**2.2. Setup and procedure**

maintain their elastic stiffness up to strains of approximately 2%, and thus contribute to the stability of the steel pipe when it is deforming plastically and help to prevent buckling. Thus, an optimal field joint coating thickness can be found where a balance is struck between steel strength being regained sooner and the contribution of the polymer material to the resistance of the joint. In the current chapter, the experimental campaign is described and results presented, followed by a description of the thermal and mechanical numerical models developed to simulate the behaviour of the hot tie-in field joints. After successful validation of the models against the experimental results, a parametric study was conducted varying the thickness of the FJC and the cooldown time prior to bending, providing a matrix of viable coating and cooldown combinations upon

which an appropriate operational envelope could be defined for the pipeline system.

Following successful qualification and commissioning of 12.75″ pipelines with 53 mm-thick five-layer polypropylene (5LPP) coating for a previous project, it was initially decided to investigate the behaviour of a pipeline with 100 mm-thick nine-layer polypropylene (9LPP) coating in order to expand the reel-lay capacity envelope for the *Aegir*. Given that the inner radius of the reel is 8 m, the outer diameter of bare or thinly-coated reel-laid pipes is typically limited to 16″ so that strains in the innermost layer of pipe around the reel are limited to 2.5% in accordance with DNV guidelines [5]. For thicker coatings, the strains in the outer surface of the coating increase accordingly, with a greater risk of damage to the pipe walls and coating. Thus, the primary aim of the experimental investigation was to assess whether these higher levels of strain could be withstood satisfactorily by the steel pipe and coating materials.

Factory-coated test specimens were prepared comprising three pipe segments of grade X65

were 3.45 m apart in order to test two FJCs at the same time on the rig. The outer diameter of the pipes was nominally 327 mm, while the wall thickness was 15.7 mm, giving a diameterto-thickness ratio of 20.8; the pipes were intentionally chosen to be this slender in order to provide a more onerous combination of pipe wall thickness and coating thickness [3]. The composition of the 9LPP coating is shown in **Figure 4**; after a thin three-layer polypropylene (3LPP) base layer is applied, alternating layers of foam and solid polypropylene are provided in order to combine the enhanced thermal insulating performance of the foam with the

), with 20 mm girth-welded field joints. The field joints

**2. Experimental investigation**

**Figure 3.** (a) Hourglass field joint coating; (b) full field joint coating.

80 Finite Element Method - Simulation, Numerical Analysis and Solution Techniques

**2.1. Specimens**

steel (yield strength *f*

<sup>y</sup> = 450 N/mm<sup>2</sup>

The tests were conducted at Heriot Watt University, Edinburgh, from November 2014 to January 2015. A coating station was installed onsite; for the specimens being used solely for temperature development measurement, the thermocouples were installed in the coating station also. The ambient temperature was recorded during each test.

The IMPP application process involves heating the bare steel substrate to temperatures around 240°C with an induction heater, then applying a thin layer of fusion bonded epoxy (FBE) followed by thin layers of chemically-modified polypropylene (CMPP) to encourage bonding between the steel substrate and the IMPP. The chamfers of the linepipe coating are reheated to encourage bonding with the IMPP, and a mould is then fitted around the field joint. The liquid polypropylene is then injected at 200°C into the mould, which is removed after some solidification of the polypropylene.

**Figure 5.** Schematic of thermocouple locations within the FJC.

**3.1. Modelling approach**

**3.2. Thermal modelling**

**Figure 7.** Modelling of field joints in COMSOL.

In order to model the behaviour of the hot tie-in field joints, a thermo-mechanical model was required. Although coupled thermo-mechanical modelling is available in commerciallyavailable finite element modelling software such as Abaqus [6], it is computationally expensive. An alternative approach employed in the current work is to separate the analysis into a thermal model to simulate the process of applying the IMPP, followed by a mechanical model simulating the process of bending the pipe that incorporates the temperature field predicted

Numerical Analysis of Hot Polymer-Coated Steel Pipeline Joints in Bending

http://dx.doi.org/10.5772/intechopen.72262

83

Thermal modelling was performed using COMSOL Multiphysics [7], with the temperature fields around the field joint exported at a number of defined intervals of cooldown. These fields were then mapped onto an Abaqus mechanical model that simulated bend testing of the pipe. Given that the cooldown times are in the order of hours and that the bending events were performed in a number of minutes, there is a difference in orders of magnitude between the cooldown rates and the strain rate during the bend tests. Thus, it is reasonable to assume that heat flow within the field joints during bending was negligible and so can be accurately modelled by assuming a static temperature field in the mechanical models, thus achieving a

A time-dependent thermal model was developed in COMSOL Multiphysics, which was chosen due to its relative computational efficiency and modelling flexibility when compared to thermal modelling in comparable finite element modelling software. A section of the pipe around a particular field joint was represented by two-dimensional models assuming axisymmetric conditions about the longitudinal axis, with symmetry also assumed at the weld plane. The models relating to the three different FJC geometries are shown in **Figure 7**. It was found from sensitivity analysis that the change in temperature was negligible at a distance of 2 m from the weld, and thus the extent of the models reflects this. Triangular elements were used

by the thermal model along with temperature-dependent material models.

considerable degree of efficiency over a fully-coupled thermo-mechanical model.

**Figure 6.** Bend test rig at Heriot Watt University, with a test specimen bent to the reel former.

The bending rig consisted of a reel former with a radius of curvature equal to 8 m, and a straightening former with a radius of curvature equal to 55.84 m (see **Figure 6**); these radii are representative of those of the reel drum and straightener employed onboard DCV *Aegir*. After coating of the field joints was completed in the coating station, the pipe specimen was installed into the bending rig. One end of the pipe was anchored with a pin, while the other end of the pipe was attached to a pull head, which was translated between the two formers by means of a cable attached to a crane.

After a pre-defined cooldown period, the test procedure was initiated, whereby the specimen underwent five full bending cycles, with each cycle consisting of a number of steps: (i) the pipe is bent to the reel former and held; (ii) the pipe is released, (iii) the pipe is bent to the straightening former and (iv) the pipe is finally released again. The pipe was held to the reel former overnight in order to simulate the effect of the IMPP cooling down on the reel prior to resumption of reeling operations. The pipe is subjected to five full cycles during qualification testing in order to ensure that pipe integrity is maintained during initial spooling, straightening, bending over the aligner wheel and pipelay, along with contingencies for weather delays or the possibility of requiring to recover the pipe back onto the reel and then to unspool again. Ovality measurements were taken at salient locations after each cycle step, where the ovality is defined according to DNV design guidance [5] as:

$$\text{ovality} = (D\_{\text{max}} - D\_{\text{min}}) / (D\_{\text{nom}}) \tag{1}$$

where *D*max is the maximum diameter of the deformed pipe, *D*min is the minimum diameter of the deformed pipe and *D*nom is the original nominal diameter of the pipe. The ovality measurements were taken using optical metrology equipment inserted inside the pipe; thus, the values used in Eq. (1) relate to the inner diameter of the steel pipes with *D*nom = 295.7 mm.
