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Manifold Technology in the Offshore Industry

Karan Sotoodeh

Valve Engineering, Baker Hughes, Oslo, Norway

Abstract: Manifolds are widely used in the oil and gas industry for the distribution of process fluids such as oil, gas, and water. Manifolds are designed to either merge multiple junctions into a single channel or divide one flow line to multiple outputs. The size of a manifold is selected by process engineers based on the flow rate passing through the manifold. Manifolds used in the offshore industry are made in 22Cr duplex instead of carbon steel to save thickness and weight. The manifolds described in this paper are either made of welding wrought tees or designed using a standard pipe as a header and welding olets to the pipe header. Mechanical joints (hubs and clamps) instead of standard ASME flanges are used for closing some of the manifold header ends as well as some of the branch connections to save weight and space. Traditionally, welding techniques and preparation of the welded ends has been done as per ASME B16.25, the common standard for butt welded connections in piping systems, including manifolds. However, narrow gap welding is an advanced welding end preparation in which the angle of the bevel end fitting is 7° to the vertical line. The advantages of narrow gap welding include using less weld electrodes, a faster welding process, and less heat input. This paper presents a method for calculating welding consumables volume and weight in one meter. The result shows that the amount of welding electrodes used for standard ASME welding is more than double the amount used for narrow gap welding.

Keywords: manifold, manufacturing process, material selection, welding, oil and gas, offshore

Received March 10, 2020; Revised April 12, 2020; Accepted April 22, 2020

Citation: Karan Sotoodeh, Manifold Technology in the Offshore Industry. American Journal of Marine Science, 8 (1). 14-19. January 2020. doi:10.12691/marine-8-1-3

1. Introduction to Manifolds

Manifolds are widely used in the oil and gas industry for the distribution of process fluid such as oil, gas, and water. [1] Manifolds are designed to either merge multiple junctions into a single channel or divide one flow line to multiple outputs. [1] As an example, a production manifold is located before the separator and it collects the oil from different flow lines coming from wellheads. It transfers the produced fluid into a single channel, including three phases of oil, gas, and water, to the separator for further treatment. On the contrary, a gas lift or water injection manifold collects the produced gas as a single line and divides them into different reservoirs for advanced or secondary production. In fact, secondary production methods are used to increase the production through boosting the pressure inside the formation or reservoir, [2] The production of oil and gas continually decreases because the pressure in the reservoir has been decreased, so one solution for advanced hydrocarbon recovery is gas or water injection to the reservoir. [2] Figure 1 and Figure 2 illustrate the production manifold in 20” header and 8” branch sizes and the gas lift manifold in 6” header and 2” branch sizes, respectively.

The size of a manifold is selected by process engineers based on the flow rate passing through the manifold. Manifolds usually handle high pressure fluid, so they may be designed based on a pressure nominal of 250Barg equal to Class 1500, based on the ASME B16.5 standard, [3] for example. The following sections of this paper will focus on the manufacturing process for manifolds.

2. Manifold Material Selection

Manifolds are made in 22Cr duplex stainless steel (DSS) in the offshore industry instead of carbon steel (CS), to save thickness and weight [4]. CS piping has higher weight per length compared to 22Cr DSS, because carbon steel has a lower value of mechanical strength. In addition, a 3mm corrosion allowance should be added to the piping thickness as per the NORSOK [5] standard, which increases the thickness and weight of the CS piping. [4] Figure 3 shows a weight comparison chart in KG between CS and 22Cr DSS for average pipe sizes from 2” to 20” in different ASME pressure classes from 150 to 2500. [4] The manifolds shown in Figure 1 and Figure 2 are both made in 22Cr DSS.

3. Manifold Manufacturing & Fabrication Process

The manifolds described in this paper are made by either welding wrought tees [6] together like the production manifold shown in Figure 1, or by using a standard pipe [7] as a header and welding olets [8] to the pipe header, such as the gas lift manifold in shown in Figure 2. The sizes of the header and branch affect the choice between a standard tee or a standard pipe with welding tees. As a rule of thumb, a standard tee cannot be used for branch connection sizes with 1/3 or less than 1/3 of the header size. [6] This is the reason why a 6”x 2” manifold is made of a pipe header with olets for branches. The optimized manufacturing solution for a manifold made of tees is to weld the long-length tees directly together without any pipe between. Figure 4 shows a manifold made of tees that are connected to each other through pieces of pipe. Alternatively, Figure 5 shows the same manifold made of long length tees connected directly together. The advantage of the optimized solution is to reduce the number of welding joints by deleting the pipe pieces and using a faster fabrication process.

