Pipeline

August 1, 2017 | Author: Narendra Singh Shekhawat | Category: Shock Wave, Pipe (Fluid Conveyance), Pipeline Transport, Bending, Earthquakes
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Short Answer 1. Enlist the factor for mechanical design of pipeline? 1. Internal Pressure 2. Stability 3. Construction Loads 4. External Damages. 5. External Pressure 6. Expansion Stresses (Frequent temperature changes) 7. Risers 8. Climatic Conditions 9. Hydraulic Shock/ surge and Water hammering in Pipes 10. Seismic effect on Design of Pipelines 11. Construction Stresses (Pipe bending etc) 12. Free Spans 2. What is the importance of Risers? Risers are vertical sections of pipe, which are used to connect an offshore pipeline on the seabed to the production facilities, normally located on a platform. Riser design must account for variations in temperature, internal pressure and external environmental loads anticipated throughout the lifetime of the system. 3. Draw diagram of a Riser?

4. What is the purpose of Mechanical Design of Pipeline? The purpose of mechanical design is to ensure that the pipeline has the mechanical strength to withstand its expected load. These normally include:  Functional loads, such as pressure and temperature,  Environmental loads, such as waves, current, wind and earthquake,  Construction loads, such as laying and testing, which may arise during the installation. 5. What are different pipeline standards?

6. Why Supports are used in pipeline? If the distance between the supports is maximized, the number of supports required throughout the length of pipeline will reduce.

Long Answer

7. Explain the role of support in pipeline? The cross-country pipelines are mainly supported on metal pipelines. The material is usually alloy metal, which is chosen based on the fluid to be transported. These pipelines are supported on different forms of supports viz, Metal in RCC supports, Metal frame supports, Small Trusses, etc. If the distance between the supports is maximized, the number of supports required throughout the length of pipeline will reduce. Thus, reducing the total cost of erection. Supports for piping must be spaced with respect to three considerations: a) Ability to place a support at some desired location. b) Keeping sag in the line within limits that will permit drainage. c) Avoiding excessive bending stresses from the uniform and concentrated loads between supports.

Need of Pipe Support :The layout and design of piping and its supporting elements shall be directed toward preventing the following: (a) Piping stresses. (b) Leakage at joints. (c) Excessive thrusts and moments on connected equipment (Such as pumps and turbines). (d) Excessive stresses in the supporting (or restraining) elements. (e) Resonance with imposed or fluid-induced vibrations. (f) Excessive interference with thermal expansion and contraction in piping which is otherwise adequately flexible. (g) Unintentional disengagement of piping from its supports. (h) Excessive piping sag in piping requiring drainage slope; (i) Excessive distortion or sag of piping (e.g., thermoplastics) subject to creep under conditions of repeated thermal cycling. (j) Excessive heat flow, exposing supporting elements to temperature extremes outside their design limits. 8.

Explain procedure of calculation of maximum span using a sample data? Design formulas for calculating bending stress and deflection between supports are derived from the usual beam formulas, which depend upon the method of support and the type of loading. Maximum Bending stress, S b = {(0.0624 wL2 + 0.124 wc L) D/ I} in N m2 [1] ---------------

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Maximum Deflection, y = 5wL4 8 w c L3 / 384 EI in meters -------------------------------Where, w = uniformly distributed weight of pipeline in N/m W c = concentrated weight on pipeline in N L = Span length in m D = Outside diameter of pipe in m d = Inside diameter of pipe in m E = Modulus of elasticity of pipe in N/m2 I = Moment of Inertia of pipe in m4 Calculation of total weight Total weight = weight of pipe (wp) + weight of fluid (wf) weight of pipe Thickness of pipe can be calculated as:

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t = P x D/ 2(S a E+ PY) ------------------------------------------------------------ (3) Where, P = Pressure of the fluid in pipe in N/m2 S a = Allowable stress in pipe in N/m2 E = Quality Factor from ASME B 31.3 Y = Coefficient of material from ASME B 31.3 Annular cross-sectional area of pipe = 4 π (D2 - d2) Hence weight of pipe can be calculated as, 4 π (D2 - d2) x density of pipe material The weight of Stainless Steel pipe can be directly calculated as----w p = 0.02466(D-t)t Calculation of weight of fluid Weight of fluid = 4 π d2 x density of fluid in N/m SAMPLE CALCULATION & RESULTS Let us calculate the maximum support span for transporting water through a seamless stainless steel pipe (ASTM A 312 TP 316 L) of 300 NPS through a distance of 15 km. Pressure in pipe is 20 bar at atmospheric temperature using the procedure described above. D = 0.3239 m P = 20 bar S b = 34.53 MPa (30% of S a = 115.1 MPa) Therefore, using equation (3), thickness of pipe comes out to be 6 mm. Note: Schedule 20 is the nearest schedule for this thickness and according to thumb rule, the next schedule of pipe is finally selected, which is schedule 30. Schedule 30 gives a thickness of 8.382 mm. Hence, d = 0.3071 m. Weight of stainless steel pipe is calculated 641.16 N/m. Weight of water = 726.64 N/m Total weight = 1367.8 N/m Moment of inertia = 1.0369 x 10-4 m 4 Modulus of Elasticity = 195122 MPa Substituting the above values in the maximum bending stress equation:

