Three years ago the American Petroleum Institute released an update to their riser design code, changing it from a recommended practice to a standard. To date, the new code has yet to be endorsed by the Bureau of Safety and Environmental Enforcement (BSEE) and the old code is still in use. Current BSEE requirements for riser design, 30 CFR 250.1002, still point to the earlier recommended practice, so projects continue to be approved in accordance with API-RP-2RD, not the new standard, API-STD-2RD.
The intent of this article is not to speculate why the latest version has not been universally adopted, but rather give an overview of the changes that will impact riser designs once it has been. The key differences are as follows:
- The design approach has changed from an allowable stress criteria to a load and resistance factor method. Four different ‘methods’ are given to evaluate combined loads and the engineer has the flexibility to choose which one they want to use.
- Exceedance of yield is allowed when stress is primary as opposed to displacement controlled secondary stress.
- For combined load, the Von Mises stress approach is replaced with a limit load approach.
- Service loads (SLS) and extreme loads (ULS) have the same factors of safety.
- Explicit burst and collapse checks are required.
- Temperature de-rating is required below as well as above 250°F.
- Integrity management is required.
- Special material testing is required when strain exceeds 0.5%.
- Checks for single event fatigue utilisation are required.
- Accidental load assessment considers events with probabilities as low as 0.0001.
The changes and available methods are all based on sound engineering and expected failure modes. However, the ability to adopt one of four different design approaches could be a cause for concern for designers. The changes are meant to result in more accurate designs, but there may be cases where it will result in changes to the riser design. For example, the introduction of burst checks combined with temperature de-rating below 250°F can increase the riser wall thickness requirement.
The requirement for integrity management is a positive change that is consistent with the direction that the industry is moving. Although the exact requirements of the integrity plan are left open to interpretation, the very fact that a plan will be required is useful.
It’s also worth noting a few things to be aware of. There is no guidance on how to apply corrosion allowance, so engineers will need to define that. Consideration of a 1000 yr design case is not explicitly required, and calculation of utilisation using the methodology of the worked examples can result in divide by zero problems.
The new standard’s lack of traction with BSEE does indicate that the code may require some more work, or at least an explanation of why the changes are beneficial. However, we expect that it will be endorsed at some point and riser engineers will need to adjust their methods and designs in order to comply.
If you have any questions, feel free to get in touch or leave a comment below.
Mark Cerkovnik, Chief Engineer, Houston
Mark has over 35 years of engineering experience with emphasis on analysis and testing of structural and mechanical systems. He has significant expertise in structural dynamics, fracture and fatigue, composites and FEA. He has 15 years of experience with deep water risers.