1. Mechanical performance requirements

In terms of strength

Compressive strength: The deep-sea environment where underwater oil pipelines are located has enormous pressure, and LSAW Steel Pipes need to have sufficient compressive strength. As the depth of seawater increases, the external pressure increases linearly (for every 10 meters decrease, the water pressure increases by about 1 atmosphere), and steel pipes must be able to withstand the pressure generated by seawater at depths of thousands of meters. For example, at a depth of 3000 meters in the deep sea, the water pressure is about 300 atmospheres, and the yield strength of steel pipes is usually above 450 MPa to prevent the pipes from being flattened or broken.

Tensile strength: During pipeline laying and operation, it is also necessary to withstand a certain amount of tensile force. For example, during the process of dragging and laying pipelines, steel pipes have to withstand the pulling force from tugboats; When pipelines undergo thermal expansion and contraction or are affected by changes in seabed topography, tensile forces can also be generated. Therefore, the tensile strength of LSAW steel pipes is generally required to be around 500 and 600 MPa to ensure the integrity of the pipeline.

Resilience and fatigue resistance performance

Resilience requirement: The underwater environment is complex and ever-changing, with various factors that may cause pipeline damage, such as underwater earthquakes, iceberg movement, and ship breakdowns. LSAW steel pipes need to have good toughness to resist sudden external impacts and avoid brittle fracture. Its impact toughness index (such as Charpy V-notch impact absorption energy) is generally required to be no less than 100J in low-temperature seawater (such as 20 ℃) to ensure that the steel pipe can maintain good performance in harsh environments.

Fatigue resistance: Due to the periodic effects of waves, currents, and internal oil flow over a long period of time, pipelines will experience fatigue stress. The weld quality and overall structure of LSAW steel pipes should ensure that they can withstand fatigue loads for at least 20 to 30 years (the design service life of general subsea oil pipelines). The fatigue strength at the weld seam should not be lower than 80% of the base metal, and it should be verified through rigorous fatigue tests, such as pressure cycling and bending cycling tests that simulate actual working conditions.

2. Corrosion resistance requirements

Protection against seawater corrosion:

Seawater is a highly corrosive medium that contains a large amount of salt (mainly sodium chloride), dissolved oxygen, and microorganisms. The outer surface of LSAW steel pipes requires an efficient anti-corrosion coating to resist seawater corrosion. It is common to use a three-layer polyolefin (3PE) coating, where the epoxy powder on the bottom layer has good adhesion to the surface of the steel pipe, and the adhesive in the middle can enhance the bonding force with the outer polyolefin layer. The outer polyolefin layer provides good resistance to seawater corrosion and wear. The thickness of the coating is generally required to be between 2.5 and 3.7mm, and the integrity of the coating must be ensured without defects such as pinholes and bubbles.

At the same time, it is necessary to cooperate with the cathodic protection system, usually using sacrificial anode protection method. Install magnesium alloy or zinc alloy sacrificial anodes on the surface of steel pipes, with a more negative potential than the steel pipe, to provide cathodic protection current for the steel pipe by consuming the anode material. The installation spacing and quantity of sacrificial anodes should be accurately calculated based on the seawater environment in which the pipeline is located (such as water resistance, flow velocity, etc.). Generally, 35 sacrificial anodes need to be installed per kilometer of pipeline.

Internal oil flow corrosion protection:

The transported oil may contain corrosive components such as sulfides, carbon dioxide, and water. The inner wall of LSAW steel pipes needs to be coated with oil resistant and chemically corrosion-resistant materials. For example, liquid epoxy coating is a commonly used internal anti-corrosion coating, which has good chemical stability and wear resistance. The coating thickness is generally between 300 and 500 μ m, and it should have good adhesion to the inner wall of the steel pipe to prevent peeling under long-term oil flow erosion. At the same time, the flow velocity design inside the pipeline also needs to consider corrosion factors to avoid excessive flow velocity causing corrosion to intensify.

3. Dimensional accuracy and welding quality requirements

Dimensional accuracy requirements:

The installation and connection of underwater oil pipelines require high-precision steel pipes. The diameter deviation is generally required to be controlled within ± 0.5%, and the wall thickness deviation is controlled within ± 10%. Such precision requirements can ensure the tightness of pipeline connections on the seabed, such as achieving good sealing during flange or welding connections to prevent oil spills.

The length of steel pipes also needs to be precisely controlled, as the length of each section of steel pipe can affect the efficiency and accuracy of laying underwater pipelines. Usually, the length error of LSAW steel pipes should be controlled within ± 50mm.

Welding quality requirements:

For straight seam welding of LSAW steel pipes, the quality of the weld seam is crucial. Welds must undergo 100% non-destructive testing, including ultrasonic testing (UT), radiographic testing (RT), etc. Defects such as cracks, lack of fusion, and slag inclusion are not allowed inside the weld seam. The excess height of the weld seam (the height of the weld metal beyond the surface of the base metal) is generally controlled between 13mm, and the transition between the weld seam and the base metal should be smooth to reduce stress concentration. At the same time, the selection of welding materials should match the base metal to ensure that the strength and toughness of the weld are not lower than those of the base metal.

4. Requirements for stability and operability of pipelines

Stability requirements:

Subsea oil pipelines need to remain stable on the seabed to avoid displacement or suspension due to factors such as ocean currents and waves. The weight and diameter of LSAW steel pipes should be designed according to the geological conditions of the seabed, such as soil type, slope, etc. For example, on soft seabed soil, it may be necessary to increase the weight of the pipeline or use special fixing devices (such as concrete counterweights or pile foundations) to ensure the stability of the pipeline.

The bending radius of the pipeline also needs to be designed reasonably, taking into account both the bending requirements during pipeline laying (such as bypassing underwater obstacles) and ensuring the safety of the pipeline during operation. The minimum bending radius of submarine oil pipelines is generally not less than 50 times the pipe diameter.

Operability requirements:

During the pipeline laying phase, the connection method of LSAW steel pipes should be convenient for construction operations. For example, when using welding connections, the welding process should be adapted to the underwater environment and can be efficiently carried out underwater or on a pipeline laying vessel. At the same time, both ends of the steel pipe need to have suitable interface designs to facilitate connection with other pipelines or equipment (such as valves, pump stations, etc.). In addition, the marking and identification of pipelines should be clear, facilitating the identification of the location, orientation, and properties of pipelines during installation and maintenance.