Page 330 - Marine Structural Design

306 Part II Ultimate Strength

API (1991) are summarized below. The design philosophy for earthquake leads in API (1991)
is illustrated in Table 15.1.

Table 15.1 Earthquake Design Philosophy, API RP2A
I Strength Level Earthquake (SLE) Ductility Level Earthquake @LE) I
~~
Prevent interruption of normal Prevent loss of life and maintain well
platform operations. control.
Ground shaking which has a Rare intense ground shaking that
reasonable likelihood of not being unlikely to occur during the platform
exceeded during the platform life. life.
No significant structural damage, No collapse, although structural
essentially elastic response. damage is allowed; inelastic response.

The AFT’S seismic design recommendation are based upon a two level design approach, these
are
Strength Requirements
The platform is designed for a severe earthquake which has reasonable likelihood of not
being exceeded during the platform life (typical return period hundreds of years, Strength
Level Earthquake SLE).
Ductility Requirements.
The platform is then checked for a rare earthquake with a very low probability of
occurrence (typical return period thousands of years, Ductility Level Earthquake DLE).
The objective of the strength requirements is to prevent significant interruption of normal
platform operations after exposure to a relatively severe earthquake. Response spectrum
method of time history approach is normally applied.
The objective of the ductility requirements is to ensure that the platform has adequate capacity
to prevent total collapse under a rare intense earthquake. Member damage such as in-elastic
member yielding and member buckling are allowed to occur, but the structure foundation
system should be ductile under severe earthquakes, such that it absorbs the imposed energy.
The energy absorbed by the foundation is expected to be mostly dissipated through non-linear
behaviour of the soil.
For some typical jacket structures, both strength and ductility requirements are by API
considered satisfied if the below listed previsions are implemented in the strength design of
these platforms:
Strength requirements for strength level earthquake loads (SLE) are in general documented.
Strength requirements are documented for jacket legs, including enclosed piles, using 2
times the strength level earthquake loads (Le. 2*SLE).
Rare, intense earthquake ground motion is less than 2 times the earthquake ground motions
applied for documentation of strength level requirements (Le. DLE < 2*SLE).