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Application of welded pipe in submarine pipeline

The sustainable development of the world economy has brought human society a high degree of concern and dependence on global energy. The improvement of the ecological environment has promoted the development and utilization of clean, low-carbon and environmentally friendly energy such as oil and natural gas, and has become the fastest-growing new energy in the disposable energy consumption. According to statistics, by 2050, the world natural gas demand will increase to 5.5 trillion m3, with an average annual growth of 1.43% in 2016-2050; the world oil demand will reach 4.91 billion T, with an average annual growth of 0.35% in 2015-2050. The proportion of non fossil energy in primary energy consumption structure in China will increase from 12% in 2015 to 20% in 2030, and the proportion of natural gas in primary energy consumption structure will increase from 5.9% in 2015 to 15% in 2030 [1-2].

With the continuous exploitation of oil and gas on land, exploitable resources are more and more restricted by environment and other factors, while marine resources can be developed and applied, and gradually become another strategic area after land. In 2013-2017, the capital expenditure of global submarine pipeline construction increased by 59.8% compared with the previous five-year plan. As the main means of offshore oil and gas development and gathering, subsea pipeline has also developed rapidly. The existing subsea oil and gas pipeline in the world has exceeded 100000 km. From 2018 to 2022, the laying of subsea pipeline in the world will also reach more than 56000 km [3-4]. In the submarine pipeline, welded pipe is widely used in the field of offshore oil and gas transportation because of its unique quality characteristics and manufacturing advantages.

What is submarine pipeline?

Subsea pipeline is the most convenient, economical and safe transportation mode in the offshore oil and gas development system.

Types of subsea pipelines

There are several types of subsea pipelines:

  • (1) Seamless steel pipe. It is generally used in small diameter subsea pipe trays with special requirements.
  • (2) Longitudinal submerged arc welded steel pipe (SAW). The diameter is generally more than 16in, and the maximum diameter is 56in.
  • (3) Straight seam high frequency resistance welded steel pipe (hferw). Due to the improvement of welding quality and low price, this kind of pipe has been more and more used in the subsea pipeline of more than 20 in.
  • (4) Threaded welded steel pipe. This kind of pipe is not generally accepted by the offshore pipeline industry because of the problems of welding quality.

Advantages of submarine pipeline

The advantage of subsea pipeline is that it can be transported continuously, hardly affected by environmental conditions, and it will not force the oil field to reduce production or stop production due to the capacity limitation of offshore oil storage facilities or the untimely transportation of shuttle tankers. Therefore, the oil transportation efficiency is high and the oil transportation capacity is large. In addition, the construction period of submarine pipeline is short, the production is fast, the management is convenient and the operation cost is low.
The disadvantages are: the pipeline is located in the seabed, most of which need to be buried in the seabed soil at a certain depth, so it is difficult to check and maintain. Some pipeline sections (especially risers) in the tidal range or wave breaking zone are greatly affected by the wind and waves, tidal current, ice and so on, and sometimes may be damaged by the impact of floating objects and ships in the sea or anchoring.

Application of submarine pipeline

With the development of offshore oil and gas exploration and development from shallow sea to mid deep sea (water depth of 100-500 m), deep sea (water depth of 500-1500 m), and even ultra deep sea (water depth of more than 1500 m), submarine pipelines are gradually applied to the deep sea. The world’s deepest submarine pipeline is an independent pipeline in the eastern Gulf of Mexico, with a maximum depth of 2454m. The deepest submarine pipeline in China is Nanhai Liwan 3-1 gas pipeline, with a maximum depth of 1409 m [4].
Compared with the land, the submarine pipeline not only bears internal and external pressure, but also is affected by various load environments such as submarine current, sediment, low temperature, corrosive medium, etc. Therefore, for the steel pipe applied to the submarine pipeline, there will be higher requirements in the aspects of composition, performance, size control, etc.

