Wind energy is starting to count, with a price of wind power being increasingly competitive. Offshore wind turbines with a monopile support structure fixed to the sea bed in shallow water, have already been industrialized, while fixed turbines in deeper water and floating wind turbines for deeper water are emerging. Various floating concepts have been proposed for offshore wind application. The criteria for lifecycle integrity management refer to serviceability and safety, with minimal cost of power. An important aspect of the integrity assessment is the prediction of the dynamic behaviour of wind turbines consisting of the sub-systems: rotor-drivetrain, support structure and mooring, under environmental and operational loads considering both intact and fault turbine conditions. This presentation briefly touches upon recent developments of concepts, design criteria and focuses on methods for integrated dynamic analysis to determine the response in different sub-systems, as well as illustrates typical features of their behaviour through numerical studies.

The presentation deals with methods for the coupled hydro – aeroelastic analysis of offshore WT, either bottom fixed or floating, using time- and frequency-domain analysis approaches. The aerodynamic and aeroelastic characteristics of the WT along with the hydrodynamic characteristics of the supporting marine structure are properly coupled in the context of the multi-body non-linear dynamics. Each of the system components: the blades, the drive train, the tower, the jacket (or floating) supporting structure and possible mooring lines will be considered in the time domain as non-linearly interconnected flexible bodies based on beam theory. A frequency – domain approximate approach will be also presented as a design tool for a first evaluation of alternative floating designs. Here, the mass, damping and stiffness characteristics of the WT are obtained through reduced order models analysis with reference to the six DOF's of the supporting floater and superposed to its frequency dependent hydrodynamic characteristics.

Fatigue of welded tubular joints is a major and critical issue for safeguarding the structural integrity of offshore tubular platforms. An experimental investigation is presented on the fatigue performance of welded tubular connections, subjected to in-plane bending loading. Ten (10) X-joint specimens are tested in this experimental program. The brace-to-chord-diameter ratio is equal to 1 (8-inch-diameter tubes are used for the braces and the chord), the brace-to-chord-thickness ratio is equal 0.6, and the brace-chord angle is 90-degrees. The specimens are made of carbon steel grade 355, and have been fabricated using three different welding techniques: (a) manual [4 specimens]; (b) robot [4 specimens] and (c) manual with post-weld treatment (hammer peening) [2 specimens]. Numerical simulations are performed, so that the critical locations around the weld toe are determined, and proper instrumentation of the tubular specimen is made in terms of strain gauge locations. The results are aimed at critical evaluation of available design standards, towards the development of more reliable design tools and reduction of the cost of the platform.

A testing method was developed to load a tubular joint close to its resonance frequency, decreasing the testing time by a factor of 20. The test loads are successively in-plane and out-of-plane with different amplitude, allowing for testing the complete circumference of the weld. Four joints have been tested, and the results demonstrate that crack growth occurs significant after the first detection of the crack. The results are also compared to the fatigue design curve.