Efficient Fatigue Testing of Tubular Joints

Tubular joints are intensively used in off-shore structures for shallow waters. Depending on the sea conditions and the type of structure, the design can be fatigue driven. This is particularly the case for off-shore wind turbines, where turbulences are generating a fatigue loading. Any improvement of the fatigue performance of the tubular joint would be beneficial to reduce the weight and the cost of the structure. To assess efficiently the fatigue resistance of the tubular joint, a testing method has been developed based on the resonance principle. The complete circumference of the welded joint can be loaded, successively in the in-plane and out-of-plane modes at a frequency close to 20Hz. Finite element computations were used to investigate the feasibility of the concept. Then, an X-node was made and successfully tested to investigate the stress distribution along the weld. The experimental results were compared with finite element computations, giving a good agreement.

Influence of Porosity on the Integrity Properties of Butt Welded Super Duplex Stainless Steel Umbilical Tube

Super Duplex Stainless Steel (SDSS) tube is the main component in steel tube umbilicals for the transportation of hydraulic fluids and chemicals, or gas injection into the subsea well. The umbilical tube is made from seamless tube joined together by girth weld. Therefore, porosity is one of the main weld defects that could be discovered within the welds, and these defects can have significant impact on the execution of the whole project, in term of both schedule and cost. This paper reviews the design codes and standards relating to the porosity acceptance criteria, and discusses the recent investigation on the influences of porosity (number, size and location) to the stress distribution within the weld. The study was performed using both FEA and practical fatigue test. The results demonstrated that the distance of the pores to the weld surface is a critical parameter to the increase and localisation of stress as the pore starts to interact with the weld root or cap. In addition, significant pore stress interaction has been observed when a bending moment is introduced to the weld. The study has demonstrated that the current porosity acceptance criteria applied to umbilical manufacture in the subsea industry are typically conservative and can be modified.

Practical Evaluation of the Strain Rate and Temperature Effects on Fracture Toughness of Steels

Overview of the quantitative evaluation procedure of strain rate and temperature effects on fracture toughness proposed by the authors is introduced. Important concept of former researches is that the fracture toughness is a function of the strain rate-temperature parameter (R), which enables to unify both strain rate and temperature effects for the mechanical properties of materials. Using this knowledge, the equivalent temperature shift values at arbitrary strain rate from static loading condition are proposed.

Effect of the Cooling Rate on Crystallinity and Tensile Strength of Two PVDF Grades

The PVDF (poly(vinylidene fluoride)) is the recommended material for pressure barrier application when the temperature is above 90 C and below 130 C (API Specification 17J). Two grades of PVDFs used in flexible raisers were processed. The grades selected were those for extrusion, but they were compression molded instead. The processing conditions were the same for each grade, from slow to fast cooling rate. The results allowed the evaluation of their performance using a simpler processing technique and, also, to observe how the mechanical properties varied with the cooling rate applied.

Fatigue Crack Growth Rate of Reeled Pipe in Sour Environments

Fatigue Crack Growth Rate (FCGR) of reeled pipe (strained & aged) in sour environments was investigated. FCGR frequency scans on different microstructures, i.e. heat affected zone (HAZ), and weld center line (WCL), revealed that, FCGR in corrosive environments increased with decreasing frequency and reached a plateau value at low frequencies of 10mHz to 3mHz. At these ‘plateau frequencies’, FCGR in the moderately sour environment that was investigated were found to be about 10–18× or 30× higher than the in-air values for the WCL and HAZ, respectively. There was no effect of the reeling cycles on the FCGR of the WCL or HAZ specimens. The FCGRs of the WCL were consistently lower than that of the HAZ by about a factor of 2–3× under various conditions. The reason for the lower FCGR of the WCL is not well understood. It is possible that it may be due to the higher yield strength (YS) of the overmatched welds, differing hydrogen concentration and/or diffusion coefficient or possibly due to the differences in the microstructure between the HAZ and WCL. Paris law curves, FCGRs as a function of ΔK (stress intensity factor range), were measured on the HAZ, and WCL (both intrados) at the plateau frequency (10mHz), representative of flowline cyclic loading. They were also measured at a higher frequency of 0.33Hz, representative of Steel Catenary Risers (SCR) cyclic loading associated with wave motion. Comparisons of measured Paris law curves in corrosive environments to those in air were consistent with the results of the frequency scans. There was no effect of number of cycles of reeling on the Paris law curves in the sour environment tested for WCL and HAZ specimens at both the plateau frequency and 0.33Hz. The results of the test program suggest FCGR of WCL and HAZ in the sour environment tested are not affected by number of cycles (up to 5) of straining on the intrados side for the strain level (1.93% per cycle) used in this study.

