A research program and API committee activity underway since 1979 have produced the draft "Recommended LRFD Practice for Planning, Designing, and Constructing Fixed Offshore Platforms, API RP2ALRFD." A companion paper by Lloyd and Karsan (1988) presents the history, objectives, and benefits of this draft Recommended Practice (RP), the first from API to have a thorough reliability basis. This paper explains the reliability calibration of the principal safety factors.


The traditional one-third allowable stress increase for environmenta1 loading found in working stress design (WSD) has been replaced in the Draft RP2A-LRFD by separate load factors (?) for dead load, live load, wind-wave-current load, earthquake load, and wave dynamic load. Resistance factors (ø) vary for pile capacity, beam bending, axial compression, hydrostatic pressure, etc. Together, these load and resistance factors provide a level of safety close to present practice, yet provide more uniform safety and economy.

The detailed documentation of the load and resistance factor calibration is described in parts of references Moses and Russell (79–22), Moses (80–22, 81–22, 82–22, 83–22, 85–22, 86–22, and 87–22). This paper highlights the methodology and data, leaving specific provisions for a later publication.

In the course of this work over several years the strategy for calibrating the LRFD factors was refined and adapted. The initial goal was to achieve safety close to present practice for each component design check. One consequence is that this can result in different computed safety levels (for example, bending as compared to tension). This calibration strategy then evolved to achieving more equal computed safety over a wider class of design situations than at present. An example is closer computed reliabilities for the various tubular joint checks. Both of these strategies are called "reliability calibration" or "Beta Calibration" for the safety index ?.

However, in some situations, such as when statistical data was unavailable, it was necessary to set aside the reliability calculations and simply achieve the same strength as present. We call this direct or "brute force" calibration. This was used for both seismic design and for the design of nontubular members.


Using the reliability calibration, the load and resistance factors were chosen by the following steps:

  1. Assemble a data base of the statistics of loads and component strengths. This includes the mean and coefficient of variation (COV) of each variable, as well as the bias that relates nominal design values to mean values.

  2. Obtain a distribution of safety indices implied in current working stress design. The safety index ? is computed for a range of environmental to gravity loads ratios typical for each type of component. From these calculations, establish a target safety index for each type of structural component.

  3. Find load factors (?) for each load condition and resistance factors (ø)for each component to provide maximum uniformity of ? ?S for each component over the range of its load ratios.

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