ experience in the laser cladding industry and is a division of parent company Apollo Machine and Welding Ltd. with over 45 years of oil and gas manufacturing experience.

Summary The total cost of ownership is an important and often overlooked factor in selecting wear protection technologies and materials for components used in demanding modern drilling applications. The analysis presented here is for the cost of a hypothetical generic drill string stabilizer, a sub-component of the bottom-hole assembly of a drilling tool that is subject to aggressive and potentially damaging operational wear. A variety of different coating materials and weld application coating technologies are employed to maximize the component service life in practice often without a full consideration of their impact on the overall total cost of ownership. Five combinations of are presented here: laser clad manufacture with laser clad repair, plasma transfer arc welding manufacture with plasma transfer arc welding repair, plasma transfer arc welding manufacture with laser clad repair, tungsten carbide brick oxy-fuel brazing manufacture with laser clad repair, and tungsten carbide brick oxy-fuel brazing manufacture with tungsten carbide brick oxy-fuel repair. Considering initial manufacturing costs, repair costs, repair cycles and a total target service life of 1000 hours, laser cladding manufacture with laser clad repair was shown to be the lowest cost of ownership option of the five potential combinations at a 50% cost compared to brazed brick with brazed brick repair option. This example supports that laser cladding can represent the lowest total cost of ownership for directional drilling components considering the entire part life cycle. For more information about laser cladding processes, products, and service please visit www.apollomachine.com or follow the QR code provided below.

http://www.apollomachine.com/

About the Authors

Gentry Wood is a recent graduate from the Canadian Centre for Welding and Joining (CCWJ) at University of Alberta (2017) where he completed his PhD in modelling of the geometry of laser clad beads under MIT trained Dr. Patricio Mendez. This research was sponsored by Apollo-Clad. He has been associated with Apollo since the summer of 2011 where he worked as a metallurgical intern, and has now returned as a research and development engineer 6 years in the making. He has 4 first author peer reviewed publications, 2 co-authored publications, and 10 conference presentations including international speaking engagements. Gentry is actively involved in the welding community and technical societies. He is an expert delegate of the Canadian Commission of the International Institute of Welding (CCIIW) in Commission IV on Power Beam Processes and a reviewer for the American Welding Societies (AWS) Welding Journal. Locally he is publicity chair for the Canadian Welding Association (CWA) in Edmonton, Alberta.

Doug attended the Colorado School of Mines in Golden, Colorado. He received a B.Sc. in 2001 and a Ph.D. in 2005 both in the field of Metallurgical and Materials Engineering. Doug returned to the Edmonton area in 2011 to join Apollo-Clad after working in the investment casting industry making Titanium and Superalloy investment castings for the aerospace and power generation industries. Doug is a Professional Engineer in the province of Alberta and currently the Research and Development Manager at Apollo-Clad Laser Cladding.

References [1] Mendez, P.F., Wood, G., Barnes, N., Bell, K., Borle, S., Gajapathi, S. S., Guest, S. D., Izadi,

H., and Gol, A. K., Welding Processes for Wear Resistant Overlays. Journal of Manufacturing Processes, Vol. 16. No. 1, pp.4-25, 2013.

Appendix Introduction to String Stabilizers With any resource based industry there is significant pressure to increase performance by pushing existing limits of the operational envelope. The drilling industry is no different with latest generation technologies being developed to drill deeper, longer, and faster to decrease well completion times and thereby increase revenue for the company and investors. The financial incentives for these performance gains push engineers and manufacturers of directional drilling equipment to implement state-of-the-art materials and manufacturing solutions. The bottom-hole assembly (BHA) is a critical part of the drill string that, as the name suggests, comprises the lowest series of components in the drill string. The BHA typically consists of a series of drill collars, stabilizers, sub-assemblies, and the bit itself that work together to both create and open the hole to allow the tool assembly to progress underground. The focus of this analysis is on a particular component called a string stabilizer: a portion of the bottom hole assembly that has “blade” features that extend outwards from a cylindrical base shaft that contact the walls of the bore hole. The drill string stabilizer serves a variety of important functional purposes. It concentrates the weight of the collars onto the drill bit, stiffens the drill string reducing tendency to buckle, keeps the collars away from the bore wall, and can be used to build, drop, or maintain the current drilling or build angle (orientation) of the tool path. Figure 3 shows a schematic of a BHA and a photo of a drill string stabilizer.

Figure 3. Left: Schematic of a typical bottom hole assembly (BHA). Right: Photo of an actual drill string stabilizer. The curved

nature of the blade is not unique to drill string stabilizers, which can also have straight blades similar to the near-bit stabilizer shown in the schematic.

The successful functioning of the drill string stabilizer hinges on the contact between the blade tip area and the bore wall. Under normal drilling conditions, whether the stabilizer is stationary or rotating, the stresses involved combined with the inherent abrasive wear between the engaged blade and wall material creates an incredibly aggressive environment for the blade to survive. The drill string stabilizers lifetime and performance are necessary to help guarantee that other issues related to stress or wear do not domino into unintended areas of the shaft. Consequence of this could be a reduction in service life of other high value components of the tool or in the worst case, catastrophic failure stopping production and rapidly increasing drilling costs. These costs associated risk of downtime are not factored into the analysis of cost of ownership presented here.

