1. High strength and toughness

Compressive strength requirement: In deep-sea environments, water pressure increases sharply with depth. For example, at a depth of 1000 meters, the water pressure is about 100 atmospheres, and at a depth of 3000 meters, the water pressure can reach around 300 atmospheres. Therefore, LSAW Steel Pipes need to have extremely high compressive strength to prevent the pipes from collapsing or breaking. Generally speaking, its yield strength should be above 400-600MPa, and its tensile strength should be above 500-700MPa to withstand the enormous water pressure in deep sea.

Resilience index requirement: The temperature of seawater decreases with increasing depth, and deep-sea environments are usually in a low temperature state. Steel pipe materials are prone to brittle fracture at low temperatures. To avoid this situation, steel pipes should have good toughness. In low temperature environments (such as below -20 ℃), the impact toughness should not be less than 100J/cm ² to ensure that steel pipes will not undergo brittle failure when subjected to external impacts (such as seabed geological activity, object collisions, etc.).

2. Excellent corrosion resistance

Corrosion protection of seawater: Seawater contains a large amount of salt, mainly sodium chloride, as well as various ions such as magnesium, calcium, and potassium, which can cause electrochemical corrosion of steel pipes. In addition, dissolved oxygen in seawater can accelerate the corrosion process. Therefore, LSAW steel pipes need to have good resistance to seawater corrosion, and their corrosion rate is generally required to be less than 0.05mm/year.

Localized corrosion resistance: In deep-sea environments, steel pipes are also prone to localized corrosion, such as pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC). Pitting corrosion is a small hole like corrosion formed by the destruction of the passivation film in a local area of the metal surface; Gap corrosion occurs at the connection or surface of steel pipes where there are deposits; Stress corrosion cracking is a crack that occurs under the combined action of tensile stress and corrosive media. In order to resist these localized corrosion, the material and surface treatment of steel pipes should be appropriate, such as using corrosion-resistant alloys or coating anti-corrosion coatings on the surface of steel pipes, with a coating thickness generally reaching 200-300 μ m.

3. Accurate dimensional accuracy and good roundness

Installation and connection requirements: During the installation of deep-sea pipelines, steel pipes need to be precisely connected together. Accurate dimensional accuracy and good roundness are helpful for on-site installation and welding of pipelines. The diameter tolerance of steel pipes is generally required to be controlled within ± 0.5%, and the ellipticity (roundness deviation) should be less than 1%, which can ensure the sealing and structural integrity of pipeline connections.

Requirements for external pressure stability: Good roundness is also very important for improving the external pressure stability of steel pipes. In deep-sea high-pressure environments, steel pipes with poor roundness are more prone to local deformation, thereby reducing their compressive strength. Accurate size and roundness can make the stress distribution of steel pipes more uniform when subjected to external water pressure, reducing the risk of damage caused by local stress concentration.

4. Good anti fatigue performance

The influence of marine environmental factors: Pipelines in the deep sea are subjected to various periodic loads, such as vibrations caused by waves and currents, as well as fluctuations in internal fluid pressure. These cyclic loads can cause fatigue damage to steel pipes. In order to ensure the long-term stable operation of steel pipes in deep-sea environments, their fatigue resistance is crucial.

Fatigue life requirement: Through fatigue testing evaluation, steel pipes should be able to withstand a certain number of cyclic loads (such as 10 ⁷ -10 ⁸ times) without fatigue failure during their designed service life (such as 20-30 years). This requires optimization in the material selection and manufacturing process of steel pipes, such as controlling the surface quality of steel pipes and reducing surface defects, as small cracks or scratches on the surface can become the origin of fatigue cracks.

5. Reliable welding quality

Performance requirements for welded joints: LSAW steel pipe welded joints in deep-sea environments should have strength, toughness, and corrosion resistance equivalent to the base metal. The tensile strength of the welded joint should not be less than 90% of the base material, and the impact toughness should also meet the requirements for use at low temperatures. At the same time, the welded joint should ensure good sealing, without defects such as pores, slag inclusion, and incomplete welding, to prevent seawater from entering the interior of the pipeline, corroding the steel pipe or causing leakage.

