Predicting Equipment Failure Due to Sand Erosion - Research Paper Example

School of Computing, Engineering and Mathematics Division of Engineering and Product Design Module Code: XE336 Predicting equipment failure due to sand erosion Naser Abdulla AlRayes Student number: 14800079 Supervisor: Dr Dal Koshal Internal examiner: Dr Oyuna Rybdylova Aim: The aim of this project is to simulate and evaluate the erosion happening in oil and gas pipelines. This is vital because if it is not determined properly then a severe hazard could occur in pipeline components which could cause the construction or manufacturing process to be delayed. Objectives: 1. Analyse the consequences of sand particles on a steel wall of a pipe. 2. Review and analyse these consequences using the Computational Fluid Dynamics software (CFD) 3. Plan different amount of studies of erosion of pipe walls using different flows and factors in many different pipelines. We use the based on the CFD software and calculate the results Introduction: In the oil and gas industries worldwide, hydrocarbons are extracted from the wells and contain a polluted mixture which includes sand and solid particles. The sand particles are existent inside the pipe as the fluid flows. This causes flow problems such as a damage of erosion to the fittings of pipelines which include chokers and elbows. Rapid erosion and wear of top side equipment is a common problem associated with sand production in the oil and gas producing wells. The loss of material due to sand erosion has the potential of causing severe damage to the production facilities (Mohsin, Majid & Yusof, 2013). If the damage is not detected on time, it can lead to leaks and raptures. It is therefore important during the design stage to ensure that the material has the ability of withstanding the sand erosion. A reliable prediction tool is required in order to ensure that the material is able to withstand the erosion. The prediction tool must correctly model the sand erosion resistance of the exposed materials and the effects of the particle impact trajectories ad velocities (Barker, Hu, Neville & Cushnaghan, 2014). An investigation of the estimation of sand erosion needs to be carried out in order to prevent the blockage of an entire production process. In order to evaluate the erosion in the pipe elements, a software package of Computational Fluid Dynamics is used. A focus on the modelling of pipes and the analysis of flow rates may be needed. Dimensions: A research was done on 4 previous simulations done on sand erosion and the dimensions of each test were compared: DNV GL Erosion model (reducer) Simulation using Anysis by Ilja Kivko (Elbow) Inlet Diameter of pipe 54mm 0.1m Outlet Diameter of pipe 20mm - Inlet Pipe length 0.27m 1.0m Outlet Pipe length 0.5m 1.0m Inlet fluid velocity 11.4m/s 10m/s Viscosity of Fluid 11.5E-3kg/m.s 11.5E-3kg/m.s Sand Particle Diameter 0.25mm 0.25mm Simulation by DNV (Elbow) Simulation by Akar Abdulla (Elbow) Pipe Diameter 0.1m 0.00508m Fluid Viscosity 1.0E-4 1.85508.E-5 Sand Particle Diameter 0.25mm 0.15 mm Fluid Velocity 15m/s 12m/s After analysing many previous simulations, some of the results were very similar. For example the simulations were all done on a fluid velocity of above 10m/s. The fluid viscosity and Pipe diameters differed due to the different dimensions and models of the pipes, different fluids were used as well for each simulation. Sand particles have a diameter which usually ranges from 0.15mm to 0.25mm. Types of Pipe fittings: Pipe fittings are used to connect straight pipes and have it adjust to different sizes and shapes. Sand erosion analysis is usually accomplished on pipe fittings. Types of fittings include the elbow, reducer, tee type and valve. Elbow: In order to shift the flow to another direction, elbow pipe fittings are used. Many different angles are applicable with angles that range from 22.5 degrees, 45 degrees and 90 degrees. Reducer: Reducers are used in order to reduce the capacity of flow from a larger amount to smaller. This is achieved by decreasing the size of the pipe. Two types of reducers are a concentric reducer and an Eccentric reducer. The Concentric reducer has a shape similar to a cone with an accumulation of air that may be viable. Air accumulation is an Eccentric reducer is not possible due to having one edge parallel to the pipe which is connected. Tee type fitting: The tee type pipe fitting is mostly used in a plumping system which has a shape of the letter ‘T’. There are two inlets and one outlet which are helpful in combining flow which comes from both inlets to the outlet. The two types are an equal and unequal tee. Valves: Valves are used in pipes in order to cease the flow or control it. There are different types of valves that have different functions: Gate valve – Used for separation of flow Choke valve- Used to create a choked flow, restricts it Butterfly valve – used for both functions of flow restriction and separation Factors that