Thermal Analysis of In-Service Welding Process for Pipeline

A numerical model had been developed using three dimensional finite element method to predict temperature history of pipe wall thickness by using a FORTRAN computer program. The developed numerical model concerned with the effect of power and velocity of moving heat source, temperature dependent material properties, and transient heat transfer and phase change transformation. Also the effect of heat flux and flow rate on cooling time was investigated numerically. An experimental setup had been designed, manufactured and instrumented to investigate the thermal history of an in-service welding process considering air as the flowing fluid inside test pipe. The burn-through was investigated experimentally under the influence of gas flow rate, heat flux and the depth of defect presented in pipeline wall. It has been concluded that the burn-through do not occur for the studied parameter range. The results showed that as the flow rate increases from (24 30 lpm) the cooling time decrease by a rate of (11.7 %) and when the flow rate increase from (24-60 lpm) the cooling time decrease by a rate of (41.1 %) without affecting peak temperature with pipe wall thickness of (6mm). Also it has been concluded that as the heat flux increases from (11101370 kJ/mm) the peak temperature increases with a rate of (22.11 %) and cooling time increase with (25.4 %) when the heat flux increases from (11101659 kJ/mm) the peak temperature increases with a rate of (46.86%) and cooling time increase with (50.7 %) with pipe thickness of (6mm).


Introduction
In long-distance pipelines, defects can occur as a result of construction faults, corrosion, third party interference, and ground movement [1]. When a segment of pipeline is found to be corroded or local damaged, one of the conventional repair methods is that the sections of the pipeline would have to be sealed and degassed before any welding process and then purged before reinstatement. However, the cost is high in terms of venting and stopping the interior media supply. In addition, some of interior fluid leaks into the atmosphere which are wasteful and also environmentally damaging actions [2,3]. Welding onto a pipeline in active operation called in-service welding, it is an advance technique employed to repair, modify or extend of pipelines. These methods are widely used in guaranteeing the safe transmission of petroleum and natural gas. There are two concerns

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E3 during in-service welding the first one is the possibility of burn-through during the welding process which is due to the localized heating, leading to the loss of material strength on the inner surface of the main pipe. The pipe wall may burst under internal pressure if the loss of the strength is significant. The second concern is the high cooling rate of the weldment or quick removal of heat from the pipe wall as a result of forced convection due to the fluid flow through the main pipe. The high cooling rate can promote the formation of heat affecting zone (HAZ) microstructure with high hardness making the weldment susceptible to cracking therefore, by implementing proper heat input and appropriate in-service welding procedure optimal (HAZ) hardness can be obtained with low risk of burn-through.
In 1999, Oddy and Mcdill [4] have tried to provide guidelines in predicting burn-through by comparing the results of a three dimensional thermo-mechanical finite element by analysis of welding on pressurized vessels with experimental data. It has been observed that burn-through does not occur as long as temperature at inside surface does not exceed 980˚C. In 2005, Wahab et al. [5] have developed a new approach to the problem of burnthrough based on the thermo-elastic plastic models and they have directly simulated the pipe bursting during welding. In 2002, Cisilino et al. [6] evaluated numerically the minimum weldable thickness needed on the pipe for avoiding burn-through during girth welding of the reinforcement to the pipe. They concluded that when the pressure is (100% OP) minimum weldable thickness was (4.65mm) and for gas pressure equal to (80% and 60% OP) minimum weldable thickness is (4.8 and 5.3 mm) respectively. They also concluded that the minimum thickness increase when the gas flow increases.
In the present work a thermal experimental and numerical study is accomplished on welding process of pipeline to study the influence of air inside the pipe on temperature history. Also the burn-through was investigated under the influence of air flow rate, arc heat flux and defect depth.

Governing Equation:
The general equation of conduction heat transfer for an isotropic material is used to describe the pipe wall thermal history [7]. α (1) where is the heat generation per unit volume, is the thermal conductivity of the test pipe material, α is the thermal diffusivity of metal at time t.

ρ (2)
H is the latent heat of fusion of the metal (due to phase change), ρ is the density.

Initial and Boundary Conditions:
The pipe metal initial temperature is assumed uniform and equal to °, i.e. ° The convection boundary condition on the upper surface of the welded pipe (which in contact with ambient air) can be formulated using Newton's law of cooling, which is Where T w is the pipe transient unknown temperature ˚C, ∞ is the ambient temperature ˚C.
The pipe ends will considered as an insulated faces no heat flow crosses these boundaries, which lead to :- Where i refer to x or z coordinate The convection boundary condition on the lower surface of the welded pipe (which is in contact with the flowing air) can be formulated using Newton's low of cooling, which is (6) Where T a is the air temperature.

Convective Heat Transfer Coefficients:
Simplified equation for the convective coefficient of heat transfer from the upper surface of welded pipe exposed to air at T ∞ is given by [8] based on natural convection by:- Where is the heat transfer coefficient, W/m 2 .˚C, ∆T is the temperature difference between pipe wall ( ) and ambient temperature ∞ ,˚C, such that ∞ L is the surface length that exposed to air, m The bottom surface is affected by forced convection (flow of air). Its heat transfer coefficient is calculated by the following equations [9, 10]: where Pr denotes Prandtl number, μ is the dynamic viscosity, the specific heat, thermal conductivity, Re Reynolds number, is the mean velocity of the fluid, d is the tube diameter, ν is the kinematic viscosity, Nu denotes the Nusselt number.

