Pembina Pipelines Trust operates approximately 8,350 km of pipelines in British Columbia and Alberta. The company was awarded the contract to supply pipeline services to the oil sands project.

The pipeline project included the completion of five pipeline loops along the Alberta Oil Sands Pipeline (AOSPL). Existing AOSPL rights-of-way (RoWs) were used wherever possible to limit the quantity of new disturbance caused by the pipeline project. One of the loops, dubbed the Horizon Pipeline Project involved the looping of approximately 129 km of 24 and 30 inch diameter pipe, construction of which started in 2006.

One of the five new pipeline loops was the 17.3 km NPS24 Pine Creek Loop, which followed the existing pipeline corridor through the environmentally sensitive La Biche Wildland Provincial Park. This loop involved the crossing of two water bodies, the La Biche River at the north boundary of the Park and Pine Creek at the south boundary.

Due to the protected status of the La Biche River Wildland Park, no additional RoW was being granted within the Park. WorleyParsons attempted to design a horizontal directional drilling (HDD) crossing with this restriction in mind but could not achieve the desired results based on the following:

 

  • The existing AOSPL RoW configuration was designed to accommodate the previously installed trenched crossing. There were changes in horizontal direction, both upstream and downstream of the crossing.
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  • A number of adjacent pipelines within the area had to be crossed along the HDD path.
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  • The minimum design radius of curvature of the drill required clearance from the bottom of Pine Creek and the required entry and exit angles.

Ledcor Pipelines was awarded the mainline contract for the Horizon Pipeline Project. Ledcor subsequently awarded the HDD subcontract to Direct Horizontal of Stoney Plain, Alberta.

Analysing the HDD path

The soil within the HDD crossing at Pine Creek was assumed to be a ‘soft formation’ based on feedback from the drilling contractor, which included the difficulty experienced with downhole steering and the results of a geotechnical investigation.

Analysis of the drill path was completed using the Von Mises’ method. There are a number of methods available for calculating the combined stress within an HDD path: the Tresca, Von Mises’, and limit-states’ approaches are all recognised within the CSA Z662 oil and gas pipeline systems standard, and offer varying degrees of conservatism. Regardless of the method chosen, stress is analysed for a pipeline within the HDD path for the maximum operational conditions.

The presence of applied loads in more than one direction results in a much more complex state of stress than for applied loads in only one direction (uniaxial). The predominant stresses in pipelines are typically biaxial, with internal pressure acting in the circumferential (hoop) direction and thermal loads and beam bending acting in the longitudinal direction. The yielding of the steel under these conditions is considerably more complex, and there are two widely accepted approaches for determining the combination of stress in various directions that result in yielding of ductile material (such as steel); the two approaches are the Tresca theory and the Von Mises’ theory.

The combined stress according to CSA Z662 is a biaxial stress, which is a combination of hoop stress due to internal pressure, and longitudinal stress due to bending of the pipe to achieve the curved shape of the HDD path added to the stress imposed on the pipeline due to thermal differential. A large positive thermal differential, together with internal pressure, is typically the worst case.

The Pine Creek crossing

The resultant minimum acceptable radius for the Pine Creek crossing was determined to be 385 m, a marginal improvement on the previous 425 m. The 385 m radius would need to be compared to the calculated minimum radius of the drill path after completion of the 36 inch ream.

The ream was completed two days later. Entec analysed the drill profile data and calculated a minimum dog-leg radius of 284 m, with a number of other radii less than 385 m. The subsequent survey tool run was completed later in the same day. Entec analysed these results and calculated a minimum radius of 170 m with a number of other radii less than 385 m. The ever-decreasing calculated minimum radius confirmed that the progression of the calculations was from conservative to more realistic.

Pembina and its construction manager subsequently decided to pull the NPS24 pipe into the hole and the project team agreed to continue analysing the available information to determine if in fact the 36 inch reamed drill path was acceptable.

First, the project team completed a sensitivity analysis to determine if reducing the maximum operating pressure of the pipeline or increasing the pipe grade would be a viable alternative. After careful review, the required maximum operating pressure reduction to 4,400 kPa, or a pipe grade increase to 545 MPa, would still not be sufficient to provide acceptability for the Pine Creek crossing.

The next and only viable alternative for analysing the crossing was to determine the absolute stress level within the pipe at this location. The Tresca and Von Mises’ methods are both stress-based, conservatively not accounting for the non-linear steel properties. Due to tight-bend radii determined in the field, the group agreed to undertake a third method of analysis: a non-linear strain analysis, which is permitted by CSA Z662 Annex C as a limit-state design approach. This strain analysis is much more complex than either of the stress methods, but it also is a more rational and usually less-restrictive approach. Determination of the longitudinal strain with non-linear steel properties and subject to biaxial stresses was performed with a finite-element computer program.

Subsequent to the very detailed review of the stresses involved in this installation, the current drill alignment was supported by WorleyParsons Calgary, based on the maximum calculated strain being below the maximum allowable strain for the crossing using limit-states analysis. WorleyParsons used the minimum drill path radius of 170 m and compared the strain at this location to the maximum strain allowable for the NPS24 crossing pipe. The results were deemed acceptable as per the above discussion.

In addition, WorleyParsons recommended running a geometry pig through the crossing within two years of the start of pipeline operation to verify the geometry of the pipe within the crossing and reconfirm the results of the limit-states’ analysis.

The purpose of this article is not to support less conservative analysis as a general rule for HDD installations but rather, given the difficult circumstances encountered at the site, to illustrate that this method of analysis is applicable and required in certain circumstances. HDD installations of pipelines beneath pipeline route obstructions are technically challenging, as are the determination of the construction stresses. These installations are such that future repair and clean-up if a problem develops are not possible or are extremely difficult due to the location and depth of these installations. Therefore, a conservative approach to the design provides some assurance that problems should not occur at this type of crossing.