influence sand erosion: These are factors which can increase the level of erosion if not properly controlled. Liability of components: There are many different parts of a system that are considered. The design of the pipe fittings and their performance is a huge factor in determining the vulnerability. They are usually parts in which: The direction of the flow alters abruptly High volumetric flow rates which cause high flow velocities High flow velocities that are generated from the restriction of flow Material properties: The properties of the materials used to construct the pipes and fittings have sizable consequences. In oil rigs, most of the pipelines are made of ductile materials such as steel. Controlling the hardness of the materials benefit in decreasing the sand erosion as many different ductile materials have different harnesses. Force: The force driving the fluid as well as the sand particles is an important aspect that influences the erosion process. Drag force is a force generated by the interaction between the solid particles and the surrounding fluid (Yu, Li & Grondin, 2013). The drag force is related to the shear stresses of the solid particles when it interacts with the fluid in the oil or gas pipeline. The viscosity and friction between the particle and the fluid is an important influential factor when dealing with the drag force. The shape of the solid is also affected by the drag force. The drag force can be calculated using and equation derived by Gosman which is as follows: The virtual mass force is the additional force that is required to accelerate the particle and the fluid surrounding it. This type of force has significant effects on the movement of particles as well as impacts on the material (Nguyen, et al, 2014). The virtual mass flow can be calculated using the following equation. In order for the sand particles to move, it requires a lift force which is a force perpendicular to the velocity direction of the flow (Islam, Alam, Farhat, Mohamed & Alfantazi, 2015). The following equation can be used for the purposes of calculating the lift force. The lift force can be calculated using the following equation: Fluid flow: Fluid flow is usually forced by a fan or pump through a flow section. Pressure drop is used to determine the pumping power requirement. A piping system involves pipes of different diameters that are connected to each other using altered pipe fittings to control the route of the fluid. Most of the fluids are transported through circular pipes. The reason of that is that circular pipes can endure large pressure differences between the outside and the inside of it. Laminar flow- The flow appears to be in level and parallel to one another. The velocity is also very low when the fluid flows without any evidence of the parts of the stream combining together. Turbulent flow- The flow has a high velocity in which the motion is not organized is mixing and the fluids mixing with one another. Turbulent flow is associated with the dispersion force that arises due to the interaction between the solid particles and the liquid eddies. It is through turbulence that the solid particles can move from a region of high concentration to a low concentration region (Zeng, Zhang & Guo, 2014). The following turbulence force equation can be used during the calculation process. The turbulence model is used as part of the tool to predict the failure of steel wall pipes. This model is dependent on the chaotic flow that is characterised in all the fluids. When modelling turbulence in CFD, various approaches can be used. One of the main approaches that are used is the Reynolds-Averaged Navier-Stoke (RANS). The RANS equations are mainly time averaged for the fluid motion (Majid, Mohsin &Yusof, 2012). During the simulations it is mainly used for modelling two equation models. The equations have been formed based on Reynolds decomposition. This is achieved by separating the time averaged part from the fluctuating parts of quantities. RANS ensures that the turbulence flow is analysed and dealt by the mean and fluctuating components (Peng Jr, Pak, Chinello, Wood & Low, 2013). This is achieved through the use of equations for pressure as highlighted below: Where,  and  represents time averaged pressure and velocity respectively. P’ and V’ represents pressure and velocity fluctuating components. Multiphase Flow: A flow which consists of more than one single phase. An example is two immovable liquids such as water and Toluene or the flow of liquid and gas such as Carbon dioxide and sodium Hydroxide. In the oil and gas industry gas dissolves from oil due to the different temperatures and pressures. Since we are analysing sand erosion alongside the flow of hydrocarbons, we are looking at a gas-liquid-solid multiphase flow. This happens in different stages of the production which include extracting the oil and gas from the wells and transporting the pipelines. Types of multiphase flows which involve Gas and liquid: Figure 1 - (Hubbard, 1966) 1. Separated flow- Smooth separated flow: The plugs join together to form a larger gas flow on the top of the pipe with the liquid flowing below on the bottom of the pipe. Wavy separated flow: The gas and liquid blend inside the pipe is not often smooth as ripples appear on the surface of the liquid. This is the cause of the gas flowing on the top of the pipe which increases in amplitude. 