Seismic Analysis Requirements in Areas with Low Seismic Activity

[jthrncir]

Does anyone have any experience with interpreting the requirements of API RP 2A with regards to performing a seismic analysis when the SLE ground acceleration is less than 0.05g? For the specific project I am working (offshore jacket/topsides structure near Equatorial Guinea), the site-specific seismic analysis estimates an SLE ground accleration of 0.03g and and a DLE ground accleration of 0.171g. API RP 2A Sect 2.3.6.b.2 states that “For areas where the strength level design ground acceleration is less than 0.05g, e.g., the Gulf of Mexico, no earthquake analysis is required, since the design for environmental loading other than earthquake will provide sufficient resistance against potential effects from the seismically active zones.” This would seem to indicate that no seismic analysis is required. However, the fact that the DLE/SLE ratio is so large (5.7) concerns me and I think that perhaps a ductility level analysis should be performed to ensure the platform will not collapse under the rare, intense earthquake. Also, the Equatorial Guinea area has low environmental loading requirements, so it can't be assumed that the design for the environmental loading will automatically mean that the seismic criteria has been fulfilled as well. FYI, if the platform was designed per ISO 19901-2, then a seismic analysis would need to be performed since ISO 19901-2 has a lower threshold for performing seismic. My main goal is to find someone who has run into this issue in the past to see what the resolution was.

Thanks,
Jason Hrncir

 

[nour75]

Hi jthrncir :
I think it is better to perform DLE analysis. This is because of 2 things:
1. in API RP 2A Sect 2.3.6.b.2 states that "platform design would normally controlled by storm or other environmental loading rather than earthquake", this means it may be not adequate for special rare case like your case. the phrase "would normally" does not mean that it must be in all cases.

2. Sect 2.3.6.b.2 gave an example of Gulf of Mexico, which is a special case, and in this gulf the design would be controlled by storm or other environmental loading rather than earthquake if the strength level design ground acceleration is less than 0.05g.

I hope it helps
Mohamed

 

[London]

At that range, it could be possible to justify the structure using the DLE level accelerations, and remain in the elastic zone. This is a simpler check that a plastic analysis and if this passes, a plastic analysis wouldn't be required, I imagine.

This is covered in the next part in Section 2.3.6.b, API RP 2A.