Selection and technical specification of steel pipe for submarine pipeline

Subsea pipeline is usually formed by butt joint of seamless pipe or welded pipe as the main pipe. Welded pipe is used in submarine pipeline construction with its excellent material performance, large diameter pipe making ability, high dimensional accuracy and good delivery mode perceived by users, and shows certain advantages in its construction.

The high maintenance cost of submarine pipeline requires the safety of pipeline

Once the submarine pipeline fails, there will be more maintenance and environmental treatment costs, so the performance stability and matching of materials for submarine pipeline are required. Generally, subsea pipeline is based on API Spec 5L (Appendix J) and dnvgl-st-f101 standard. The standard puts forward the upper and lower limit requirements for the tensile properties of marine welded pipe in service to maintain the stability of performance. For example, if the grade is x60mo or above, Rt0.5 is required to be controlled over 120 MPa, RM is required to be controlled over 225 MPa, as shown in Figure 1 [5-6]. In the practical engineering application, the requirements are more strict, such as the yield strength of x65mo / x70mo steel grade of the South China Sea Liwan deepwater pipeline is required to be controlled over 120 MPa, and the tensile strength is required to be controlled over 190 MPa, so as to improve the pipeline’s isostrength and reduce the risk of the pipeline’s invalidity during the service period.

20200409214506 83665 - Application of welded pipe in submarine pipeline
Figure.1 Requirements of API Spec 5L / dnv-os-f101 for tensile properties of submarine pipeline

Requirements for composition and size of pipe materials in offshore construction environment

Submarine pipelines are usually laid by laying vessels. Due to the influence of the environmental conditions and rental costs of the laying on board, the cost pressure of the construction process often requires to improve the efficiency of the laying operation to shorten the construction period, and the butt welding of the steel pipe on board is one of the key factors affecting the construction efficiency. Therefore, in the product specification of welded pipe for marine use, there are strict requirements for carbon equivalent (cepcm, ceiiw) and pipe end size (ovality) that affect weldability. The PCM of commonly used x65mo steel shall not exceed 0.22%. When the diameter of welded pipe is 610 mm < D ≤ 1422 mm, the ovality of pipe end shall be less than 8 mm. See Table 1 for the requirements of different standards for carbon equivalent and pipe size of pipe components.
In the actual project, there are also strict requirements according to the environmental conditions of the project, such as the world’s longest Beixi pipeline project, the PCM requirement is 0.21%, and the ovality of Φ 1219mm welded pipe is not more than 5.0mm. For the deepest Nanhai Liwan 3-1 submarine pipeline project in China, the PCM of welded pipe is required to be 0.17%, and the ovality of Φ 762mm welded pipe end is required to be no more than 3.5mm. The average ovality of actual control pipe end of welded pipe supplied by Baosteel UOE is 1.7mm, Cpk = 1.43, which is convenient for butt joint and welding of offshore steel pipe [7-8].
Table.1 Requirements of different standards for carbon equivalent of pipe components

20200409214854 24032 - Application of welded pipe in submarine pipeline

Table 2 requirements for pipe size of different marking values

API SPEC 5L API SPEC 5L (Appendix J) DNVGL-ST-F101 (2007)
Diameter of welded pipe D / mm Pipe end tolerance / mm Ellipticity of pipe end / mm Pipe end tolerance / mm Ellipticity of pipe end / mm Pipe end tolerance / mm Ellipticity of pipe end / mm
168.3<D≤610 005D, the maximum is not more than ± 1.6 0.005D ± 0.5 or 0.005d, whichever is greater, but the maximum is not more than ± 1.6 When D/t≤ 75, it is 0.01d, when D/t> 75, it is determined by negotiation It is ± 1.6 when 320 < D ≤ 1422 0.01D
610<D≤1422 ±1.6 When D /t≤ 75, it is 0.01d, but the maximum is no more than 13; when D /t> 75, it is determined by negotiation ±1.6 When D/t≤ 75, it is 0.0075d, but the maximum is no more than 8; when D/t>75, it is determined by negotiation 0075d for 610 < d < 1066.7 and 8 for 1066.7 ≤ d < 1422
D>1422 Determined through consultation

