Fatigue Design Criteria as Function of the Principal Stress Direction Relative to the Weld Toe

For fatigue design it is necessary to provide guidelines on how to calculate fatigue damage at weld toes based on S-N data when the principal stress direction is different from that of the normal direction to the weld toe. Such stress conditions are found at details in different types of plated structures. Some different fatigue criteria for these stress conditions are presented in design standards on fatigue design. Criteria used by the International Institute of Welding (IIW), Eurocode, British Standard and in the DNV standards have been assessed against some relevant fatigue test data presented in the literature. Only proportional loading conditions have been considered here. (By proportional loading is understood that the principal stress direction is kept constant during a load cycle). An alternative equation for calculation of an equivalent or effective stress range based on stress normal to the weld toe and shear stress at the weld toe has been proposed. The proposed methodology can be used for nominal S-N curves and it can be used together with a hot spot stress S-N curve with stresses read out from finite element analysis. The different design criteria are presented in this paper together with recommendations on analysis procedure.

Novel Measurement System With Optical Fiber Sensor for Strain Distributions in Welded Tubular Joints

Defects and damage in a structural joint can trigger a critical degradation or collapse of the structure. Among joints, welded joints have many uncertainties, such as remaining stress, initial defects, and heat-affected zone. Such uncertainties in a welded joint can also induce a strain fluctuation, when they are subjected to a load. Conversely, the strain fluctuation along the weld line may represent the presence of uncertainties in the welded joint. This means that monitoring strain distribution along the weld line can be good way to assess their integrity and improve reliability of the structure. We have developed a novel distributed strain measurement technique which can measure strain distributions along fiber Bragg gratings (FBG) in an optical fiber and has the high spatial resolution and we applied it to strain monitoring of a welded tubular joint. The spatial resolution o is less than 1 mm and it was confirmed by measurement simulations and experiments. Then, we measured the strain distributions along the weld line of the tubular joint of a steel pipe subjected to a tensile load by the developed measurement system. We could successfully measure the development of the strain distribution along the weld line, where the sharply fluctuating strain distributions resulting from some defects were observed. In this paper, we describe the measurement principle and performance of the optical fiber sensor, and then show the results of strain monitoring in the welded joint.

Effects of Aging on the Mechanical Properties of Pipeline Steels

The intelligent design of a given pipeline system intended for operation beyond the elastic limit should incorporate specific features into both the base material (line pipe) and girth weld that enable the affected system to deform safely into the plastic regime within the intended strain demand limits. The current paper focuses on the mechanical properties known to influence the strain capacity of the base material (i.e., line pipe steel independent of the girth weld). Line pipe mechanical properties of interest include: longitudinal yield strength, tensile strength, yield to tensile strength ratio, reduction of area, elongation and uniform elongation. Of particular interest (in consideration of the conventional thermally applied corrosion protection coating systems to be employed), are the longitudinal mechanical properties in the “aged” condition. The present study investigates six (6) different pipeline steels encompassing grades X60 (415 MPa) to X100 (690 MPa), and includes both UOE Submerged Arc Welded - Longitudinal (SAW-L) and seamless (SMLS) forming methods.

Strain Based Failure Assessment for Offshore Structures Under Seismic Loading

Under seismic loading, structural hot spots can experience very high levels of stress and many random stress reversals. Conventional stress based methods cannot assess the failure state in detail when stress is beyond the elastic limit and nominal stress reversals are more than double the yield stress. A method has been created to fully reproduce the true stress/ strain history by using 1) generalised Masing’s rule with equivalent cyclic energy dissipation to model cyclic stress/strain relation, 2) Neuber’s method to calculate inelastic strain concentration factor, and 3) relative effective notch factor determined from comparing S-N curves of different joint classes. From this reproduced strain history, strain cycles can be counted and low cycle fatigue analysis can be conducted by using Miner’s rule and by estimating damage from the strain based failure criteria such as Coffin-Mason method. This method has been implemented in a numeric procedure and coded in a FORTRAN program called CYSTRA (as for CYclic STRain Analysis). It takes input of “nominal” random stress history directly from general structural software, linear or non-linear, local or global, and calculates extreme strain and strain cycles at multiple hot spots for the whole structure efficiently. Thus it greatly facilitates failure assessment for offshore structures which can have a large number of hot spots within the structure, unlike mechanical devices commonly assessed in strain based analysis where detailed FE based methods can be used.

Production of Metallurgically Cladded Pipes for High End Applications in the Oil and Gas Industry

The paper describes the different industrially used options to produce a clad pipe and explains in detail the manufacture of metallurgically cladded pipes starting with the production of roll bonded plates. In plate manufacturing the advantages as well as the limitations of thermo-mechanical (TM) rolling are discussed. The TM-technology is shown to improve weldability, HIC-resistance, strength and toughness properties of the carbon steel section of the pipe. Moreover, it also improves corrosion resistance of the CRA layer. The pipe manufacturing procedure, which involves two welding technologies for longitudinal welds is described. The carbon steel parts of the pipe are joined using double-sided multi-pass Submerged-Arc-Welding (SAW). The single-pass Electroslag-Welding (ESW) is subsequently used for recladding of the CRA layer. The multi-pass SAW results in excellent mechanical properties of the weld joint, whereas the ESW technique ensures low dilution of CRA with the carbon steel, a smooth weld bead shape and a high corrosion resistance of the deposited layer. With the aid of thermodynamic modeling and numerical simulations it is shown, that the high corrosion resistance is promoted by an intensive mixing within the ESW weld pool and relatively low segregation level of Cr and Mo during solidification. Furthermore, FEM analysis is applied to examine the plastic deformation and residual stresses distribution in the pipe during forming, welding and final calibration. The obtained information assists in optimization of manufacturing procedure, and can also be included in prediction of resulting pipe fatigue during operation.

