What Are Support Hose?

For 500ft jack-up drilling platforms, most of the seamless steel pipes for leg support pipes are large-diameter thick-walled pipes, and the steel grades are generally super high-strength steel pipes of NVE550, X80 and above.

At present, JU2000E is one of the most advanced jack-up drilling platform designs in the world. The leg is the key equipment of the jack-up drilling platform. It consists of chords, racks and support tubes. Its role is to lift and support the entire platform for drilling operations. The support pipe adopts a larger wall thickness and a smaller pipe diameter-to-wall thickness ratio in the structure, and X80 steel grade seamless steel pipe is used in the material selection to ensure the load capacity, reduce the platform's own weight, reduce the moving resistance, and weld with the chord tube. Form X-shaped or K-shaped pipe joints to form a trussed high-strength bearing structure. Taking into account the safety and reliability of platform pipes in extremely harsh marine environments (such as marine storms, variable loads, seawater corrosion, accidental impacts and fires).

Support tube test material

A domestic steel pipe company developed and produced the JU2000E jack-up drilling platform X80 steel-level pile leg support pipe, which solved many key technical problems such as material composition design, high-purity homogenization smelting-continuous casting process, high-precision pipe rolling process, and heat treatment process. , And the product batch production performance is stable, various performance indicators reach or exceed similar foreign products. The company chose to use the 2,000.85mm × 28.58mm specifications of the leg support pipe of the JU2000E jack-up drilling platform. The yield strength Rt0.5 and tensile strength Rm of the support pipe were 610MPa and 700MPa, respectively, and the elongation A after breaking was 28%, the reduction in area is 85%, and the average full-scale impact energy at -40 ° C is 251J.

Metallographic structure and low temperature toughness of support tube

1. Metallographic organization

X80 steel grade leg support pipe adopts quenching + high temperature tempering quenching and tempering heat treatment process. X80 steel grade leg support pipe is quenched at 960 +650 to form fine granular bainite and acicular ferrite as The matrix, a multi-phase structure with a small amount of M / A component brittle phases and a large number of dispersed fine precipitated particles, the grain size can reach more than 9 grades. This heterogeneous structure comes from steel grades designed with reduced carbon, low carbon equivalent, multiple alloying, and microalloying. It has low temperature impact resistance, fatigue performance, and hydrogen embrittlement resistance for X80 steel grade leg support pipe materials. It has a positive meaning.

2.Low temperature toughness

The ductile-brittle transition temperature FATT determines the low-temperature performance of the steel and is an important indicator for measuring the brittle transition of steel. Its size directly affects the use range of steel. The ductile-brittle transition temperature of X80 steel grade leg support pipe is lower than -80 . This is related to the design of its steel composition: lower carbon content is beneficial to reduce the number of brittle phases of the M / A component; the addition of Ni element is helpful to improve the low temperature toughness; and Nb and Ti element is helpful to improve the toughness. In addition, the heat treatment process of quenching and high temperature tempering can refine the structure. The above process also makes the X80 steel-level pile leg support pipe produced by the steel pipe company have a lower toughness-brittle transition temperature, which can be used in a lower temperature environment and has better safety.

3. Performance

(1) Welding performance

The supporting structure of the pile leg of the jack-up drilling platform is an important force-bearing structure, and the pile leg supporting pipe must have good girth welding performance. Welding procedure qualification tests were performed in accordance with the relevant welding codes of the American Welding Society (AWS) and American Bureau of Shipping (ABS). The test results show that the strength, toughness and hardness of all girth welded joints meet the requirements of relevant technical conditions under the conventional welding manufacturing conditions of the jack leg supporting structure of the jack-up drilling platform of X80 steel grade leg support pipe. Welding performance.

(2) Fatigue performance

According to GB / T 30752008 "Method for Controlling Axial Force of Fatigue Test of Metallic Materials", in PLG-200 fatigue testing machine and series of symmetrical tensile and compression loads (stress ratio R = -1), similar frequency, maximum number of cycles 1 × The fatigue performance of the X80 steel leg support pipe was tested and evaluated under the conditions of 10 ⁷ . The fatigue performance test data of the X80 steel leg support pipe shows that when the stress amplitude S is 384 MPa, the sample has failed when the cycle does not reach 1 × 10 ⁷ ; when S is reduced to 369 MPa, the sample is cycling When the week reached 1 × 10 ⁷ , none of them failed. Because the difference between the stress amplitudes of the two sets of tests is less than the fatigue failure ratio of the high-strength structural steel, the fatigue limit ratio of the high-strength structural steel ( ^-1 / Rm) is usually 0.3 to 0.5. High, reaching 0.54; therefore, X80 steel leg support tube material has excellent fatigue performance.

According to the fatigue fracture morphology, the reasons for the excellent fatigue performance of the X80 steel leg support pipe are believed to be:

(1) X80 steel grade leg support pipe steel has high purity, few inclusions, small particles in the second phase, and it is difficult to initiate fatigue cracks;

(2) The grain size of the X80 steel leg support pipe is small and the number of grain boundaries is large, which has a significant hindrance to the fatigue crack propagation in the first stage.

3.Hydrogen embrittlement resistance

Structural steel pipes above X80 are sensitive to hydrogen embrittlement. Hydrogen-induced cracks have been found in the leg support pipe, which caused the collapse of the offshore platform. For long-term fixed drilling platforms in the seawater environment, it is difficult to detect hydrogen-induced cracking of structural pipes, and special attention must be paid to the hydrogen embrittlement of materials. HSE recommends that the offshore high-strength structural steel pipe should use the slow strain rate tensile test of the "seawater + CP + sulfide" environment to test the hydrogen embrittlement index EI, and the offshore high-strength structural steel pipe should meet the EI60% requirement. EI is defined by formula: EI = (RAa-RAe) / RAa

In the formula, RAa Sectional area shrinkage measured by stretching the material in air environment,%;

RAe Section shrinkage measured by tensile test of material in test environment,%.

