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2 Application of Casing Wear Theory to Top Tensioned Risers David Saldaña, 2H Offshore Inc. Suneel Patel, 2H Offshore Inc. Shankar Sundararaman, 2H Offshore Inc. Adam Szucs, 2H Offshore Inc. Pete Padelopoulos, 2H Offshore Inc. Kamal Raghavan, Chevron Energy Technology Company Metin Karayaka, Chevron Energy Technology Company Copyright 2015, Deep Offshore Technology International This paper was prepared for presentation at the Deep Offshore Technology International Conference held in The Woodlands, Houston, Texas, USA, October This paper was selected for presentation by the DOT Advisory Board following a review of information contained in an abstract submitted by the author(s). Contents of the paper may not have been reviewed by the Deep Offshore Technology International Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Deep Offshore Technology International Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Deep Offshore Technology International Conference is prohibited. Abstract During drilling and/or workover operations through a production top tensioned riser, the riser will experience contact with the rotating drill string, resulting in wall loss from the riser. This wall loss from the contact between the rotating drill string and riser is defined as riser wear. Riser wear is primarily a function of riser curvature, which is dependent on riser stiffness, environmental loading, vessel offset, drilling operational parameters and drilling mud properties. Wear allowance and windows for conducting drilling operations can be defined prior to wear analysis. This paper discusses a complete methodology to determine riser wear, through application of casing wear theory, and thus confirm the wear allowance for the given drilling operating windows. Environmental loading conditions are selected based on drilling windows and a global analysis is performed with FEA software to obtain riser curvatures. As casing wear theory and associated software are based on survey parameters, the riser curvatures obtained from FEA software are converted to survey parameters (i.e., total vertical depth (TVD), inclination, and azimuth) using the Minimum Curvature Method. Contact forces between the drill string and riser are determined based on torque and drag theory, and a cumulative wear volume is calculated considering drilling operational parameters. Various approaches for converting this cumulative wear volume to wall thickness loss are presented and compared. Introduction Drilling and workover top tensioned risers (TTRs) are subject to wear, or material loss of the riser pipe due to contact with the drill string, as shown in Figure 1, during drilling and/or workover operations considering sidetrack drilling through a production riser. As wear is inevitable, it is important to quantify this material loss to ensure that the riser will still meet its design requirements. However, most available wear theory and commercial wear software are not aimed specifically for riser wear, but rather for downhole casing wear. While downhole casing curvature depends on well geometry, riser curvature depends on environmental loading and vessel offset. Throughout a drilling campaign, the riser will experience a range of deflected shapes with varying current loading and vessel offsets. As the drill bit penetrates the seabed, the drill string rotation and travel through the riser results in wall thickness loss of the riser pipe. Factors affecting the wear in the riser include the riser curvature due to current loading and vessel offset, drilling operational parameters (including RPM, weight on bit (WOB), rate of penetration (ROP), and drilling depth), and drilling mud properties. As the environmental loads may vary during a drilling operation, the magnitude and circumferential position of the drill string contact forces also vary. 1

3 This paper presents and discusses a complete methodology for the application of casing wear theory to quantify the wear that will occur in a riser under given drilling operability windows, in order to confirm a previously defined wear allowance. Environmental loading conditions are chosen based on operability windows and a global analysis is performed with Finite Element Analysis (FEA) software to obtain riser curvatures. As casing wear theory and software are based on survey parameters, the riser curvatures obtained from FEA software are converted to survey parameters (i.e. total vertical depth (TVD), inclination, and azimuth) through the Minimum Curvature Method. Contact forces between the drill string and riser are determined based on torque and drag theory, and a cumulative wear volume is calculated considering drilling operational parameters. The methodology provided in this paper also includes different approaches of varying levels of conservatism when combining wear wall thickness loss from various environmental loading conditions. A detailed case study in which this methodology is applied to a drilling TTR is incorporated into this paper. Tension Bare Drill String (Drill Pipe) Riser Normal Force Tool Joint Wear Groove Tension Figure 1 - Riser Wear from Drill String (Hall et al., 1994) Wear Assessment Theory Survey Parameters (Minimum Curvature Method) Most commercial casing wear software requires input of survey parameters that define the riser deflected shape. These survey parameters are determined by the minimum curvature method, shown in Figure 2 and described by Eq. 1 to Eq. 5 (Bourgoyne et al., 1986). The minimum curvature method originates from converting measured well survey data (inclination, azimuth, and measured depth) to 3D coordinates and dogleg / curvature. For riser wear analysis, the method is applied in reverse to determine survey parameters given the global analysis output of nodal 3D coordinates and nodal curvatures. 2

