Fatigue of welded tubular X-joints in offshore wind platforms

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. Copyright © 2019 ASME

Fatigue resistance of welded steel tubular X-joints

High-cycle fatigue experiments are performed on welded tubular steel X-joints, with braces and chord of equal diameter. They are scaled-down joints, used extensively in offshore wind platforms. Three different welding procedures are considered in specimen fabrication: manual, fully-automatic and manual with HFMI post-weld treatment. Τwo possible locations for crack initiation were identified: chord “crown” and “in-between location”, also verified by numerical calculations and fractography of failed specimens. Monotonic loading tests on fatigue-cracked specimens showed good performance in terms of ultimate strength and deformation capacity, despite the presence of through-thickness cracks. The results are compared with predictions from relevant design standards. © 202