Standards for Seamless Steel Pipes for Submarine Reel-Laying

Abstract: This paper compares standards and specifications for seamless steel pipes used in submarine pipeline reel-laying, focusing on key parameters such as chemical composition, mechanical properties, geometric dimensions, and surface quality. It also outlines the specific requirements for carbon-manganese line pipes across different standards. The study found that all these standards include strain aging requirements and impose strict performance criteria for steel pipes after deformation. Therefore, ensuring consistent steel pipe performance after repeated bending and deformation is critical for both design and manufacturing. This study serves as a reference for the development and production of seamless steel pipes for reel-laying applications.

Submarine long-distance pipelines are a crucial means of transporting oil and gas. The reel-lay method has seen widespread adoption in recent years due to its high efficiency, cost-effectiveness, continuous laying capability, and low operational risk. In this method, pipes are wound onto reels after onshore processing and then laid on the seabed by pipe-laying vessels. Due to the plastic deformation that occurs during the reel-laying process and the stringent welding requirements, steel pipes must meet strict dimensional and performance standards. At present, no manufacturer in China can consistently supply seamless steel pipes for reel-laying.

This article compares major international standards for seamless steel pipes used in reel-laying, including DNV-ST-F101-2021 (Submarine Pipeline Systems), IOGP S616-2022 (API Spec 5L FLE and ISO 3183 Supplementary Specification), DEP 31.40.20.37-Gen-2017 (Line Pipes in Extreme Conditions), and a foreign user specification. These standards are adaptations of API Spec 5L, and comparing them provides insights into the overall requirements for carbon-manganese line pipes. In this article, DNV, IOGP, DEP, and the foreign specification refer to their respective standards.

1. Comparative Analysis of Standards

1.1 Chemical Composition

DNV’s chemical composition requirements are largely consistent with Table N.1 of API Spec 5L 46th Edition, but the hydrogen content in the final product must not exceed ≤2×10⁻⁶. IOGP expands on Table N.1 of API Spec 5L 46th Edition by introducing additional requirements for chemical composition variation ranges and setting limits on harmful elements and hydrogen content: w(Sn) ≤ 0.020%, w(Sb) ≤ 0.010%, w(Bi) < 0.010%, w(Pb) ≤ 0.010%, w(As) < 0.020%, w(H) < 2×10⁻⁶. DEP primarily refers to Appendix J of API Spec 5L 46th Edition and specifies the allowable range for chemical composition variations. A foreign specification builds upon DNV standards by introducing limits on carbon equivalent, sulfur content, and the Ca/S ratio, specifying CEIw≤0.38%, CERm≤0.21%, S≤0.003%, and Ca/S≤3. IOGP and DEP define the chemical composition variation ranges for pipes used in non-acidic environments (Table 1). The ranges for CEnw and CEPam should be symmetrically distributed around the agreed target values, with allowable deviations of CEIw ±0.030 and CEPen ±0.020. The CEN calculation results are provided for reference only and do not have specified acceptance criteria. The calculation formula is as follows:

Table 1: IOGP and DEP Requirements for the Range of Variation of Non-Acidic Pipe Composition

1.2 Mechanical Properties

Plastic deformation generally occurs during the winding of the pipe onto the reel. The primary material property requirements for the reel-laying method include uniformity in material properties before coiling, along with good impact toughness and hardness. A strain aging test is required to replicate the plastic deformation that occurs during installation and operation. Strain aging is typically conducted through tension-compression or compression-tension cyclic plastic deformation. Certain standards or technical specifications mandate a specific proportion of tensile plastic deformation. The specimen’s performance is assessed after aging at 250°C for one hour to verify compliance with the requirements.

(1) DNV requires that plastic deformation during installation be simulated, with supplementary requirements (P) typically applied. The test may involve full-size steel pipes or strip specimens extracted from steel pipes. The final strain must be tensile, and the specimen must be aged at 250°C for one hour. If supplementary requirements (S) and/or (F) are specified, the corresponding criteria for impact toughness, the ductile-brittle transition curve, the drop-weight tear test (DWTT) and its transition curve, hydrogen-induced cracking (HIC), and sulfide stress corrosion (SSC) must be satisfied.

(2) IOGP specifies two cumulative strain levels for simulating strain in seamless steel pipes used for coiling:
① Strain Level 1: Two cycles of ±2.5% plastic strain, resulting in a total cumulative plastic strain of 10.0%.
② Strain Level 2: Two additional cycles, following Strain Level 1, resulting in a total cumulative plastic strain of 20.0%.

In both cases, strain aging tests are required, and specimens must be evaluated after aging at 250°C for one hour. The first case begins with tension and ends with compression, while the second case begins with compression and ends with tension. The test may use either full-size or sub-size specimens, and detailed procedures for simulating coiling must be provided to the customer. If requested, HIC and SSC tests must be performed following strain aging in accordance with the IOGP specification, and a crack tip opening displacement (CTOD) test must also be conducted.