After forming and welding, the welded pipe should be calibrated or expanded, and the tolerance control of outer diameter and dimension is higher. In addition, the tolerance control of wall thickness is smaller when the hot-rolled steel coil and thick plate are used as raw materials. The inner diameter of welded pipe can be controlled according to the outer diameter and wall thickness, which is beneficial to the long-distance submarine pipeline. In the design of Beixi submarine pipeline, which is the longest in the world, the inner diameter of the whole pipeline is required to be 1155mm constant, while in the first section, middle section and last section of the pipeline, 34.6mm, 30.9mm and 26.8mm different wall thicknesses are used to consider the pressure loss, so as to meet the requirements of Engineering diameter, operation and maintenance.

The length of HFW welded pipe is delivered according to the user’s requirements, and the length of UOE welded pipe is also applied for thick plate raw materials according to the user’s requirements, so the welded pipe can achieve the delivery of more than 95% of the fixed length according to the maximum pipe length required by the user, minimize the number of girth welds in offshore construction, and save costs.

Large diameter, high class and thick wall pipes become the choice direction of submarine pipelines

With the development of a large number of oil and gas resources in the deep sea and the gradual improvement of pipeline transportation capacity, large-diameter, high class thick wall welded pipes are widely used in subsea pipelines. The world’s longest Beixi subsea pipeline project has been applied to welded pipes with a diameter of 1219 mm (48 in), x70mo steel grade and a wall thickness of 26.8 mm / 30.9 mm / 34.6 mm, and the inner diameter of the whole pipeline is consistent. The outer diameter of the welded pipe used in the South China Sea Liwan 3-1 deepwater submarine pipeline is 762 mm, the steel grade is X65 / X70 Mo, and the wall thickness is up to 31.8 mm, so as to increase the transportation capacity.
At present, the main HFW welded pipes used for submarine pipelines are 219-610 mm in outer diameter, 12-19.1 mm in wall thickness and x65mo in steel grade. The longitudinal submerged arc welded pipe is mainly 508-1219 mm in outer diameter, 20-41 mm in wall thickness and X65 / x70mo in steel grade.

The extension of the pipeline to the deep sea contributes to the demand for thick walled pipes

With the increase of the laying depth of the submarine pipeline, the collapse failure of the pipeline in the deep sea environment is one of the main forms of the failure of the offshore pipeline. Once the pipeline is partially collapsed, it will induce the buckling propagation and lead to the overall failure damage. Pipeline collapse is mainly affected by initial structural defects and complex loading conditions in deep water environment. In reference [9-10], the reliability analysis of initial ovality and diameter thickness ratio (D / T) and other parameters of the pipeline is carried out, and it is found that the increase of ovality reduces the reliability of the pipeline; the increase of diameter thickness ratio reduces the reliability of the pipeline. The selection of thick wall pipe with small diameter thickness ratio can enhance the ability of resisting buckling and increase the reliability of the pipe. The same is true for the laying depth to diameter thickness ratio of some submarine pipelines that have been laid in the world, as shown in Table 2. Therefore, with the increase of laying depth, the pipe diameter is decreasing, but the diameter thickness ratio is significantly reduced, and thick wall pipe will become the main field of deep-sea pipeline application [11].

