Collapse Safety Margin Of Jacket Platform Subjected To Strong Earthquakes

Offshore platforms can be divided into fixed platforms and mobile platforms, jacket platforms is the typical representative type of fixed platform, and jacket platform is currently the most popular type of offshore platforms. Jacket platform suffers complicated environmental loads, such as wind, wave, stream, ice and seismic loads, in addition to bears its own weight and gravity of deck equipments. Among these environmental loads, the reaction of jacket platform structure under wind, wave, stream and ice loads has been researched by a lot of people, seismic load is researched relatively less, the standard is also rare involved about the earthquake loads. The earthquake is an abrupt energy release process, structures destroyed by strong earthquakes brings huge economic losses and human casualties. Three China’s offshore oil reserves regions (the Bohai Sea, the south of Yellow Sea and the South China Sea) are seismic activity zone, which means that the jacket platforms in work may suffer earthquakes. In order to ensure the safety of the platform, to avoid the earthquake casualties and economic losses, it is necessary to get the collapse margin of jacket platforms. This article does some research as follows:In this paper, the overall collapse margin of jacket platforms was studied based on the Collapse Margin Ratio (CMR) presented by Applied Technology Council (ATC) and Reserve Strength Ratio (RSR) proposed by Health and Safety Executive (HSE),The basics of the response spectrum theory was briefly introduced, standards about seismic analysis of International Organization for Standardization (ISO), the American Petroleum Institute (API) and the China Classification Society (CCS) were learned, the relevant design response spectrum requirements were compared to select a suitable one for China’s Bohai Sea, and the design response spectrum has been designed in accordance with standard steps.The analysis process of Pushover analysis was described, focusing on the effects of lateral force distribution patterns and capacity spectrum method, which is the basis of nonlinear static analysis method, each step in detail and some theoretical background of capacity spectrum method is given, prior knowledge for the method evaluating CMR on the basis of the pushover analysis procedures was provided.The process of evaluating CMR based on Incremental Dynamic Analysis (IDA) described in the ATC-63 report (Quantification of Building Seismic Performance Factors) and applying the method into evaluating the collapse margin of a jacket platform in Bohai were introduced, and Finite Element Analysis program OpenSees (Open System for Earthquake Engineering Simulation) was chosen to build the finite element modeling and analyse the platform. According to the earthquake selection principles.20 earthquakes were chosen to do the IDA of the platform, and get CMR of the platform. A relatively simpler nonlinear static analysis method to get CMR was presented to replace the complicated IDA, and this method has been validated to be reasonable by comparing the results from two methods. RSR of jacket platform was calculated based on Pushover analysis. Factors in the process of CMR and RSR through Pushover analysis were analyzed to decide which safety reserve evaluation index is better.A new method was proposed to quickly find CMR. The basic thinking and analysis procedures of this method was introduced, this method was demonstrated rationality, and the advantages and disadvantages of this method were summarized. A set of jacket platforms with different top quality were analyzed througth this method, then the trends of CMR, RSR and ductility factor μ were obtained, and the pros and cons of the three safety reserve evaluation indexes were studied based on the trends. According to the relationship between CMR and RSR, a thought determining the CMR limits was proposed.