(Since the pipe is not considered to carry flanges, it will not carry any concentrated load; hence 2nd element of equation is eliminated) Maximum Span between supports is calculated as 11.38 meters, which is rounded back to 11.0 meters. Hence number of supports required for 15 km pipeline is approx. 1364. With the above values, deflection comes out to be 12.89 mm, which is less than 600 L; hence the calculated span is also safe in deflection.

COMPARATIVE ANALYSIS Table 1 shows a comparative analysis of the span shown in different tables marked in references. It can be seen that for the sample pipeline of 15 km length, the minimum number of supports required is calculated by the procedure described ABOVE.

CONCLUSION Efforts are made to maximize the distance between supports keeping the values of stresses and deflection within safe limits. The aim is to reduce the number of supports to reduce the total cost of erection. A saving of supports will have a great effect on the total cost of erection. The cost of erection can further be reduced if the schedule of pipe (i.e., thickness of pipe) is raised. This will increase the cost of material but at the same time reduce the cost of erecting supports. Hence, a comparative study of cost is required before changing the schedule of pipe.

9.

Explain briefly Pipeline Span?

Freely suspended parts of the pipeline are usually referred to as ’free spans’. This type of structure may also be found closer to the coast when crossing rough topography or in the rivers. The hydrodynamic loading on free span pipelines below the shelf edge is mostly due to current flow. It is well known that current flow may cause vortex-induced vibrations (VIV) in free span pipelines. Parameters such as turbulence in the flow, proximity of the sea bed, pipe sagging, flow inclination angle relative to the longitudinal axis of the pipe, pipe-soil interaction and the dynamic coupling between adjacent free spans all influence the vortex shedding induced response of the pipe. Pipelines oscillations may occur in the cross-flow directions and the in-line direction of the flow. By far the more serious oscillations are those, which occur, in the cross-flow direction. In-line oscillations are not generally considered to cause serious oscillation problems in the pipe, although some exceptions to this have been reported. Pipeline failure that may be caused by vortex-excited motions can be prevented if the vortex-shedding frequency is sufficiently far from the natural frequency of the pipe span such that dynamic oscillations of the pipe are minimized. Based on that maximum allowable free span length is calculated for a particular pipeline. The maximum allowable free span length of a pipeline depends on the length and wall thickness e.g. For 36” diameter of 28 mm thickness pipeline, 35 meters is the maximum allowable free span. Any span of greater length is critical. Free spans occur in the in-field pipelines and only those which are critical are taken are rectified supporting the pipe with sand bags. The pipeline has no oscillation and vibrations. Such pipeline crossing needs to be monitored closely as per the safety codes.

10.

What is the effect of Hydraulic Shock/ surge and Water hammering in Pipes?

For high-pressure pipes, analysis and design are generally focused on the stress, deformation, and failure caused by high internal pressure. In cases where water hammer is a common occurrence in a pipeline, not only must the highest pressure generated by water hammer be added to the steady pressure in the calculation of the hoop tension, the dynamic effects of the water hammer, including vibration and material fatigue, must also be carefully analyzed. The essence of hydraulic shock in pipelines is that the stationary flow of fluid in a pipeline is disturbed by the abrupt closing or opening of a gate valve, the switching on or switching off of a pump and so on, resulting in hard braking or acceleration of the fluid and shock compression of the fluid particles. The front at which the variation of the hydrodynamic parameters of the fluid takes place has a relatively small extent and propagates in the form of a pressure wave down-stream and up-stream of the fluid.

