Design and Construction Challenges of a Roped Insulated Pipeline

Thermally insulated hot buried pipelines pose a unique set of challenges. This paper discusses those challenges and how they were met during design and construction of the 150 km long Husky LLB Direct Pipeline, the longest thermally insulated oil pipeline in Canada. Thermal insulation materials are soft and can be easily damaged during construction and backfilling, and by large restraining forces at bends when the pipeline is operating at high temperatures. The large temperature difference between pipeline installation temperature and maximum operating temperature leads to large axial compressive forces that can cause movement at bends, crush insulation, increase temperatures at ground surface, cause loss of restraint, and in the worst case, lead to upheaval buckling and loss of containment. Special design and construction features to deal with these challenges, including insulation specifications, insulation of pipe bends, pipeline pre-straining, long radius bends, deeper burial, and pipeline roping, were therefore necessary. After pipe has been insulated with polyurethane foam it cannot be bent in standard field bending machines used for uninsulated pipes because the foam is too soft. The induction bends and cold bends that are shop insulated after bending are expensive. The Project minimized the number of these expensive insulated bends by using an engineered ditch bottom profile. This meant that shop bends were only needed to reduce excavation depth at sharp changes in ground surface elevation where the roped profile required excessive grading. Care was therefore necessary in the selection and development of specifications for the insulation system and shop fabricated bends, and to design and construct a ditch profile to minimize forces on the insulation and control upheaval buckling. Close co-ordination with vendors and the construction contractor was crucial for a successful and timely completion.

Experimental studies on the possibility of manufacturing cold bends with increased bending angles

Used in order to turn the line of trunk pipelines in horizontal and vertical planes, cold bends are the simplest and most economical elements to be manufactured. Their smaller rotation angle compared with other types of bends is, however, a disadvantage. The article presents the results of cold bends, experimentally manufactured from K56 strength class pipes to have an increased bending angle compared to that stated in regulatory requirements; also, it contains test results regarding metal samples taken from deformed and undeformed sites of the experimental bends. It was found that 720×8 mm and 1020×17 mm bends at rotation angles of up to 13 and 9.1°, respectively, retain satisfactory geometric parameters and are not prone to corrugations. A study on how hardening at cold deformation affects the change in the mechanical properties of deformed bend sites showed that the values of temporary resistance, yield strength, elongation, and impact strength comply with regulatory requirements. It was revealed that the delivery state of the rolled stock of the initial bare pipe affects the level of impact strength and the cold brittleness threshold attributed to the bends: thus, when testing the bent-out metal after high tempering, a higher level of toughness together with a higher cold brittleness threshold were revealed compared to those obtained when testing the bent-out metal after controlled rolling. In general, experimental studies confirmed that cold bends with an increased bending angle could be manufactured from Russian-made pipes of K56 strength class. Such bends used to construct a trunk pipeline will contribute to reducing the total number of bends, the amount of work and, consequently, the cost of construction.

Pipeline Integrity Analyses for Construction in Mountainous Areas

The construction of a pipeline in mountainous terrain often exposes great challenges compared to that on flat land. To accommodate the terrain and resultantly complex route, the pipeline design must incorporate a large quantity of cold bends and elbow fittings. A recently constructed project provides a prime example of a pipeline crossing such terrain. The challenging construction conditions and the bends and elbows make the assessment of stress impacting long-term pipeline integrity critical, yet difficult. This paper focuses on three specific aspects of long-term integrity for construction in mountain areas using advanced finite element analysis (FEA). The first scenario is tie-in welding. Tie-in welding connects separate pipeline segments constructed independently. In general practice, considerable lengths of pipe are left unburied to reduce the potential resultant stress due to the misalignment between the pipes at the tie-in weld location. However, in mountainous terrain the length of unburied pipe may be constrained by field conditions of the tie-in location. The implications are amplified at a tie-in adjacent to bends or elbows. The second scenario is hydrostatic testing. The gravitational weight of water generates additional internal pressure in the pipeline segments at low elevations. In areas of significant elevation change, hydrostatic test section design defines the segments based on the maximum allowable hoop stress level calculated for straight pipe. However the bends and elbows often encounter increased combined stresses at such locations that may not be adequately considered. The last scenario is ratcheting. Exacerbated by complex routing and profile, pipelines constructed in mountainous areas are at risk to develop significant uplift in the soil at bend locations during hydrostatic testing and initial operating cycles. If such uplift displacement accumulates during subsequent operating cycles, a phenomenon known as ratcheting, the pipe may eventually fail by upheaval buckling. This paper evaluates the above scenarios of a NPS 30 section of pipeline consisting of several segments with wall thicknesses varying from 12.0 mm through 19.6 mm, and contains frequent bends and elbows. The pipeline route is mountainous with slopes exceeding 70 degrees, and includes a tunnel immediately adjacent to water crossings and steep slopes. Tie-in welds are made in tight confines at either end. Analysis based on this project profile provides detailed information and insight into the design and construction of pipelines in mountainous terrain.

