Fatigue of Welded Joints in Steel Marine Structures

Abstract

Fatigue failure is a governing limit state for marine structures. Marine structures are composed of the numerous structural members connected by the welded joints which are the typically governing fatigue sensitive locations. In this thesis, attention has been paid to fatigue of welded joints from design and from testing perspective. The double sided longitudinal attachment is a common structural detail in marine structures and available resistance information, SN data, has been used to investigate the life time estimate accuracy in case the total stress is adopted as fatigue damage criterion. The single sided butt joint is a common structural detail in marine structures like steel catenary risers and a specimen has been designed for testing using the hexapod in order to ensure fatigue induced failure at the required location.Fatigue design of double sided welded longitudinal attachments using a Total Stress criterion
The recently developed total stress concept considers both the weld notch and far field characteristic contributions. The total through-thickness weld notch stress distribution has been adopted to establish the total stress fatigue damage criterion and corresponding fatigue resistance curve. Thanks to a semi-analytical formulation, it is easy to determine the total weld notch stress distribution. The involved weld load carrying stress component is a characteristic one and unique for each weld notch location. A double weld element beam model has been developed to replace the original single weld finite element beam model in order to capture correct estimates for the double sided longitudinal attachments. Alternatively, a parametric has been established as well. The weld notch angle has turned out to be a governing parameter. Evaluation of the fatigue resistance of the welded double sided longitudinal attachment using the Total Stress concept shows a better accuracy in comparison to the nominal stress, hot spot structural stress and effective notch stress concept results. A governing factor for double sided longitudinal attachments is investigated which is the base plate thickness tb. Although, the IIW classifies the longitudinal attachment based on the attachment length la, the base plate thickness tb is much more dominant. Fatigue testing of single sided butt joints using a Hexapod
The riser system forms a significant part of the development costs for floating offshore oil production facilities. The SCReen joint industry project aims to optimise the fatigue design and maintenance costs for Steel Catenary Risers (SCR’s). Generally, published fatigue resistance data of SCR welded joints are performed using resonance bending tests, which have two major limitations: a very high mean stress state and unrealistic variable amplitude loading. In order to obtain realistic fatigue resistance data, the SCReen project plans to do the fatigue tests with the Hexapod. A dedicated specimen containing the critical single sided butt joint has been designed  in order to obtain fatigue failure at the required location, involving two girth welds at the flange-pipe connection and the single sided butt joint at the middle of the specimen. With a post welding improvement, fatigue failure at the intended weld root location can be obtained.