Corrosive fatigue failure of submarine pipelines is very common because the pipeline is immersed in a sea environment. In Bohai sea, many old pipelines are made of API 5L X56 steel materials, and it is necessary to provide an accurate method for predicting the residual life of these pipelines. As Paris law has been proven to be reliable in predicting the fatigue crack growth in metal materials, the two constants in Paris law for API 5L X56 steel materials are obtained by using a new proposed shape factor based on the analysis of experimental data measured from fatigue tests on compact tension specimens immersed in the water of Bohai sea. The results of the newly proposed shape factor show that, for a given stress intensity factor range (ΔK), the fatigue crack growth rate (da/dN) in seawater is 1.6 times of that that in air. With the increase of fatigue crack growth rate, the influence of seawater on corrosive fatigue decreases gradually. Thereafter, a finite element model for analyzing the stress intensity factor of fatigue crack in pipelines is built, and the corrosive fatigue life of a submarine pipeline is then predicted according to the Paris law. To verify the presented method, the fatigue crack growth (FCG) behavior of an API 5L X56 pipeline with an initial crack under cyclic load is tested. Comparison between the prediction and the tested result indicates that the presented method is effective in evaluating the corrosive fatigue life of API 5L X56 pipelines.

To predict the static strength of a welded tubular joint at elevated temperature using finite element simulation, two methods in the literature were reported. The first method aims to analyze the static strength of a tubular joint at a specified elevated temperature, and a routine mechanical analysis is carried out by defining the material properties at the specified elevated temperature according to some specifications. This method does not consider the heat transfer process of the tubular joint in a fire condition. The second method is used to determine the static strength of a tubular joint using a combination of transient state heat transfer analysis and mechanical analysis. The tubular joint subjected to a specified load is heated in accordance with ISO 834-1 standard fire curve to fail at a critical temperature, and the specified load is considered as the static strength of the joint at the critical temperature. In this study, a detailed parametric study on the failure process of circular hollow section tubular T-joints at elevated temperature is carried out using finite element method. The static strengths of the circular hollow section T-joint models obtained from the two methods are compared. The comparison shows that the first method produces a higher estimation on the static strength compared to the second method. Finally, the effect of some geometrical parameters, chord stress ratio, and elevated temperature on the difference of the two methods is also investigated.

This study presents a design method for predicting the static strength of a Circular Hollow Section (CHS) tubular K-joint at elevated temperature. The presented method is obtained based on a parametric study by the authors (He et al., 2015a). In determining the static strength of a tubular K-joint at elevated temperature, a deforming rate criterion is proposed, and it is proved to be much safer and more reasonable for design purposes rather than the conventional definition on that static strength by using 3% of chord ovalisation for the K-joint at ambient temperature. In the presented design method, a strength reduction factor k is proposed, and the static strength of a CHS K-joint at elevated temperature can be obtained simply by multiplying the static strength at ambient temperature with k. The strength reduction factor k is a product of two factors, a revised reduction factor of elastic modulus (λ) and a revised chord stress ratio (k _{n′, θ} ). From the finite element results of overall 57 K-joint models, the accuracy of the presented design method is assessed to be reliable and accurate.