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Industry Headlines

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David Paradis Named President of Weir Flow Control Division

2 DAYS AGO

The Weir Group PLC has appointed David Paradis to its group executive as president of the Weir Flow Control Division. He will take up this new role on January 23, 2017. Paradis succeeds John Heasley who was appointed Weir Group CFO in October 2016.

Paradis is currently president of Weir Oil & Gas&r...

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GE, Transocean Announce Performance-Based Service Agreement

4 DAYS AGO

GE Oil & Gas has secured a new contractual service agreement (CSA), valued at approximately $180 million, with Transocean. Under the agreement , GE will provide condition-based monitoring and maintenance services for pressure control equipment on seven of Transocean’s rigs over the next 10 t...

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$4.17B Canadian Trans Mountain Pipeline Expansion Approved

3 DAYS AGO

Kinder Morgan’s Trans Mountain Expansion Project has received its environmental certificate from British Columbia, Canada. The proposed $4.17 billion Trans Mountain Expansion Project would increase the capacity of the pipeline to 890,000 barrels per day. The current capacity of the pipeline is 3...

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New Upstream Oil & Gas Industry Projects to Double in 2017

4 DAYS AGO

Wood Mackenzie forecasts the investment cycle will show the first signs of growth in 2017 since 2014 and final investment decisions (FIDs) will double , compared with 2016.

Malcolm Dickson, a principal analyst for Upstream Oil and Gas for Wood Mackenzie, said: "2017 will demonstrate how efficient the o...

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World Bank: U.S. Could Boost Global Economy in 2017

3 DAYS AGO

Global growth for 2017 is projected at 2.7%, 0.1% lower than the June 2016 forecast, and 0.4% higher than the estimate for 2016. Going forward, according to the World Bank, global growth is projected to pick up modestly, reaching 2.9% by 2018.

Downside risks to global growth include increasing policy u...

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Implementing Factory of the Future Will Reduce Conversion Costs 40%

4 DAYS AGO

Investments in the factory of the future will pay off, and industrial companies that begin implementation today will save up to 40% of their conversion costs in ten years. To succeed, however, manufacturers have to leverage the potential of modular production concepts and new technologies, as well a...

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Materials Selection for Deepwater Gate Valves

spr11_deepsea_fig1Figure 1. A typical subsea installation showing valves, manifolds and jumpers

With the discovery of oil and gas in water depths thousands of feet below the surface, selection of valves is more important, difficult and complicated. Gate valves, which are often used in subsea applications, are available today in a wide selection of materials, but choosing them requires knowledge of new challenges and established standards.

In years past, the materials used to handle corrosive service in the sea faced mainly the challenges of hydrogen sulfide (H2S), carbon dioxide (CO2) and chlorides. With deepwater well drilling, the newer subsea systems being drilled also need to handle chemicals that will minimize paraffin, asphaltene, hydrates and scale formation as well as provide corrosion inhibitions. These chemicals, however, have adverse effects on metallic and non-metallic materials, and the problem is compounded when materials have to handle produced fluids, annular fluids and the injected chemicals. Also, with subsea systems, the effects of hydrogen embrittlement from the cathodic protection system have to be taken into account. For this reason, choosing the materials to be used in gate valves for subsea is especially challenging.

WHAT GOES INTO THE CHOICE

In selection of materials for subsea gate valves, the following must be considered:

  • Composition of produced fluids in contact with valves and internal parts—all wetted parts
  • Service temperatures
  • Operating pressure ranges
  • Galvanic effects from contact of dissimilar materials
  • Crevice corrosion at seal and flange faces
  • Temperature and chemical resistance for non-metallic materials
  • Cathodic protection (CP) on materials
  • Effectiveness of coatings on materials
  • Weldability for weld overlay
  • Material availability and cost
  • Compatibility of materials with injected fluids

 


VALVE BODY MATERIALS

Several organizations provide recommendations for the selection of materials for valves. These include the National Association for Corrosion Engineers (NACE) and American Petroleum Institute (API).

NACE only covers metallic material requirements for resistance to sulfide stress cracking (SCC) for oilfield equipment, which is not intended to include design specification. (Other forms of corrosion and other modes of failure are outside the scope of NACE’s standard and should be considered in design and operation of equipment.) NACE also has requirements for low-alloy materials exposed to sour service. For example, the organization requires that hardness for alloy materials be limited to HRC 22 maximum. Nickel content is limited to 1% maximum, and NACE also has proposed heat treatment such as normalized, normalized and temper, and quench and temper.

API has several standards, such a specification 17D “Specification for Subsea Wellhead and Christmas Tree Equipment,” which uses the material requirements of API 6A.

Specification API 6A covers a number of specific areas for subsea valves, including strength, impact and quality testing. Strength level depends on the pressure rating of the equipment. For example, for flanged end connections, equipment used to pressure levels of 10,000 psi must be manufactured from material having a minimum yield strength of 60,000 psi. Equipment exceeding 10,000 psi pressure must be designed using equipment with specified yield strength of 75,000 psi (refer to API 6A Table 5.2).

Once the fluids that will be produced have been determined, valve selection can occur. Besides the challenges the fluids will produce, as well as the temperatures and pressures involved, the service conditions must also be considered. This includes how long the equipment might be exposed to seawater. Alloy steel will handle most benign conditions, including low CO2 for short periods of time, but even short seawater exposure can cause corrosion of critical components. This is especially true if seawater is trapped in those components and cannot be flushed out in a timely manner. Even with benign conditions, there is need for long-term life—in many cases over 25 years.

Valves as specified using API and NACE standards to handle strength and corrosive requirements can be grouped as follows with typical materials and applicable service conditions:


Stainless-steel Valves
spr11_deepsea_fig2Figure 2. Typical subsea tree assembly

When environments call for stainless steels such as 410 and F6NM, they may have similar corrosion resistance in oilfield environments; however, they have significant differences in weldability. Stainless 410 in the wrought and welded condition has lower impact toughness than F6NM. Welds of 410 have lower toughness, and depending on the operation, F6NM is often used if there is a risk of Joule Thomson effect (the temperature change of a gas or liquid forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment) at the wellhead. Even though stainless steels such as 410 and F6NM have good corrosion resistance and can handle mild corrosive conditions, weld overlay of critical sealing surfaces with corrosion-resistant alloy (CRA) is used to minimize pitting.


Duplex Stainless-steel Components

Although duplex stainless steels have good corrosion resistance in most environments, the use of these materials is limited for wellhead equipment because of the danger associated with sigma formation during heat treatment in large section thicknesses. Improper heat treatment not only results in poor corrosion resistance, but also poor toughness property. Duplex stainless steels require a satisfactory balance between ferrite and austenite both in the wrought and welded structures. Super duplex is specified where the Pitting Resistance Index (PRE) exceeds 40, whereas duplex is specified for thin components.

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