Subsea Pipelines

What Every Pipeline Engineer Should Know?

These are the 11 things I think every pipeline engineer should know, for a start:

  1. Design Codes and Standards
  2. Pipeline Material Selection
  3. Pipeline Wall Thickness Selection
  4. Pipeline Routing
  5. Pipeline On-Bottom Stability Design
  6. Corrosion Control (Cathodic Protection)
  7. Pipeline End Expansion
  8. Pipeline Free Span Analysis
  9. Pipeline Lateral Buckling Analysis
  10. Pipeline Walking Analysis
  11. Pipeline Installation Analysis



 1.0  Design Codes and Standards

Pipeline engineers should, first and foremost, be familiar with the pipeline design code relevant to the project. The pipeline standard/design code is typically governed by the country the project is being executed.

1.1     What is the American standard for offshore pipeline?

If the project is executed in North America, the governing code would be API RP 1111 – Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design).

 1.2     What is the British standard for offshore pipeline?

If the project is executed in the United Kingdom, the governing code would be the British Standard PD 8010 Part 2: Subsea Pipelines. This is typically supplemented by DNV-ST-F101, Submarine Pipeline Systems.

 1.3     What is the standard for offshore pipeline used in the rest of the world?

The rest of the world, outside North America and United Kingdom, relies on DNV-ST-F101, Submarine Pipeline Systems as the primary offshore pipeline code. This code is quite comprehensive and covers pipeline design, installation, storage, linepipe selection and much more. This is the most common standard for offshore pipeline design.

  1.4     What is the difference between these codes/standard?

The differences are in the safety factors each code places on certain aspects of the design. Some codes assume the material’s tensile strength reduces after 50°C (DNV) while another code (API), doesn’t consider any material reduction below 100°C. There are other differences which I may later tabulate.

2.0 Pipeline Material Selection

Pipeline Material Selection is one of the first analysis performed in order to determine the pipeline outer diameter, wall thickness and material type. The goal of this analysis is to ensure that the chosen pipeline has the strength and durability to withstand the harsh environmental conditions that it is going to endure for its entire design life.

Typically, these are the main considerations:

Design Pressure and Temperature: The material must be able to withstand the highest pressures and temperatures it will be exposed to during operation.

Corrosion Resistance: Subsea environments are often corrosive because of the presence of seawater and various substances in the oil or gas being transported. The chosen material must be resistant to this corrosion to ensure the longevity of the pipeline.

Physical Properties: The material’s physical properties, such as tensile strength, yield strength, and hardness, are also critical considerations. These properties will affect the pipeline’s ability to withstand the pressures and strains encountered during installation and operation.

Chemical Composition of the Transported Fluid: The chemical composition of the oil or gas being transported can also affect the selection of the pipeline material. Some substances can be corrosive or erosive to certain materials, and this needs to be considered.

Environment Conditions: Factors such as water depth, seabed conditions, wave and current actions, and seismic activity are considered, as these can impact the pipeline’s structural integrity.

Cost and Availability: The cost of the materials and their availability can also affect the decision. It’s important to strike a balance between the ideal material and what is practically and economically feasible. In most projects, this is actually one of the main factors in pipeline material selection.

Generally, carbon steel (API grades like API 5L X65, X70, etc.) is widely used because of its good strength and cost-effectiveness. But, in corrosive environments, Corrosion Resistant Alloys (CRA) such as duplex stainless steel, Inconel, Alloy 316, etc., may be used. However, instead of the entire pipeline being of CRA, the pipeline is just cladded with CRA — this means only 3 mm of the pipeline will be constructed using CRA material, the remaining is still carbon steel. To make this clearer, if your pipeline is 22 mm thick, the linepipe manufacturer will make the linepipe with 19 mm carbon steel and then glue 3 mm of CRA material on the inner side of the linepipe. The CRA is always on the inner side as this is the side exposed to corrosive contents.

Ultimately, the specific choice of materials will be influenced by the factors above, along with regulatory requirements, project-specific considerations, and engineering judgment.