Evaluation of Local Joint Flexibility Effects in Fixed Oil Platforms

Abstract

Most of the fixed offshore platforms installed in Brazil are reaching or exceeding their design life. Even with the sharp fall in oil prices and the discovery of new oil fields, many of these installations remain economically viable in spite of their age; therefore, there is a need to extend their operational life. To ensure the platform structural integrity, the most sensitive structural joints and elements are systematically inspected. These inspections are based on structural analyses that identify all elements in which the working stress or design life do not comply with adopted codes. Underwater inspections can be difficult and very expensive. The use of local joint flexibility can lead to a reduction in the number of joints to be inspected. For that reason, this work aims to present an evaluation of the effects using methodologies of local joint flexibility.This work was conducted for six fixed oil platforms installed in Brazil between 1988 and 2012 with water depths ranging from 82m to 177m and was fully developed using finite element software. The analysis results were obtained taking into account three distinct methodologies of local joint flexibility and compared to the standard model, without considering local joint flexibility. The influence of different parameters were evaluated, such as water depth, natural frequency, maximum displacement, ratio between brace to chord diameter (d/D), ratio between chord diameter to chord wall thickness (D/T), angle between chord and brace axial and bending stresses.The results allowed a better understanding of each parameter influence and the impact that each methodology has in punching shear verification and in the structural behavior of the global model. In some analyzed offshore platforms the results indicated that the consideration of local joint flexibility may produce significant reduction in punching shear stress ratio, while at the same time enables an identification of critical joints that may have been over looked otherwise.