API 5L Research Articles

Failure Investigation of a Defective Weldment of an Oil Product Transmission Pipeline

An API 5L X52 oil product transmission pipeline experienced a sudden failure at a portion which had been buried in the bed of a river crossing after 33 years of service. Results showed that mechanical properties of the weldment had been reduced due to weld defects. The defects had been formed during construction welding and installation of the pipeline in the bed of the river crossing. No failure had occurred until this portion of the buried pipeline came out of its place in the bed of the river crossing. Under this condition, defective weldment (due to hydrogen stepwise cracking, incomplete fusion, porosity formation, and entrapped slag inclusion) was not capable of enduring stress imposed by the strong river stream and consequently failed. Metallographic and SEM studies as well as stress analysis were used for pipeline fracture investigation.

Evaluation of the influence of post welding heat treatments on microstructure and mechanical properties of API 5L X70Q weld joints

Steels that meet the API 5L standard are employed in pipeline construction, and there is considerable interest in those of higher strength, such as grade API 5L X70Q. There have nevertheless been few studies relating to seamless pipes. During installation, the pipes may be welded with covered electrodes (SMAW) and may optionally undergo inductive heating for adjustment to the topography of the terrain, processes that may alter the microstructure of the joints. In this work, welded joints were obtained by the SMAW process, from seamless pipe of grade API 5L X70Q. The joints were submitted to post-weld heat treatments at temperatures of 920 and 600 °C. Tensile, Charpy impact and microhardness tests were carried out, besides metallographic characterization in specimens of base metal and in the joints, as-welded and after the heat treatments. Heat treatment at 920 °C promoted formation of a ferrite/pearlite structure, with consequent reduction in the mechanical strength of the joints to values below those specified for grade X70Q. However, heat treatment at 600 °C proved to be beneficial owing to the reduction of the volume fraction of the martensite/austenite and martensite-austenite-bainite constituents, without significantly reducing the mechanical strength and with a relative gain in impact toughness in the heat-affected zone and in the fusion zone (FZ).

Use of a Ring DWTT Specimen for Determination of Steel NDT from Pipe of Diameter Less than DN500

It has become recognized that the drop weight tearing test (DWTT) energy better represents the ductile fracture resistance of pipe steels since it utilizes a specimen that has the full thickness of the pipe and has a fracture path long enough to reach steady-state fracture resistance. However, the API 5L code does not require it for pipe sizes less than DN500. The aim of this paper is to propose a DWTT specific to small diameter pipes based on a new specimen, the ring drop weight tearing test (RDWTT) specimen; to evaluate the transition temperature T t, DWTT and nil ductility temperature of the pipe steel API 5L X65; to introduce the transition temperature T t, DWTT in the transition temperature material master curve of the API 5L X65 steel; and to compare the prediction of the crack ductile extension in a pipe based on the RDWTT’s energy and crack tip opening angle in the case of the steel API 5L X65.

Microstructure and wear behavior of Fe-based amorphous HVOF coatings produced from commercial precursors

Abstract Wear resistant highly amorphous Fe 60 Cr 8 Nb 8 B 24 (at.%) coatings of about 280 μm thickness were prepared through high velocity oxygen fuel (HVOF) thermal spray process onto API 5L X80 steel substrate. Feedstock powders were produced by gas atomization with low purity precursors by modifying AISI 430 stainless steel with additions of niobium (Fe-Nb) and boron (Fe-B). It was found that the coatings were mostly amorphous with some embedded FeNbB and Fe 2 B borides. The formation of a large fraction of amorphous phase was attributed to the high cooling rates of molten droplets combined with a proper powder composition. The average Vickers hardness of the coating (HV 0.3 = 838 ± 23) was about four times higher than that of the API 5L X80 substrate (HV 0.3 = 222 ± 5). The excellent wear resistance of the amorphous coating in the pin-on-disc measurements was attributed to its large fraction of amorphous phase (~ 66%) with reinforcing hard Fe 2 B and FeNbB borides, high Vickers hardness, low oxygen content ( − 5 and 8.5 × 10 − 4 mm 3 ·N − 1 ·m − 1 , respectively). API 5L X80 steel exhibited dominant adhesive wear at low sliding speed and oxidative wear at high sliding speed. HVOF coatings presented oxidative wear regardless of the sliding speed.