Figure 1. 20” x 8” production manifold during welding

Figure 2. 6” x 2” gas lift manifold during the pressure test

Figure 3. Piping average weight values from 2” to 20” size in Kg comparing CS and DSS in different pressure classes

Figure 4. Standard manifold fabrication made of tees and pipe pieces

Figure 5. Optimized manifold manufacturing process through welding the long length tees directly together without any pipe pieces

The wrought made tees are based on ASME B16.9 standard [6] through forming or a hot extrusion process illustrated in Figure numbers 6 and 7 respectively. [9] In the forming method, the piece of pipe is placed in a hydraulic die, liquid is poured into the pipe, and hydraulic pressure pushes out the branch in the fixed-opening die. [9] The alternative method for thick tees is a hot extrusion method in which the branch outlet is extruded from the pipe with the assistance of an extrusion tool. [8] Tees will be heat-treated and machined for making bevels at three ends after forming or extrusion.

Figure 6. Forming a Tee

Figure 7. Hot Tee extrusion

This method cannot be used for very thick tees. Seamless pipe with a heavy wall is selected for manifolds that are made of pipe. Seamless pipe has a joint efficiency equal to 1 since there is no longitudinal seam weld on that which is higher. [10] On the other hand, welded pipes have welded seams, which reduces the joint efficiency of the pipe to 0.8 or 0.85, for example, depending on the method of welding. [10] Seamless piping is made of a solid cylinder named a billet shown in Figure 8, which is pierced through the centre with a mandrel.

Figure 8. Seamless pipe and billet

The mechanical joints (hubs and clamps) instead of standard ASME flanges [3] are used for closing some manifold header ends as well as some of the branch connections (see Figure 9) to save weight and space in the offshore industry (see Figure 10).

Figure 9. Applications of mechanical joints on Manifolds

Figure 10. Comparison between ASME flange and mechanical joint assembly

4. Manifold Header Welding Process

The bevel ended long length tees on a manifold should be welded together through butt welding. Traditionally, the welding technique and preparation of the welded ends was done as per ASME B16.25, [11] the common standard for butt welded connections in piping systems including manifolds. Figure 11 shows the butt weld end preparation as per ASME B16.25 standard. Two scenarios of the bevel end preparation based on the piping thickness are shown in Figure 11, one for wall thickness (t) up to and including 22mm, and one for thicknesses greater than 22mm. On average, 1.6mm root face is prepared, and the bevel end fitting or pipe has 37, 5 ° angle on average to the vertical line for wall thicknesses up to and including 22mm, according to ASME B16.25 (see Figure 11). The angle should be reduced to 10° on average for thickness values above 22mm on the extra thickness over 19mm on average. Narrow gap welding is an advanced welding end preparation in which the angle of the bevel end fitting has 7° to the vertical line, as shown in this example. The advantages of narrow gap welding (see Figure 12) include using fewer weld electrodes, faster welding processes, and less heat input. The advantage of less heat input production during the welding of 22Cr DSS material is a lowered risk of sigma formation. In fact, DSS may undergo different structural transformations due to high temperature and heat, such as temperature ranges between 600°C to 1000°C. [12] One of those structural transformations in the form of intermetallic compounds is sigma phase made of chromium and molybdenum. [12]

Figure 11. Butt weld end preparation according to ASME B16.25

Figure 12 A-B. Narrow gap welding with 7 degree angle

It is possible to calculate and compare the volumes and weights of consumed electrodes for standard ASME B16.25 end preparation and narrow gap welding based on the method explained in this paper. [13] There are two assumptions in welding consumable calculation based on this method. The first is that there is no root face for the bevel ends, and the second is that the wall thicknesses of the welded joints are maximum 22mm. Parameter b is the angle of the bevel with a vertical line that is 37, 5 ° for a standard bevel end and 7 ° for narrow gap welding. The total amount of welding consumable is represented in Figure 13 by the two darker orange triangles on both sides, the lighter orange rectangle in the middle, and the pink area on the top.

[picture] | (1)

Figure 13. Four areas for estimation of welding consumables

It is possible to calculate the area of each darker orange triangle using Equation #2.

[picture] | (2)

[picture] | (3)

The area of the light orange rectangle in the middle is calculated using Equation #4. The pink area of excess metal on the top is calculated using Equation #5.

[picture] | (4)

[picture] | (5)

Where:

[picture] | (6)

Thus, the total area of the welding consumable is calculated using Equation #7.