4 Figure 2 - Minimum Curvature Method (Bourgoyne et al., 1986) cosβ = cos(α 2 α 1 ) {sin(α 1 ) sin(α 2 ) [1 cos (ε 2 ε 1 )]} (1) F = 2 β tan (β 2 ) (2) M i = ( MD 2 ) [sin(α 1) sin(ε 1 ) + sin (α 2 )sin (ε 2 )] F (3) L i = ( MD 2 ) [sin(α 1) cos(ε 1 ) + sin (α 2 )cos (ε 2 )] F (4) TVD = ( MD 2 ) [cos(α 1) + cos (α 2 )] F (5) Where, β is the dogleg [radians] between survey points A1 and A2; α is the inclination: angle of the wellbore from the vertical; ε is the azimuth: angle of the wellbore course in the North-East plane, measured clockwise from North; F is the ratio factor of the straight line section vs. the curved section; Mi is the East/West displacement between survey points A1 and A2; MD is the measured depth: actual length along the wellbore path between survey points A1 and A2; Li is the North/South displacement between survey points A1 and A2; TVD is the total vertical depth: vertical displacement between survey points A1 and A2. Side Force (Torque and Drag Theory) For a given drill bit elevation, the side force from the contact between the drill string and riser is determined in accordance with torque and drag theory, shown in Figure 3 and described by Eq. 6 to Eq. 8 (Johancsik et al., 1983). At a given drill bit elevation, the torque and tension (drag) at the bottom of the lowest elevation drill string element are equal to the torque on bit (TOB) and weight on bit (WOB), respectively. The magnitudes of TOB and WOB are controlled during operation. The side force, tension increment, and torque increment are determined by Eq. 6 to Eq. 8 for this lowest element and each subsequent element of ascending elevation up to the drill string hook. A side force profile along the drill string is thus obtained for this given drill bit elevation, and the process is repeated for a finite number of intermediate bit elevations, between the initial and final bit elevation of the drilling operation. The inclination and azimuth are obtained per the Minimum Curvature Method. 3

5 T+ΔT α+δα, ε+δε M+ΔM N fn M W T α, ε Figure 3 - Side Force on Drill String per Torque and Drag Theory N = (T ε sin α avg ) 2 + (T α + W sin α avg ) 2 (6) Where, N is the net normal force on the drill string (side force); T is the axial tension at the lower end of the element; W is the buoyant weight of the element; α is the inclination angle [radians] at lower end of the element; ε is the azimuth angle [radians] at lower end of the element. T = W cos θ avg + fn (7) Where, ΔT is the tension increment of the element; f is the coefficient of friction. M = rfn (8) Where, ΔM is the torque increment of the element; r is the radius of the drill string / tool joint. Wear Volume For a given bit elevation, the wear volume per foot of riser pipe is determined by Eq. 9 to Eq. 11 (Poss et al., 1995). A wear volume profile is calculated for a finite number of intermediate bit elevations, between the initial and final bit elevation of the drilling operation. The wear volumes from the intermediate bit elevations are then summed to obtain a total wear volume profile. WV = WF SF tj SD (9) Where, WV is the volume of wall thickness loss per unit length of riser pipe; WF is the wear factor, defined as the ratio of friction factor to specific energy, which is the energy required to remove 1 in 3 (16.4 cm 3 ) of steel (Hall et al., 1994); SF tj is the side force on tool joint per unit length; SD is the sliding distance travelled by the tool joint against riser wall thickness. SD = π D tj N t L tj L dp (10) 4