Evaluation of Pressurized Cold Bend Pipe Body Tensile Fractures Under Bending Loads

Cold bending is applied at locations where the pipeline direction has to be changed in a horizontal or vertical plane. The process of cold bending usually results in residual stresses as well as changes in the material properties at the vicinity of the cold bend location which makes the study of the mechanical behaviour of cold bends indispensable. Due to discontinuous permafrost in arctic regions as well as slope instabilities and earthquakes cold bends within pipelines constructed in such locations can be subjected to significant tensile or compressive forces. Experimental studies were carried out by Sen et al [1][2][3]in order to investigate the buckling behaviour of pressurized cold bends. In these experiments the curvature of the cold bend is increased in the presence of a constant internal pressure. In their experimental study a total of 8 full scale tests were conducted with a variety of pipe diameters, diameter to wall thickness ratio and steel grade. In this set of full scale tests one of the pipes with grade X65 failed due to fracture at the extrados after buckling and formation of wrinkles at the intrados[1]. Our previous work [4], [5] on this subject showed the simulations of this case using finite element analysis. These simulations demonstrated that indeed pipe body tensile side fracture can be observed for this particular pipe specification. Whereby the tension side fractures are expected starting from a specific internal pressure level. The simulation results showed that the equivalent plastic strain values at the cold bend extrados increase dramatically starting from a certain level of applied curvature in load cases with an internal pressure higher than a transition value. In this paper the effect of steel grade on this transition from compressive to tensile failure is investigated. Parametric studies are conducted for the entire range of steel grades tested in the experimental study of Sen et al. It is found that there is a linear proportionality between the steel grade and the transition internal pressure for steel grades between X60 and X80.

Finite Element Analysis of Cold Bend Pipes Under Bending Loads

Cold bends are required in pipelines at locations of changes in horizontal or vertical direction in the right of way. Due to this change of direction, pipeline deformations caused by geotechnical or operational loading conditions tend to accumulate at the site of cold bends. These deformations may become sufficient to cause local buckling at the bend. For pipeline design, it is important to understand the level of deformation that a cold bend can accumulate prior to local buckling so that the strain capacity can be compared to the expected pipeline deformations. Evaluating the buckling strain of cold bends is extremely complex due to the residual stresses, ripples, and material transformations cause by the cold bending process. Accordingly a finite element model was developed herein. This model accounted for the cold bend geometry, initial imperfections, and the material transformations caused by the cold bending process. This model was validated against 7 full scale tests of cold bend pipes that were subjected to bend loading and internal pressure. The global and local behavior of this model exhibited reasonable correlation against the tests.

Mechanical Properties of Cold Bend Pipes

Cold bends are frequently required in energy pipelines in order to change the vertical and horizontal orientations of the pipeline route. They are produced by plastically bending a pipe joint in a cold bending machine by creating a series of uniformly spaced incremental bends. This procedure acts to reduce the moment capacity and buckling strain of the pipe, and studying the changes in pipe properties caused by cold bending is valuable in assessing the level of this strength reduction. Accordingly, the initial imperfections and material transformations of five full-scale cold bend pipes were assessed in this research program. The imperfections were measured at several locations around the circumference of the specimens, along the entire bend length. It was determined that the distribution of imperfections was similar in shape to a sine function, and their amplitude ranged from 0.3mmto1.0mm. Tension coupon tests were conducted on the intrados, extrados, and virgin materials of the specimens. It was revealed that the extrados material exhibited an increase in yield strength due to work hardening and that the intrados material demonstrated a reduction in yield strength due to the Bauschinger effect. It was established that the imperfections, and material transformations in the specimens were predominantly unaffected by the incremental-bend magnitude or spacing that was employed during the cold bending procedure.