Whitening Generation in PVDF Under Cyclic Loading

Whitening is defined as the change in the opacity of a polymeric material due to microvoid nucleation and/or grow, resulting in volume increase and possible modification of the mechanical properties. Its formation may arise from processing conditions or mechanical stresses acting in the polymers. The understanding of the whitening phenomenon and its effect on mechanical behavior is of great importance for engineering application; for instance, poly (vinylidene fluoride) (PVDF) is widely used as pressure barrier in flexible raisers, where high pressure and aggressive media are presents and the presence of whitening may increase the gas permeation rate. The present study, cyclic loading experiments were performed on PVDF to generate whitening in a controlled manner. Subsequently, the whitening effect on the mechanical properties were evaluated using tensile tests and the structure was evaluated using X-ray diffraction, while the fracture surface was analyzed via scanning electron microscopy. It was observed that cyclic load was able to nucleated the voids and there was an increase in the crystallinity and in the mechanical properties of the material.

Hydrogen Induced Corrosion Cracking and Crevice Corrosion Behavior in Natural Sea Water of Super Duplex Seam Welded Tubes for Umbilicals Applications

Umbilicals are important tubes assembly essentially used for subsea equipment controlling functions and fluid injection. Super duplex stainless steel 2507 (UNS 32750 / EN 1.4410) has been the historical grade used for its corrosion resistance and mechanical properties. The Oil and Gas industry is gradually working on the exploration and exploitation of offshore and deep offshore resources. These operating conditions lead to new philosophy in the design of subsea umbilicals, which are evolving to answer the needs of new functionalities. Recent years development supported by O&G operators, have seen innovative super duplex stainless seam welded tube solution being used for their high mechanical characteristics (YS0.2≥ 750 MPa; UTS≥ 900 MPa), tight dimension tolerances, and their ability to generate cost saving during installation. The specific metallurgical manufacturing route for the 2507 strip together with an efficient roll forming process and NDT control allows to obtained tubes for umbilicals which push the usual boundaries of utilization of this super duplex grades. In this paper, Hydrogen Induced Stress Cracking (HISC) behavior, possible failure mode of the super duplex stainless steel tubes exposed to cathodic protection in the termination unit, was extensively studied. A stress load of 110% YS0.2 was applied using a dead weight bench and the tubes performance was characterized after 500h of exposure in synthetic sea water under cathodic protection. Crevice corrosion is also a reported failure mode for umbilical tubes. Corrosion resistance of the 2507 seam welded tubes was also assessed in natural sea water in crevice conditions at open circuit potential (OCP). No failure was observed during the HISC testing in synthetic sea water after 500hours of exposure and no micro-crack was detected on the tubes after characterization. The metallurgical microstructure of the laser weld is not preferential initiation zone for crevice corrosion.

An Experimental Investigation About Fatigue Crack Growth Rate of Aluminum Alloy Considering Various Mean Stress

In general, aluminum alloy is a commonly used for liquefied natural gas (LNG) storage systems. In this regard, it is important to know exact mechanical properties at cryogenic temperature. There are many researches to assess mechanical properties of aluminum alloy, such as tensile strength, fatigue performance and fracture toughness. Fatigue crack growth rate (FCGR) is important to predict the service life. In particular, mean stress effect can significantly affect the fatigue life. In this regard, this study carried out by a series of FCGR test at five different stress ratios (R=0.1, 0.3, 0.5, 0.7 and 0.85). The major objective of this paper is to suggest a new model that can consider the mean stress effect on FCGR of aluminum alloy in a unified manner. A mean stress equation is incorporated into Paris’ law. In order to validate the model, FCGR test data of aluminum alloy is compared with Walker’s relationship. Compared to the other existing model, the new model is found to exhibit more accurate result compared to Walker model.

Evaluation of Aged PVDF by Instrumented Indentation Technique

Instrumented indentation (IIT) is proposed as an alternative mechanical evaluation technique to identify ageing of semicrystalline thermoplastic polymers such as PVDF. This technique does not require special geometries such as standard tensile experiments and allows following changes in mechanical properties as deeper penetration depth are imposed. Therefore, it is able to detect differences in mechanical properties along the thickness of the material. For PVDF aged in crude oil, it was found that for shallow indentations the viscoelasticity effects are minimized. For deeper indentations it was possible to observe a loss of rigidity with aging time. The technique also allows to determine yield behavior.

Current and Future Tendency of Monitoring Technology for Integrity Assessment and Life Extension of the Hull Structure of Floating Production, Storage and Offloading (FPSO) Vessel

This paper critically reviewed commercially available hull stress monitoring systems (HSM) and summarized notations for vessels with hull stress monitoring systems from major classification societies. The authors recommended that, for every FPSO vessel, at least one HSM should be equipped in compliance with regulation from the International Maritime Organization. Additionally, sixteen types of sensing technologies were identified for monitoring of FPSO Vessel. Depending on the technology matureness and readiness for application in marine structures, they were categorized into three groups: Proven, Un-Proven and State-of-the-art technology. Furthermore, challenges associated with FPSO hull monitoring including Sensor Systems, Communication and Data Analysis techniques were examined. It is concluded that multiple sensing technologies should be systematically combined to provide more accurate information. Based on these discussions, a feasible solution that integrates proper wireless sensor network technology and Bayesian Network modeling was proposed as the future direction of FPSO hull monitoring system.