Ultra-high-strength drill pipe expands the drilling envelope (Jul-2009)

A new lightweight steel alloy will allow wells to be drilled farther and deeper.  

Michael Jellison, Andrei Muradov and Lucien Hehn, NOV Grant Prideco; Brandon Foster, K&M Technology Group; Greg Elliott and Leianne Sanclemente, Workstrings, LLC 

Drilling teams are evaluating alternative materials and advanced technologies to expand the Extended Reach Drilling (ERD) envelope, since present and future ERD projects exceed the capabilities of conventional steel S-135 drillstring assemblies. One solution under development incorporates a new Ultra-High-Strength Steel (UHSS) with 165-ksi minimum yield strength used in thinner-walled, lighter-weight tubes, compared to conventional steel drillstrings.

By using UHSS in thin-walled, lightweight tubes, steel drillstrings can offer strength-to-weight ratios in air that exceed those of Aluminum Drill Pipe (ADP), are comparable to those of Titanium Drill Pipe (TDP) and have hydraulic performance that surpasses both materials. Buoyancy tends to improve the strength-to-weight ratios of ADP and TDP relative to steel drillstings and should be considered during engineering of any drillstring for critical drilling applications.

ADVANCED MATERIALS

When considering strength-to-weight ratio, many publications have neglected to factor in the steel tool joints attached to non-steel alternative material tubes. Rather, they have focused only on the strength-to-weight ratio improvement of material density. This is somewhat misleading and errs on the side of promoting non-steel alternative materials.

Table 1 provides a summary of advanced material comparisons alongside S-135, Z-140, V-150 and UD-165 steel drill pipe products.¹ It factors in the presence of steel tool joints and uses a conservative (optimistic) approach by analyzing Range III drill pipe. Range III product lengths can offer handling efficiency advantages, but may present wear issues and higher potential for fatigue damage versus Range II designs.

Aluminum drill pipe. Drill pipe made from aluminum has been used for decades. Most of this experience comes from the former Soviet Union, where ADP is used extensively. It offers several advantages over conventional steel drill pipe including: lower weight, higher strength-to-weight ratio, superior corrosion resistance and enhanced fatigue resistance. The pipe costs about twice that of conventional steel DP, although this is dependent on specifications.

ADP may have application for drilling in some ERD and horizontal drilling applications, but it also has disadvantages. It has relatively low yield strength of about 69,000 psi. Consequently, it has a lower strength-to-weight ratio in air than UHSS drill pipe when factoring in the steel tool joints. It generally requires a greater wall thickness than steel drill pipe, adversely affecting hydraulic performance. In addition, its yield strength in service can drop dramatically at temperatures above 250°F. This can be a problem in some ERD applications.

Titanium drill pipe. Drill pipe made from titanium as been successfully manufactured on a limited scale for ultra-short-radius drilling applications. But, manufacturing cost is very high, some seven to ten times more than that of conventional steel drill pipe. TDP offers significant performance advantages for ERD and critical deep-drilling applications.

Tianium has a density that is slightly more than half (56%) that of steel, is highly resistant to corrosion and erosion, and has good fatigue resistance. A standard titanium alloy that would be suitable for drill pipe has minimum yield strength of 120,000 psi, resulting in strength-to-weight ratio improvement (including steel tool joints) of about 37% over S-135 steel drill pipe. However, TDP can be notch sensitive in fatigue-inducing situations and it has significant wear rates when exposed to prolonged and stressed contact with steel.

There is little question that titanium could be used to make a high-performance drillstring that provides an innovative technical solution to push the ERD envelope. The question that must be answered before TDP will be seriously adopted is: Does the high cost of TDP preclude it from offering greater value than UHSS drill pipe or other alternative technologies?

Ultra-high-strength steel. Using UHSS represents a near-, mid- and long-term technology solution for ERD and other critical drilling applications. The development of the UD-165-grade with 165-ksi yield strength provides a 22% improvement in strength-to-weight ratio compared to S-135. This represents an alternative, second only to TDP by 15% in strength-to-weight ratio. It is likely that the cost of UD-165 is substantially less than TDP.

