Evaluation of Global Positioning System Data for Offshore Platform Deformation

Abstract

Problem statement: Reservoir compaction and shallow gas migration phenomena may cause offshore platform to experience deformation which if happens excessively will affect their structural integrity. Approach: Hence it is crucial to monitor and quantify the magnitude of the deformation especially if they are not uniform throughout the platform structure. However since mostly the offshore platforms are few hundreds kilometers away from shore, the precise monitoring of their deformation is limited to very few sophisticated instruments, in which GPS technology is one of them albeit using very special GPS data processing technique such as Long Baseline Relative Positioning. Results: Using this technique and employing GPS data observed on one of PETRONAS own platform, Pulai, its deformation magnitude will be determined with various options such as number of reference stations used, configuration and their geographic location. This study presents initial deformation processing result using scientific software GAMIT/GLOBK and their analysis utilizing postfit nrms and chi-squared statistics. The result indicated that for the period of two months there was displacement as big as 0.0094 m with standard deviation of 0.0106 m. However following congruency statistical test using t-student distribution with 95% confidence level, indicated that this displacement is insignificance. Analysis of the output result with postfit nrms also indicated that the data were of good quality, the processing procedure was correct and the output for each processing epoch is internally and externally consistent. Conclusion/Recommendations: It could be concluded with correct data processing strategy GPS data could be used to determine deformation magnitude which consequently could be utilized as input to assess structural integrity of an offshore platform.