Reliability estimation of pressurized API 5L X70 pipeline steel under longitudinal elliptical corrosion defect

The probabilistic approach is the best way to give realistic answers for design and maintenance and it is considered as a powerful decision-making tool. In the present paper, assessment of the failure probability of corroded pipeline subjected to internal pressure is estimated by using the first order reliability method (FORM). Measurements of defect dimensions have been achieved changed into collected by using ultra sound inspection probe through a 75-km pipeline portion on one of the main Algerian high pressure gas transportation that has been in service for 30 years. The sensitivity analyses have been carried out on random variables to identify the importance of the parameters within the reliability mechanical model. The modified B31G code has been coupled as a mechanical model, with Rackwitz optimization algorithm by using PHIMECA Software. The limit state function resumes the difference between the pipeline burst pressure and the pipeline operating pressure. The aim of the present work is to make in evidence the competitive importance of pressure service, wall thickness and the defect depth with regards to defect length and to determine the influence of the coefficients of variation on the failure probability and the remaining life of the pipeline. A diagram based on the reliability index results is proposed to predict the pipeline degradation and it can be used as a decision tool for maintenance program.

Effects of alternating current interference on the cathodic protection for API 5L X60 pipeline steel

In this work, the effect of alternating current (AC) interference on the cathodic protection (CP) of API 5L X60 pipeline steel was investigated using electro-chemical and mass-loss methods, as well as surface analysis techniques. The results demonstrate that, the corrosion potential of API 5L X60 steel shifts, and the polarization current density increases, with the AC interference. At a lower AC interference density, the existing standard CP potential (−850 mV (vs. a CSE)) can provide full protection for a buried API 5L X60 steel pipeline. However, at a higher AC interference density, the potential cannot ensure the safety of the pipeline. The results of a CP of −950 mV (vs. a CSE) demonstrate that a more negative CP potential can relieve the AC corrosion risk for buried API 5L X60 pipeline at higher AC interference densities. This finding benefits pipeline safety design and operation and weakens the AC-induced corrosion often observed in buried pipelines.

Effect of Stress on Corrosion at Crack Tip on Pipeline Steel in a Near-Neutral pH Solution

In this work, the local corrosion at crack tip on an API 5L X46 pipeline steel specimens was investigated under various applied loads in a near-neutral pH solution. Electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy, combined with micro-electrochemical technique and surface characterization, were conducted to investigate the effect of stress on local anodic solution of the steel at the crack tip. The stress corrosion cracking of the steel was dominated by an anodic dissolution mechanism, while the effect of hydrogen was negligible. The applied load (stress) increased the corrosion rate at the crack tip, contributing to crack propagation. The deposit of corrosion products at the crack tip could protect somewhat from further corrosion. At sufficiently large applied loads such as 740 N in the work, it was possible to generate separated cathode and anode, further accelerating the crack growth.

Residual Stress Evaluation by X-Ray Diffraction and Hole-Drilling in an API 5L X70 Steel Pipe Bent by Hot Induction

The API 5L X70 steel is used in high-pressure gas transmission pipelines. Because of this, knowledge of presence of residual stress and their magnitude is important to assess the material integrity in service. For the pipeline manufacturing, tubes need to be curved which is often made using the hot induction bending process. This process can introduce different residual stress depending of tube position. For this research, in order to evaluate the residual stress, was used an API 5L X70 tube that was previously curved by hot induction process. Samples were taken from the extrados, intrados, neutral line and straight section of the curved tube. Residual stresses were studied by two conventional methods: X-Ray Diffraction (XRD) and Hole-Drilling, which are destructive and non-destructive methods, respectively, in order to assess their qualitative responses. Each of these methods presents particular methodologies in sample preparation and material analysis, but also they differ in factors such time consumption and cost of the analysis. The qualitative responses obtained by the two different methods were comparable and satisfactory and pointed out the existence of a compressive residual stress state in steel pipe.