Tensile Properties of Cold Bends for Line Pipes

To comprehensively investigate the tensile properties of cold bends, full-scale cold bending experiments, tensile tests using prestrained small-scale specimens, and finite element (FE) analyses of the cold bending processes were conducted on API 5L X60 and X80 grade line pipes. The tensile tests revealed that the tensile properties of the cold bends were comparable to the uniaxially prestrained specimens machined from the straight part of the pipes. A FE model simulating the cold bending process was verified with the full-scale experimental results in terms of the distributions of residual strains. These results supported a procedure for estimating the tensile properties of the cold bends with a combination of the FE model and the tensile tests using the prestrained specimens; the residual strains obtained from the FE model are transformed into the tensile properties based on the relationship between the residual strains and the tensile properties. This study clarified that the tensile properties come close to being uniformly distributed by reducing the distance between the bending locations; the distance between the bending locations has a significant influence on the overlap of adjacent deformed areas, which governs the distribution of the tensile properties of the cold bends.

Mechanical Properties of Cold Bend Pipes

Cold bends are frequently required in energy pipelines in order to change the vertical and horizontal orientation of the pipeline route. They are produced by plastically bending a pipe joint in a cold bending machine, by creating a series of uniformly spaced kinks. This procedure acts to reduce the moment capacity and buckling strain of the pipe, and studying the changes in pipe properties caused by cold bending is valuable in assessing the level of this strength reduction. Accordingly, the initial imperfections and material transformations of five full-scale cold bend pipes were assessed in this research program. The imperfections were measured at several locations around the circumference of the specimens, along the entire bend length. It was determined that the distribution of imperfections was similar in shape to a sine function, and their amplitude ranged from 0.3 to 1.0 mm. Tension coupon tests were conducted on material from the intrados, extrados, and virgin material of the specimens. It was revealed that the extrados material exhibited an increase in yield strength due to work hardening, and that the intrados material demonstrated a reduction in yield strength due to the Bauschinger Effect. It was established that the imperfections, and material transformations in the specimens were predominantly unaffected by the kink magnitude or spacing that was employed during the cold bending procedure.

A Unique Pipeline Fault Crossing Design for a Highly Focused Fault

This paper describes the development of a unique pipeline fault crossing design upgrade for a 22-inch (559 mm) diameter Pacific Gas & Electric Company (PG&E) gas transmission line where it crosses the Calaveras fault near Sunol, California. The new design is capable of withstanding significant levels of horizontal fault offset while minimizing the deformation demands experienced by the pipeline. This unique design concept is applicable to fault crossings with well defined fault locations and highly localized fault offset profiles (e.g., for this fault, 85% of the offset is expected to occur within ±5 feet (±1.5 m) from the center of the fault trace, which was precisely located by field trenching studies). Relative to the original fault crossing design, the new design provides a more favorable “local” fault crossing angle “β” (β = 73° for the original design vs. β = 95° for the new design). The angle change is accomplished by installing an offset section of the pipeline adjacent to the fault such that the fault crosses the pipeline in the middle of a tangent section in the nearest offsetting leg. The four bends used to fabricate the offset section are cold bends with an average radius of 76.4 feet (23.3 m). The entire mitigated section of the pipeline is buried in a select sand trench. For this design configuration, right lateral fault motion results in (a) a “closing” action within the two adjacent cold bends located on either side of the fault and (b) a net tension force in the pipe (due to the obtuse β value) centered on the tangent section of the offsetting leg containing the fault crossing. The net tension force in the offsetting leg results in an “opening” action within the two adjacent cold bends on either side of the fault. By adjusting the local fault crossing angle β, the “bend opening” action that results from pipe extension across the fault can be made to nearly offset the “bend closing” action induced by the transverse component of the fault offset. The use of a select sand backfill in the retrofit section allows the bends to engage the soil with relatively low transverse and longitudinal resistance thereby enhancing the overall flexibility/compliance of the fault crossing design. Implementation of this unique design concept at the Calaveras fault crossing increased the amount of fault offset required to damage the pipeline from about 7 inches (18 cm) for the “as-built” design to well over 90 inches (2.3 m) for the retrofit.