Structural Design Considerations
Authors: Marshall Peter , Chang Ben 
Published: October 2008

This paper is one of seven presented al a special OTC session on international standards for the design of offshore platfotms for earthquakes. Companion papers describe an overview of proposed ISO provisions, seismic exposure, foundation design, performance characteristics, a site-specific example, and probability-based LRFD. The focus of this paper is upon results of interest to structural designers, such as:–simplified load and resistance factors–lifetime reliability estimates–ductility analysis using "API jolts"–example North Sea application

Introduction

1S0 / TC 67/ SC 7 /WC 3 / Panel 5- Seismic Loading and Response has been working on a major revision to the API LRFD earthquake design guidelines. Load and resistance factors are no longer constant, but vary in response to different loading uncertainties in different tectonic settings, different structural reserve strengths and ductilities, and different consequence-bawd sttucture classifications. Annual versus lifetime reliability implications have been reexamined. Although nominal design is based on elastic SLE (200-year strength level earthquakes), the survival of larger DLE (ductility level earthquakes) should also be checked, A new efficient dynamic approach for analyzing the latter is presented, to complement traditional push-over and multiple time-history methods.

The proposed 1S0 provisions are in load and resistance factor design (LRFD) format. Separate safety factors are applied to the various load components or structural actions, and to the various structural element resistances, depending on their respective bias and uncertainty. Each element is cheeked for the factored loads or actions using the following equation:

Mathematical equation (Available in full paper)

The Draft 3 earthquake design proposal [1S0, Sept. 1995] is reliability-based, in that three different structural safety levels (SSL'S) are provided for, following the philosophy of consequence-based oceanographic criteria and different performance specification levels (PSL'S) in ongoing API standardization efforts [Marshall, 1994].

The definition of SSL'S in terms of the safety index, ? (beta), and other attributes of the platform is summarized below:

Definition of SSL's (Available in full paper)

Here, safety index (?) is the margin between unbiased mean resistance andexpected annual extreme load, expressed in units oftotal uncertainty [API RP2A-LRFD, 1993].

Each designer derives his own safety factors, depending upon thetarget reliability level and other prescribed factors, using a 12-step process [Bea, 1995].

Most analysis and design steps focus on elastic behavior in the 200-yr. strength level earthquake (SLE). However, the real design point (closest approach to a failure condition) relates to ultimate strength and plastic deformation in a more rare, intense ductility level earthquake (DLE). Furthermore, instead of actually analyzing the DLE, its effects are represented by a reserve strength ratio (RSR) and the mean effective earthquake loading factor, F?, which reflects ductility and residual strength, as would be exhibited in a push-over analysis. Structures which meet prescribd architectural guidelines are presumed to have RSR of 2.0 and F? of 0.5 (corresponding to ductility of 2.5 and residual strength ratio of 0.8), making them capable of absorbing 4 times the SLE energy.