6 Where, D tj is the tool joint outer diameter; N is the rotary speed, RPM; t is the rotating time; L tj is the drill string tool joint length; is the drill string joint length L dp Where, SF dp is the side force between drill string and riser, per unit length. SF tj = SF dp L dp L tj (11) Wall Thickness Loss from Single Spot Wear The wall thickness loss from wear is calculated from the conservative assumption of single spot wear, which assumes the wear volume is distributed among a concentrated area of contact, as shown in Figure 4 and described in Eq. 12 to Eq. 15 (Poss et al., 1995). Single spot wear only relates to the method of converting a wear volume to a wall thickness loss for a given environment. The assumption of single spot wear does not imply that drill string contact positions from various environment directions occur in a single spot. The actual circumferential position of the contact between the drill string the riser may vary with environmental loading direction, as described in the later section, Levels of Conservatism for Environment Direction and Contact Position. Riser Drill String S γ R r θ h Figure 4 - Single Spot Wear WV = θr 2 γr 2 + RS sin γ (12) h(2r h) cos γ = 1 2RS (13) sin θ = R sin γ (14) r S = R (r h) (15) Where, WV is the wear volume per unit length; R is the inner radius of the riser; r is the outer radius of the drill string; h is the wear wall thickness loss; S is the center-to-center distance between the riser and the drill string. 5

7 Case Study The global analysis and wear assessment of a single casing drilling TTR connected to a floating production vessel in an arbitrary location and water depth is considered for this case study. At the beginning of this case study, the wear allowance and operability windows for drilling have already been determined by previous assessment. This section details a complete methodology for applying casing wear theory to confirm the wear allowance under the given operability windows. Included in this methodology are different approaches, with varying levels of conservatism, to combine wear from various environmental loading conditions. These approaches are presented and their results are compared. Design Basis Riser and Drill String. The single casing drilling TTR configuration is shown in Figure 5. The drilling riser is a high pressure system with a surface blowout preventer (BOP). Cross section views of the riser at a bare drill string (drill pipe) section and at a tool joint (larger outer diameter than bare drill string) section are included in Figure 5. A view of the drill string configuration along its length is shown in Figure 6. For a given drill string, tool joints only exist at a given spacing, and the remainder of the string is bare drill string (drill pipe). The bare drill string and tool joint each has its own unique outer diameter (OD) and inner diameter (ID). The internal wear allowance for the riser is given in Table 1. This wear allowance is determined as part of a separate wall thickness sizing assessment, which is conducted before the wear analysis. The wear allowance assigned to a given joint is based on typical industry standard values and expected curvature at the joint. Forged joints (lower tapered stress joint, tension joint, and BOP spool) are placed in regions of higher curvature and are assigned a higher wear allowance of 2 mm. Standard riser joints are in regions of lower curvature and do not require as high of a wear allowance; thus they are assigned a lower wear allowance of 1 mm. 6

8 DRILL FLOOR MAIN DECK TTR FRAME DECK MWL Diverter Telescopic Joint Surface Tree BOP BOP Spool Wellhead Tension Joint Tension Cross-over Jt Splash Zone Jt Pup Jt Straked Jt Slick Jt (Fairings) Buoyant Jt (Fairings) Stress Jt Conductor Anti-Torque Conductor Guidance Rollers Tensioners Bare Drill String (Drill Pipe) Drilling Mud Drilling Riser Pipe Section A-A, Riser at Bare Drill String (Drill Pipe), Fairing Not Shown Drilling Mud Drilling Riser Pipe A A B B Tool Joint MUDLINE Section B-B, Riser at Tool Joint, Fairing Not Shown Figure 5 - Single Casing Drilling TTR Configuration (with Drill String) Tool Joint Bare Drill String (Drill Pipe) Tool Joint Length Tool Joint Spacing Figure 6 - Drill String Configuration 7

