API REPORT 83-22
Implementation of a Reliability-Based API RP2A Format
|Publication Date:||1 January 1985|
2. OBJECTIVES AND SCOPE OF WORK
The objective of this study is to provide basic data and parametric study data on extreme wave dynamic design of a deepwater platform to the Technical Advisory Committee and its principal investigator, Dr. Fred Moses. This analysis effort, using a Gulf of Mexico deepwater platform in 935 feet of water, will assist in the further development of the Reliability Based API RP2A LRFD format. The developed data consists of platform static and dynamic global responses (base and level shears and overturning moments), resulting dynamic amplification factors (DAFs) and a breakdown of member forces for approximately fifty selected platform members.
The base case study will provide base case data on the dynamic response of a deepwater platform (called Platform 'D') to extreme waves by using a hybrid time and frequency domain method. The use of this method will demonstrate the bases for the derivation of γdyn and allow others to derive appropriate γdyn for another analysis method.
Base case data obtained will form the basis for the sensitivity study. This sensitivity study, including the evaluation of variations in platform damping, fundamental mode, wave spectra, etc. is intended to support Dr. Fred Moses' effort in the safety index (Beta) calculations.
The code check of the base case will include documentation and evaluation of both working stress design (WSD) and load and resistance factor design (LRFD) based interaction ratios for approximately fifty selected platform members.
2.2 Base Case Study
The base case study is undertaken to form the basis of the sensitivity study to follow. The structure analyzed along with its base case parameters is summarized below. Methodology used is the same for all studies and is discussed in Section 2.5.
The studied platform is an eight-legged structure in 935 feet of water at M.L.W. in the Gulf of Mexico. The structure was designed in accordance with API RP2A, 10th Edition. The main characteristics of the platform are:
The analysis parameters to be considered in the parametric studies and their base case values are:
These parameters were established by Committee representatives as the study progressed.
2.3 Sensitivity Study
Once the base case study has been completed, a further sensitivity study will be undertaken to determine the sensitivity on DAF, resulting member inertial loads, and WSD and LRFD results. The parameters to be studied and their range of values are as follows:
Not all combinations of parameter variations will be studied, but sufficient variations will be studied to establish the sensitivity of the analysis results to these parameters. These parameters were established by Committee representatives as the study progressed.
2.4 Code Checking
Once the design base case inertial loads have been determined for selected members, code checking will be undertaken using working stress design (WSD) procedures and the proposed LRFD checking equation for dynamic loading (Reference 1, Equation 8.3). Two values of the dynamic factor, γdyn, will be used. Code checking of sensitivity study results will be by others.
2.5 Overall Methodology
The analysis procedures (Figure 2-1) invoked for both the base case and sensitivity study follow seven basic steps:
1) Develop a detailed three dimensional space frame model of Platform 'D'. Perform an eigenvalue analysis of the structure. Generate loadings for static dead, live and wind conditions and for the design wave and current. Perform stiffness analysis of these loadings and determine member forces.
2) Determine the static global response (shears and overturning moments) of the platform for selected wave heights and periods, excluding current.
3) Determine the associated dynamic global response of the platform for the selected wave heights and periods, and variations in dynamic parameters (e.g. structure damping, structure periods).
4) Develop transfer functions for static and dynamic global response of the platform.
5) Perform spectral analyses using the above transfer functions to obtain overall dynamic amplification factors (DAFs) for global shears and overturning moments.
6) Using the design wave and current global static response, the overall DAFs, and a modal (eigenvalue) analysis of the platform, determine the inertial forces for selected members of the platform.
7) Perform WSD and LRFD member column interaction code checks for the generated member forces and compare member utilization results.
Detailed discussion of these steps is presented in the following sections and appendices.