Design and construction method alternatives

Routing to avoid environmentally sensitive areas and features that include cultural heritage, munitions and infrastructure is a primary impact avoidance strategy as discussed above.

In addition to routing aspects, Nord Stream 2 AG has considered the following mitigation measures in the planning and design process:

• Alternative construction methods for the shore crossings in Russia and Germany;

• Alternative approaches to pre-commissioning;

• Selection of the pipe-laying vessel.

These topics are addressed below.

Shore crossings in Russia and Germany 5.5.1

The region in which a pipeline transitions from offshore to onshore is called a shore crossing. In shallow nearshore areas, marine pipelines require protection from wave action and ice scoring and are normally buried in a trench created by dredging prior to pipe-laying. The wet pipeline continues in a trench through a transitional zone incorporating the beach and dunes. Typically, a

temporary cofferdam is used to maintain an open trench across the dunes, beach and shallow water during the installation period. This approach may be described as “conventional open cut”.

5.5.1.1 Germany

In Germany, the shore crossing point is characterised by a 200 m wide belt of sensitive coastal forest. A conventional open cut construction methodology through the forest belt would lead to a permanent loss of habitat and changes in landscape character, as the forest would not be reinstated due to the need to protect the pipelines from tree roots. Nord Stream 2 AG has explored the alternative of twin 700 m long micro-tunnels, with entry pits located within the onshore gas receiving facility and exiting in shallow waters.

The micro-tunnel shore crossing method, which has been assessed to be technically feasible, has been selected as the preferred construction method and is described in Chapter 6 – Project description. The advantages of micro-tunnelling as opposed to open cut pipeline installation in Germany include:

• Eliminating temporary environmental disturbance along the pipeline routes during construction with impacts limited to the tunnel portals;

• Avoiding the need for reinstatement of forest habitat in the temporary working corridor;

• Eliminating the need for a cofferdam for the shore crossing and associated construction impacts at the beach–sea interface;

• Avoiding direct impacts on tourism use of the beach area, as disturbance is confined to construction of the exit portal which is both small scale and of short duration;

• Avoiding permanent disturbance of habitat for the onshore pipeline section, as the tunnel would be beneath the root base, allowing for trees to be left in place without risk to the buried pipelines.

5.5.1.2 Russia

In Russia, the preferred landfall location is Narva Bay, subject to final approval by the Russian Federaion authorities.

A wide-ranging series of trenching options were initially considered including various trenchless techniques. A shortlist of four technical options is being investigated in more detail by a team comprising environmental experts and engineers. For each option, vulnerability of the habitats that would be affected by the onshore section of the pipeline system and constructability constraints are being assessed. The habitats are identified in the figure below.

A = nearshore area. B = coastal dune. C = forest. D = secondary forest. E = relict dune. F = swamp. G = modified habitat.

Figure 5-7 Habitat types along the pipeline onshore section in Russia.

The base case method is for conventional open cut construction with an approximately 3,800 m open cut with 85 m wide right of way (ROW) from the pig trap area (PTA) to the shoreline. As an alternative to this base case, an optimisation is being considered. The optimised open cut alternative maintains an 85 m wide ROW through habitats G and F to the relict dune formation (habitat E) and then the ROW narrows to 56 m to traverse through the secondary forest and

forest (habitats D and C). Both open cut solutions cross the shoreline via a 300 to 500 m long cofferdam, which transitions into a trench extending some 3’300 m offshore.

Various trenchless options that are also being considered as an alternative to the base case method are:

• Option 2: open cut from PTA to east of dune (2 km) with a pipeline corridor width of 85 m. 1.5 km micro-tunnel through dune and forest shore crossing with cofferdam and nearshore trench.).

• Option 4a: open cut from PTA to west of dune (2.3 km) and pipeline corridor width of 85 m. 2.0 km micro-tunnel through forest and tunnel exit pit 500 m from shore dredged flotation channel for pipe-laying vessel.

• Option 4e: open cut from PTA to east of dune (2 km) and pipeline corridor width 85 m.

2.4 km micro-tunnel through dune and forest and tunnel exit pit 500 m from shore.

Dredged flotation channel for pipe-laying vessel.

While it has been possible to select a micro-tunnel crossing for the German landfall, the significantly longer trenchless section involved at the Russian landfall poses a substantially greater risk with respect to constructability. The base case conventional open cut construction method is being evaluated by the NSP2 engineers and environmental experts in parallel with the trenchless alternatives. A decision on the construction method will be taken later in the year once engineering feasibility and constructability studies are complete.

Pre-commissioning concept (offshore pipeline section) 5.5.2

Pre-commissioning activities are undertaken to confirm the integrity of the pipelines and ensure that they are airtight and that they are ready for safe operational use with natural gas.

Wet pre-commissioning (for offshore pipeline section)

Hydrostatic tests on pipelines are normally undertaken to test for strength and leaks. The test involves the filling of the pipe system with a liquid, usually water, and the pressurisation of the pipeline system to the specified test pressure. This approach is a standard approach to confirming pipeline integrity and is referred to as ‘wet’ commissioning. For wet pre-commissioning, the pipeline would be tested as three separate sections which would be subsequently joined (utilising hyperbaric welds) on the seabed at locations in Finland and Sweden to create a continuous pipeline.

As an alternative to the wet pre-commissioning concept, Nord Stream 2 AG is considering a ‘dry’

pre-commissioning approach as described below.

Dry pre-commissioning (for offshore pipeline section)

The offshore pipelines will not be pressure tested with water. Cleaning and gauging will be performed using dry air as the pigging medium. An internal inspection will be carried out by intelligent pigging, also using dried air as the pigging medium. In addition, leak detection will be carried out through an external survey using a remotely operated vehicle (ROV). For dry pre-commissioning, the required air will be dried and compressed at the German PTA by means of a temporary air compression facility, and subsequently all pigs will be launched from Germany towards Russia. Thus the pipelines will not be filled with water and, consequently, no dewatering and subsequent dedicated drying will be required.

The comparative environmental aspects of the dry pre-commissioning method compared with the wet pre-commissioning method are:

• In the case of conventional pressure testing, seawater would be used to fill and pressurise the pipelines. Not performing a pressure test would avoid the filling of the pipelines with water (approximately 1,300,000 m3 for each pipeline). Seawater contains dissolved

oxygen (DO) and bacteria, including sulphate reducing bacteria (SRB). Both DO and SRB, if not controlled, have the potential to cause corrosion and compromise the integrity of the pipeline system. Water treatment additives would be required to mitigate this risk. By applying dry pre-commissioning, the potential risk of corrosion would be eliminated. Since there would be no discharge of oxygen-depleted and treated water, potential impacts associated with the discharge of test water would be avoided.

• Another significant benefit of the dry pre-commissioning option is that it allows for the pipelines to be installed continuously and hence eliminates the need for subsea test heads and subsequent subsea (hyperbaric welds) tie-ins. Only above-water tie-ins to connect the shallow water sections in Germany and Russia would be required. The opportunity to avoid subsea tie-ins eliminates a critical operation from the construction sequence.

Consequent environmental impacts are also eliminated, as the intervention works necessary to construct large rock berms otherwise required to prepare the subsea tie-in sites would not be required.

• In the case of dry pre-commissioning, a survey type vessel would be operating along the pipeline route for one month (for each pipeline). This results in significantly reduced emissions produced offshore compared with wet commissioning. Wet pre-commissioning would require a construction type vessel with a pumping spread on board to operate at the subsea tie-in locations in Finland and Sweden for approximately six weeks on each line. Additionally, a dive support vessel would be required to operate at these locations for approximately four weeks on each pipeline during the hyperbaric welding process that would be required to create a continuous pipeline.

• For the dry concept, there are marginally higher emissions in Germany associated with the operation of compressors.

It should be noted that the onshore pipeline sections and PTAs are subject to conventional hydro-testing. This is addressed in the following chapter.

Selection of the pipe-laying vessel 5.5.3

Pipeline installation for various sections of the pipeline route will be undertaken with two different types of pipe-laying vessel: an anchored lay vessel and a DP (dynamically positioned) lay vessel.

The position of the anchored lay vessel is controlled by a mooring system that consists of up to 12 anchors, anchor wires and winches. DP vessels utilise thrusters to maintain their position, which avoids the need for anchors and anchor-handling tugs. The selection of vessel type will depend upon the following factors:

• Water depth (DP vessels are limited to deeper waters);

• Presence of munitions on the seabed;

• Presence of cultural heritage;

• Presence of shipping lanes.

DP vessels will be selected for, e.g., areas in the Gulf of Finland, where there is a high concentration of munitions from World War I and World War II and where there is a risk of anchors coming into contact with munitions. Utilisation of a DP vessel in these areas avoids the munitions clearance activities required for a pipeline anchor corridor. Where NSP2 runs close to other Baltic Sea pipelines, the selection of a DP lay vessel can reduce the risk of contact with existing infrastructure. Conversely, in shallow waters, anchored lay vessels will be utilised as, amongst other things, their use avoids the potential scouring of the seabed associated with DP thrusters.

The final selection of the pipe-laying vessel type used in particular areas will depend on technical and environmental considerations.