9 Section Forgings (Lower Tapered Stress Joint, Tension Joint, BOP Spool) Internal Wear Allowance (mm) 2.0 Standard Riser Joint 1.0 Table 1 Wear Allowance Operational Parameters and Drilling Mud. During drilling, the drill string is strung through the riser at the drill floor location per the operational parameters given in Table 2. The selected drilling mud and drill string material properties result in the wear factor and friction factor given in Table 3. The effects of drilling mud properties, additives, and other operational actions taken (e.g., use of pipe protectors) on wear factors and/or friction factors have been studied and published in Hall, et al., Parameter Value ROP (ft/hr; m/hr) 50; 15.2 RPM (rpm) 90 WOB (kips; kn) 20; 89.0 TOB (kip-ft; kn-m) 30; 40.7 Table 2 Operational Parameters (1), (2) Value Parameter Friction Factor, f 0.20 Wear Factor, WF ( in 2 /lb; cm 2 /N) 2.00; / The friction factor and wear factor are selected per the casing wear software built-in database. 2/ The friction factor and wear factor are based on drilling mud with any additives (e.g., limestone) Table 3 Wear Factor and Friction Factor Environmental Loading. During drilling, the riser is assumed to be under global static loading due to co-linear current and associated vessel offset. A total of 8 environment directions are considered (4 cardinal and 4 intermediate). While there are a range of current magnitudes for a given direction, the specific current magnitudes selected for the global analysis are based on operability windows. An operability window is shown in Figure 7 for the North environment, as an example. In this case study, a separate operability analysis is conducted before the wear analysis. This operability analysis determines, for each of the 8 environment directions, the limiting current and offset conditions (operability window) that maintain riser curvature below a required magnitude. The wear analysis is conducted based on current and offset loading that does not exceed these operating windows, as drilling is not to take place beyond these windows. 8

10 Non-Exceedance Current Non-Exceedance Current 100% 90% 80% 70% Do not drill (Minimum riser curvature is exceeded) 60% 50% 40% 30% Drill (Minimum riser curvature is not exceeded) 20% 10% 0% Vessel Offset from Nominal (m) Figure 7 - Operability Window, North Environment as Example Analysis Methodology Riser Global Analysis. The riser is analyzed under current and offset loading (static loading) through finite element analysis (FEA) in order to determine the riser deflected shape (curvature). This deflected shape is assumed to remain constant for a given non-exceedance current and offset. The global riser analysis is carried out using the non-linear time domain analysis program, Flexcom, Version 7.9 The current profiles selected for global analysis are obtained by assessing the operability window. While there is a range of varying incremental current and offset pairs for each environmental direction, performing global analysis for a large number of current and offset pairs for every direction may not be necessary. Instead, for each environment direction High, Medium, and Low current and offset pairs are selected with respect to the operability envelope, as shown in Figure 8. The High current is obtained by selecting the current and offset pair nearest to the envelope. Medium and Low current pairs are then chosen. The offset associated with each given current is determined by a vessel motion timetrace analysis. 100% 90% 80% 70% 60% 50% "Medium" Current "High" Current Do not drill (Minimum riser curvature is exceeded) 40% 30% Drill (Minimum riser curvature is not exceeded) 20% 10% "Low" Current 0% Vessel Offset from Nominal (m) Figure 8 - Operability Envelope with High, Medium, and Low Currents Superimposed, North Environment as Example 9

11 Determining Survey Parameters from Finite Element Global Analysis Output through The Minimum Curvature Method. The riser global finite element analysis (FEA) results (riser deflected shape) are in the form of nodal 3D coordinates and element curvature. However, survey parameters are required for most commercial casing wear software. Thus the Minimum Curvature Method is applied in reverse in order to utilize the FEA output as input to the wear analysis. The Minimum Curvature Method was originally intended to obtain well path/curvature from measured well survey parameters, and when used with this original intent it is a closedform solution. However, when the method is applied in reverse with given riser FEA global analysis output, as described in Table 4, the system becomes over-determinate (No. of Equations > No. of Unknowns) and the problem must be solved iteratively. There are further complications with numerical stability and convergence when solving these equations when they are applied to finely meshed nodes, as opposed to survey points that can be greater than 50 meters apart. However, there exist numerical methods to resolve these issues and ultimately check the results by solving the equations forwards and comparing the output of TVD, Mi, Li, and β to ΔX, ΔY, ΔZ, and 1/R from the riser global FEA output. Output from Riser Global FEA Analysis Assumptions Equations, per Wear Assessment Theory section (5 Total) Eq. 1 Eq. 2 Eq. 3 Eq. 4 Eq. 5 Unknowns (4 Total) Global XYZ coordinates at each node ΔX = TVD ΔY = Mi α 2 ε 2 Bending radius at each ΔZ = Li MD element (Curvature = 1/R = β (accounting F 1/R) for unit conversions) Riser is vertical at top-most element (α 1 = 0, ε 1 = 0) Table 4 Over-Determinacy of Inverse Minimum Curvature Method Wear Wall Thickness Loss Analysis. To determine the wear wall thickness loss, a simulation of the drill string travel through this deflected riser shape is assessed. This simulation is achieved in the following steps by implementing the casing wear analysis program, CWPRO, Version 5.0, developed by Pegasus Vertex Inc.: 1. Inputs are provided, including the following: a. Riser and drill string geometry b. Global analysis output parameters that represent the riser deflected shape and have been transformed into survey parameters by the Minimum Curvature Method c. Initial and final drill bit elevations d. Operational parameters (Table 2) e. Wear factor and friction factor (Table 3) 2. For a given drill bit elevation, a side force profile is determined to represent contact between the drill string and riser through torque and drag theory. 3. For a given side force profile (obtained from Step 2), a wear volume profile is determined. 4. Steps 2 and 3 are repeated for incremental drill bit elevations until the final drill bit elevation is reached. 5. Wear volumes at incremental drill bit elevations (obtained from Step 4) are summed to obtain a total wear volume profile. 6. The total wear volume profile (obtained from Step 5) is converted to a remaining wall thickness, or wear wall thickness loss, profile by assuming single spot wear. During the wear analysis, the drill string curvature profile is assumed to match the riser curvature profile (i.e. the drill string is conservatively assumed to always be in contact with the riser throughout the riser profile and throughout the duration of the drilling operation.) Moreover, the bare drill string is assumed to be in contact throughout the riser profile as it slides against the riser, even when the tool joint OD is larger than the bare drill string OD and the drill string curvature matches the riser curvature. Further, the drill string bending stiffness is assumed to be negligible and therefore assumed not to affect the riser curvature. 10

