ASSESSMENT OF EXISTING OFFSHORE STRUCTURES WITH EMPHASIZING ON NON LINEAR ANALYSIS AND STRUCTURAL RELIABILITY ANALYSIS

• Main Author: Ferry Setyadi (NIM 25005018), Eka
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/6035
• Institution: Institut Teknologi Bandung

ABSTRACT: <br /> <br /> <br /> <br /> <br /> <br /> <br /> Assessment of an existing offshore structure is performed if the utilization of the structures is to be extended beyond design life or due changes in safety parameters such as load or strength of the structure. From an economical point of view the continued use of an existing installation in many cases is preferable, compared to a new installation. The purpose of an assessment of an existing structure is to ensure that probability of failure of the structure is still within accepted criteria. <br /> <br /> <br /> <br /> <br /> <br /> <br /> There are several assessment procedures for an existing offshore structure. In general, it based on 1) Linear analysis and component check, 2) Non linear analysis and component check, 3) Structural reliability analysis for the ultimate state check. In this thesis assessment of existing offshore structures based on non linear analysis and structural reliability analysis is explored. <br /> <br /> <br /> <br /> <br /> <br /> <br /> Offshore platform for study case is a 4 (four) legs conventional jacket type with length 113.3 meters that operated on Natuna Sea. In the case study, the operational area is given a scenario of subsidence about 1.93 meters. The jacket structure is assessed for its safety to continue operations with the changed environmental criteria. <br /> <br /> <br /> <br /> <br /> <br /> <br /> Non linear structural analysis is performed to get the value of RSR (Reserve Strength Ratio) which is ratio of ultimate base shear to 100 years design base shear. The non linearity included in the analysis includes geometric and material non linearity. Static pushover analysis is performed to the jacket platform. From the analysis we can study structural behaviors from first loading until collapse. Important events noted are first yield, plastic hinges, elements buckling, maximum cross section capacity utilization, and maximum load factor could be resisted by the structures. Load case used for pushover analysis is wave load in 8 (eight) directions. Reserve Strength Ratio (RSR) is calculated for 8 (eight) wave loading directions. Critical RSR is compared to ISO 19902 acceptance criteria. <br /> <br /> <br /> <br /> <br /> <br /> <br /> The analysis shows that critical wave load direction is 270 degrees, with corresponding RSR 1.84. ISO/DIS 19902 recommends minimum RSR to be 1.85, so the jacket structure is not acceptable based on non linear assessment stage. <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> With insufficient RSR value, the structural reliability analysis is performed to get reliability index and probability of failure. Structural reliability analysis considers resistance and load parameter as statistical parameters. Probability of failure is compared to an accepted criteria references to decide if the structure is acceptable or not. The structural reliability analysis used is First Order Second Moment (FOSM) method. The FOSM is an approximation method based on first moment, mean, and its second moment, variance. <br />