3. PROJECT DESCRIPTION
3.3 Pipeline design
The following sections describe the mechanical design activities for the Baltic Pipe and Section 3.3.4 presents the estimated inventory of materials.
3.3.1 Gas composition
The design and construction of the pipeline have been carried out to allow for the gas
composition shown in Table 3-2 (gas from Denmark to Poland) and Table 3-3 (gas from Poland to Denmark).
Document ID: PL1-RAM-12-Z02-RA-00003-EN 10/433 Table 3-2 Gas composition for gas export from Denmark to Poland. Expected gas composition (mole-%) and range in the Baltic Pipe pipeline, with an expected average flow of 8.8 BCM/year.
Component Symbol Expected composition Expected range
Methane C1 89.65 84 – 97
Table 3-3 Gas composition for gas export from Poland to Denmark. Expected gas composition (mole-%) and typical parameters of gas in the Baltic pipe pipeline, based on examples from the LNG Terminal Świnoujście in Poland, for expected average flow of 3 BCM/year.
Component Symbol Natural gas from LNG
Terminal (4.9.2017)
Min. gross calorific value MJ/Nm3 41.84 42.39
Wobbe Index MJ/Nm3 54.47 54.73
Relative density - 0.59 0.60
Molecular weight g/mole 16.98 17.44
Document ID: PL1-RAM-12-Z02-RA-00003-EN 11/433
3.3.2 Wall thickness
The pipeline system will be designed in accordance with the DNVGL offshore standard F101 Submarine Pipeline Systems (DNVGL-ST-F101, 2017), and any other national requirements that the authorities may have or disclose during the liaison process (Rambøll, 2017).
The following assumptions have formed the basis for the design of the wall thickness of the pipeline:
• Pipeline size: 36" (fixed inner diameter of 872.8 mm);
• Estimated annual transfer volume: up to 10 billion m3/year;
• Expected input pressure to the onshore network in Poland: 46-84 barg;
• Design pressure: 120 barg.
The offshore pipeline will be constructed using high-quality carbon steel, commonly used for the construction of high-pressure pipelines. Pipe joints with a length of 12.2 m will be welded together during a continuous pipe-lay process. Steel pipes with standard thickness will be used.
The selected wall thicknesses are shown in Table 3-2, and have been calculated according to the risks to the pipeline integrity along the pipeline route. With the required wall thickness, no buckle arrestors are required to prevent propagating buckling (Rambøll, 2018d).
Table 3-4 Selected wall thickness for the 36’’ diameter Baltic Pipe. The safety zone 2 is the highest safety class, applied onshore at the Danish landfall (and Polish landfall), extending 500 from the shore.
The rest of the pipeline is zone 1, i.e. medium safety class (Rambøll, 2017).
Wall thickness criteria Safety Zone Unit Wall thickness [mm]
Selected API wall thickness Zone 1 mm 20.6
Zone 2 mm 23.8
3.3.3 Coating
Internal flow coating
The line pipe joints will be coated with internal flow coating to limit flow friction. The coating will consist of 0.1 mm epoxy paint.
External anti-corrosion coating
External anti-corrosion coating will be applied to the pipeline to prevent corrosion. This coating consists of 4.2 mm polyethylene (PE).
Concrete weight coating
The on-bottom stability design complies with the requirements from DNVGL’s recommended practice On-bottom stability design of submarine pipelines (DNVGL-RP-F109, 2017).
Concrete weight coating with a thickness ranging between 50 mm and 140 mm will be applied over the pipeline’s external anti-corrosion coating to provide on-bottom stability. While the primary purpose of the concrete coating is to provide stability, the coating also provides additional external protection against external load, e.g. trawl gear.
To assess the on-bottom stability of the offshore part of the Baltic Pipe as subject to wave and current loading, calculations have been made of how thick a of concrete weight coating is required, and to identify where seabed interventions are required.
Document ID: PL1-RAM-12-Z02-RA-00003-EN 12/433 Figure 3-2 External concrete coating on top of the three-layer anticorrosion coating covering the steel line pipes.
While the concrete thickness ranges from between 50 mm and 140 mm, the concrete density is between 2,250 and 3,300 kg/m3. In this report, the average concrete weight coating is assumed to be 100 mm @ 3,040 kg/m3.
