S13 ORME 7 2016 Penspen_Layout 1 26/10/2016 11:09 Page 58
A strain-based approach for
pipeline design Matthew Laing, pipeline engineer, Penspen, discusses the strain-based design of buried horizontal cold formed bends under high temperature loading. HE DESIGN OF pipelines using a strain-based approach takes advantage of steelâ€™s ability to deform plastically without loss of integrity. Originally, the process was deployed offshore to allow for high strains caused during pipe laying. Onshore applications of such designs have been limited to areas where high external loading is expected, such as ground movement. Consequently, there is limited guidance for onshore applications; DNV OS F101 and CSA Z662 partially cover the topic. For a strain-based design to be carried out, a framework must first be established based on the limited code guidance and performance limits available. Onshore pipelines are commonly designed using allowable stress methods. These provide a simple framework that limits the stress in the pipeline to a fraction of the yield stress, traditionally defined by the specified minimum yield stress (SMYS). At high temperatures, when designed using a stress-based approach, the required wall thickness is governed by the magnitude of the equivalent stress. The rate of increase in wall thickness with an increasing operational temperature is shown in Figure 1; there is a rapid increase in the required wall thickness for high temperatures, where the equivalent stress criterion governs. Note that elastic bending is also considered here. If plastic deformation is allowed to occur, the stress does not need to be limited and the required wall thickness can be significantly reduced, creating a significant cost benefit to the project. The key question for strain-based design is: what magnitude of strain is considered acceptable? The point of the onset of local buckling is a key performance limit under high thermal loading. An assessment of the strain level when local buckling occurs can be calculated using finite element analysis, and compared DNV-OS-F101 calculations for critical buckling. Local buckling is more likely to occur at points of out-of-straightness in the pipe, such as bends.
using full scale testing by the American Society of Civil Engineers.
Issue 7 2016
Influence on bend geometry and design
Cold formed bends, high temperature operation and soil response Cold formed bending is a popular mechanical process of plastically deforming the pipeline to give the required bend angle for the routing of a pipeline. Such bends are formed to specific curve radii, typically between 35 to 40 pipe diameters in the onshore environment. The process involves a series of small incremental bends to create the required angle. The resulting plastic bending strain in the pipeline, b, can be calculated by: b=D/2R, where D is the pipeline diameter and R is the bend radius. High temperature operation of buried pipelines induces moment loads at bends. A reduction in axial restraint at the bend location allows axial expansion of the pipe in the direction of the bend. The bend then displaces into the surrounding soil, where additional local bending can occur. The bend develops both longitudinal tension and compression loads in addition to ovalisation of the cross-section. The amount of movement is governed by the frictional restraint of the straight pipe adjacent to the bend and the lateral resistance to movement. The loading response of the soil in each direction can be represented by a simplification of the hyperbolic curve for modelling purposes. This method has been previously validated
The movement at bends causes high strains in compression at the inside of the apex, due to the high thermal loads and the extra bending caused as the bend moves into the surrounding soil. If the magnitude of the developed strain is significant, then local buckling at the intrados of the bend could occur. The strain at the bends under operational loading must be calculated and compared to the limit for the onset of local buckling. This can be completed using finite element analysis. Where geometrically possible, bends are constructed within a single pipe length at the required bend angle and radius, see Figure 2.
When the bend is located in a single pipe length, the extra bending and movement is concentrated in the apex of this single bend and the lateral restraint of the soil may not be high enough to resist lateral movement, leading to strains beyond the limit for the onset of local buckling. A method of preventing high strains exceeding this limit, and also limiting the displacement of the bend, is to separate the bend between adjacent pipe lengths. This allows the correct routing of the pipeline to be achieved, but reduces the compressive strain at the intrados of the bend