[picture] | (7)

Assuming that the thickness of the welded tee (t) is 20mm, root gap (parameter g) is 7mm, cap height (parameter h) is 5mm, and is equal to 0.1228 and 0.767 for narrow gap welding (b=7°) and standard welding (b=37.5°) respectively. Now it is possible to calculate c for both welding conditions through equation #1.

The next step is to calculate W in both welding conditions using Equation #6.

The next step is to calculate the total volume of the welding for both conditions using Equation #7.

To calculate the volume of the weld, the length of the weld should be multiplied by the area. Assuming the length of the weld equal to 1m is equal to 100cm, the volume of the weld will be 251.37 and 573.5 for narrow gap welding and standard ASME welding, respectively.

Therefore, the volume and area of welding electrode in this example is more than double for standard ASME butt weld ending compared to narrow gap welding. The 22Cr DSS welding electrodes are used for welding the manifolds in this paper. The density of 22Cr DSS is equal to 7.8 Thus, it is possible to calculate the weight of welded consumables for each end preparation using Equation #8.

[picture] | (8)

Where:

Weight (gr);

Density ;

V: Volume

Note: The values of weight and volume are given in one meter.

5. Conclusions

Manifolds are widely used in the oil and gas industry for the distribution of process fluid such as oil, gas, and water. The sizes of manifolds are selected by process engineers based on the flow rate passing through the manifold. Manifolds normally handle high-pressure fluid, so they may be designed based on pressure nominal of 250Barg, as an example. In the offshore industry, manifolds are made in 22Cr DSS instead of carbon steel to save thickness and weight. The manifolds described in this paper are made of either welding wrought tees or using a standard pipe as a header and welding olets to the pipe header. Bevel ended long length tees on the manifolds should be welded together through butt welding. Traditionally, welding technique and preparation of the welded ends was done as per ASME B16.25, the common standard for butt welded connections in piping systems including manifolds. However, narrow gap welding is an advanced welding end preparation in which the angle of bevel end fitting has 7° to the vertical line in this example. The advantages of narrow gap welding include using fewer weld electrodes, having a faster welding process, and requiring less heat input. The method shown in this paper for calculation of welding consumables volume and weight in one-meter shows that the amount of welding electrodes used for standard ASME welding is more than double the amount of narrow gap welding.

Acknowledgements

I would like to express my gratitude to my partner, Ms. Tamara Zhunussova for her constant support.

References

[1]   | Honiron. (2018). Manifold application in the oil and gas industry. [Online]. Available from: https://www.honiron.com/manifold-applications-oil-gas-industry/.
[2]   | Rigzone. (2018). How does gas injection work? [Online]. Available from: https://www.rigzone.com/training/insight.asp?insight_id=345.
[3]   | American Society of Mechanical Engineering B16.5. (2012). Piping flanges and flanged fittings. New York, NY: ASME.
[4]   | Sotoodeh, K. (2018). Analysis and improvement of material selection for process piping system in offshore industry. American Journal of Mechanical Engineering, Vol.6, No. 1, p. 17-26.
[5]   | NORSOK L-001, (2017). Piping and valves. 4th revision, Oslo, Norway.
[6]   | American Society of Mechanical Engineering B16.9. (2012). Factory made wrought buttwelding fittings. New York, NY: ASME.
[7]   | American Society of Mechanical Engineers (ASME). (2004). Carbon, alloy and stainless steel pipes. ASME B36.10/19. New York, NY: ASME.
[8]   | Manufacturers Standardization Society of the Valve and Fittings Industry, MSS SP 97. (2002). Integrally reinforced forged branch outlet fittings- socket welding, threaded and butt-welded ends. Vienna, VA: MSS.
[9]   | Hardhatengineer. (2018). Pipe fitting manufacturing process [Online]. Available from: https://hardhatengineer.com/pipe-fittings/pipe-fittings-manufacturing-process/
[10]   | American Society of Mechanical Engineers (ASME B31.3). (2012). Process piping. New York, NY: ASME.
[11]   | American Society of Mechanical Engineers (ASME B16.5). (2017). Butt welding ends. New York, NY: ASME.
[12]   | Cristini S. I., Sacchi, B., Guerrini, E., & Trasatti, S. (2010). Detection of Sigma phase in 22Cr duplex stainless steel by electrochemical methods. Russian Journal of Electrochemistry. Vol.46, No.10, pp.1168–1175.
[13]   | The Welding Institute (2018). Calculating weld volume and weight. [Online]. Available from: https://www.twi-global.com/technical-knowledge/job-knowledge/calculating-weld-volume-and-weight-095/.