**1. Introduction**

In the field of subsea engineering, the reel-lay method offers logistical and technical advantages over alternative pipe laying methods such as J-lay as S-lay [1]. A considerable length of pipeline is fabricated and coated onshore from pipe segments, which are welded together to form

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

individual stalks, as shown in **Figure 1a**. The stalks are spooled continuously onto a reel situated on a barge (see **Figure 1b** and **c**) docked at an onshore production facility, with tie-in welds performed to join consecutive stalks together to form a contiguous pipeline typically numerous kilometres in length. The barge then transports the loaded reel out to a construction vessel offshore, such as Heerema Marine Contractors' deepwater construction vessel DCV *Aegir* (see **Figure 2a**), where it is installed onto the vessel. The pipe is fed over the aligner wheel on the lay tower, through a straightener and through the moonpool in the deck of the vessel, then subsequently unspooled and laid on the seabed (see **Figure 2b**). In the case of the *Aegir*, logistical advantages are offered by three reels being available. While a reel is being unspooled onboard the vessel and pipe being laid offshore, another reel can be loaded with another pipeline back onshore, with yet another reel in transit either to or from the vessel. Compared to the J-lay method where the pipeline is fabricated onboard the vessel by welding successive 50 to 75 m-long segments together, the onshore fabrication process is far less susceptible to the suspension of operations due to adverse weather conditions, and the overall process of unspooling and laying the pipe is considerably quicker and thus also less likely to be affected by scheduling or weather delays [2].

impetus for the investigation described in the current chapter was to ascertain whether this capacity could be extended to considerably thicker coating systems. Extending the envelope to encompass these coating solutions would allow Heerema to employ the reel-lay method to lay pipeline in ever more challenging environments and greater water depths. The current chapter

**Figure 2.** (a) DCV *Aegir*, owned and operated by Heerema Marine Contractors, capable of reel-lay and J-lay; (b) schematic

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In order not to damage the polymer coating when welding pipe segments together, it is necessary to cut back the coating approximately 300 mm either side of the weld, typically with a 30° chamfer to provide a smooth transition between the linepipe coating and the field joint coating and avoid stress raisers due to geometric and material discontinuities. After the weld has been performed and passed inspection, an injection-moulded polypropylene (IMPP) field joint coating (FJC) is applied around the weld in order to replace the coating material that had been cut back prior to welding. The IMPP application process (described in more detail in Section 2.2) involves heating the steel to above 200°C. Since steel loses strength and stiffness with increasing temperature, the field joint is left to cool down so that the steel pipe can regain

It is thus necessary to determine how long is required for the field joint to cool down sufficiently before reeling can take place safely. For field joints within a single stalk this is not an issue since the stalks are fabricated well in advance of reeling and so the field joints will have cooled down and regained full strength. However, during continuous reeling the stalks are joined together

The pipeline is thus subject to barge motions and the associated fatigue effects which can further weaken the welds. It is advantageous that reeling is paused for a short time as possible so that allowances made for fatigue effects need not be too onerous; current best practice is to

One of the primary factors dictating the cooldown time is the thickness of the insulating IMPP field joint coating, which can be controlled by using a specially-shaped mould for the IMPP, creating either hourglass or full FJCs (see **Figure 3**). A thinner coating allows for a quicker cooldown time and hence the strength of the steel is regained sooner. However, thicker FJCs contribute noticeably to the overall resistance of the field joint, even though the elastic stiffness and the strength of steel are orders of magnitude greater than those of the polymer coatings; although the steel yields at a strain of around 0.2% and loses much of its stiffness thereafter, the polymers

focuses on pipelines with a 9LPP coating with an overall thickness of 100 mm.

its strength and be bent to the reel without buckling or deforming excessively.

while the pipeline is being spooled with reeling paused while the weld is performed.

pause reeling at least overnight.

of reel-lay system onboard the *Aegir*.

In order to provide mechanical protection, corrosion protection and thermal insulation for the product conveyed within, the steel pipes are coated with a polymer linepipe coating. In order to maintain the temperature of production fluids during operation and shutdown conditions, thicker coatings have had to be employed to enhance the thermal performance of the pipeline solution [3]. There are considerable challenges inherent to installing pipes with such thick coatings; it has been found previously [4] that combinations of coating thickness, pipe wall thickness and mismatch in material properties across a field joint can result in buckling. These challenges are especially present when employing the reel-lay method where the pipeline, welds and coatings typically undergo a number of bending events during fabrication, spooling, unspooling and eventual touchdown on the seabed. Previous projects have seen Heerema successfully reel and lay pipelines with a 53 mm-thick multiple layer polypropylene (MLPP) coating; the

**Figure 1.** (a) Stalks prepared onsite; (b) empty reel drum onboard a barge; (c) reeling of pipeline stalks.

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individual stalks, as shown in **Figure 1a**. The stalks are spooled continuously onto a reel situated on a barge (see **Figure 1b** and **c**) docked at an onshore production facility, with tie-in welds performed to join consecutive stalks together to form a contiguous pipeline typically numerous kilometres in length. The barge then transports the loaded reel out to a construction vessel offshore, such as Heerema Marine Contractors' deepwater construction vessel DCV *Aegir* (see **Figure 2a**), where it is installed onto the vessel. The pipe is fed over the aligner wheel on the lay tower, through a straightener and through the moonpool in the deck of the vessel, then subsequently unspooled and laid on the seabed (see **Figure 2b**). In the case of the *Aegir*, logistical advantages are offered by three reels being available. While a reel is being unspooled onboard the vessel and pipe being laid offshore, another reel can be loaded with another pipeline back onshore, with yet another reel in transit either to or from the vessel. Compared to the J-lay method where the pipeline is fabricated onboard the vessel by welding successive 50 to 75 m-long segments together, the onshore fabrication process is far less susceptible to the suspension of operations due to adverse weather conditions, and the overall process of unspooling and laying the pipe is considerably

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

quicker and thus also less likely to be affected by scheduling or weather delays [2].