Fatigue Resistant Threaded and Coupled Connectors for Deepwater Riser Systems: Design and Performance Evaluation by Analysis and Full Scale Tests

Threaded and Coupled (T&C) riser connectors with High Strength Steels have been developed for deepwater top tensioned riser (TTR) applications up to 10,000ft Water Depth. These developments have been ongoing for a decade, and the resulting solutions are now becoming the standard in the industry. Due to the stringent fatigue requirements involved, new design and performance evaluation methods were needed and have been built over time. In this article, we will demonstrate how these methods were implemented into the standard development process of T&C connectors, with a focus on finite element analysis (FEA) techniques. This process includes full scale tests programs on resonant fatigue frames, statistical post treatment of the resulting data, and fatigue cracks expertise for failure mechanism analysis. These elements are a key for the evaluation of T&C connectors’ fatigue performance and for the determination of influencing parameters, leading to the proper design optimization possibilities. The application of these methods will be illustrated with actual examples on T&C connectors’ recent developments. Namely, we will describe FEA methodologies, testing methods and results post-treatment techniques. We will show how the connectors’ performance is eventually derived after such analysis and test data accumulation. The reader will see that innovative and effective fatigue enhancement techniques have resulted, along with premium fatigue compliant sealing devices. The experience and expertise gained, together with a continuous improvement process of our methods have made T&C riser connectors a viable solution to meet emerging needs within deepwater industry, including xHP-HT, SCR and flow lines.

Sustained Load Cracking Resistance of Grade 23/29 Titanium GTA-Welded Pipe Joints

Grade 23 and/or 29 titanium alloy pipe and forgings are typically butt-welded together in the fabrication of offshore riser components such as tapered stress joints (TSJs) for top-tensioned risers and as hang-offs for dynamic catenary risers. Although Grade 29 titanium base metal in a relevant wrought/forged product form has already been evaluated in regards to sustained-load cracking (SLC) resistance, minimal data is available to ensure that the SLC resistance of typical GTA butt-welded joints in these thicker-wall titanium alloy components will also meet design requirements. As part of a TSJ production weld qualification, conservative fracture-mechanics based SLC tests were conducted at room temperature on 1G-position machine GTA butt-welded Grade 23 titanium pipe utilizing Grade 29 titanium filler metal. Test results revealed no significant SLC susceptibility in the weld and a minor effect in HAZ metal, producing KSLC values similar to KQ values. These values safely meet typical TSJ fracture mechanics requirements, and are consistent with published SLC information on this alloy system.

Cyclic and High Strain Behavior of Titanium Alloys Used for Riser Components

Grades 23 and 29 titanium alloys are becoming more commonly used offshore for critical components of riser systems. Many of these components, such as compact flanges, operate at high mean strains due to make-up loads, often close to or above yield. Hitherto, weld fatigue data collected at low mean strains have been used for prediction of fatigue lives of such components. However, these analyses have resulted in short but adequate lives. The objective of this work was, therefore, to derive fatigue data that could be applied to components operating at high strains and over a large range of stress ratios. This necessitated strain-controlled fatigue testing. Cyclic material properties evaluated from the tests are presented. In addition, it was found that sustained strain load, a type of stress relaxation that, unlike normal stress relaxation, does not require elevated temperatures, affected the cyclic behavior It also affects the distribution of make up stresses. This phenomenon was also investigated and it was shown that the cyclic stress/strain curve, readily derived from strain-controlled fatigue tests, accounted for sustained strain load effects.

The Importance of Mode Number on In-Line Amplitude of Vortex-Induced Vibration of Flexible Cylinders

Most empirical codes for prediction of vortex-induced vibrations (VIV) has so far been limited to cross-flow response. The reason for this is that cross-flow amplitudes are normally larger that in-line amplitudes. Additionally the in-line response is considered to be driven by the cross-flow vibrations. However since the in-line frequency is twice the cross-flow frequency, fatigue damage from in-line vibrations may become as important and even exceed the damage from cross-flow vibrations. A way to predict in-line vibrations is to apply traditional methods that are used for cross-flow VIV and establish an empirical relationship between the cross-flow and in-line response. Previous work suggests that the ratio between the in-line and cross-flow amplitudes depends on the cross-flow mode number, Baarhom et al. (2004), but the empirical basis for this hypothesis is not strong. The motivation for the present work has been to verify or modify this hypothesis by extensive analysis of observed response. The present analysis uses complex data from experiments with wide variations in the physical parameters of the system, including length-to-diameter ratios from 82 to 4236, tension dominated natural frequencies and bending stiffness dominated natural frequencies, sub-critical and critical Reynolds numbers, different damping coefficients, uniform and sheared flows, standing wave and traveling wave vibrations, mode numbers from 1–25th, and different mass ratios. The conclusion from this work is that the cross-flow mode number is not the important parameter, but whether the frequency of vibration in the cross-flow direction is dominated by bending stiffness of tension.

Benchmark Studies Using Virtual Model Basin for Resistance and Diffraction: A RANS CFD Approach

Virtual Model Basin (VMB) developed based on RANS CFD Approach along with VOF model to simulate free-surface has been used to perform benchmark studies and the results are presented in this paper. The VMB based on general purpose CFD solver ANSYS FLUENT has been used to simulate resistance and diffraction problems for a Navy surface combatant hull and the results are validated against experimental data. The resistance simulations are done to assess two turbulence models and best among the two is used to solve the diffraction problem. The validation results suggest that the VMB approach reproduces the flow features, forces and moments accurately.