Fatigue of Welded Tubular X-Joints in Offshore Wind Platforms

Abstract

Abstract The paper is part of the European research program JABACO (2015–2018), on the optimization of design and construction of offshore jacket platforms for supporting large wind turbines (5–10 MW) in water depths ranging from 30m to 80m. In particular, the paper describes an experimental investigation on the high-cycle fatigue performance of welded tubular connections, subjected to in-plane bending loading. Experimental results from seven (7) X-joint specimens are presented. The specimens were manufactured with 18-inch-diameter tubes and a brace-to-chord-diameter ratio equal to 1. Furthermore, the brace-to-chord-thickness ratio is equal 0.6, and the brace-chord angle is 90-degrees. The specimens are made of regular carbon steel grade 355, and have been fabricated using two different welding techniques: (a) manual (semi-automatic) welding (5 specimens); and (b) robot (automatic) welding (2 specimens). The comparison of the fatigue design life of those welding methods is a major objective of the present study. Prior to testing, numerical simulations have been performed to determine the critical locations around the weld toe, for proper instrumentation of the tubular specimens in terms of strain gage locations. This research work aims at a critical evaluation of available design standards, towards the development of more reliable design tools and reduction of the construction cost of the platform.