The Effect of Carbon Dioxide Pressure in Carbon Steel Corrosion in 5 Wt.% NaCl Solution Using Flow Loop

The pipelines transport a raw, unrefined well stream, usually consisting of a multiphase flow of gas, oil and water contained carbon dioxide, hydrogen sulfide and chlorides at elevated temperatures. Internal corrosion is the important factor affecting the reliability and longevity of subsea oil and gas pipelines in the oil and gas field. In the pipeline design, the measurement of corrosion rate was evaluated under similar corrosion environments using flow loop. The flow loop installed at KITECH located in Korea was made of UNS 31603 with internal diameter 1 inch and corrosion tests can be performed up to 7 m/s liquid velocity and up to 12 m/s gas velocity under maximum pressure 10 MPa. The flow loop system was comprises of a progressive cavity pump, separator, corrosion test section and corrosion monitoring equipment such as electrical resistance probes, linear polarization resistance probes. Corrosion inhibitor performance test, gas hydrate test etc. can be conducted with the system. Material of the coupon was API 5L X52 and weight loss coupon method was adopted based on NACE Standard RP 0775-2005. Measurement of corrosion rate was conducted under test conditions, 0 and 0.5 MPa, dissolved oxygen concentration less than 10 ppb, temperature 313 K, for 24 hours in NaCl 5wt. % solution at the velocity of 3 and 4 m/s respectively. The test results showed that corrosion rate increased at higher flow rate and the predominant factor of corrosion was not chloride concentration but pH of the internal liquid of the flow loop.

Influence of Hydrogenation on Residual Stresses of Pipeline Steel Welded Joints

Hydrogen embrittlement is a phenomenon that affects the performance of steel used for oil and gas pipelines. This paper presents a study of the effect of hydrogenation on the residual stresses of an API 5L X65 steel pipe. Residual stresses were analyzed by X-ray diffraction technique using the sin2Ψ method. The hydrogenation of base metal and welded joint specimens was performed by electrochemical tests in a simulated soil solution NS4. Results show that the hydrogenation led to significant changes in residual stress configuration and in the mechanical properties of steel. The hydrogenation increased the magnitude of the longitudinal residual stress of base metal and fusion zone, without changing the tensile/compressive nature. On the other hand, the hydrogenation increased the intensity of the tangential stress of base metal, and changed from compressive to tensile the residual stress of the fusion zone. The microstructural characterization by optical and scanning electron microscopy was used to complement this study.

Simulation of Cooling Curves of the Heat Affected Zones of API 5L X80 Steel of Welds Made by Flux Core Arc Welding Self Protecting

The simulation of cooling curves of the heat affected zones (HAZ) of API 5L X80 steel was based on the cooling rates of welds made by the process named FCAW-S (Flux Core Arc Welding Self-Protecting). The specimens were analyzed by macroscopy, light microscopy (LM) and scanning electronic microscopy (SEM) to investigate the effects of cooling conditions on the tensile test and Vickers hardness. A thermo-mechanical simulator showed be able to reproduce the different thermodynamics conditions of points in the HAZ and allowed the reproduction of the thermal cycle obtained by welds were applied to the specimens of API 5l X80 steel. The values of the yield strength, ultimate strength and hardness found in the specimens were lower than those presented by the material as received.

Strategies for corrosion inhibition of slurry pipelines prior to commissioning

A practice used to prevent corrosion in buried slurry pipelines is the hibernation of pipelines with an aqueous solution containing corrosion inhibitors before beginning the mining operation. The aim of this work is to evaluate the corrosion resistance of API 5L X70 steel used in the slurry pipelines in hibernation solution containing sodium sulphite, sodium hydroxide and glutaraldehyde in a synthetic river water. The electrochemical techniques used are the Tafel analysis and electrochemical impedance spectroscopy. The synergistic effect of the components of hibernation solution on the corrosion resistance of the API 5L X70 steel was evaluated. A lower corrosion resistance was obtained for the API steel in aqueous solution of sodium sulphite, and in solution of sodium sulphite with glutaraldehyde. The hibernation solution was efficient in inhibiting steel corrosion but the highest corrosion inhibition efficiency was obtained with the glutaraldehyde aqueous solution.

Thermo-mechanic and Microstructural Analysis of an Underwater Welding Joint

Abstract The aim of this research is to present a comparative analysis between theoretical and experimental thermal fields as well as a microstructural behaviour and residual stresses applying multiple weld beads in the joint of two API 5L X52 pipe sections. The thermal field, microstructural and residual stresses were numerically modelled through the finite element method (FEM) and compared to experimentally. The simulation conditions used in the FEM analysis were similar considerations to the underwater welding conditions. The finite element analysis was carried out, first by a non-linear transient thermal analysis for obtaining the global temperature history generated during the underwater welding process. Subsequently, a microstructural behaviour was determined using the temperatures distribution obtained in the pipe material by calculating the structural transformations of the material during the welding process, and finally a stress analysis was developed using the temperatures obtained from the thermal analysis. It was found that this simulation method can be used efficiently to determinate with accuracy the optimum welding parameters of this kind of weld applications.