UHSS in thin-walled, lightweight tubes can offer strength-to-weight ratios that exceed ADP, are comparable to TDP and have hydraulic performance that surpasses both materials. Compared to standard-weight 57⁄8-in. drill pipe (a size often used for critical ERD applications), the UD-165 design can potentially reduce the weight per joint by over 30%. This significantly reduces torque and drag forces on long ERD and highly directional well applications. In addition, since the lightweight pipe features a larger ID, hydraulic performance is greatly improved.

HIGH-STRENGTH DRILL PIPE

The 1963 API specifications included only grades D-55 and E-75 drill pipe. The 1973 API specifications included X-95, G-105 and S-135 as well as D and E grades, although X, G and S grades were in use between 1963 and 1973. Grade D was not included in the 1981 API specification. Grades Z-140 and V-150 have not yet been adopted by API, but have been in use for many years; cumulative footage sold to date is 939,413 ft and 415,381 ft, respectively.

Impact requirements for grade S-135 were increased under Shell SQAIR in the early 1990s.² The increase was needed to satisfy a “leak before break” fracture toughness criterion. This ensures that a pressure drop from a growing crack that reached a size spanning the distance between the outer and inner diameter would not proceed to an unstable fracture before drilling operators noticed the pressure loss.

Through the succession of higher-yield-strength drill pipe from S-135 to Z-140 to V-150, it became apparent that each successive generation was accomplished by raising the yield strength, but only marginally raising the ultimate tensile strength. A limit seemed to be approached with low-alloy steels in which the higher yield strength values were shifted toward the ultimate tensile strength. Yield-to-tensile-strength ratios increased to over 90% with V-150 grades and higher. In addition, as yield strength is increased, a corresponding decrease in fracture resistance and increase in hardness may occur.

Driven by the need for higher load capacity, an ultra-high-performance drill pipe grade with minimum yield strength of 165 ksi was developed.

The starting point was the base alloy used in the S-135 (NS-1) and V-150 grades. Refinements in the triple alloy and the addition of micro-alloying constituents led to the current UD-165 chemistry.

FRACTURE TOUGHNESS

A size-independent fracture toughness (K_IC) gives a measure of the “crack size tolerance” ability of a material to withstand catastrophic failure under expected loading (i.e., with stresses at yield strength or below) with a crack of a particular size. A minimum sample size over 1 in. (in the pipe wall direction) is required for K_IC. This is too large to obtain from standard drill pipe wall thicknesses (i.e., about 0.5 in.).

Crack Tip Opening Displacement (CTOD) measurement tests are generally accepted in place of K_IC. The crack opening is measured as the load is increased until failure. The crack opening at maximum loading gives a fracture energy measure. A larger crack opening size indicates a higher absorbed fracture energy. CTOD measurements are best used as a comparative tool, providing relative toughness between grades, rather than as an absolute measure of toughness, which a K_IC measure provides.

K_IC measurements require a minimum sample size. Notched impact or Charpy impact tests provide an impact fracture resistance measure in a notched specimen. The Charpy test is simple, inexpensive and highly repeatable. The energy absorbed by the impact is a measure of toughness, which follows the trend in the steel’s K_IC values. Direct conversion of Charpy impact energies to K_IC is not possible, due to differences in their definitions. Charpy impacts therefore are generally taken in place of K_IC, Table 2.

CTOD TESTING

CTOD tests were taken on representative S-135, V-150 and UD-165 drill pipe from recently manufactured drill pipe lots. Three tests were taken for each grade and the results averaged. The average CTOD value was largest for the S-135 grade (0.006 in.) and nearly equal for V-150 (0.0034 in.) and UD-165 (0.0035 in.). The trend of decreasing CTOD with increase in yield strength was expected. However, the nearly equal values for V-150 and UD-165 are highly favorable for UD-165, considering the extensive, safe and proven field history of V-150 over the last decade.

FATIGUE TESTING

Cyclic fatigue testing was performed per ASTM E466 on the same samples of V-150 and UD-165 material. The sample geometry was cylindrical with a total sample length of 3.5 in. including the threaded ends. The samples tapered down to a testing gage 0.75-in. length and 0.218-in. diameter. The testing gage section was polished to a 32-rms (root mean square) finish. The gage sections were polished to create an identical surface finish to compare fatigue resistance of the two grades.