Solidification Equilibrium :
For equilibrium solidification, the solid fraction f s is related to the temperature such that ° (12) where T L is the liquids temperature and T f is the freezing temperature of the pure material of the pipe wall. The second case leads to Scheild equation relating the fraction to temperature. [11] ˳ (13) For linear relationship the value of ° is equal to 1, and equation can be written

Finite Element Method (FEM) Formulation:
The unknown temperature T is approximated in the elemental solution domain at any time by: Is the instantaneous temperature value of node I, and p is the number of elemental nodes, and is a linear interpolation functions. Discretizition of equation (1)

Grid Generation:
The physical domain is divided into sub divisions (1920) and (3069) nodes including the work piece and the backup metal. Fig.1. shows subdivision of the volume.

Transient Solution by -Method :
The , where is the time step. The recursion formulas make it possible for the solution to be "marched" in line, starting from the initial conditions at time t=0 and continuing step by step until reaching the desired duration. Let in denote a typical time in the response so that [12].
where superscripts (n,0) refer to new and old respectively.
A general family algorithms result by introducing a parameter such that:- where Introducing the approximations: Substituting equation (27) and (28)

Experimental setup:
The experimental rig consists of compressor, test section and cooling system. A compressor type Balma 150 compressor which is compress to (10 bar) as maximum is used to supply air to the rig.
The Test section was a low carbon steel pipe with (101.6mm) inside diameter, (6mm) wall thickness and (1800 mm) in length. The pipe receives air from the compressor and discharges it to the environment. A defect is machined in the test section and then it is welded in order to make the experimental tests. A Cooling System was also used in the experimental rig which is consists of three parts the first one is the Cooling coil which is made of copper pipe of diameter (9.525mm). It designed according to the equation: Where L denotes coil length, N is the number of turns, r is the coil radius, and P is the pitch. It is used to cool the main pipe to decrease the temperature of the air so it will cool the molten pool during the process of welding. The second part is the Pump which is used for pumping the water through the cooling coil from the tank. The head of the pump is (9-12 m). The operating voltage required is (220 V) and the current is (2.5 Amp). The third part is galvanized water storage of dimension (500 * 500 *500 mm 3 ) with a thickness of (1.8mm) is used to feed water to the pump. The BX1 series AC arc welding machine was used in the welding process which is a kind of AC arc welding power supply.
Thermocouples type (k) is used to measure the temperatures at selected points. Type (k) thermocouple consists of chromel -alumel with a sensitivity of (41μv/C˚) and temperature range of (-200C˚ to +1350 C˚). The thermocouples connected to the data logger which records the temperatures varying with time and save them in an SD card in the form of excel sheet. The data logger used consists of 12 channels temperature recorder.
The pressure transducer type DRUCK LTD S/N 18770 of (0 to 5000 mbar) range with pressure indicator is used in order to show the pressure value. Two pressure transducer were used to measure the inlet and outlet pressure of test pipe. A flow meter with a reader is used in order to measure the rate of air or gas flowing through the test pipe which is

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E10 installed at the test pipe inlet. The flow meter and reader used is S-meter TFB-100 which is designed for LPG in liquid phase. The flow rate rang is (0.04-0.25 m 3 /h of liquid LPG) with measurement accuracy of ( 0.5%). Figure (3) Shows experimental rig.

Experimental Work
An experimental rig was designed, manufactured and instrumented in order to make the experimental tests which is representing by an in-service welding process for a non-through holes of (4mm) in diameter machined by a manual drilling with different depths and welding them by supplying different values of heat flux and different air flow rates to study the influence of these parameters on the burn-through.

Results and Discussion
x Experimental Results: Welding a non-through holes (dead end holes) of (4mm) in diameter with different depths (2, 3 and 4mm) and apply different current (64, 79, and 94 Ampere) with different air flow rate (24, 30 and 60 lpm). These tests were carried out in order to study the burn-through possibility which is affect by three parameters which are internal pressure, heat input by welding arc and thickness of the wall or depth of defect.
Thermocouples are fixed around the hole as shown in Figure (4).
x Comparison between Experimental and Computational Results: Figure   (9a) shows a comparison between computational and experimental temperature history for nodes (1 and 1'). The figure shows that the computed maximum temperature is slightly below the experimental maximum temperature for node (1'), and slightly above the experimental maximum temperature for node (1). Figure  Conclusions A 3D FE model was developed to simulate in-service welding process. The model was used to predict temperature history of pipe wall. Good agreement was obtained with local thermocouples locations. During in-service welding, the flowing air will take away

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E13 mach of the heat, which accelerate cooling rate of the welding zone. It was concluded that burn-through do not occur for the ranged parameter studied under the influence of internal pressure, heat flux and defect depth variation. The results showed that as the flow rate increases from (24 -30 lpm) the cooling time decrease by a rate of (11.7 %) and when the flow rate increase from (24-60 lpm) the cooling time decrease by a rate of (41.1 %) without affecting peak temperature when the pipe thickness is (6mm). Also it has been showed that as the heat flux increases from (1110-1370 kJ/mm) the peak temperature increases with a rate of (22.11 %) and cooling time increase with (25.4 %) when the heat flux increases from (1110-1659 kJ/mm) the peak temperature increases with a rate of ( 46.86%) and cooling time increase with (50.7 %) when the pipe thickness is (6mm).