According to the above method, the resistance to environmental hydrogen embrittlement of the X80 steel leg support pipe material was tested and evaluated. The seawater in the test environment uses natural seawater; the selection of CP potential (-1100mV) refers to relevant specifications and engineering requirements; H ₂ S is used to simulate the biological sulfide caused by sulfate reducing bacteria SRB, and the actual concentration of the latter in seawater is referred (Generally 0.06%), the concentration range of H ₂ S is selected; the tensile strain rate is 4.0 × 10 ^-6 s ^-1 based on previous work experience. The results show that the X80 steel-level pile leg support pipe is in the "sea water + CP potential -1100 -750mV (Ag / AgCl) + no sulfide" environment and "sea water + CP potential -1000 m V (Ag / Ag Cl) +0.01 % 0.06% sulfide environment hydrogen embrittlement index meets the requirements of EI60%, that is, X80 steel grade leg support pipe material has a lower tendency to hydrogen embrittlement, which can meet the offshore platform The requirement of material's resistance to hydrogen embrittlement has excellent resistance to hydrogen embrittlement. The related mechanism of X80 steel grade leg support pipe for hydrogen embrittlement is:

(1) X80 steel grade leg support pipe steel has high purity, small number of inclusions and small size, thus reducing the source of hydrogen-induced cracks

(2) After quenching + high temperature tempering, the X80 steel grade leg support tube has formed a fine granular bainite and ferrite matrix structure, and a large amount of dispersed alloy carbide particles and a few dislocations are distributed on the matrix The number of hydrogen traps is large, and the diffusion channels of hydrogen are reduced, which greatly reduces the hydrogen absorption of steel.

4. Anti-collision performance

Pile leg support pipes for offshore platforms may be subject to accidental collisions by tankers and engineering ships during service. According to calculations, when a ship collides, the pile leg support tube undergoes deformation at a rate of 10 ^-2 to 10 s ^-1 . Therefore, the tensile test was used to evaluate the anti-collision performance of the X80 steel leg support pipe material by testing the dynamic tensile properties. The test results show that when the tensile strain rate is 0.002 to 5s ^-1 , the yield strength, tensile strength, and strain strength (Rp2 and Rp5) of the X80 steel leg support pipe increase with the increase of the strain rate, and the elongation decreases after breaking. In addition, the yield strength and tensile strength are 612-670 MPa and 695-790 MPa, respectively, which still meet the requirements of the standard. Therefore, the X80 steel leg support pipe material has good anti-collision performance.

5, fire resistance

Support pipes for offshore platforms may also serve in high temperature environments caused by oil pool flames, jet flames, or accidental fires, and the loss of material strength may cause partial or complete collapse of pile legs. Therefore, the structural tube can maintain a high strength under short-term (1 to 3h) high temperature conditions, which is sufficient for all personnel to evacuate, which is an important safety technical indicator. In the refractory design, based on the analysis of the heat conduction and thermal deformation process in the steel when the fire occurs, the refractory temperature of the refractory steel is generally set at 600 ° C, and the refractory steel should comply with 600 ° C. The high temperature strength Rp600 is not lower than 2/3 of the room temperature strength Rp.

That is, Rp600 2 / 3Rp, or the strength coefficient SF = Rp600 / Rp2 / 3. With reference to GB / T 43382006 "Methods for High Temperature Tensile Tests of Metallic Materials", the yield strength and strength coefficient of X80 steel grade leg support pipes decrease with the increase of ambient temperature, but the temperature of the steel at 100 600 The high temperature strength is not less than 2/3 of the room temperature strength, that is, the strength coefficient is not less than 2/3; especially the strength coefficient reaches 0.70 at an ambient temperature of 600 , which is higher than the relevant technical requirements of refractory steel. Therefore, the X80 steel leg support pipe material has excellent fire resistance.

The fire resistance mechanism of the X80 steel leg support pipe is that the material of the leg support pipe adopts Mo-Nb-V alloy design, and the content of Mo, Nb and V is actually controlled at 0.20%, 0.04% and 0.08, respectively. Compared with ordinary refractory steel, the Mo content of X80 steel is equivalent, but the total content of Nb and V is two to three times that of the former, so a greater amount is observed near the high temperature tensile fracture of the steel (Nb , V) (C, N) particles.

The fine particles precipitated on the dislocations play a pinning role, which significantly delays the recovery of dislocations at high temperatures; the fine particles precipitated on the ferrite boundary prevent the ferrite from recovering. Therefore, the high-temperature strength of the X80 steel-level pile leg support pipe is significantly improved.

Support tube summary

The X80 steel grade leg support pipe for the legs of the JU2000E jack-up drilling platform has the following characteristics:

(1) Uniform structure, good low-temperature toughness, ductile-brittle transition temperature below -80 ; excellent welding performance; fatigue limit ratio of 0.54, excellent fatigue performance;

(2) The hydrogen embrittlement index EI is less than or equal to 60% under the test conditions. It has a low tendency to hydrogen embrittlement, and can meet the requirements for the material's resistance to hydrogen embrittlement.

(3) When the tensile strain rate is 0.002 to 5 s ^-1 , it has good anti-collision performance;

(4) The strength coefficient reaches 0.70 at an ambient temperature of 600 , which has excellent fire resistance. ^[1-2]