A study on the application of modern corrosion resistant alloys in the upstream Oil and Gas industry - Cost optimization

Kukuh W. Soerowidjojo

Sandvik Materials Technology SEA, Singapore kukuh.soerowidjojo@sandvik.com

Keywords: Stress corrosion cracking (SCC), Sulphide stress cracking (SSC), PRE, Pitting corrosion, crevice corrosion, CRA, duplex, super duplex stainless steel (SDSS), hyper duplex, ASTM G48, ASTM B31.3(2002), total cost of ownership, Enhanced Oil Recovery (EOR). INTRODUCTION The challenges in future oil and gas field development are getting tougher. Easy producing fields have become scarcer and oil and gas operators must move to more difficult operations such as Enhanced Oil Recovery (EOR), High Pressure and High Temperature (HPHT), sour and deepwater fields. Each method acquires its own complications and especially when it is an offshore field, these operations are expensive. Safety and environmental issues are top priorities in equipment design and impact on the importance of material selection without compromising costs. In offshore oil and gas industry, duplex and super duplex materials application are common today especially for critical and low maintenance components, such as umbilical tubes, drilling risers, manifolds, valves, wellheads, etc. With the benefits of the new duplex grades – higher corrosion resistance, mechanical properties, easy fabrication and cost effectiveness – there are limitations on their application. Regardless of the environment, duplex cannot be used at temperatures above 250o C due to the risk of 475o C embrittlement. Limited critical crevice temperature and critical pitting temperature in sea water induced corrosions. In this proceeding, we review corrosion types in chloride induced corrosion with different materials so that we can group those in the same level of corrosion resistance and compare alloying costs in order to predict the cost efficiency of selected materials. Considering both corrosion resistance and mechanical strength of materials, we could optimize the material spending by understanding the alloying element price dynamic.

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NEW ALLOYS – DEVELOPMENTS AND PROPERTIES The aim of new alloy developments is to replace existing grades with more cost effective materials i.e. to reach stronger material that benefits to the design and less alloyed material to reduce the alloy surcharges whilst keeping or even increasing the corrosion resistance. Both approaches are fulfilled by the duplex family of materials especially in offshore environment. Austenitic steel grades, which have relatively high nickel contents, have been outperformed by duplex in terms of alloying cost effectiveness and mechanical properties. Duplex stainless steel To describe briefly duplex stainless steel grades, the duplex microstructure comprises two different microstructures, namely austenite and ferrite, ideally with a 50/50 ratio. The matrix of the microstructure is ferrite with islands of austenite structure in between. However, in reality the ferrite content can vary between 40 to 60 percent in base metal and can be up to 70 percent in the weld. In pipe and plate products, the grains are either elongated or flattened due to the manufacturing processes. Duplex stainless steel was first developed in 1930 but due to limited knowledge and metallurgical techniques at that time, further major developments were not undertaken until the 1970’s. In the 1990’s duplex gained market share over austenitic grades due to cost, improved weldability, metallurgical techniques and availability so that today, duplex materials have become as much a commodity product as the austenitic grades.

(a) (b)

Figure 1. (a) shows FCC crystal configuration that forms an austenitic structure and (b) shows BCC crystal configuration that forms a ferritic structure.(12) Modern duplex stainless steels are developed in two contrasting ways to save costs. Lean duplex UNS S 32101 was developed to keep its corrosion resistance similar to the previous version of lean duplex, UNS S 32304 and the austenitic grade UNS S31603 but with lower nickel content to reduce alloying element costs and compete in the austenitic UNS S31603 market. The higher yield strength of lean duplex compared with UNS S31603 is also very beneficial to reduce weight by thinner designs and also cost. On the other hand, hyper duplex was developed for more aggressive environments by increasing the alloying elements than the existing super duplex to achieve both higher mechanical strength and corrosion resistance and competes in the highly alloyed materials market. There are conditions in severe chloride containing environments and high temperature, hyper duplex is a good candidate for replacing nickel alloy and titanium as a more cost effective alternative. For example, in sea water cooled heat exchanger where the operating temperature is too high for super duplex UNS S32750, the grade of hyper duplex UNS S32707 could be an alternative to more expensive titanium or nickel base tubes.