12 Combining Wear from Various Environments and Directions. A combined wall thickness loss profile is obtained from all the load cases analyzed for wear. The combined profile is calculated from Eq. 16. h combined = p dir,i ( p ne,i,j h i,j ) i=environ.dir. j=high,med,low (16) Where, h is the wear wall thickness loss; p dir is the probability of environment direction (set to 1.0 if and only if the environment direction is fixed and thus assumed occur 100% of the time) p ne is the weight factor, for High / Medium / Low non-exceedance current profiles for a given environment direction. The values assigned to the factor p ne for a given environment direction depend on the non-exceedance probabilities within the operability window. As an example, the p ne factors for the North environment, which uses a 70% non-exceedance current as the High current, are given in Table 5 ( High / Medium / Low currents are shown against operability window in Figure 8). As drilling is not planned for currents > 70% nonexceedance (outside the operability window), the probability of the non-exceedance currents < 70% nonexceedance (within the operability window) are normalized to sum to 100% probability (Table 5). Current Non- Exceedance Probability (per 70% max Non- Exceedance) Representative Current Non- Exceedance Probability, Normalized to Operability Window (p ne ) >70% 0.0% - 0.0% 70% 14.3% 60% 14.3% 70% 28.6% 50% 14.3% 40% 14.3% 30% 14.3% 50% 57.1% 20% 14.3% 10% 14.3% 10% 14.3% Sum 100.0% % Table 5 Factor for Combining Wear Wall Thickness Loss, Varying Current Non-Exceedance, North Environment as Example Levels of Conservatism for Environment Direction and Contact Position. There are three levels of conservatism, as detailed in Figure 9, considered for riser wear analysis. These different levels are controlled by the fixed vs. variable assumptions of the environment direction and drill string to riser circumferential contact position (clock position of contact). Level 1 does not require assessment of the contact position and should therefore be applied first. For Levels 2 and 3, the contact position of the between the drill string and riser can be determined by global analysis modelling of the riser and drill string as separate strings. In this case study, all 3 levels of conservatism are applied to discuss and compare all 3 sets of results. However, in practice, the levels of conservatism are applied in the order of least complexity to greatest complexity (order of increasing time and effort), per Figure 9, to optimize allocation of time and effort. When applied in order of increasing complexity, once an acceptable wear wall thickness allowance is obtained from a given level, per Figure 9, the subsequent levels of greater complexity and less conservatism are not necessary. 11

