Dynamic Performance And Safety Analysis Of Offshore Jacket Platforms

As a common type of offshore structures, jacket platforms are assigned to achieve accomplishment of operations, such as drilling, oil extraction and oil storage. Meanwhile, these platforms are exposed in marine environments, complicated environment and some other accidental loads, such as ship impacting, blast and fire, can be quite a threat to structural safety, causing collapse of the structure, which not only causes serious environmental pollution, but also brings huge economic losses. In order to guarantee adequate collapse resistance under severe loads, extend service period, it is necessary to study the Overall safety performance of the platform structure from perspective of overall structural level. Currently, the studies of overall level safety of jacket platform are focused on analysis under static-type loads, research on structural collapse caused by dynamic loads, and the embodied safety reserve has just arose people’s attention. Meanwhile, influence of yielding process within the structure and plastic development characteristics accompanied with collapse to safety reserve have rarely been discussed. This article will launch the research from following several aspects:Implementation procedures of nonlinear method in finite element analysis method were explored. For common collapse analysis, it is generally required to consider the material and geometric nonlinearities, which provides a more realistic simulation of the collapse process, therefore it is necessary to understand the implementation process and analysis methods in nonlinear structural analysis. With the derivation of geometric and material nonlinear two-dimensional beam finite element model, influences of nonlinear structural analysis to the calculation results are discovered, using Newton-Raphson method.Static collapse safety of a jacket platform was conducted. Pushover analysis was carried out to a jacket platform, using environmental factors of an ice condition, as well as the method of increasing return periods to elevate load levels. Bearing capacity and plastic development process under in different directions of the structure was obtained, then the structure of the overall safety performance was discussed based on Reserve strength ration (RSR); Regressive response surface program was complied, which provides a fast means to obtain the structural reliability index. JC method with pre-fitted equations was also carried out to verify the reliability of the program.Dynamic performance of the jacket platform under seismic loads was analyzed.26three-dimensional seismic records with different spectral characteristics were selected; IDA method was conducted for collapse dynamic analysis. During each calculation step, plastic status and response information of the structure was recorded. Development patterns of dynamic parameters, such as structural displacement responses and interstory drifts were disscussed. It was discovered that the development trend characteristics of dynamic parameters under different seismic loads are not consistent. Then, plastic development processes of a platform subject to seismic loads were analyzed, a most likely to occur collapse mode was explored. The suggested collapse mode is similar to all the actual collapse modes, and the similarity discretenesses are small, therefore the collapse mode obtained has statistical significance.Dynamic collapse failure characteristics of platform structure were studied. To begin with, collapse related parameters were summarized; collapse resistance and safety reserve of platform structure were analyzed from perspective of deformation energy, base shear and displacement response. In addition, dynamic characteristics of structure collapse limit states were discussed form the perspective of frequency spectrum characteristics of seismic records and structural accelations. It was discovered that for loads with strong dynamic characteristics like earthquakes, when the base shear has reached its collapse bearing limits, the platform may still maintain its stability for a while before collapse occurs, i.e., the overall safety reserve is provided by both its bearing and deformation capacities. The capacities of the structure under different seismic loads are similar, the collapse can be considered as dynamic strength failure process. When the load level is low, spectral components in main response frequency vibration area is highest. With the increasement of load level, forced vibration is enhanced, proportion of resonance effects are decreased. When the spectral component near a certain range of resonance frequency of Seismic spectrum characteristic is large, there will be an adverse effect on the structure.