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Table 1. Main alloying elements from different CRA’s (13),(14),(15)

Ni Cr Mo N OtherAISI 304L UNS S30403 8.0 - 12.0 18.0 - 20.0 max 0.1 20AISI 316L UNS S31603 10.0 -14.0 16.0 - 18.0 2.0 - 3.0 max 0.1 25Lean duplex 2304 UNS S32304 3.0 - 5.5 21.5 - 24.5 0.05 - 0.60 0.05 - 0.20 25Lean duplex 2101 UNS S32101 1.35 - 1.70 21.0 - 22.0 0.10 - 0.80 0.20 - 0.25 26.5AISI 904L UNS N08904 23.0 - 28.0 19.0 - 23.0 4.0 - 5.0 > 35Duplex 2205 UNS S32205 4.5 - 6.5 22.0 - 23.0 3.0 - 3.5 0.14 - 0.20 >35Austenitic 6Mo UNS S31254 17.5 - 18.5 19.5 -20.5 6.0 - 6.5 0.18 - 0.22 > 40Superduplex 2507 UNS S32750 6.0 - 8.0 24.0 - 26.0 3.0 - 5.0 0.24 - 0.32 >40Ni based alloy 625 UNS N06625 min. 58 20.0 - 23.0 8.0 - 10.0 Nb: 3.15 - 4.15 > 50Hyperduplex 2707 UNS S32707 5.5 - 9.5 26.0 - 29.0 4.0 - 5.0 0.30 - 0.50 Co: 0.5 - 2.0 > 50Hyperduplex 3207 UNS S33207 6.0 - 9.0 29.0 - 33.0 3.0 - 5.0 0.40 - 0.60 > 50

PREN : %C + 3.3 x %Mo + 16 x %N

Main alloying elementsUNS #Grade name PRE ave.

(a) Austenitic structure (b) Ferritic structure (c) Duplex structure Figure 2. Microstructures of austenitic, ferritic and duplex materials. CORROSION PROPERTIES Pitting and Crevice corrosion The most interesting property to know about for a new material is its corrosion resistance. In offshore corrosion, seawater dictates the level of attack where chlorides lead to localized attacks like pitting corrosion, crevice corrosion and stress corrosion cracking. The Pitting Resistance Equivalent (PRE) number can be used to rank the resistance of materials in chloride induced corrosion environments where the number is calculated from its chemical composition i.e. chromium, molybdenum, tungsten and nitrogen. PRE = %Cr + 3.3 x (%Mo + 0.5 W) + 16 x %N

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The higher the PRE number the more resistant is the material to chloride induced corrosion. This trend is seen when we look at each Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) of materials in ferric chloride solution (ASTM G48 method E and F, FeCl3 6% w.t.). From table 1, lean duplex UNS S32304 and UNS S32101 are in one group with austenitic UNS S31603. This group shows that both pitting and crevice corrosion resistance are at the same level and practically, lean duplex is a good alternative to replace austenitic UNS S31603 grade. In the same table, hyper duplex UNS S33207 and UNS S32707 are in the same group with nickel UNS N06625 with PRE 50. Hyper duplex can be used in environments where super duplex is not serviceable due to the risk of pitting and crevice corrosion attack in higher operating temperature. In selected conditions and applications, hyper duplex is a good alternative to titanium as well.

Figure 3. Critical pitting temperature (CPT) and critical crevice temperature (CCT) in 6% FeCl3 solution, 24 hours. The idea of this graph is to show a trend that an increasing PRE number also demonstrates an increasing resistance to pitting and crevice corrosion. Stress corrosion cracking (SCC) There are three prerequisites for chloride induced stress corrosion cracking to occur i.e. temperature (roughly above 50°C for standard austenitic grades), chlorides presence and stress in the material. Stress can be present in many different ways, such as applied force, the tube or pipe’s own weight or internal pressure. Cold working and welding contraction can also be considered as internal stresses. Ferrite structure in duplex stainless steel is more SCC the resistant than it austenitic structure due to its higher PRE number. Once the necessary conditions are met and a crack in the material starts, failure will follow crack propagation through the wall thickness very quickly and cause a premature and catastrophic failure. For materials sensitive to SCC, the cracking process could take several days up to months depending on the temperature, chloride content and stresses present. In stress corrosion cracking tests, lean duplex UNS S32304 performed much better than austenitic grade 304L/316L. In the higher group such as at PRE 35 and 40, austenitic and duplex performed similarly such as UNS N080904 and UNS S32205.

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CCT and CPT versus PRE graph

CCTCPTPRE

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PRE number of each tested material: UNS N08904 : 35 UNS S32205 : 35 UNS S31254 : 40 UNS S32750 : 42.5 UNS S32707 : 49.5 UNS S33207 : 51

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Fig. 4. PRE comparison in ferrite and austenite in duplex UNS S32205 structure.

