Session: MF-15-01 Fatigue and Fracture of Welds and Heat Affected Zones-1

Paper Number: 155026

155026 - High-Cycle and Low-Cycle Fatigue Resistance of Welded T-Joint Plate Connections Made of Mild and High-Strength Steel for Offshore Energy Applications 

Abstract: 

The present paper examines the fatigue performance of eighteen (18) welded plate T-joint specimens, commonly used in offshore tubular steel structures. The research work is part of an extensive national research program with the acronym SIRENES, that aims at improving the structural performance of offshore floating platforms installed in deep-water offshore locations. It extends the work performed by the research team in previous European research projects for developing a hybrid tension-leg-platform (TLP), suitable for combined offshore wind and wave energy exploitation. This TLP constitutes the basis for the present research and has a triangular configuration, consisting of one cylinder that supports the wind turbine tower, and three corner cylinders, connected with tubular members (braces). Each corner cylinder is capped with a double-skin shell that traps the air, operates as an oscillating water column (OWC), producing wave energy.

Towards extending the fatigue life of the steel platform, the application of high-frequency mechanical impact (HFMI) post-weld treatment and the use     of high-strength steel are considered. The main objective is to study the effect of HFMI on the structural performance of welded joints under high/low-cycle fatigue conditions, especially for high-strength steel. HFMI post-weld treatment is a newer and user-friendly version of hammer/needle peening, with advantages over conventional hammer peening. Its implementation on welded connections could be beneficial, extending their fatigue life and, consequently, the life of the entire offshore platform.  

Dedicated fatigue tests on full penetration welded T-joint specimens are performed, representing the brace-to-cylinder connections of the prototype offshore floating structure. The specimens are made of mild steel S355 and high-strength S700 grade steel, and subjected to four-point bending cyclic loading under constant load amplitude histories, in high- and low-cycle regime (R=0.5 and -1, respectively). Two different welding processes for the connection have been employed: (a) manual (semi-automatic) weld; (b) manual weld with High-Frequency Mechanical Impact (HFMI) post-welding treatment. Crack propagation along the weld toe is monitored during testing using magnetic particle inspection method and strain gauges are placed at critical locations to monitor the local strains in low-cycle fatigue tests.

The experiments are supported by a fractographic examination and a finite element analysis developed in ABAQUS/Standard. A three-dimensional numerical model has been employed for the numerical simulation of the tests, which simulates the welded connection. The constitutive model employs von Mises flow theory of plasticity, with isotropic hardening calibrated through uniaxial tests of coupon specimens extracted from the same steel sheet used for the specimens’ fabrication.

 

Presenting Author: Theocharis Papatheocharis University of Thessaly

Presenting Author Biography: Theocharis Papatheocharis, is a Civil Engineer, PhD and Research Associate at the U Thessaly, with expertise in experimental methods and laboratory testing in structural engineering and mechanics, focusing on fatigue of metal components.

Authors:

Theocharis Papatheocharis University of Thessaly
Christos Mourlas University of Thessaly
Ilias Gavriilidis University of Thessaly
Spyros Karamanos University of Thessaly
Anna Zervaki National Technical University οφ Athens