2. Intermittent flow- Elongated bubble flow: This is called a plug flow. The collisions between the tiny bubbles of gas form a larger bubble which is the cause of the increased gas flow rate. Slug flow: The high amplitude of the wave that travels along reaches the top of the pipe. The gas flow as unsteady slugs with smaller bubbles heading in the path of the liquid. 3. Dispersed flow- Annular flow: Due to the large gas flow rate, it supports sheets of liquid which are around the pipe walls. Due to the gravity effect, the sheet of liquid at the bottom of the pipe is thicker. Liquid is transported via bubbles in the middle of the pipe throughout the stream of gas. Bubble flow- Small bubbles flow alongside the top of the pipe. Type of multiphase flows which involve Gas/solid/liquid systems: Since the sand particles are heavier than the carrying fluids, they are usually transported at the bottom of the pipe. Figure 2 - (multiphase design handbook, 2005) 1. Plug flow- As gas bubbles move along the top of the pipe, the sand flow is unaffected. Once the amount of gas is increased, the bubble depth increases as well. The unsteady speed of the velocity affects the transport. 2. Slug flow- The sand is transported alongside the slug body. This causes the motion to become unsteady. A bed of sand can be formed if the sand sticks to the slug and doesn’t get transported. 3. Annular flow- The sand could be transported alongside the sheet of liquid or the base of the gas at the top or bottom of the pipe. The velocities are high in an annular flow so the concern is how excessive the erosion might be. Sand Erosion: The cause of sand erosion in pipelines is due to the clash of sand particles, flowing dependently on the pipeline wall. Sand usually exist in the hydrocarbon wells and are pumped along the fluid in to the pipelines. Due to the malformation which is repeated, the material is removed. A cutting action which erodes the material. The sand particles are usually fixed in a flow of either liquid or gas or in a multiphase flow. The volumetric percentage of sand in the fluid may range from 5 to 10%. The figure above shows the relationship of the impact of the particle angle and the ratio of erosion. These are for brittle and ductile material. So basically this shows that ductile materials experience high erosion rates at angles of 20 to 30 degrees and brittle materials experience erosion rates at an impact angle of 90 degrees. Brittle materials endure elastic deformation and ductile materials suffer very large strains before any fracture develops. Factors affecting erosion:- 1) Velocity, Viscosity and density of fluids – The greater the velocity, the greater the erosion. In dense, viscous fluids the particles are most likely to be carried around barriers from the flow rather than having an impact on them. A low viscosity and density increases erosion. 2) Size – As the size increased, the erosion increases 3) Shape – Determines contact area between particle and metal surface during impact 4) Impact angle or angle of incidence – As explained in the graph above, the angles of impact play a big role in the erosion rate. 5) Material properties a) Temperature b) Surface hardness c) Erodent hardness 6) Sand Transport – Gas can flow in high velocities, sand particles may be trapped in slugs as discussed earlier and with the high velocity the erosion may be increased. This equation above is used to calculate an estimate of the rates of erosion. In order to calculate the equation above we may need the particle impact velocity, particle impact angle and the mass of the sand particle having an impact on the pipe. All the factors we mentioned depend on the component geometry and the characteristics of flow. Erosion in Pipe bends: Pipe bends are usually the part which is most prone to erosion. When the flow direction is changed with the bend, the sand particles do not follow the fluid but hit the wall instead. For example vulnerable pipe fittings may be Chokers, reducers, blind tees and elbows. We can compare sand erosion in different pipe bends and see the impact of each one. I. Small light particles require very little drag to change its direction in highly viscous, dense fluid therefore it avoids hitting the pipe wall. II. Medium particles are typically size grains of sand III. Large particles will hardly be deflected by the flow of fluid at all due to the high force and therefore it will travel in a straight line and bounce of the pipe wall. CFD modelling: For non-standard geometries which are very detailed, we need to use advanced