RESIDENCIAL
3.2. Riesgo de exclusión social: los Programas de Preparación para la Vida Independiente (ILPs)
In buried piping applications, such as pipelines, the piping system is continuously supported by the soil, and anchors are used to absorb the thrust loads that result from restrained thermal expansion and limit pipe end movements. There are special considerations for anchoring underground piping that the engineer must consider in pipeline applications. Specifically, the resistance of the anchor to movement (discussed below), and external loads at roads and railroad crossings, must be considered.
Thermal expansion calculations are needed for buried lines if large temperature changes are expected. Anchors are needed to limit movement at the ends of the pipeline, at changes in direction, and at changes in pipe size. Excessive movement of a buried pipeline can cause shifting of the soil that supports the pipe, subsidence of the cover, or damage the external pipe coating (if one is installed). In extreme cases, excessive movement can eventually lead to overstress in the pipe and/or inadequate cover depth, or external pipe corrosion.
For buried, restrained pipelines, the net longitudinal compressive stress in the pipe due to temperature rise and pressure is calculated as follows, in accordance with ASME/ANSI B31.4:
S L = = E α
( (
T 2 − − T 1) )
− − ν S hwhere: SL = Longitudinal compressive stress, psi.
Sh = Hoop stress due to pressure, psi.
T1 = Temperature at time of installation, °F.
T2 = Maximum or minimum operating temperature, °F.
E = Modulus of elasticity, psi.
α
= Coefficient of thermal expansion, in./in./°F.ν
= Poisson's ratio.ASME/ANSI B31.4 limits SL to 90% of the pipe SMYS. The equation shows that it is preferable to have the tie-in temperature as close as possible to the operating temperature to minimize SL. Multiplying this calculated stress by the pipe cross-sectional area yields the force that must be resisted by the anchor to limit pipeline movement during operation.
However, the maximum force for which the anchor must be designed occurs when there is no pressure in the line (i.e., Sh is zero). This situation occurs when the line is empty, or is filled with liquid but is not in operation.
ASME/ANSI B31.4 does not require that the bending stress in buried pipelines, Sb, be included in the calculation of SL. However, SAES-L-003 requires that all sustained loads and constraining forces also be included in the calculation of SL. These loads result in an additional bending stress, and are typically caused by pipe misalignment during installation and elastic bends in the pipe that are required to conform to the ground profile. Saudi Aramco limits the maximum bending stress, Sb, as follows:
Sb ≤ 0.9 (SMYS) - 0.7 Sh - E
α
(T2 - T1)The maximum permitted bending stress can then be calculated based on the known values of the other parameters. In some cases, it may be necessary to increase the pipewall thickness beyond what is required for internal pressure alone. This thickness increase might be needed to provide a high enough value of Sb to permit pipeline installation using practical limits on misalignment tolerances and elastic bend requirements.
Saudi Aramco often uses computer programs to calculate SL, Sb and the resulting forces for which the pipeline anchors must be designed because multiple parameters are involved. The Consulting Services Department should be contacted when these calculations are required.
Types of Anchors
Before discussing the forces available to resist anchor movement which may result from Sb, the following describes Saudi Aramco requirements for the primary types of anchors for restrained end and intermediate anchors, as stated in SAES-L-011.
• Aboveground restrained pipelines shall be provided with end anchors that are designed to withstand the full thrust and pull forces due to thermal expansion and contraction, and due to internal fluid pressure considering Poissons ratio, with a maximum anchor deflection of 6 mm (0.25 in.).
• The potential end movements of buried pipelines shall be conservatively estimated. If these movements exceed 50 mm (2 in.), a full-thrust or drag anchor shall be provided and be designed per SAES-L-044 to limit the end movement to less than 25 mm (1 in.).
• Differential-thrust anchors shall be provided on aboveground restrained pipelines where there is a change in thrust, such as due to a change in pipe diameter or wall thickness, except when the associated local axial movement of the line can be shown to be less than 6 mm (0.25 in.).
• Any change in direction in the horizontal and/or vertical plane of aboveground restrained pipelines requires one or more deflection anchors designed to resist the resultant forces on each side of the deflection. Deflection anchors may be required on buried pipelines if the passive soil restraint against the pipe alone is not sufficient to fully restrain the line.
• Saudi Aramco Standard Drawing AB-036415 illustrates typical details for concrete-thrust block anchors.
Forces Available to Resist Anchor Movement
The previous discussion described the pipeline forces that must be resisted, the anchor types that may be used, and limitations on anchor movement. This section will conclude with discussion of the forces that are available to resist movement of an anchor. A combination of the soil cover depth, soil friction, and anchors are used to restrain thermal expansion.
The full-thrust anchor force, or differential anchor force, without any reduction on account of soil friction due to assumed movement, shall be used for the design and stress calculations of the anchor. This includes structural steel design, welding details, attachment to the pipe, stability of the anchor against overturning, concrete stresses, and reinforcing bar selection.
Solely for sizing a concrete-block drag anchor, credit may be taken for the soil friction on the length of pipe assumed to be a maximum of 25 mm (1 in.) at the drag anchor.
The forces that are available to resist anchor movement come from the direct bearing loads between the anchor and the surrounding material, friction loads between the anchor and the surrounding material, and friction between the soil and pipe. SAES-L-044, Anchors for Cross-Country Pipelines, provides design criteria for sizing concrete anchors. These design criteria consider the following factors:
• Whether the anchor is in a rock area or in granular, well-compacted soil that is above the water table. This determines the bearing load and friction factors that are applicable at the anchor.
• Anchor depth beneath the surface.
• Anchor size (height and width). This determines the effective anchor bearing load based on specified values of bearing pressure.
• Friction on both sides of the anchor assuming a specified active soil pressure and friction coefficient.
• Friction on the bottom of the anchor assuming a specified friction coefficient.
• The axial friction force that accumulates along the moving length of pipeline that is adjacent to a drag anchor. This pipe friction reduces the load that must actually be absorbed by the anchor.
• In order to maximize the stability of anchors, the resultant of all anchor forces shall have a line of action that is close to the centerline of the pipeline. This will minimize the tendency for the applied loads to rotate the anchor. Any resulting overturning moment on buried anchors shall be resisted by the weight of the anchor times the distance from the center of gravity to the leading face.
WORK AID 1: CRITERIA FOR DETERMINING MAXIMUM SUPPORT SPACING