13 More Conservative, Less Complex Level 1 (Environment Direction Fixed and Contact Position Fixed): Assume environment direction resulting in greatest curvature, occurs 100% of the time. The drill string contact position against the riser is fixed, because only one direction is occurring Level 2 (Environment Direction Variable and Contact Position Fixed): Assume all environment directions occur per Metocean probabilities, but the drill string contact position against the riser is fixed for all environment directions. Wear wall thickness loss values from each environment direction are summed. Level 3 (Environment Direction Variable and Contact Position Variable): Assume all environment directions occur per Metocean probabilities, and the drill string contact position depends on each environment direction. Less Conservative, More Complex Figure 9 - Levels of Conservatism for Environment Direction and Contact Position Drill String Circumferential Contact Position. To determine the circumferential contact position of the contact between the drill string and the riser pipe for a given environment, a pipe-in-pipe model is developed to model the drill string and find the contact position when considering environment directionality. The drill string is modelled as an independent string. The tool joints along the length of the drill string are spaced so that a tool joint is modelled near the critical wear location. Furthermore, a point load is applied to the bottom of the drill string to model the additional weight of the drill string that is not captured directly in the model (the model has size limits in the FEA software). Based on whether the non-linear springs at the critical wear location are in tension or compression, the circumferential position of contact on the outer riser can be determined. There is a critical angle difference for two given contact positions to be considered coinciding, which will require their wear to be summed. This critical angle difference, for two given contact positions, is calculated by Eq. 13 and Eq. 15. Two contact positions are considered to be coinciding when their angle difference is equal to twice the angle corresponding to the arc length of contact (2γ, per Eq. 13), with a conservative assumption of the wear wall thickness loss (h, per Eq. 13) equal to the wear allowance. Results Global Analysis. The riser curvatures are determined from the global analysis. The resulting curvatures are given in Table 6 for the North environment as an example, as it is found to be the critical environment for curvature. A complete set of curvatures is obtained from the global analysis for all 8 directions, High, Medium, and Low current profiles. These curvatures are converted into survey parameters per the Minimum Curvature Method and input into the casing wear software CWPRO, along with the operational parameters given in the Design Basis section of this paper, to perform the wear analysis. 12

14 Wall Thickness Loss (mm) Direction Current Description Non- Exceedance Offset (m) Curvature (1/m) N High 70% E-03 Medium 50% E-03 Low 10% E-03 Table 6 Wear Allowance Wear Analysis. Wear analysis is performed and 3 levels of varying conservatism, per Figure 9, are applied when combining wear wall thickness loss from various environments. The results from each approach are compared for the critical region for wear (dependent on curvature and wear allowance): the lower standard riser joints and lower tapered stress joint (TSJ). Level 1 conservatism is applied. This approach assumes a fixed environment direction and fixed contact position. The environment direction resulting in the greatest curvature is assumed to occur 100% of the time. The contact position is assumed not to change as the environment direction is not changing. The resulting wear wall thickness loss profile with Level 1 conservatism is shown in Figure 10. The wear wall thickness loss exceeds the wear allowance in the lower standard joints TSJ AND LOWER STANDARD JOINTS WALL THICKNESS LOSS FROM WEAR Environment Direction Fixed and Contact Position Fixed North Environment Elevation Above Mudline (m) Wear Wear Allowance Figure 10 - Combined Wear Wall Thickness Loss, Level 1 Conservatism (Environment Direction Fixed and Contact Position Fixed, per Figure 9) Level 2 conservatism is applied. This approach assumes a variable environment direction and fixed contact position. All 8 environment directions are assumed to occur at their respective probabilities, but the drill string is assumed to contact at the same circumferential position of the riser 100% of the time, regardless of the environment direction. The resulting wear wall thickness loss profile with Level 2 conservatism is shown in Figure 11. Compared to the Level 1 profile in Figure 10, the Level 2 wear wall thickness loss profile in Figure 11 is improved near the bottom of the standard joints and just below the wear allowance. This improvement is largely due to the inclusion of wear results from the various directions weighted by probability, as these other directions result in lower curvatures than the North environment. 13