Engineering project Depth/mm External diameter/mm Thickness/mm Dlt
Gulf of Mexico ITP pipeline 2 412 610 24.13-34.29 17.9
Black Sea 2 160 610 31.8 19.2
Gulf of Mexico 1 600 457 19.1-20.6 22.2
Nanhai Liwan 3-1 deepwater pipeline 1 409 762 28.6/30.2/31.8 24
Mediterranean Sea 610 508 19.1-20.6 24.7
The North Sea 365 1 016 26.1-31.3 33.5
Nord Stream 210 Inner diameter 1 153 26.8/30.9/34.6 35.3

Table.2 Laying depth and diameter thickness ratio of submarine pipeline

Higher requirements for corrosion resistance of submarine pipeline materials

The submarine pipeline is affected by the complex environment factors such as current, quicksand, low temperature and corrosive medium, which put forward higher requirements for the pipeline’s low temperature toughness, crack sensitivity (CTOD), corrosion resistance (HIC, SSC), etc. Generally, δ CTOD of welded pipe weld is required to be ≥ 0.15 mm. For the environment with corrosive medium, HIC and SSC tests are required according to the specification to enhance the service life of the pipeline.

Technical discussion on Application of welded pipe in submarine pipeline

Control of longitudinal and transverse performance of pipe body

Due to the characteristics of the construction process and the service environment of the submarine pipeline, the upper and lower bend sections of the pipeline are subject to large axial tension, and are affected by the transverse external forces such as current during the service period. Therefore, special requirements will be put forward for the transverse and longitudinal performance of the welded pipe used in the submarine pipeline.
At present, high frequency straight seam welded pipe (HFW) and straight submerged arc welded pipe (SAW) are the main welding pipes used in submarine pipeline. The tube making process is formed by bending the steel coil / plate horizontally, which will cause the work hardening effect of the material; while in the process of making the transverse specimen of the tube body, it needs to be flattened to induce the Bauschinger effect. Therefore, the transverse yield strength of the pipe body is affected by the pipe making method, the diameter thickness ratio, the inherent characteristics of the material and so on, while the longitudinal performance is not significantly affected. According to the comparative analysis of welded pipes (Φ 508mm, steel grade x65m) of the same material and different pipe making methods (JCOE and UOE) in reference [12], it is found that the transverse yield strength of welded pipes formed by JCOE is higher than that of welded pipes formed by UOE, and the yield strength ratio of welded pipes formed by JCOE is also higher than that of UOE, which shows that the work hardening effect of JCOE is stronger than that of UOE. The analysis of HFW welded pipe with different diameter thickness ratio shows that with the decrease of diameter thickness ratio, the transverse yield strength of HFW welded pipe gradually approaches to the longitudinal [13].
Table.3 is the mechanical property data of welded pipe which has been used in submarine pipeline engineering. From the mean value, the mean value of transverse yield strength is slightly lower than that of longitudinal, while the transverse tensile strength is slightly higher than that of longitudinal, showing the result that the longitudinal yield strength ratio is slightly higher than that of transverse yield strength ratio. Therefore, the longitudinal yield strength ratio is usually 0.02 higher than the transverse yield strength ratio in design application. This is closely related to the Bauschinger effect and work hardening effect of thick wall welded pipe materials in the process of pipe making and testing.

Project Depth/m External diameter/mm Thickness/mm D/t
Gulf of Mexico ITP pipeline 2412 610 24.13-34.29 17.9
Black Sea 2160 610 31.8 19.2
Gulf of Mexico 1600 457 19.1-20.6 22.2
Nanhai Liwan 3-1 deepwater pipeline 1409 762 28.6/30.2/31.8 24
Mediterranean sea 610 508 19.1-20.6 24.7
North sea 365 1016 26.1-31.3 33.5
Nord Stream 210 Inner diameter 1153 26.8/30.9/34.6 35.3