**Figure 1.** (a) Stalks prepared onsite; (b) empty reel drum onboard a barge; (c) reeling of pipeline stalks.

In order to provide mechanical protection, corrosion protection and thermal insulation for the product conveyed within, the steel pipes are coated with a polymer linepipe coating. In order to maintain the temperature of production fluids during operation and shutdown conditions, thicker coatings have had to be employed to enhance the thermal performance of the pipeline solution [3]. There are considerable challenges inherent to installing pipes with such thick coatings; it has been found previously [4] that combinations of coating thickness, pipe wall thickness and mismatch in material properties across a field joint can result in buckling. These challenges are especially present when employing the reel-lay method where the pipeline, welds and coatings typically undergo a number of bending events during fabrication, spooling, unspooling and eventual touchdown on the seabed. Previous projects have seen Heerema successfully reel and lay pipelines with a 53 mm-thick multiple layer polypropylene (MLPP) coating; the

**Figure 2.** (a) DCV *Aegir*, owned and operated by Heerema Marine Contractors, capable of reel-lay and J-lay; (b) schematic of reel-lay system onboard the *Aegir*.

impetus for the investigation described in the current chapter was to ascertain whether this capacity could be extended to considerably thicker coating systems. Extending the envelope to encompass these coating solutions would allow Heerema to employ the reel-lay method to lay pipeline in ever more challenging environments and greater water depths. The current chapter focuses on pipelines with a 9LPP coating with an overall thickness of 100 mm.

In order not to damage the polymer coating when welding pipe segments together, it is necessary to cut back the coating approximately 300 mm either side of the weld, typically with a 30° chamfer to provide a smooth transition between the linepipe coating and the field joint coating and avoid stress raisers due to geometric and material discontinuities. After the weld has been performed and passed inspection, an injection-moulded polypropylene (IMPP) field joint coating (FJC) is applied around the weld in order to replace the coating material that had been cut back prior to welding. The IMPP application process (described in more detail in Section 2.2) involves heating the steel to above 200°C. Since steel loses strength and stiffness with increasing temperature, the field joint is left to cool down so that the steel pipe can regain its strength and be bent to the reel without buckling or deforming excessively.

It is thus necessary to determine how long is required for the field joint to cool down sufficiently before reeling can take place safely. For field joints within a single stalk this is not an issue since the stalks are fabricated well in advance of reeling and so the field joints will have cooled down and regained full strength. However, during continuous reeling the stalks are joined together while the pipeline is being spooled with reeling paused while the weld is performed.

The pipeline is thus subject to barge motions and the associated fatigue effects which can further weaken the welds. It is advantageous that reeling is paused for a short time as possible so that allowances made for fatigue effects need not be too onerous; current best practice is to pause reeling at least overnight.

One of the primary factors dictating the cooldown time is the thickness of the insulating IMPP field joint coating, which can be controlled by using a specially-shaped mould for the IMPP, creating either hourglass or full FJCs (see **Figure 3**). A thinner coating allows for a quicker cooldown time and hence the strength of the steel is regained sooner. However, thicker FJCs contribute noticeably to the overall resistance of the field joint, even though the elastic stiffness and the strength of steel are orders of magnitude greater than those of the polymer coatings; although the steel yields at a strain of around 0.2% and loses much of its stiffness thereafter, the polymers

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

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.

relative stiffness and strength of the solid polypropylene layers. Material fingerprinting was undertaken in parallel to the experimental investigation in order to characterise the thermal

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While the majority of specimens prepared were intended for full mechanical bend testing, a number of specimens were reserved in order to measure the heat evolution occurring within the liquid IMPP after injection. The temperatures were recorded using thermocouples, which were arranged as shown in **Figure 5**, in order to calibrate and validate thermal numerical models.

Three different FJC geometries were investigated: full, with a nominal thickness of 108 mm to include a 50 mm long, 8 mm thick overlap at the top of the chamfer, a thick hourglass with a nominal thickness of 50 mm, and a thin hourglass with a nominal thickness of 40 mm. A bare pipe with no FJCs was also tested in the bend rig as a control specimen. Temperature readings

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 sta-

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

and mechanical properties of the various materials used in the pipeline system.

**Figure 4.** Chamfer section of 9LPP linepipe coating and IMPP field joint coating.

recorded by the thermocouples are presented and discussed in Section 3.2.3.

tion also. The ambient temperature was recorded during each test.

after some solidification of the polypropylene.

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

**2.2. Setup and procedure**

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.