15 Local z (mm) Wall Thickness Loss (mm) 3.50 TSJ AND LOWER STANDARD JOINTS WALL THICKNESS LOSS FROM WEAR Environment Direction Variable and Contact Position Fixed All Environment Directions Elevation Above Mudline (m) Wear Wear Allowance Figure 11 - Combined Wear Wall Thickness Loss, Level 2 Conservatism (Environment Direction Variable and Contact Position Fixed, per Figure 9) Level 3 conservatism is applied. This approach assumes a variable environment direction and variable contact position. This approach requires an auxiliary global analysis with the drill string modelled so that the circumferential contact position can be determined for each environment direction. The resulting contact positions from this more detailed global analysis are given in Figure 12, at the critical riser elevation for wear. These contact positions are assessed for overlap, per the methodology described in the previous section, Drill String Circumferential Contact Position, and the wear is summed wherever contact positions coincide. Overlap is observed at the SE/NW and S/SW pairs. The resulting wear wall thickness loss with Level 3 conservatism is given in Table Local y (mm) N NE E SE S SW W NW Riser ID Face Figure 12 - Drill String and Riser Contact Position for Various Environment Directions at Critical Riser Elevation for Wear (per Figure 11) 14

16 Maximum Wear Wall Thickness Environment Driving the Drill String and Riser Contact Position (1) Loss (2)(3) at Critical Elevation for Wear (4) (mm) N 0.21 NE 0.19 E 0.14 SE and NW 0.15 S and SW 0.17 W / Per the contact positions shown in Figure 12 2/ Wear wall thickness loss is summed where contact positions coincide 3/ Wear wall thickness loss from each environment is weighted by probability of environment 4/ Critical elevation for wear per Figure 11 Table 7 Wear Wall Thickness Loss, Level 3 Conservatism (Environment Direction Variable and Contact Position Variable, per Figure 9) at Critical Riser Elevation for Wear The critical wear wall thickness loss when applying Level 3 conservatism: occurs from approximately 16m above the mudline in the lower standard riser joints; is due to the North environment alone (no coinciding contact from other environments is observed), even compared to the summations of SE/NW and S/SW environments; improves to 21% of the wear allowance, while the wall thickness losses in the Levels 1 and 2 conservatisms are > 100% and just under 100% of the wear allowance, respectively; cannot be any greater than wear wall thickness loss results from Level 2 conservatism, at a given elevation, and thus the remainder of the wear wall thickness loss profile remains below the wear allowance. Conclusions All risers undergoing drilling and/or workover operations considering sidetrack drilling should be assessed for wear. This paper presents a complete methodology for confirming a wear allowance when drilling operations are governed by an operability analysis which is performed before the wear analysis. A case study which applies this method is examined and discussed. Casing wear theory can be applied to risers in order to determine a wear wall thickness loss. As most casing wear software requires survey parameters as input, riser global analysis to determine riser curvature during drilling may likely need to be converted from nodal XYZ coordinates and element curvatures to survey parameters through the Minimum Curvature Method. This method of obtaining survey parameters can be challenging as it involves solving an inverse problem that is over-determinate and numerically unstable at finer meshing (nodal spacing). A solution through this method is achievable through an iterative numerical methods approach. As a riser experiences current and offset loading of various magnitudes during drilling, a representative set of environments are selected from the operability windows for the global analysis, and the resulting wear wall thickness losses are weighted and combined. The methodology in this paper includes options of differing levels of conservatism when combining wear wall thickness loss from various environments. Among these different levels of conservatism, the complexity and effort required increases as the conservatism decreases. To maximize efficiency of effort, the levels of conservatism should be applied in order of increasing complexity and the next level should only be assessed if wear wall thickness loss is exceeding the maximum wear allowance. 15

17 References Bourgoyne, AT, Jr, Chenevert, ME, Millheim, KK, Young, FS, Jr (1986) Applied Drilling Engineering, Society of Petroleum Engineers, Richardson, TX, SPE Textbook Series, Vol. 2. Hall, RW, Jr, Garkasi, A, Deskins, G, and Vozniak, J (1994) Recent Advances in Casing Wear Technology, IADC/SPE Drilling Conference, Dallas, TX, IADC/SPE Johancsik, CA, Friesen, DB, and Dawson, R (1983), Torque and Drag in Directional Wells Prediction and Measurement, IADC/SPE Drilling Conference, New Orleans, LA, IADC/SPE Poss, GT, Hall, RW, Jr (1995) Subsea Drilling Riser Wear: A Case History, IADC/SPE Drilling Conference, Amsterdam, IADC/SPE