Offshore pipeline installation criteria

[48inch]

I'm looking for some design criteria limiting stress/strain/buckling for offshore pipeline installation. Usually we use DNV-OS-F101. When using API linepipe however, alternative standards must be applied. Any suggestion where to find installation criteria? As the criteria for operations, for example according to ASME B31.8 A842.2 are too conservative for use in installation phase.

 

[GregLamberson]

48inch

Calculating the stresses is straightforward and will depend on the installation method. For acceptability of the stresses, the codes give guidelines, but you casn set more conservative criteria.

Personally, for HDD's, for installation stresses, I use calculations that are based from a variety of sources, for example DNV, Appendix B for acceptable combination of stresses, for hoop stress design factors - 49 CFR Part 192.111, acceptable shear stress - B31.4, 403.3.

You might want to look at VERITEC E305 - On Bottom Stability Design of Submarine Pipelines. Sect 5.3 is a good fundamental approach to the use of Morison's equation for on-bottom stability design calcs. This will also help determine if the pipeline weight will remain stable on the seabed with stesdy cyurrent or comnbined current & wave cases.

For concrete coated pipe, There is always a concern the concrete will fail under the bending stress. Due to its low tensile strength, the concrete will fail in tension at the interface of the steel and concrete. If the concrete has a good bond to the pipe, it will crack at the pipe surface. This will eliminate the stress in the concrete, and as long as the concrete coating thickness is thicker than the crack is deep, there will be no cracking visible on the concrete surface. If the bond to the pipe is weak, the pipe and concrete will separate which will reduce the stress in the concrete and there will be no visible cracking at the concrete surface.

If there is visible cracking on the concrete surface, the concrete can be repaired to seal the cracks. If cracking is a concern, you might want to look at the severity of the problem and look for a solution.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BigInch]

API RECOMMENDED PRACTICE 1111

Design, Construction, Operation,
and Maintenance of Offshore
Hydrocarbon Pipelines
(Limit State Design)

http://virtualpipeline.spaces.msn.com

 

[48inch]

Thanks Greg and BigInch!
I'll dig through these. Good to know that I'm looking in the right place.
Another thing, if to consider corrosion with respect to installation calcs(laying analyses), are there any additional important standards/practices, or can I find that also in the sources you have suggested? And if you know, will this be simply reduction in wall thickness, or are there more complex checks required?

 

[BigInch]

I would not (normally) include a corrosion allowance for stress calculations for the installation condition. Pipe should be free from corrosion during that time. Leave corrosion allowances for the operating stress condition.

If you're not the laying contractor, once you determine that it is indeed possible to lay the pipe using at least one of several typical lay methods, there is little to worry about in that respect, as its the contractor's responsibility to control stresses to less than maximum allowables during installation. Its not the design engineer's job to specify any installation methods. The design engineer's job is to ONLY specify what shall be an acceptable result of the installation process. (If you specify construction methods, you presume responsibility for a successful outcome of that method and any associated risk and have little recourse, if any, in case of deficiencies.) Have the laying contractor do the detailed installation calcs according to whatever actual installation method he chooses to use. I suggest you give all your contractors a call now and discuss what installation methods can be used for your pipe size/ wall thickness /specific water depth, wave and currents and season of construction in regards to his available construction equipment in your project location. The potential contractor will work with you to arrive at a mutually agreeable installation method that will assure stresses are acceptably controlled.

http://virtualpipeline.spaces.msn.com

 

[48inch]

BigInch,

Appretiate your points, but I do work for the laying contractor, and the pipe is not new, but from long term storage.

cheers!

 

[GregLamberson]

48inch

Agree with BigInch on the corrosion allowance.

If you're in the tendering stage, then I would use BigInch's advice in reverse. I would certainly state in the proposal the specific method of installation and the equipment to be used and if this is a pure installation contract (not an EPC), then the owner's engineer will need to look at the proposed method and equipment and approve.

Some owner companies will require you, the contractor, to verify the installation methods meet certain design requirements, that may be what prompted the question???

For installation ,there are a few others you might want to look at (not sure if this is US/GOM), you may fall under 1104, but there is a good British standard - BSI BS 4515-1 Specification for Welding of Steel Pipelines on Land and Offshore - Part 1: Carbon and Carbon Manganese Steel Pipelines (Q)



Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[48inch]

Greg,

Yes, our main requirement is the DNV-OS-F101 as a governing standard. But the linepipe is API5L spec. This suggests that the limit state criteria of section 5D, may not be applicable for this pipe. One can modify dimensions input to account for the wider dimensional tolerances. However i believe other things like test requirements etc. are still less stringent for the API pipe. So the first question is if any of the limit state equations of the F101 can be used, or if alternative codes should be applied, when calculating laying bending stress/strain, and local buckling. That was my assumption, leading to the request for guidance to alternative codes.

 

[SJones]

Check the specification and materials certificates of the API pipe. The purchaser may have supplemented API 5L sufficiently to achieve parity with ISO 3183-3/DNV.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[GregLamberson]

48inch

Regardless of the pipe spec used, the acceptability of the installation stresses are determined by the parameters contained in the codes and if the pipe that is spec'd meets those parameters, it is acceptable, if not, then a higher yield or a heavier wall pipe may need to be used.

The keys are the hoop stress and shear stress. Not that you shouldn't also check the longitudinal stresses (from benindg, hoop stress, and thermal expansion), but if the hoop stress is within your parameters (again, I use 49 CFR Part 192.111 which is 72% for Class I and 60% for Class II) and the maximum shear is within B31.4 limits (45% of SMYS) and the installation method doesn't exceed the max rad of curvature, then you should be okay.


Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[narenr]

Regardless of which linepipe specification you use, DnV OS F101 can be used for offshore pipeline design.

Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.