Welding cooling rate effects on microstructure of an API 5l x100 steel

The aim of this work was to investigate the influence of five different cooling rates on properties of weld metal (WM) and heat affected zone (HAZ) of an API 5L X100 steel welded by GMAW. Bead on pipe (BOP) welds were made on 100x300 mm sections of a 15.8 mm thickness and 580 mm outside diameter API 5L X100 pipe, through five preheat temperatures (-30, 34, 100, 150 and 200 °C), aiming to obtain different cooling rates. Thermocouples were used to obtain the cooling time from 800 to 500 ºC (Δt8/5) during welding. The microstructures of base metal (BM) and HAZ were analyzed by light microscopy and scanning electron microscopy (SEM). Hardness measurements were made on the BOP welds in order to identify maximum and minimum values. As expected, due to the different cooling rates during welding, the distinct preheat temperatures have influenced the weld properties. Maximum and minimum HAZ microhardness, and weld metal hardness decrease as the preheat temperature increases. HAZ area, grain size of fine grained HAZ and coarse grained HAZ increase with increasing preheat temperature.

Soil Stresses on Carbon Steel Pipe Undergoing Uplift

This paper presents a three-dimensional numerical study to evaluate the variations in stresses in the soil mass surrounding a carbon steel pipe class API 5L X60 submitted to uplift due to ground elevation. Analyses were carried out for soil relative density, pipe stiffness and surficial surcharge loading. Results have shown that stress variations due to uplift are lower for looser backfill soils and flexible pipes. Stress variations in pipe invert are meaningful in the vicinity of the region between stable and unstable soil masses.

Evaluation of the Simultaneous Effects of Sulfate Reducing Bacteria, Soil Type and Moisture Content on Corrosion Behavior of Buried Carbon Steel API 5L X65

Evaluation of the Simultaneous Effects of Sulfate Reducing Bacteria, Soil Type and Moisture Content on Corrosion Behavior of Buried Carbon Steel API 5L X65

Kinetics of Strain Aging Behavior of API 5L X65 and API 5L B Steel Types on Long-Term Operations

Kinetics of Strain Aging Behavior of API 5L X65 and API 5L B Steel Types on Long-Term Operations

Kinetics of Strain Aging Behavior of API 5L X65 and API 5L B Steel Types on Long-Term Operations

Kinetics of Strain Aging Behavior of API 5L X65 and API 5L B Steel Types on Long-Term Operations

The Effect of Temperature and Acid Gas Loading on Corrosion Behavior of API 5L X52 Carbon Steel in Amine Unit

The effect of temperature and H2S concentration on amine corrosion of API 5L X52 carbon steel in a CO2-saturated 25 wt.% diethanolamine solution was investigated via electrochemical techniques. It was found that increase in temperature from 25 to 80 °C resulted in severe increase in corrosion rate from 0.88 to 16.24 mpy due to increase in degradation rate of amine. Also, it was concluded that increase in H2S concentration led to increase in corrosion rate because of formation of more heat stable amine salts. The effect of temperature on corrosion rate was more significant than acid gas loading.

Numerical Evaluation of Temperature Field and Residual Stresses in an API 5L X80 Steel Welded Joint Using the Finite Element Method

Metallic materials undergo many metallurgical changes when subjected to welding thermal cycles, and these changes have a considerable influence on the thermo-mechanical properties of welded structures. One method for evaluating the welding thermal cycle variables, while still in the project phase, would be simulation using computational methods. This paper presents an evaluation of the temperature field and residual stresses in a multipass weld of API 5L X80 steel, which is extensively used in oil and gas industry, using the Finite Element Method (FEM). In the simulation, the following complex phenomena were considered: the variation in physical and mechanical properties of the material as a function of the temperature, welding speed and convection and radiation mechanisms. Additionally, in order to characterize a multipass weld using the Gas Tungsten Arc Welding process for the root pass and the Shielded Metal Arc Welding process for the filling passes, the analytical heat source proposed by Goldak and Chakravarti was used. In addition, we were able to analyze the influence of the mesh refinement in the simulation results. The findings indicated a significant variation of about 50% in the peak temperature values. Furthermore, changes were observed in terms of the level and profile of the welded joint residual stresses when more than one welding pass was considered.