DETERMINATION OF LRFD ENVIRONMENTAL LOAD FACTORS OF OFFSHORE PLATFORM IN THE NORTH OF JAVA SEA AND MAKASSAR STRAIT
Keywords:Environmental Load Factor, Fixed Offshore Structure, Reliability Analysis, Pushover Analysis, LRFD, WSD
The environmental load factor in commonly used offshore platforms design code, API RP-2A (American Petroleum Institute - Recommended Practice 2A), is developed based on the environmental conditions of American waters, especially the Gulf of Mexico, which have relatively extreme environmental conditions when compared to Indonesian waters. Case studies were conducted to determine environmental load factors in Indonesian waters, particularly the North Java Sea and Makassar Strait, categorized as shallow seas. This analysis was carried out on the performance criteria of pushover failure. In this study, the base shear was analyzed to describe the strength of the structure in the form of a collapse base shear (CBS) and the load in the form of a wave base shear (WBS). CBS was obtained through pushover analysis with yield strength randomness. WBS was obtained through in-place analysis with wave height randomness. This concept was applied to the structure of the Monopod and Braced Monopod types of offshore platforms located in the North of Java Sea and Makassar Strait waters which had been optimized for the WSD and LRFD design methods. The reliability of the structure was analyzed based on the CBS and WBS values using the First Order Reliability Method (FORM) II. The reliability analysis results were in the form of a reliability index (β). North Java Sea gives a reliability index in the range from 3.58 to 4.38 for every design criteria. While Makassar Strait gives a reliability index in the range from 3.17 to 3.54 for every design criteria. With a high target safety level for the North Java Sea location, a 1.10 environmental load factor is recommended for further offshore structure design. But, for the Makassar Strait location, more studies need to be done to get better environmental load factor recommendations.