Verification of a Simplified Method to Evaluate the Capacities of Template-Type Platforms

Abstract

ABSTRACT This paper summarizes development of simplified procedures toEvaluates form loadings imposed on template-type Platforms and to evaluate the ultimate limit state lateral Loading capacities of such platforms, Verification of these Procedures has been accomplished by comparing results porn the simplified analyses with results from three dimensional, linear and nonlinear analyses of a variety of template type platforms. Good agreement between results from the two types of analyses has been developed for the evaluations of both loadings and capacities. The verifications platforms have included four-leg well Protector and quarks structures and eight-leg drilling and Production Gulf of Mexico structures that employed a Variety of types of bracing patterns and joints. Several of These structures were subjected to intense hurricane storm Loadings during hurricanes Andrew, Carmen, and Fredric, With in the population of verification platforms are several That failed or were very near failure. The simplified loading And capacity analyses are able to replicate the observed Performance of these plat forms. Realistic simulation of The brace joints and foundation capacity characteristic are Critical aspects of these analyses. There is a reasonable Degree of verifications of the simplified methods with the Observed performance of platforms in the field during intense hurricane storm loadings. These methods can be used to help screen platforms that are Being evaluated for extended service. In addition, the results From these analyses can be used to help verify results from complex analytical models that are intended to determine the ultimate limit state loading capacities of platforms. Lastly, and perhaps most importantly this approach can be used in the preliminary design of new platforms. INTRODUCTION During the past three decades, an immense amount of effort has been devoted to development of sophisticated computer programs to enable the assessment of storm wind, wave, and current loadings and the ultimate limit state capacitycharacteristics of conventional, pile-supported, template type offshore platforms. I-3 The= programs acquire high designs of expertise to operate properly, are expensive to purchase and maintain, and requite large amounts of manpower and time to complete the analyses. Due to thesophistication of these programs, experience has shown that it is easy to make mistakes that ate difficult to detect nd that can have significant influences on the results. This paper summarizes the second phase of verification of simplified procedures to evaluate environmental loadings and ultimate limit state lateral loading capacities of template- type platforms. Reasonable simplifications and high degrees of "user friendliness" have been employed in cW14- opment of the computer software to reduce the engineering effort expertise, and costs associated with the analyses. The computer program that has been developed to perform the simplified analyses has been i&ntit3ed as ULSLEA (Ultimate Limit State Limit Equilibrium Analyses).5,6 The first phase of development and verification of these has been documented7,8 The first phase developments were verified with comparisons of observed and computed loadings and capacities from five 8-pile self contained drilling and production platforms and one 5-pilewell protector, The simplified static capacity bias managed from 0.80 to 1.07 with a mean value of 0.95.