Analysis of fatigue behaviour of drill pipe on pin-box connection

Drilling is one of the costliest and risky activities in oil and gas industry due to complexity of interactions with downhole formation. Cyclic loads while drilling cause the initiation and growth of cracks in oil tubulars. This phenomenon, known as fatigue, results in permanent reduction of the failure-free service envelope of a certain tubular. Further, most of the drill string failures are triggered by fatigue, which results from repetitive cyclic bending loads and stresses in tensile or buckled drill strings. Fatigue is a cumulative and non-reversible condition induced by repetitive cyclic bending loads and tensile or buckled drill pipe stresses. Fatigue exists even though cyclic tension of the drill pipe material is much lower than static strength limit. Present work investigates fatigue capacity of the 4” WT38 drill pipe connection where cracks have been observed. In accordance with the geometry of connection, a hot spot stress from bending moment is calculated in the crack location in the first thread (upper) of the pin. The DNVGL-RP-C203 SN-curve B1 “in air” is used as relevant fatigue SN curve for the drill pipe thread location. Finite element method (FEM) is employed in modeling and analyzing of drill pipe on pin-box connections. With this method, various connections can be investigated relatively faster and cheaper compared with experimental tests. It is found that the fatigue failure may have been caused as a result of the cyclic load level and number of load cycles. A detailed discussion of the fatigue damage assessment concludes the paper.

Scaling of SN Curves for Varying ‘Initiation’ Crack Definitions From Striation Counted Environmental Fatigue Specimens: A 250 Micron Austenitic Stainless Steel SN Curve and Associated Fen Factors

Future code development to adopt a risk-informed design methodology will require improved accuracy of fatigue initiation predictions. The ability to account for through wall strain gradients in plant components, particularly over the first 3 mm of wall thickness is one area where conservatism can be reduced. This is due to extant design fatigue curves being derived from strain controlled membrane loading tests where the 25% load drop definition for end of test equates to approximately a 3 mm crack. Being able to define initiation fatigue curves for much shorter crack depths would enable fatigue crack growth methods to then predict the additional cycles, taking into consideration the strain gradient, required before the defined end of life crack size is reached. The R5 procedure provides a method to adjust an existing Stress-Life (SN) curve representing an initiation crack depth, to a smaller depth. This method was developed for materials at higher temperature and for a CO2 cooling environment, thus its validity was uncertain for application to a Pressurised Water Reactor (PWR) plant. This paper details the development of best estimate and design basis SN curves and environmental fatigue enhancement factors (Fen) for crack initiation to a depth of 250 μm. It is concluded that the general methodology in R5 was found, through this work, to adequately describe fatigue initiation lives for stainless steel in a PWR environment when augmented with a crack size dependent Fen equation and with modified coefficients determined through regression. The resulting method is similar to R5 in that an SN curve can be derived for any crack size, however the current data set only provides validation down to a crack size of 250 μm, as striations at shorter depths were not visible with existing methods.

A Method to Calculate the Multi-Axial Fatigue of Subsea Rigid Jumper due to VIV

Prediction of vortex induced vibration (VIV) and estimation of fatigue damage for subsea multi planar structures exposed to significant current condition is an important topic for subsea engineering. Due to the multi-planar characteristics of structure, the VIV induced stress states of subsea jumper are normally with the combination of flexural and bending stresses. Consequently, the associated fatigue is a multi-axial fatigue problem. In this study a novel energy-critical plane method by Farahani [1][2] has been used for fatigue assessment instead of the traditional method with utilizing the stress range and SN curve, for example the 1st principle stress method as recommended by DNVGL-RP-F105 (2017) [3]. By using this method, the phase change of the stress from different modes is considered. A case study based on previous jumper experiments by ExxonMobil is included. Fatigue results have been compared with DNVGL-RP-F105 (2017) [3] approach. Advantages of energy-critical plane approach have been discussed. Phase change between flexural and torsional stresses has been successfully captured and its effect on fatigue damage has been presented. Additionally, some discussions have been performed with respect to the application of DNVGL-RP-F105 (2017) [3] to calculate the VIV fatigue to subsea rigid jumpers.

A CRITICAL ASSESSMENT OF KASSAPOGLOU'S STATISTICAL MODEL FOR COMPOSITES FATIGUE

Kassapoglou has recently proposed a model for fatigue of composite materials which seems to suggest that the fatigue SN curve can be fully predicted on the basis of the statistical distribution of static strengths. The original abstract writes expressions for the cycles to failure as a function of R ratio are derived. These expressions do not require any curve fitting and do not involve any experimentally determined parameters. The fatigue predictions do not require any fatigue tests for calibration". These surprisingly ambitious claims and attractive results deserve careful scrutiny. We contend that the result, which originates from the reliability theory where exponential distributions is sometimes used to model distribution of failures when age (or wearout) has no influence on the probability of failure, does not conform to a fatigue testing with the resulting SN curve distribution. Despite Kassapoglou's attempt to use a wearout law which seems to confirm this result even with wearout, we contend that a proper statistical treatment of the fatigue process should not make wear-out constants disappear, and hence the SN curves would depend on them, and not just on scatter of static data. These concerns explain the large discrepancies found by 3 independent studies which have tried to apply Kassapoglou's model to composite fatigue data.

Failure Analysis of Automatic Access Pedestrian Gate Turnstile using SolidWorks Simulation Model

This research work is focused on the failure analysis of Automatic Access Pedestrian Gate Turnstile using SolidWorks Simulation Model. Failure analysis was carried out on both static and dynamic failure of the system putting into consideration the yield stress, resultant displacement, failure cycle, percentage damage, maximum strain and most importantly factor of safety. The results of Von Mises stress analysis showed that a maximum value of 5.77Mpa yield strength was obtained. A displacement of 0.257737m and a strain value of 2.03989×10-5 were obtained respectively. The fatigue simulation damage factor was not up to 100% though is up to 50%. At the endurance limit or fatigue limit, the SN curve flattens at 106 cycle. A safety of 61 was recorded with the system which is above 1, thus the system is safe and efficient and can be used for security check.