Similar phenomena occur in the pipeline in other cases when the velocity (flow rate) of the fluid varies in a stepwise manner. The possibility of hydraulic shock should be taken into account in the exploitation of pipeline safety and maximum efficiency, since shock pressure can far exceed permissible standards, leading to pipe breakage and an emergency situation. A pressure surge is any change in pressure in the pipeline with no set limits of magnitude or rate of change of pressure. Pressure surges travel through the liquid in the pipeline at a sonic velocity which varies from 3000 to 4000 ft / Sec in most pipelines, which depends upon the diameter and thickness of the pipe. The important factor in pressure surge control is the rate of change of pressure than the magnitude of the pressure. The main reason for surge analysis is to determine if at any point in the pipeline maximum allowable transient operating pressure exceeds the maximum allowable operating pressure. If it exceeds, the action needs to be taken to reduce its magnitude. The explanation of hydraulic shock was given by Joukovski in his article ‘‘on hydraulic shock in water-supply pipes’’ (1899). He connected the magnitude of the pressure jump [p] with the properties of fluid compressibility and the elasticity of the pipe and obtained the following Formula [p] = ρ0 D · [v] Where, D is the velocity of shock wave propagation in the pipeline and [v] the magnitude of the stepwise change in the fluid. It should be noted that the introduction of stepwise variations (jumps) of hydrodynamic flow parameters is nothing more than a model of the phenomenon under consideration. In fact each such discontinuity has a transition region, though very narrow, from the value of parameter A+ to the left of the discontinuity front up to the value A− of the same parameter to the right of the front. The quantity [A] = A+ − A− is called the jump of parameter A at the discontinuity front. To describe the structure of this transition zone needs as a rule a more complicated model than the given one. An example of surge pressure action, assume a system includes a segment of pipeline with a centrifugal pump and check valve at the upstream end and a check valve in the downstream. In the steady state condition, flow rate and pressure gradient are constant. If the block valve is closed, flow rate at the valve decreases, pressure increases. The surge pressure is the amount by which the pressure exceeds the steady state pressure. The surge pressure is propagated upstream until it reaches the pump, which responds to the change in pressure according to characteristics of its head versus flow rate curve. As the pressure wave moves upstream, its characteristics are changed by a friction in the pipe line and elasticity of the liquid and the pipe material. Original waves and reflected waves reinforce each other. Many critical external loads, such as those due to earthquake (in earthquake region), high winds (for elevated pipes), ocean current (for submarine pipes), thermal stresses (for pipes welded in hot weather), etc., must be

considered. In contrast, because the pressure due to earth load in this case is much lower than the internal pressure, it can be safely ignored without risk in most cases.

11.

What is the effect of seismic in Pipelines?

Designing a pipeline to withstand earthquakes is complicated because a strong earthquake can damage the pipe in many ways. Large vibrations and differential settlement of the ground can cause large bending and shear stresses in parts of the pipe. In the case of submarine pipelines, the earthquake can cause rapid movement of the pipe relative to the surrounding water, which in turn can cause large drag and fluidinduced vibration of the pipe. It is not possible for this elementary text to treat such a complex subject. Suffice it to mention that in order to minimize vibration damage to elevated pipes, the supports of pipes must be spaced at a sufficiently small distance so that the structure’s natural frequency, fn, will be much higher than the dominant frequency of the ground movement induced by earthquakes. This prevents resonant vibration, which can destroy pipe and its supports. To be safe, the spacing between supports, L, must be within the following limit: L = (Π / 2f) 2 [g E p I p /W] 1/4 ------------------------ (a) Where f is the design frequency of the pipe in hertz (cycles per second); Ep is the Young’s modulus of the pipe material; Ip is the moment of inertia of the pipe wall cross section; and w is the weight of the pipe (including the fluid in it) per unit length. Note that Equation is dimensionally homogeneous. Thus, all the units used in the equation must be consistent. For instance, if L is given in feet, g must be given in ft/sec2, w in lb/ft, and EI in lb-ft2 [E in psf (pounds per square foot) and I in ft4]. With a span determined from Equation 13.23, the corresponding seismic stress generated due to vibration is: σe = 0.000488 I w GD0 L2 / I p ------------------------ (b) Where I w is a stress intensification factor, and G is the seismic acceleration in gs, which is dimensionless 12. Make a note on Pipeline vs Facilities piping? Pipelines and facilities piping have essentially the same purpose: to transport fluid from a source to a delivery point. Beyond this, however, they exhibit some fundamental differences. While facilities piping are normally confined to privately owned areas and are usually secured, pipelines (except for their start and end points) are often built in the public

domain. This introduces additional risks due to third party activities. It also requires additional planning and negotiations to obtain the necessary permits. Pipelines are usually buried, and are not readily accessible for inspection. Facilities piping are normally installed above ground, and so are much easier to access and inspect. Pipelines are usually allowed to operate at higher stress levels than facilities piping. The facilities piping code (B31.3) is much more conservative in allowable stress levels (B31.4/B31.8), primarily because facilities piping are unburied and has proximity to fired equipment, thus creating a greater hazard potential. Pipelines have no standard system of pressure rating. Unlike facilities piping, pipelines do not use a system of standard piping classes (e.g., Class 150, 300, 600, etc.) Pipelines instead are designed according to a specific design pressure and design temperature. There is also a difference in the relationship between the design pressure (DP) and the maximum allowable operating pressure (MAOP). Figure -1illustrates the differences between pipelines and facilities piping.

Figure-1 As can be seen from this diagram, piping design codes do not permit any pressure above the design pressure. In facilities piping, the process trip pressure is usually set at 90 % of the design pressure and the process alarm and maximum operating pressure are set at 85 % of the design pressure. But in pipelines, the MAOP is allowed to be set equal to the design pressure, with incidental pressures allowed up to a maximum of 110 % design pressure.

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