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Market Outlook 2018: A Sunnier Mood with a Few Caveats

Market Outlook 2018: A Sunnier Mood with a Few Caveats

The atmosphere at VMA’s 2018 Marke...

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Is it Time to Toss Those Commissions?

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Young Valve Professionals: Megan Johnston

Young Valve Professionals: Megan Johnston

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Industry Headlines

Crude Oil, Petroleum Product Exports Reach Record Levels

Thursday, 19 October 2017  |  Chris Guy

According to the U.S. Energy Information Administration ( EIA ), crude oil exports in the first half of 2017 increased by more than 300,000 barrels per ...

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Industry Headlines

Emerson Agrees to Acquire Paradigm

4 DAYS AGO

Emerson has agreed to acquire Paradigm for a purchase price of $510 million, reflecting a multiple of 13 times expected 2017 EBITDA. Paradigm will be joined with Emerson’s existing Roxar automation software. The acquisition is expected to close within the next 60 days, subject to various regulat...

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ValvTechnologies Receives ISO 15848 Certification

5 DAYS AGO

ValvTechnologies’ EcoPack technology has received ISO 15848-1:2015 certification from Odin Heavy Industries. To earn ISO 15848 certification, ValvTechnologies underwent a stringent evaluation process that included a series of 17 helium leak tests and eight thermal cycles on a fully assembled v...

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Crude Oil, Petroleum Product Exports Reach Record Levels

3 DAYS AGO

According to the U.S. Energy Information Administration ( EIA ), crude oil exports in the first half of 2017 increased by more than 300,000 barrels per day (b/d) from the first half of 2016, reaching a record high of 0.9 million b/d. Petroleum product exports also grew over the same period with propan...

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Energy Intensity in U.S. Manufacturing Decreased

3 DAYS AGO

Energy intensity in manufacturing in the U.S. decreased from 2010 to 2014. U.S. manufacturing overall fuel intensity decreased by 4.4% from 3.016 thousand British thermal units (Btu) per dollar of output in 2010 to 2.882 thousand Btu in 2014. According to the Manufacturing Energy Consumption Survey (M...

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Fed Reports Economy Growth in September, Early October

3 DAYS AGO

Reports from all 12 Federal Reserve Districts indicated that economic activity increased in September through early October, with the pace of growth split between modest and moderate. The Richmond, Atlanta, and Dallas Districts reported major disruptions from Hurricanes Harvey and Irma in some areas a...

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Industrial Production Up 0.3% in September

4 DAYS AGO

Industrial production rose 0.3% in September. The rates of change for July and August were notably revised; the current estimate for July, a decrease of 0.1%, was 0.5% lower than previously reported, while the estimate for August, a decrease of 0.7%, was 0.2% higher than before. Manufacturing output...

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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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