STRENGTH COMPARISON IN THE STAINLESS STEEL FAMILY High strength in duplex structure is achieved by the presence of ferrite structure, smaller grain size and nitrogen content. Since the strength of duplex family materials is much higher than austenitic grades, significant thinner tube and pipe wall thickness can be designed for the same pressure rating. Within its own family, the yield strength of hyper duplex UNS S33207 is 20% stronger than super duplex UNS S32750 as in table 2.

Figure 5. Stress corrosion cracking resistance profile of different materials in oxygen-bearing (approximately 8 ppm), neutral chloride solutions. Testing time 1,000 hours. Applied stress is equal to proof strength at testing temperature.

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Alloying elements price dynamic More alloyed stainless steels do not necessarily have more expensive alloying element costs but depend on which alloying elements are used. Nickel prices have always been high and fluctuate quite a lot. It causes CRAs with high nickel contents to become expensive. CRA manufacturers are trying to find new grades to reduce the total production cost. Table 2. Strength of CRAs (14),(15),(16) in solution-annealed condition for austenitic and duplex grades, annealed condition for UNS N06625.

Tensile strength

(min,

Yield strength

(min,

Elongation min. in

2" or Hardness

max.(Brinell)AISI 304L UNS S30403 485 170 35 192AISI 316L UNS S31603 485 170 35 192Lean duplex 2304 UNS S32304 690 450 25 290Lean duplex 2101 UNS S32101 700 530 30 290AISI 904L UNS N08904 490 215 35 192Duplex 2205 UNS S32205 655 485 25 290Austenitic 6Mo UNS S31254 650 300 35 220Superduplex 2507 UNS S32750 800 550 15 300Ni based alloy 625 UNS N06625 827 414 30 N.AHyperduplex 2707 UNS S32707 920 700 25 318Hyperduplex 3207 UNS S33207 950 770 15 336

PREN : %Cr + 3.3 x %Mo + 16 x %N

Grade name UNS #

Mechanical propeties

Volatility of alloying element prices leads to volatile CRA prices as well. (17) Alloying element cost index The production cost of CRAs is dominated mainly by the alloying elements in the material that can reach between 50%-70% of the total production cost. Hence, it is relevant to look at alloying element cost comparisons among CRAs to indicate the price level. In this study, an alloying element cost index chart was built by using austenitic 304L SS as a base to rank CRA prices taking into account the main alloying element prices i.e. nickel, chromium and molybdenum. There are wide spread of alloying element indices in all PRE groups where CRAs with higher nickel show a higher index. That indicates austenitic SS has higher alloying element costs than the duplex stainless steel family. In this analysis, UNS S31603 is taken as the base of calculation. Percentage of each important alloying element in the grade is multiplied by its price per ton. Thus, we aquire the total alloying element cost by adding up all of those components. Value from UNS S31603 is taken as the base with index equal to 1. The alloying price index is taken as reference in the consideration to eliminate the factor of expected return of each manufacturing, different manufacturing , distribution and other local costs involved.

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Figure 6. Nickel and iron prices development from May 2015 to May 2016.

Figure 7. Individual alloying element cost indices with the base of nickel price at USD 9,460/ton, molybdenum at USD 13,558/ton and chromium at USD 1,807 /ton on May 2016. CASE STUDY Examples of material selection by utilizing modern corrosion resistant alloy properties. A case study was conducted to propose material for a 2 ½” (outer diameter 73.0.3 mm) drilling riser hydraulic line for 6000 psi and 8000 psi internal pressure with austenitic UNS S31603 as the base. In the study, several other materials were included as alternatives by considering both mechanical and corrosion properties. Wall thicknesses selected are standard wall thicknesses available. Selected materials acquire the same level or higher corrosion resistance than austenitic UNS S31603 in

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seawater environments i.e. lean duplex UNS S32304, duplex UNS S32205 and super duplex UNS S32750. Other austenitic grades are not included since they all have higher alloying element cost indices. Pressure calculations are based on ASME B31.3 (2002) and it has revealed that duplex and super duplex grades can save significant weight and could lead to cost saving.

Figure 8. Drilling riser (18) Table 3. Weight saving obtained from altering the wall thickness of materials according ASME B31.3(2002) calculation. 6000 psi pressure designGrade 316L SAF2304 SAF2205 SAF2507Schedule XXS 160 160 80Wall thickness (mm) 14.021 9.525 9.525 7.01Allowable internal pressure (ksi) 6,479.63 7,284.14 7,535.32 6,969.04 Weight (kg/m) 20.27 14.82 14.82 11.34

8000 psi pressure designGrade 316L SAF2304 SAF2205 SAF2507Schedule XXS XXS 160Wall thickness (mm) 17.145 14.021 14.021 9.525Allowable internal pressure (ksi) 8,093.86 11,252.06 11,640.06 9,720.56 Weight (kg/m) 23.48 20.27 20.27 14.82 This case study – just looking at weight saving and alloying elements cost indices – gives a strong indication that lean duplex UNS S32304 is the most cost effective material in such conditions to replace austenitic UNS S31603. It is supported by the actual price comparison indices. Lean duplex has the benefits of withstanding the corrosion environment designed for austenitic UNS S31603, a price reduction due to thinner pipe wall, reduced weight and less alloy cost. Table 4. Actual price comparison indices (include alloy surcharge in May 2016).