A servo-hydraulic test frame was used with a sinusoidal loading curve applied to the specimens at a stress ratio of R = 0.1. The test frequency was nominally run at 10 Hz, testing until the sample failed or the run-out condition reached 1 million cycles.

A plot of stress vs. number of cycles for the fatigue testing program is shown in Fig. 1. Several samples failed in the threaded grip section and are not considered valid (open symbols). In all, 13 samples were tested. Three UD-165 and two V-150 samples resulted in failures in the threaded grip region.

Fig. 1. The fatigue study yielded a surprising S-N plot showing that UD-165 grade was superior under fatigue.

UD-165 and V-150 performed well under fatigue testing, reaching the run-out condition at stress values near their yield strengths. Since testing at stresses above the yield stress is not practical, testing was carried out with both grades at progressively higher stresses until yield strength was approached.

The maximum testing stress for V-150 was 154.6 ksi. At this level, three samples broke in the gage, two grip failures occurred and two survived run-out. All UD-165 materials tested at or above 154.6 ksi survived to run-out with the exception of two grip region failures. In addition, the UD-165 tests at 158.5 ksi, 162.4 ksi and 166.3 ksi survived to run-out except for one grip area failure at 158.5 ksi.

UD-165 FIELD HISTORY

After laboratory testing confirmed desirable and consistent material properties, the decision was made to build about 50 joints of 5-in., 0.5-in.-wall-thickness UD-165 drill pipe. Thirty of these joints were deployed to Patterson UTI Rig 30 in South Texas to be run in combination with 5-in., 19.5-lb/ft, S-135 (NC50) drill pipe. The wells in this area typically range in depth from 16,000 ft to 17,000 ft. The UD-165 pipe was successfully run in two wells.

The first use was not planned. The UD-165 drill pipe arrived on location as the rig was approaching TD. The rig did not have sufficient pipe to reach TD, and the contractor contacted NOV Grant Prideco for permission to top out the string with the UD-165. This was not a case of insufficient planning (additional 5-in. S-135 drill pipe was readily available); rather, a deliberate decision was made to run the UD-165 drill pipe sooner. Subsequently, 12 joints of the UD-165 pipe were used to reach TD. The drill pipe performed as expected with no unusual running or handling issues.

The second case was a re-entry well in the same area. This well was kicked off at 3,450 ft, and 13,274 ft of hole was drilled to a 16,724-ft MD. The well had a maximum inclination angle of 23°. Average rotational speed was 65 rpm running with a mud motor and 12,000–15,000-lb WOB. Penetration rates averaged 30–35 ft/hr with rotary table and 15–20 ft/hr in sliding mode with mud motor. Activity time included drilling (363 hr), circulating (105 hr), reaming (54 hr) and tripping (395 hr). The drill pipe was visually inspected after completing this well. No unusual or unexpected conditions were observed, and the pipe showed signs of normal use.

Since then, 57⁄8-in., 0.5-in.-wall-thickness UD-165 pipe has been manufactured and is awaiting field trials. The first lot of lightweight 5 7⁄8-in., 0.3-in.-wall-thickness UD-165 drill pipe is being processed for evaluation and field trials. In addition, there are plans to build two ultra-high-capacity 6 5⁄8-in. UD-165 landing strings this year. As water depths in deepwater drilling increase and casing string lengths and weights rise, landing strings with ever-higher axial load-carrying capacities are required. The new UD-165 material grade enables the construction of 2.5 million-lb rated landing strings for these critical applications.

DOUBLE-SHOULDER CONNECTION

In today’s rig market, deepwater, ERD and ultra-deep wells dictate large spread rates that can benefit significantly from reduced tripping times. These same wells often have mechanical and hydraulic load demands that require optimized high-torque connections. A third-generation Double-Shoulder Connection (3G DSC) for drill pipe and drillstem components has been developed and deployed. A key objective for the 3G DSC was to significantly improve connection make-up/breakout speeds relative to 2G DSC.³ Mechanical and hydraulic gains were also dictated based on the industry’s trend toward deeper and further well programs.