(3) DEP, as a supplementary specification to API Spec 5L 45th Edition for marine seamless steel pipes, specifies that the strain aging test should involve a single strain of 3%. The specific requirements are as follows:

Specimen 1: Compression followed by tension, for a total of 3 cycles
Specimen 2: Tension followed by compression, for a total of 3 cycles
Performance evaluation is performed after aging the specimen at 250°C for one hour. If requested, an SSC test may also be required.

(4) The strain aging requirements in a foreign specification differ. The strain test involves a total strain of 5%, with performance evaluation conducted after the specimen is aged at 250°C for one hour.

The specific technical requirements for the acceptance of coiled pipes according to DNV, IOGP, DEP.

1.3 Dimensional Requirements

Since the dimensions of steel pipes can change during the coiling process, various standards and technical specifications impose stricter requirements, particularly for pipe-end dimensions and wall thickness. DNV specifies that for pipes with an inner diameter of 150 mm or more, both the inner diameter and its non-roundness must meet the required specifications, while acceptance criteria for the outer diameter and its non-roundness may be subject to negotiation. DEP mandates that for pipes with a diameter of 219.1 mm or larger, both the inner diameter and its non-roundness must comply with the specified requirements, while the outer diameter and non-roundness at the pipe end must also remain within the defined limits. IOGP imposes stricter requirements on the inner diameter and its non-roundness at the pipe ends, with implementation based on project specifications and classified under Class A and Class B accuracy standards. For Class A, accuracy requirements can be achieved through appropriate mechanical processing of the pipe ends, and each end must be measured. If requested by the customer, measurement data for each pipe end must be recorded and reported, with laser measurement explicitly specified. A foreign customer requires that for DNVSMIS 450 steel grade, each pipe end’s data be recorded using laser measurement. 

1.4 Surface Quality

The coiling process places stringent requirements on the surface quality of steel pipes, especially with respect to grinding and pitting.

(1) DNV specifies that the ground area on each steel pipe must not exceed 10% of the total internal and external surface area. Full-length machining is also permitted if it has been qualified by procedure and does not result in circumferential grooves or other defects deeper than 0.5 mm. Pitting at the pipe ends must not exceed a depth of 1 mm.

(2) IOGP specifies that the cumulative grinding of new defects using grinding wheels must not exceed 15% of the total quantity in the contract, and the ground area on any individual steel pipe must not exceed 15% of its surface. Grinding or machining at the pipe ends requires prior approval from the customer. Ground steel pipes must be identified with a green ribbon, grooves marked with the letter "R", and corresponding notations recorded on the tally sheet. Pit depth at the pipe ends must not exceed 1 mm, while pits on the pipe body must be no deeper than 2 mm. If more than 15% of the steel pipes are rejected due to surface defects, the surface of the rolled billet must be treated before further production continues. No pitting or defects greater than 0.70 mm are allowed within 200 mm of the pipe ends.

(3) DEP specifies that the depth of any pit on the pipe body must not exceed 1.6 mm, and such pits must not be removed or minimized by hammering or peening. Pitting greater than 0.25 mm is not permitted within 200 mm of the pipe ends.

1.5 NDT Requirements

DNV mandates surface inspections of both the pipe body and pipe ends, including magnetic particle testing (MT), electromagnetic inspection (EMI), and ultrasonic testing (UT). UT must provide 100% coverage for wall thickness measurement and delamination detection. IOGP does not require electromagnetic inspection; however, its UT requirements are consistent with those of DNV. Electromagnetic inspection and angled beam flaw detection may be conducted if specified in the contract. DEP likewise does not require electromagnetic inspection but utilizes the same UT techniques, albeit with slightly reduced coverage. Both delamination detection and wall thickness measurement must cover no less than 50% of the pipe surface. Furthermore, DNV and IOGP mandate that NDT personnel possess ISO 9712 or ASNT ACCP Level II certification, while DEP requires UT Level III personnel to approve all NDT procedures.

3. Conclusion

(1) The DNV standard specifies broader tolerances for the chemical composition of coiled pipes. Both IOGP and DEP specifications define narrower permissible fluctuation ranges for the chemical composition of coiled pipes, necessitating stricter control over steelmaking processes.

(2) Each standard or specification establishes strain aging criteria for the deformation of simulated coiled pipes and imposes stringent regulations on post-strain aging performance. Controlling the performance of steel pipes after repeated bending deformation is crucial for designing and manufacturing seamless steel pipes used in coiled pipe laying.

(3) Each standard or specification establishes stringent requirements for wall thickness deviation, inner diameter deviation, and out-of-roundness of pipe ends. To facilitate on-site team welding, providing customers with data on each pipe end may be necessary.

(4) Stringent requirements for flaw detection and surface quality necessitate the use of ultrasonic, electromagnetic, or magnetic particle testing methods for inspecting pipe bodies and ends.