Table.3 Mechanical performance index of welded pipe for submarine pipeline

Transverse tensile properties (mean) Longitudinal tensile properties (mean)
Project Specification / mmxmm Grade Molding process Raw material Rt0.5/Mpa Rm/Mpa Rt0.5/Rm Rt0.5/Mpa Rm/Mpa Rt0.5/Rm
Nord Stream Φ1219×30.9 X70MO UOE Thick plate 526.2 615.3 0.86 522.5 592.5 0.88
Φ1219×34.6 X70MO UOE Thick plate 511.5 610.6 0.84 520.7 591 0.88
Nanhai Liwan 3-1 Φ762×28.6/30.2 X65MO UOE Thick plate 480 587 0.82 497 568 0.87
Φ762×38.1 X70MO UOE Thick plate 514 622 0.83 525 605 0.87
Fanyu – Huizhou submarine pipeline Φ508×14.3 X65MO UOE Uncoiling of hot rolled coil 506 603 0.83 541 575 0.91
EPCI project of Yuedong Oilfield Φ406.4×19.1 X65MO HFW Hot rolled coil 520.7 610.1 0.85 523.5 592.8 0.88

Table 4 mechanical property index of welded pipe for submarine pipeline

Buckling resistance of submarine pipeline

The submarine pipeline is subjected to multi-directional loads such as external pressure and current, and the failure caused by pipeline bending is one of the problems that must be considered. In addition to the use of thick walled, high-precision welded pipe to improve the collapse resistance, the maximum bending capacity (Ultimate compression buckling strain) of the pipe is concerned. It is considered that the main factors affecting the ultimate compressive strain are the defects of the base metal (staggered edge, uneven wall thickness, etc.), internal pressure and the stress-strain curve of the pipe. The influence of literature on different materials has been tested and analyzed [14]:

  • (1) The critical buckling strain increases with the internal pressure. The analysis of different material curves shows that when there is no internal pressure in the pipeline, the axial stress-strain curve is closer to the actual situation; when there is internal pressure in the pipeline, the circumferential stress-strain curve is closer to the actual situation. When the internal pressure increases, the ultimate compression strain of the pipeline also increases.
  • (2) The effect of stress-strain curve on the buckling of pipeline. For materials with yield plateau stress-strain curves, the buckling behavior of pipes is mainly caused by the yield plateau. In the arc stress-strain curve, the pipeline has elastic-plastic buckling, and the buckling behavior is affected by the hardening index. The hardening index is large and the compressive strain resistance is strong.
  • (3) The larger the diameter thickness ratio is, the smaller the ultimate compressive strain is.

This is also one of the problems that need to be studied and discussed when using welded pipe materials for submarine pipelines.

The effect of strain aging on the application of submarine pipeline

From the construction and installation to the service of submarine pipeline, it is not only affected by external loads, but also by environmental temperature changes. It is helpful for engineering application to master the strain timeliness of pipeline materials.
The strain aging characteristics of x65mo welded pipe (T / D 4.9%) were studied in reference [15]. After artificial aging at 0.5% strain and 250 ℃ / h, it is observed that the longitudinal yield strength increases about 30-40 MPa, further increases to 1% strain, and the yield strength increases about 20 MPa. In the strain range from 1% to 2%, the yield strength basically remains unchanged. When aging without strain, the average yield strength ratio is 0.85, and the maximum value is lower than 0.90. After aging with 1% – 2% strain, the average yield strength ratio is 0.94. This behavior shows the characteristics of low carbon microalloy controlled rolling and controlled cooling pipeline steel, and the aging behavior of welded pipe material in extreme marine environment is a problem that needs to be further concerned and discussed in this field.
The research on the performance control of welded pipe for subsea pipeline, the analysis of material buckling behavior and the timeliness in extreme environment will be concerned by the industry.


Welded pipe is more and more used in subsea pipeline because of its good comprehensive performance, excellent geometry size and close to application delivery mode. Its lower carbon equivalent, stable mechanical properties, smaller dimensional tolerance and ovality, and higher sizing ratio become the first choice of offshore pipeline engineering.
With the expansion of offshore engineering to the deep sea, the development and utilization of high class thick wall welded pipe will become a new application market. With the extension of laying distance of submarine pipeline, new requirements will be put forward for the size control and measurement technology, performance stability, test means and delivery capacity of welded pipe, and new opportunities will be provided for the technical development of welded pipe manufacturing.

Source: Network Arrangement – China Submarine Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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