Double-start threads have two threads spaced 180° apart, reducing the number of turns to assemble the connection by 50%. Conservative estimates suggest that the new connections will save some 7.5 hr in planned trip time per 20,000-ft well. The thread also provides a unique dual-radius thread root that offers a step-change improvement in fatigue resistance.

API tool joints are produced with Specified Minimum Yield Strength (SMYS) of 120,000 psi. The new connection employs 130,000-psi SMYS tool joints, and provides increased mechanical and hydraulic performance compared to previous connections, while also providing fatigue performance greater than standard API connections.

The 3G DSC is designed to increase target depths for deepwater and ultra-deep drilling and reach on critical ERD wells, while improving efficiency and lowering well cost by significant reducing handling, makeup, running and tripping speeds. The combination of lightweight UHSS drill pipe and the new 3G DSC represents powerful enabling technology for ERD, deepwater and ultra-deep drilling applications.

3G DSC FIELD HISTORY

Just over 417,000 ft of drill pipe with 3G DSC has been produced since the connection was introduced in late 2006. Drill pipe OD sizes 3 1⁄2-in., 4-in., 5-in., 5 1⁄2-in. and 57⁄8-in. have been manufactured in material grades SS-105 (sour service), S-135, Z-140 and V-150, Fig. 2. The pipe has been deployed in the deepwater GOM, North Sea and Asia. A string of 5 1⁄2-in., 21.9-lb/ft, S-135 3G DSC is expected to be deployed in Nigeria later this year.

Fig. 2. Four pipe grades of 3G DSC are now manufactured, with the industry favoring the 585 connection pitch diameter.

The first 3G DSC drillstring run was 5 7⁄8-in., 26.3-lb/ft S-135 for ChevronTexaco in the GOM. The vertical well was located in 7,016 ft of water and had a total depth of 28,000 ft. 3G DSC drill pipe has also been used to drill wells for Shell Oil, Eni and an additional well for ChevronTexaco in the GOM.

The drillstrings have demonstrated cost-efficiency compared to 2G DSC drill pipe through reduced time to make up and run, as well as break out and trip, Table 3. Physical comparison testing shows an 11-sec. time savings in stab-to-shoulder for 5 7⁄8-in. 3G DSC compared with 2G DSC. Initial field applications showed a significant reduction in connection handling and running damage resulting in lower repair costs.

ULTRA-EXTENDED REACH DEVELOPMENT PLAN

An ultra-Extended Reach Drilling (uERD) development plan for a well located in the North Sea has been extensively studied, but the project has not yet been drilled, Fig. 3. With a total measured depth of 47,000 ft, the project represents a world-class ERD application. The justification for this uERD project is a classic case of exploiting additional, more remote reserves from an existing platform to avoid constructing a second platform. This uERD project exceeds the current ERD worldwide drilling envelope by a significant margin.

Fig. 3. A uERD development plan for a well located in the North Sea has a total measured depth of 47,000 ft, which significantly exceeds the worldwide extended reach drilling envelop.

A design study was executed for a 5 7⁄8-in.-OD drillstring considering various materials including: S-135, the new UHSS UD-165, ADP and TDP. Since the well has a BHT near 300°F, ISO Group III ADP with a SMYS of 49,000 psi was selected. ISO Group II is limited for use to temperatures no higher than about 250°F, Table 4.

Maximum loads correspond to drilling the long 12 1⁄4-in. hole section to 45,000 ft. Each design was then evaluated and rated either unacceptable, marginal or acceptable for torque, tension, surface pump pressure and stretch based on the following criteria:

• Torque: Unacceptable—torque load exceeds connection makeup torque; Marginal—makeup torque exceeds torque load by a small margin; Acceptable—makeup torque exceeds torque load with a reasonable safety margin

• Tension: Unacceptable—tension load exceeds drill pipe tube tension capacity; Marginal—drill pipe tube tension capacity exceeds tension load by a small margin; Acceptable—drill pipe tube tension rating exceeds tension load with a reasonable safety margin

• Surface pump pressure: Unacceptable—6,500 psi or greater; Marginal—greater than 5,800 psi and less than 6,500 psi; Acceptable—less than or equal to 5,800 psi

• Stretch: Unacceptable—greater than one stand of drill pipe; Marginal—less than or equal to one stand and more than one joint of drill pipe; Acceptable—less than or equal to one joint of pipe.