Welding process control requirements: During the welding process, welding parameters such as welding current, voltage, welding speed, etc. must be strictly controlled. For steel pipes used in deep sea, the welding heat input should be controlled within an appropriate range, generally 10-30kJ/cm, to avoid the coarsening of the microstructure in the welding heat affected zone, which affects the performance of the welded joint. In addition, strict non-destructive testing should be conducted after welding, including ultrasonic testing, radiographic testing, etc., to ensure that the welding quality meets the high standard requirements of deep-sea environments.

The performance differences of LSAW steel pipes with different wall thicknesses in deep-sea environments are as follows:

Mechanical properties

Pressure resistance: Thick walled LSAW steel pipes can withstand higher external water pressure, such as in the deep sea of 3000 meters. Steel pipes with a wall thickness of over 30mm have stronger resistance to water pressure and can effectively avoid being crushed. Thin walled steel pipes have relatively weak compressive strength in the same environment. When the wall thickness is less than 10mm, it may be difficult to withstand deep-sea high pressure.

Anti deformation ability: Thick walled steel pipes are less prone to deformation when subjected to external impacts or stress caused by seabed geological activities. For example, in situations such as underwater earthquakes, thick walled pipes can maintain a good shape, while thin-walled steel pipes are prone to local deformation, which affects the normal use and sealing of the pipeline.

Corrosion resistance performance

Comprehensive corrosion: Thick walled steel pipes have a relatively long service life in seawater due to their large corrosion allowance. If the same material and anti-corrosion coating are used for thin-walled steel pipes and thick walled steel pipes, thin-walled steel pipes may experience a dangerous reduction in wall thickness due to corrosion in a short period of time due to their small corrosion allowance.

Localized corrosion: As the wall thickness increases, localized corrosion such as pitting and crevice corrosion in thick walled steel pipes becomes more difficult to penetrate the pipe wall. Once local corrosion occurs in thin-walled steel pipes, they are more likely to quickly penetrate the pipe wall, causing accidents such as leaks.

Fluid transport performance

Resistance characteristics: When conveying fluids of the same flow rate, the inner diameter of thick walled steel pipes is relatively small. According to the principles of fluid mechanics, the flow velocity of the fluid inside the pipe will increase, which may lead to an increase in resistance. Thin walled steel pipes have a relatively large inner diameter, low fluid flow velocity, and relatively low resistance, which is beneficial for reducing transportation energy consumption.

Flow restriction: For a given pipeline system, the maximum allowable flow of thick walled steel pipes may be limited due to their smaller inner diameter. Thin walled steel pipes can transport larger flow rates under the same pressure conditions.

Installation and construction performance

Weight and handling: Thick walled steel pipes are heavy and require larger equipment and more manpower during transportation and installation. For example, in deep-sea pipeline laying, large crane ships and professional laying equipment are required to lift and install thick walled steel pipes. Thin walled steel pipes are relatively lightweight, easy to transport and install, and can reduce construction difficulty and costs.

Welding difficulty: The welding of thick walled steel pipes requires higher welding techniques and more welding passes. During the welding process, welding parameters must be strictly controlled to ensure the quality and performance of the weld seam. Thin walled steel pipe welding is relatively simple, with fast welding speed and a relatively low probability of welding defects.

What deep-sea environmental factors should be considered when selecting the wall thickness of LSAW steel pipes?

1. Water pressure factor

The water pressure intensity corresponding to depth: In deep-sea environments, water pressure significantly increases with depth. For example, at a depth of 1000 meters, the water pressure is about 100 atmospheres; At a depth of 3000 meters, the water pressure can reach around 300 atmospheres. Therefore, it is necessary to choose the appropriate wall thickness based on the actual depth of steel pipe laying. Generally speaking, the deeper the laying depth, the greater the water pressure it needs to withstand, and a thicker steel pipe wall is required to resist the water pressure and prevent the steel pipe from being crushed or broken.

Safety factor consideration: In order to ensure the long-term safe and reliable operation of steel pipes in deep-sea environments, a certain safety factor also needs to be considered. This safety factor is usually determined based on the importance of the project, the service life of the steel pipe, and the ability to withstand risks. For example, for LSAW steel pipes that transport important energy resources such as subsea oil and gas pipelines, the safety factor may be set higher, and accordingly, thicker wall thicknesses may be selected to cope with possible abnormal fluctuations in water pressure.