computer simulations. This can be found in the Computational Fluid Dynamics (CFD) software. We use the flow field when calculation the curve and path of the sand particles. We then use the impact angles and velocities to solve the erosion attacks. In one of the case studies to predict the sand erosion in oil and gas pipes, the Computational Fluid Dynamics method was used. An  elbow with a diameter of 0.0508 and a curvature of 1.5 was simulated through the use of STAR-CCM+. The following elbow was generated though the use of 3D CAD and used for the simulation. During the erosion modelling using the CFD, four main steps were undertaken. The steps included grid generation, flow solution, particle track calculation and erosion rate calculation. During the grid mesh generation, special attention should be taken due to the direct effect that it has on the quality (Nguyen, et al, 2014). Mesh diagnostics was carried out during the simulation in order to ensure that it is accurate. A polyhedral mesher was selected during the process and it was used for the creation of 0.2m length pipe on both sides. Polyhedral mesher is much easy to develop and it also has fewer cells and hence improving on its effectiveness (Abuali, Smith, Brown & Pontaza, 2013). A prism layer was used for obtaining accurate results next to the wall boundary. This is useful in the simulation of turbulence as it takes place next to the wall. Surface mesh for the elbow Volume mesh for the elbow The following table was also obtained after the configuration process. Table indicating the number of cells, faces and vertices The physics model is the step that follows after the generation of the meshes (Della Rovere, Silva, Moretti & Kuri, 2013). This was carried out during the simulation by creating physics continua that represents different materials. The physics continua have to be assigned to the correct region in the CFD model. Appropriate conditions and values for the boundaries were set up during the simulation. It is a model for predicting erosion in air sand, methane sand and mixed gas sand. Implicit unsteady time model was used during the simulation for controlling the iteration as well as unsteady time stepping. Air was used as the gas that was used for continuous fluid case. The density during the simulation was 1.18415 Kg/ while the dynamic viscosity was 1.85508  Pa. In order to model the turbulence model, the Reynolds-Averaged Navier-stokes Equation. The multiphase flow was used for the purposes of simulating the multiphase mixture. The particle trajectory during the simulation was obtained by integrating the force balance on the particle. This is useful in terms of determining the particle impact velocity on the wall of the pipe and elbow (Nguyen, et al, 2014). The force balance aspects that were integrated include drag force, virtual mass force, gravity force, lift force, pressure gradient force and turbulence dispersion force. During the simulation, the spherical particles were released at the inlet domain of the extruded pipe. The software was used for the purposes of storing the calculations about the particle trajectory, particle impact on the wall and the particle impact angle. The erosion rate was obtained by applying the erosion equation on the properties of the material. The table below summarizes the particle properties as well as the properties for setting up the model: The results indicate that the sand erosion has the ability of taking place throughout the surface of the elbow. The velocities as well as the volume of the sand are the main factors that influence the rate of sand erosion (Nguyen, et al, 2014). The graph below shows the erosion rates in relation to the velocity. The rate of erosion is much higher when the velocity of the sand particles in contact with the steel pipe is high (Della Rovere, Silva, Moretti & Kuri, 2013). The prediction of sand erosion is mainly based on the velocity of the particles as indicated in the table below. The presence of mixed gases in the pipes is an important aspect that influences the velocity as well as the other parameters. This has a direct impact on the sand erosion rate. A high drag force is usually applied on the sand particle by the oil and gas (Della Rovere, Silva, Moretti & Kuri, 2013). This forces the particles to follow the flow streamlines. This in most cases cause the maximum erosion to occur at the end of the elbow as well as the extruded pipe. The particle relaxation time is one of the main aspects that influence the sand erosion. Particle relaxation is defined as how fast the particles can react to changes in the fluid velocity. The sand particles usually react faster to the changes in the flow velocity. This means that the erosion rate may be high when the fluid is flowing at a low velocity. The information from the simulation can be used to make predictions about the failure of steel (Della Rovere, Silva, Moretti & Kuri, 2013). 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