For some sections of the pipeline, stability cannot be proven by weight coating alone. In these areas, the pipeline will be trenched and/or rock dumped for stability purposes. Ideally it will be trenched, but if trench depths cannot be achieved, rock dumping may be used. In addition, in the very nearshore region, rock backfill may be used within the trench (instead of sand backfill). This is further detailed in Section 3.5.
Field joint coating
To facilitate welding of the 12.2 m long steel pipe joints on the installation vessel, the pipe coating is stopped before the steel pipe ends. The cut-back lengths are estimated at 240 mm for the anticorrosion coating and 340 mm for the concrete coating. After completion of the
circumferential weld, the bare steel area is protected by a heat shrink sleeve, and the void between the adjacent concrete coatings is filled with moulded polyurethane (PU), either solid or foam.
3.3.4 Corrosion protection design
The design of corrosion protection has been made to comply with the requirements of DNVGL-ST-F101, 2017, DNVGL-RP-F106, 2017, and DNVGL-RP-F103, 2016. The operating temperature is conservatively assumed to equal the maximum design temperature with respect to the technical design, and the external barrier coating is envisaged as 4.2 mm, 3-layer PE coating in
accordance with DNVGL-RP-F106, 2017.
External coating will be applied to the pipeline to prevent corrosion. Further corrosion protection will be achieved by sacrificial anodes of aluminium alloy. The sacrificial anodes are a dedicated and independent protection system to that of the anticorrosion coating. The cathodic protection shall provide sufficient anode mass to protect the pipeline during the entire design life, and sufficient exposed surface to deliver the required protective current in the final end-of-life situation (Rambøll, 2017). For concrete coated pipelines, it shall be ensured that the anodes do not protrude from the coating. Therefore, an anode thickness of 45 mm will be adopted,
irrespective of the concrete coating thickness (Rambøll, 2017). The dimensions and properties of the anodes are shown in Table 3-3.
Document ID: PL1-RAM-12-Z02-RA-00003-EN 13/433 Table 3-5 Anode properties (Rambøll, 2017). The anodes consist of aluminium alloy (Al-Zn-In).
36 inch pipeline amount ensures a sufficiently large anode surface; the anode consumption has been calculated to be a maximum of 495 kg/km during the 50-year design life of the pipeline. This corresponds to a maximum anode consumption of 7.9 kg/km/year.
In practice, the release will be much lower as the role of the anodes is to provide back-up protection in case the coating of the pipeline is degraded or damaged; only a small fraction of this amount will be released.
The recommended composition of the anode material is outlined in Table 3-4.
Table 3-6 Recommended compositional limits for anode materials (DNVGL-RP-F103, 2016).
Element Al-Zn-In anodes
The geotechnical survey has identified a 15 km section close to the Polish coast (at kilometre point (KP) 255 – 270) where the seabed resistivity is very high, reducing the current output of the anodes. Therefore, the anode spacing has been reduced from six to four pipe joints,
increasing the anode mass by 50% to 1,771 kg/km on this 15 km section. This will not affect the annual anode consumption during the 50-year lifetime, but will of course prolong the duration of anode dissolution, if the pipeline is left on the seabed at the end of the design life.
The flooded section of the pipeline inside the tunnel at the landfall (see Section 3.4) will also be protected by the sacrificial anode system, possibly with a reduced spacing to deliver the required current in the confined space. For the grouted section of the tunnel, corrosion protection will be ensured by the alkalinity of the grout, possibly supplemented by an Impressed Current Cathodic Protection (ICCP) system as it is not submerged and encased in grout. This system will have cabling leading back to the valve station where the control / monitoring equipment will be located.
Document ID: PL1-RAM-12-Z02-RA-00003-EN 14/433
3.3.5 Inventory of materials
Table 3-5 summarises the expected inventory of materials to be used for construction of the offshore pipeline.
Table 3-7 Use of materials for construction of the offshore pipeline (approximate amounts).
Material Total route and route in Danish waters Total offshore
route Route in Danish waters
Length of pipeline [km] 273.7 137.6
Steel [t] 125,000 63,000
Internal flow coating, 0.1 mm epoxy paint [t] 85 45
External epoxy coating, 4.2 mm, 3 layer PE [t] 2,900 1,500
Field joint coating, Heat shrink sleeve [no.] 22,500 11,500
Concrete weight coating 100 mm, 3,040 kg/m3 [t] 253,000 127,000
Field joint coating PU [t] 5,900 3,000
Concrete (tunnel elements) [t] 6,000 4,000
Steel, landfalls (tunnel element reinforcement,
sheet piles) [t] 1,100 700