2. Corrosive environmental factors

Corrosion degree of seawater: Seawater contains a large amount of salt, such as sodium chloride, as well as various ions such as magnesium, calcium, and potassium, which can cause electrochemical corrosion of steel pipes. Meanwhile, dissolved oxygen in seawater accelerates the corrosion process. If it is expected that the seawater in the deep-sea area where the steel pipe is located is highly corrosive, or if there are some special corrosion factors (such as acidic environments near hydrothermal vents), it is necessary to increase the wall thickness appropriately to provide sufficient corrosion allowance.

Localized corrosion risk: In deep-sea environments, steel pipes are prone to localized corrosion, such as pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC). Pitting corrosion is a small hole like corrosion formed by the destruction of the passivation film in a local area of the metal surface; Gap corrosion occurs at the connection or surface of steel pipes where there are deposits; Stress corrosion cracking is a crack that occurs under the combined action of tensile stress and corrosive media. Considering the risk of localized corrosion, increasing wall thickness can prolong the safe use time of steel pipes after localized corrosion occurs.

3. Factors affecting the medium of pipeline transportation

Medium pressure: If LSAW steel pipes are used to transport high-pressure media (such as high-pressure oil and gas in subsea oil and gas fields), in addition to considering external water pressure, the effect of internal medium pressure on the pipe wall also needs to be considered. The combined effect of internal medium pressure and external water pressure may cause significant stress on the pipe wall. In this case, it is necessary to choose the appropriate wall thickness according to the magnitude of the internal medium pressure to ensure that the steel pipe can withstand this internal and external pressure difference.

Corrosivity of the medium: The corrosiveness of the transport medium itself is also a factor that needs to be considered. For example, when transporting oil and gas containing acidic components such as carbon dioxide, hydrogen sulfide, etc., these acidic components may react chemically with the inner wall of the steel pipe, leading to corrosion. If the medium is highly corrosive, increasing the wall thickness appropriately can enhance the corrosion resistance of the steel pipe and extend its service life.

4. Installation and construction factors

Installation methods and equipment limitations: The installation methods of steel pipes in deep-sea environments mainly include laying by pipeline laying ships and installation by underwater robots. Different installation methods have different requirements for the weight and size of steel pipes. If the load-bearing capacity of the installation equipment is limited, too thick steel pipes may not be able to be installed smoothly. Therefore, when choosing wall thickness, it is necessary to consider the installation method and the load-bearing capacity of existing installation equipment.

Construction difficulty and cost: Thicker steel pipe walls will increase the weight of the steel pipes, which will increase the difficulty and cost during transportation and installation. For example, in the process of laying deep-sea pipelines, large crane ships and professional laying equipment are required to lift and install heavier steel pipes. Meanwhile, the welding difficulty of thick walled steel pipes is relatively high, requiring higher welding techniques and more welding passes, which will also increase construction costs. Therefore, it is necessary to comprehensively consider the construction difficulty and cost to select the appropriate wall thickness while ensuring that the performance of the steel pipe meets the requirements of deep-sea environment.

5. Marine Geology and Environmental Factors

Submarine geological activity: If the underwater geological activity in the steel pipe laying area is frequent (such as earthquakes, volcanic activity, submarine landslides, etc.), the steel pipe may be subjected to significant external forces. In this case, increasing the wall thickness appropriately can improve the impact resistance and deformation resistance of the steel pipe, and reduce the risk of damage caused by geological activities.

Marine organisms and water erosion: The attachment of marine organisms may alter the surface condition of steel pipes, increase the risk of local corrosion, and may also affect the weight and stability of steel pipes. In addition, strong water flow erosion may cause wear and vibration on steel pipes, increasing the risk of fatigue failure. Considering these factors, when selecting wall thickness, it is necessary to comprehensively evaluate the impact of marine organisms and water flow erosion, and appropriately increase the wall thickness to improve the anti erosion and anti fatigue ability of steel pipes.