Calculation with External Coatings and Tension
Depending on the engineering application, a pipeline can be coated with a variety of materials in order to improve, or protect, its inherent properties. If present, coatings are applied uniformly along the pipe joints with the exception of a small length near the joint ends, which remains uncoated for welding purposes. These uncoated sections are known as field joints. A typical layer stack-up consists of bare steel pipe coated with anti-corrosion material plus a layer of concrete, as shown in the figure below.
Typical Layer Stack-Up on a Coated Pipe Joint
The overall bending response of a coated pipe depends on the bending stiffness of its individual layers. In the previous section, it was explained that high tension reduces the bending capacity of a bare pipe. This is also true in the coated case, but here a new level of complexity is introduced due to the different properties of the coatings themselves and how the coating layers interact with one another.
To elaborate on the latter point, as the bending curvature increases on a coated pipe joint there is a distinct possibility that the bond between the coatings can fail leading to slippage, which in turn reduces the overall bending moment for the cross section. This slipping behaviour can particularly occur between concrete and its underlining layer. Note that slippage of the concrete layer generally starts at the field joint and progresses towards the middle of the pipe joint.
Euler-Bernoulli beam theory assumes that plane sections remain plane during bending. However, this is not true for a joint whose coatings are experiencing slippage. The physical formulation for beam bending with non-planar sections is complex and computationally expensive to calculate. To ensure efficient analysis, PipeLay assumes slippage can occur only in the outermost coating (typically where the concrete coating is) when the stress at its inner boundary exceeds a specified bond bending resistance (BBR). The slippage itself is represented as a deviation in the planar sections of the outermost coating and the other layers in the cross section for a given moment load. The amount of slippage is controlled internally by means of a slippage coefficient, which is used to scale down the curvature in the outermost coating when compared to the global pipe curvature. The slippage coefficient can range from 0 to 1 and is used to decrease the amount of curvature in the outermost coating in order to reduce the boundary stress under the BBR. Note that a slippage coefficient of 1 represents a fully non-slipped scenario whereas a coefficient of 0 is a fully slipped case.
The moment-curvature calculation procedure presented in the previous section can be adapted to consider multiple layers (bare pipe and external coatings) with also the potential of slippage. An algorithm without slippage is presented first, followed by the additional steps needed to incorporate the slippage of the outermost coating. It is worth noting that the algorithm developed in PipeLay uses a similar approach to that published in Hardjanto and Walker  and has been validated against the data described in Nourpanah and Taheri .
1.Hardjanto, F., Walker A., “SEMI-ANALYTICAL MODEL FOR PIPELINE STRAIN INTENSIFICATION FACTOR”, OMAE 2013-10044, Proceedings of the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering.
2.Nourpanah, N., Taheri, F., “FINITE ELEMENT ANALYSIS OF STRAIN CONCENTRATION IN FIELD JOINT OF CONCRETE COATED PIPELINES”, OMAE 2009-79047, Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering