Pipeline technical design and materials

The development of the technical design is an ongoing process in which input from investigations of the route corridors, basic engineering, stakeholder consultation, environmental and social impact assessments and regulatory review are continuously used to optimise the design. Therefore, minor changes to the below description may be made during the detailed design period. The design de-velopment will not, however, change the project significantly, i.e. result in new or worse environ-mental impacts, as determined in this document.

Technical specifications

The design basis of NSP2 is the same as for the existing NSP. NSP2 will consist of two parallel, 48-inch, steel pipelines with a total capacity of 55 bcm per year. The pipelines will be divided into three pressure segments according to the pressure drop along the pipelines from the Russian land-fall to the German landland-fall.

The main characteristics of the pipelines are shown in Table 6-1.

Table 6-1 Design operating conditions and technical specifications for the NSP2 pipelines.

Property Value (range)

Throughput 55 bcm per annum (27.5 bcm per annum per pipeline)

Gas Dry, sweet natural gas

Design pressure Kilometre point (KP) 0 – ~KP 300: 220 bar

~KP 300 – ~KP 675: 200 bar

KP 675 – ~KP 1230.4 (NSP2 route with NSP2 route V1) / 1248.1 (NSP2 route with NSP2 route V2): 177.5 bar (Denmark)

Design temperature +40C (max)/-10C (min) for the offshore sections

Pipeline inner diameter 1,153 mm

Pipeline wall thickness 41.0 mm, 34.6 mm, 30.9 mm and 26.8 mm

(depending on pressure range, 26.8 mm in Denmark) Buckle arrestor thickness 34.6 mm / 41.0 mm (34.6 mm in Denmark)

Line pipe and buckle arrestor material C-Mn steel

Internal flow coating Low solvent epoxy, average roughness RZ <= 3 µm, thickness minimum 90 µm

External corrosion coating Three-layer polyethylene (3LPE) of 4.2 mm minimum thickness CWC thickness and density 90 mm to 110 mm, 2,400 kg/m³ to 3,040 kg/m³

Corrosion protection anodes Zinc-based anodes in low-salinity water; aluminium anodes in other ar-eas (in Denmark, only aluminium anodes are expected to be used) Standards, verification and certification

The pipelines will be designed, constructed and operated in accordance and in compliance with the international offshore standard DNV OS-F101, Submarine Pipeline Systems, along with its associ-ated Recommended Practices, issued by Det Norske Veritas (DNV).

Nord Stream 2 AG has appointed DNV GL as its independent third-party expert to confirm that the pipeline system, from pig trap to pig trap, has been designed, fabricated, installed and pre-com-missioned in accordance with the applicable technical, quality and safety requirements. When DNV GL has completed third-party verification of all project phases and the pipeline has been success-fully pre-commissioned, a DNV GL Certificate of Conformity will be issued for each of the NSP2 pipelines.

In addition to the above, the Russian and German authorities, within the respective territorial areas of competency, will independently verify the integrity and safety of the pipelines.

Materials and corrosion protection

In this section, the pipeline material design, manufacture and construction will be described in general terms. Furthermore, the expected material utilisation required for the pipeline sections in Denmark is presented.

6.2.3.1 Line pipe

The pipelines will be constructed of individual steel line pipes with a length of 12.2 m (average length is 12 m) that will be welded together in a continuous laying process. The line pipes will be internally coated with an epoxy-based material (see Figure 6-7). The purpose of the internal coat-ing is to reduce hydraulic friction, thereby improvcoat-ing the natural gas flow conditions.

An external, three-layer polyethylene (PE) coating will be applied over the line pipes to prevent corrosion. The three-layer PE external anticorrosion coating consists of an inner layer of fusion-bonded epoxy, a middle adhesive layer and a top layer of PE (see Figure 6-7). Further corrosion protection will be achieved by incorporating sacrificial anodes of aluminium or zinc (see section 6.2.3.4 describing anodes for cathodic protection). The sacrificial anodes are a dedicated and in-dependent protection system in addition to the anti-corrosion coating.

A concrete weight coating (CWC) containing iron ore (for additional density) will be applied over the external anti-corrosion coating (see Figure 6-7). The primary purpose of the CWC is to provide on-bottom stability of the constructed pipeline. Bare line pipes are already designed against exter-nal impact loads; however, the coating will provide additioexter-nal protection above and beyond project requirements.

Once the single line pipe joints are transferred onto the pipe-lay vessel, they may either be directly transferred into the vessel firing line for welding into the pipeline string or welded into double joints before being transferred into the vessel firing line for welding, Automated Ultrasonic Testing (AUT), field joint coating and subsequent pipe-lay.

Figure 6-7 Line pipe design.

6.2.3.2 Field joint coatings

After the pipe joints are welded together, and non-destructive examination (via AUT) of the weld has been performed, a field joint coating (FJC) system will be installed to prevent corrosion of the uncoated welded pipe ends, and to fill the space between the CWC sections on either side of the field joint to facilitate safe tension control (see Figure 6-8).

The field joint area will be cleaned using grit blasting before the steel is pre-heated using an in-duction heating coil, prior to the application of a PE heat shrinkable sleeve (HSS) that covers the entire exposed steel surface area. The HSS will be wrapped around the bare pipe area and shrunk onto the pipeline surface using either flame torches or the same induction coil as was used for the pre-heating.

Once the HSS has been installed, a PE former will be installed circumferentially around the field joint and secured onto the CWC on each side of the field joint using a maximum of five banding straps (three of carbon steel and two of stainless steel; the latter is used to protect against band corrosion). Polyurethane foam (PUF) will then be injected into the annular void created by the former. After a short period of time, the PUF will solidify and the coated field joint will become an integral part of the pipe, maintaining a constant pipeline outside diameter and facilitating passage of the pipeline string over the rollers as it advances down the stinger and into the water.

Figure 6-8 Field joint coating, schematic representation.

6.2.3.3 Buckle arrestors

To minimise the length of pipeline damaged by a buckle during installation, buckle arrestors (pipe reinforcement) will be installed at specific intervals in susceptible areas (see Figure 6-9). The pipe-line is at risk of collapse when it is empty, i.e. mainly during installation. Buckle arrestors are full-length pipe joints with overdimensioned thickness that are installed in the deep-water sections,

typically with a 927-m separation. The buckle arrestors are made of the same steel alloy as the pipelines. The buckle arrestors are machined at each end down to the wall thickness of the adjacent pipes to allow for welding offshore. The material requirements and properties for the buckle arres-tors are generally the same as for the line pipe.

Figure 6-9 Buckle arrestor, schematic representation. Left: full length view; right: zoomed-in view.

6.2.3.4 Anodes for cathodic protection

In addition to the three-layer PE external anti-corrosion pipe coating, secondary anti-corrosion protection will be provided by sacrificial anodes (aluminium and zinc alloys) to ensure the integrity of the pipelines over their operational lifetime (see Figure 6-10). This secondary protection will be an independent system that will protect the pipelines in case of damage to the external anti-cor-rosion coating.

The performance and durability of different sacrificial anodes in the environmental conditions of the Baltic Sea have been evaluated with dedicated tests for the construction of NSP. The tests showed that the salinity of seawater has a major effect on the electrochemical behaviour of alu-minium anodes. In light of the test results, zinc alloy anodes are foreseen for sections of the pipe-line route with very low average salinity. For all other sections, indium-activated, aluminium alloy anodes will be used.

Figure 6-10 Bracelet anode. Left: a half shell; right: two half shells trial fitted to a pipe.

In Denmark, only aluminium alloy anodes are expected to be used. The chemical composition of the aluminium anodes is shown in Table 6-2. The anodes will typically be spaced every eight to ten pipe joints apart, but this may be adjusted to suit the local environmental conditions. A total of approximately 3,000 anodes are expected to be installed in the Danish sector if the combination of

the proposed NSP2 route with V1 is selected, and approximately 3,370 anodes are expected to be installed in the Danish sector if the combination of the proposed NSP2 route with V2 is selected.

The number of anodes is subject to further optimisation.

Table 6-2 Aluminium anode composition.

The expected material consumption required for the pipeline sections in Denmark is summarised in Table 6-3 below. Quantities are approximate and subject to final optimisation.

Table 6-3 Summary of material consumption in Denmark.

Material Denmark

Total length of two pipelines (km) 293.1 328.6

Steel (t) (including buckle arrestors) 228,745 256,530

Concrete weight coating (t) 330,250 370,200

Anodes, aluminium (t) 1,105 1,240