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Table 4. Actual price index in May 2016, alloy surcharge is included. Price indices of materialsGrade UNS S31600 UNS S32304 UNS S32205 UNS S32750Design pressure 6000 psi 100% 70% 76% 76%Design pressure 8000 psi 100% 83% 89% 86% Discussion and summary Assessing the risk of corrosion for the selected material in the given environment is the first step in the material selection process. At the very early stage in the process, it is possible to acquire a group of materials that can withstand SSC, crevice and pitting corrosion attacks. Mechanical properties should be included in the material selection equation because of the opportunity to alter the design without compromising safety. Design alterations can contribute significantly to cost efficiency. From a range of selected materials, we can obtain an indication of the most cost effective alloy by looking at the alloying element indices where both the content of alloying elements and the spot price of the elements are included in the index calculation. In the case study above, it shows that lean duplex UNS S32304 is the most cost effective candidate to replace austenitic UNS S31603 material. However, riser manufacturers normally still need to coat austenitic UNS S31603 tube to increase the reliability of the component. The fact that lean duplex UNS S32304 has the same corrosion resistance level as austenitic UNS S31603, then they would need to coat lean duplex UNS S32304 as well. The other candidate is a higher grade like duplex UNS S32205. However, its crevice corrosion resistance is still border line and coating may still be required. In this case, these duplex grades are not the best alternative for UNS S31603. The next option is to increase the grade further to super duplex UNS S32750 where the risk of crevice corrosion attack is reduced significantly with higher PRE and CCT (Figure 3). Coating cost is not described specifically in this study since it varies widely depending on the coating specification. Whenever total cost of ownership to acquire duplex UNS S32304 plus coating is more expensive than super duplex UNS S32750, it would conclude that the most cost effective material is super duplex UNS S32750. The decision to coat duplex UNS S32205 can be argued but then it is the call of the manufacturer and end user, since there are examples that duplex UNS S32205 is used successfully without coating. Coating lower grades of stainless steel also brings risks. Coating can be easily broken or scratched and this small exposure will cause a concentration of a small anodic area with high corrosion current density that leads to premature failure. From this discussion, other methods of corrosion protection like coating, cathodic protection and corrosion inhibition should also be considered to find out the total cost of ownership. CONCLUSION

1. There are several factors to consider in material selection to optimize spend on offshore upstream oil and gas facilities i.e. corrosion resistance of the material in marine environments, mechanical properties to meet design demands and alloying elements.

2. The spot price of the alloying elements that move over time dictates the direction of the material selection process from the cost perspective.

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3. Lesson learned: The perception that more advanced and higher alloyed materials are more expensive than the traditional grades like austenitic UNS S31603 is not necessarily true. It depends on the corrosion environment, the alloying element prices at the time of project commencement and how willing are the designers to utilize the advantages of advanced material properties. Super duplex UNS S32750 grade is more cost efficient than austenitic UNS S31603 in the case study above.

4. This approach can also be used for other pipe or tubing system designs.

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REFERENCES

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11. Sandvik Materials Technology AB, “Sandvik welding guidelines SAF2707HD rev.1”. 12. Website for crystal structure figures: http://www.ndt-

ed.org/EducationResources/CommunityCollege/Materials/Structure/metallic_structures.htm 13. ASTM A213/A213M: Standard Specification for Seamless Ferriic and Austenitic Alloy-Steel Boiler,

Superheater, and Heat-Exchanger Tubes 14. ASTM A312/A312m-11: Standard for Seamless, Welded, and Heavily Cold Worked Austentic

Stainless Steel Pipes. 15. ASTM B 444 – 06: Standard Specification for Nickel-Chromium-Molybdenum-Columbium Alloys

(UNS N06625 and UNS N06852) and Nickel-Chromium-Molybdenum-Silicon Alloy (UNS N06129) Pipe and Tube

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17. Website for metal prices: http://www.metalprices.com 18. Website for drilling riser figure: http://en.wikipedia.org/wiki/Drilling_riser

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