Stress Analysis and Life Predictions of Ship Propulsion System Gas Turbine Uptake Structures

Gas turbine uptakes in some FFG-7 class ships have undergone cracking, a consequence of cyclic thermal stresses that occur during startup and powered operation. Because of this cracking, mainly in duct walls and some stiffeners, NavSea redesigned the FFG-7 class uptake, and ship designers have improved designs for new ship classes such as DDG-51 and MCM-1. These new designs employ insulation schemes and other features to reduce the thermal stresses associated with a gas turbine startup transient. This paper discusses the finite element methodology and thermal transient stress results of analyses on the redesigned FFG-7 class uptake, and the DDG-51 class and MCM-1 class gas turbine uptakes. It briefly describes the applicable ASME Code criteria and follows this discussion with a fatigue life evaluation with regard to crack initiation. Some design features that would mitigate thermal transient stresses due to startup, such as placement of insulation, stiffeners, and thicknesses are presented. A description is given of the dynamic shock analysis methodology used for shock qualification, and the application of computational fracture mechanics to predict the remaining life of the uptake at a location with an initial crack.