Steel tool joints are assumed. Drill pipe tube tensile and torsional limits are based on 90% of API Premium Class under uni-axial loading. Torque loads are based on a 0.25 average friction factor. Tension loads were calculated using a 0.30 friction coefficient assuming no rotation while lifting. The surface pump pressure assumes a 750-gpm flowrate with 13.0-ppg mud (PV/YP = 37/25) and 1,300-psi bit/BHP pressure. Stretch is defined as the relative change in total length of the drillstring occurring when changing from moving downhole to moving uphole.

Optimum designs incorporate Range III pipe in the lateral section of the well to reduce side loads and decrease torque and drag forces. However, in the build section of the well, only Range II is used to limit side loads on the tool joints. Using Range III vs. Range II in the build section increases side forces on the tool joints by 50%, can result in accelerated wear and can also cause increased tube body wear. As a rule of thumb, side loads on the tool joints should be limited to no more than 2,000 lb to limit wear.

COLLAPSE CONSIDERATIONS

Much consideration was given to the collapse rating of the lightweight, UD-165 drill pipe. Due to its thin wall, this pipe has a relatively low collapse rating, Table 5. Collapse failures of drill pipe are very rare. In addition, the API collapse rating for tubulars is very conservative. Nevertheless, any engineer planning to run this drill pipe should be aware of the issue and take special care to avoid situations where the drill pipe could be exposed to significant differential external pressure loading.

The lowest collapse rating of the drill pipe used in the study is 2,090 psi for the 5 7⁄8-in., 0.25-in.-wall UD-165. A 2,100-psi collapse load is equivalent to 3,100 ft (more than 30 stands of pipe) of fully evacuated drill pipe in 13-ppg drilling fluid.

Collapse strength is primarily driven by the relationship of outside diameter to wall thickness. Consequently, although the lightweight UD-165 has high yield strength, the collapse rating is relatively low. Using Premium Class wall thickness to calculate collapse strength also adds a level of conservatism. Collapse properties are a function of the average pipe geometry over a length of several inches to a foot or more. Consequently, one localized area of thin wall will not greatly impact the collapse strength.

The relatively low collapse rating for the thin-walled, ultra-high-strength drill pipe requires:

• Diligent inspection of tubes to better than premium class (the “new” collapse rating for the 0.25-in.-wall UD-165 is 3,940 psi)

• Modified tripping procedures to fill the drillstring more frequently on trips to reduce differential external pressure loads to about 1,000 psi.

CONCLUSION

A new drill pipe material grade that combines ultra-high strength with excellent fracture toughness has been developed. This new drill pipe can help expand the ERD envelope by enabling drillstrings with excellent strength-to-weight properties. Combining the UD-165 material with lightweight, thinner-walled drillstring designs can reduce torque and drag loads, while providing the high axial tension capacities and torsional strengths necessary to drill the next generation of ERD wells.

The UHSS material is also well suited to landing string applications where the loading demands to run longer and heavier casing strings in deepwater continue to increase. The authors anticipate that innovative drilling professionals may discover additional critical drilling applications that can benefit from this new drill pipe technology.   

ACKNOWLEDGMENTS

The authors thank the management of National Oilwell Varco, K&M Technology Group and Workstrings, LLC, for their support and encouragement to publish this material. This article was adapted from a paper presented at the IADC World Drilling Conference and Exhibition held in Dublin, Ireland, June 17–18, 2009.

LITERATURE CITED

  ¹  Jellison, M. J., Chandler, R. B. and J. Shepard, “Challenging drilling applications demand new technologies,” IPTC 11267 presented at the International Petroleum Technology Conference, Dubai, Dec. 4–6, 2007.
  ²  Shell SQAIR AA 03.30.00.1110 Revision 04-95, 1995, p. 7.
  ³  Chandler, R. B., Muradov, A., Jellison, M. J., Gonzalez, M. E. and J. Wu, “Drill faster, deeper and further with ultra-high torque, third generation double-shoulder connections,” SPE/IADC 105866 presented at the SPE/IADC Drilling Conference, Amsterdam, Feb. 20–22, 2007.