Introduction to Wear Protection Overlays To maximize the wear performance of the drill string stabilizer, a high value wear protection coating (also referred to as an overlay) is applied using a welding process to permanently connect the coating to the part and ensure it remains attached while in use. The wear protection materials of choice are either sintered tungsten-carbide bricks in a nickel-based braze, or spherical tungsten carbide particles in a primarily nickel matrix. Both material systems rely on tungsten carbide, an incredibly hard, abrasion resistant material to take the brunt of resisting wear. Tungsten carbide however is heat sensitive, and its performance in the coating (both in brick and powder form) depends on its exposure to heat (temp and duration) during the welding application process. The state of the art coating materials come at a premium price, and it is imperative from a cost of ownership perspective to try and maximize their in-field performance.

The tungsten-carbide bricks, hereafter referred to as “bricks” or “carbide bricks”, are advantageous for their relatively large exposed surface area on the order of several square centimeters per brick. In these types of multi-material systems, the risk is the bricks will detach prematurely because of wear that occurs in the inter-brick spaces. To mitigate this risk additional smaller spherical tungsten carbide particles are often combined with the nickel-based braze to fill in the gaps between the larger carbide bricks. The braze is designed with materials that combine to have a lower melting point than the individual metals. This lower melting temperature minimizes the total heat required to melt the braze to create a continuous coating that solidifies to lock in the bricks and additional carbide fill. Decreased process heat reduces the likelihood that the carbide based materials will degrade or even melt themselves, which drastically reduces their effectiveness in mitigating wear. Figure 4 shows a micrograph of the structure of the tungsten carbide bricks embedded in a nickel-based braze containing spherical tungsten carbide particles. Figure 5 shows sample images of in-service wear between bricks where the braze without tungsten carbide spheres has been washed away by erosive and abrasive drilling conditions.

Figure 4: Sample micrograph of a tungsten carbide brick coating. The image is taken by slicing through a coated blade stabilizer, polishing the section to a mirror finish.

Figure 5: Post run tungsten carbide bricks with a washed out braze filler without spherical tungsten carbide included in the inter-brick spaces. The exposed sides of the brick pose significant risk of premature brick detachment.

The second material system is called nickel-tungsten carbide, which in the case considered here consists of a small (0.1 mm) spherical ceramic particle that is embedded in a primarily nickel

matrix. The nickel matrix acts as the glue that allows the ceramic particles to be attached to the metal stabilizer, but also contains some additional elements that allow it to provide an additional degree of wear resistance with hardness comparable to hardened steel. The same risk exists with the spherical tungsten carbide overlays as with the bricks: process heat can damage, and in some cases, destroy the carbides the wear resistance of the overlay. The matrix materials are alloyed with element like boron that help depress the melting temperature to reduce the heat exposure of the ceramic particles, but also contribute to the hardened nature of the solidified overlay. Figure 6 shows a cross section of a typical laser clad tungsten carbide overlay.

Figure 6: Typical laser clad nickel-tungsten carbide overlay microstructure. The coating is two subsequent deposited layers.

While both materials exhibit superb wear resistance capabilities, each has their own niche application where they are superior. Tungsten-carbide bricks have a reputation for marginally improved performance in harder drilling formations where impact is a more prominent concern in additional to abrasive and erosive wear. Softer formations where the dominant mechanism is continuous erosion or wash-out (“death by a thousand paper cuts”) is where high fraction tungsten carbide powders with small inter-particle spaces (on the order of microns) perform best. The selection of the overlay material is often made based on experience and preference of the end-user with minimal if any consideration of long-term cost of ownership.

Introduction to Weld Overlay Processes The three weld overlay processes considered for the application of high performance wear resistant coating materials in this work are: oxy-fuel brazing, plasma transferred arc welding (PTAW), and laser cladding. Not all welding processes are created equal, and the three processes

have fundamentally different operating spaces based largely on the amount of heat imparted to the component to which the weld overlay is applied. The process selection is critical from a cost perspective because the welding process can dramatically impact the coating performance even for the same coating material! Figure 7 outlines the hierarchy of process heat input, which plays the largest role in process related costs regarding early replacement. The inherent heat sensitivity of both the stainless steel substrate and the tungsten carbide based coating materials makes the welding process the largest contributing factors to the requirement for early replacement, which is the largest contributing factor to total cost of ownership for these parts.

Figure 7: Relative heat input comparison between oxy-fuel brazing, plasma transfer arc welding, and laser cladding processes.

Table 1 below summarizes the relevant contributors to cost between the three weld overlay processes. Ranking is done with three levels: low, medium, and high as a comparison between typical process conditions and capabilities.

Table 1: Relative welding process factors contributing to cost of coating application

Process Heat Input

Degree of Automation

Coating Application

Time

Coating Application

Costs

Oxy-Fuel Brazing

High None High High

Plasma Transferred Arc Welding

Medium High Low Low

Laser Cladding Low High Medium Medium