Failure Analysis Of Welded Offshore Jacket Platform Under Localized Fire

Welded Offshore jacket platforms have been applied as the major oil and gas exploitation platform in the shallow sea of China due to its simple structure,strong flexibility and reliability.As the main material for constructing offshore jacket platforms,Steel would be seriously weakened under high temperature considering reduced of the elastic modulus and yield strength,posing a grave threat to the safety and sustainability of offshore jacket platforms in conflagration.Current finite element study of offshore jacket platforms in conflagration is majorly carried out in two ways.One is to build an offshore platform model with solid elements or shell elements to conduct the fire resistance analysis;the other is to simplify the offshore jacket platform as space beam elements and view the connection between the tubular members to be a rigid joint without considering the local deformation of tubular joints.The former method has a high preciseness but takes a long time and a huge cost while the latter fails to reflect the local flexibility effect of joints though analyzing fast enough.Based on the local flexibility,the paper puts forward a fire resistance fictitious beam element,which is introduced in the finite element analysis model of offshore jacket platforms to conduct the failure analysis in local conflagration.First of all,the concept of fire resistance fictitious element is proposed on the basis of local flexibility of tubular joints.The fire resistance fictitious beam element is placed at the joint of chord and brace tubes,perpendicular to the axis of chord tube with a length of radius of the chord tube.The joint area is isolated to define the material based on the steels’ constitutive relation curves under different temperatures,thus to build the finite element static analysis model of the joint area.Through converting the load deformation curves obtained from the analysis,the stress-strain curves of the fire resistance fictitious beam element under high temperature could be acquired.Shell element transient finite element model and traditional rigid transient finite element model are built for 5 T-joints plane tubular trusses and 3 Y-joints plane tubular trusses respectively,along with the introduction of FBE transient finite element model of fire resistance fictitious beam element.After the analysis of three models,the obtained temperature displacement curves of loading points are compared.The study suggests that traditional rigid models cannot precisely reflect the displacement of tubular truss in the conflagration and overestimate the fire resistance ability of the structure.The temperature displacement curve obtained from the analysis of FBE model adopting the fire resistance fictitious beam element well matches with the analysis result of shell element model.Its prediction for the structural failure temperature of the tubular truss is also closer to that of the analysis result of shell element model compared with the rigid models.For the tubular truss structure,different fire conditions are considered to study the suitability of fire resistance fictitious beam elements in local conflagration.The fire resistance fictitious beam element is used to conduct the failure analysis of wellhead platform of CB25 A Wellhead groups of Shengli Oil Field in local conflagrations.The structural response of the platform is studied according to different fire conditions,offering the reference for the fire resistance design of the welded offshore jacket platform.