Step 5: Selection of preferred option
6. PROJECT DESCRIPTION
6.7 Construction at the landfalls Landfall Russia
6.7.1
A number of construction activities will be carried out in the landfall areas in order to bring the offshore pipelines ashore and create the onshore facilities.
NSP2 will start at the PTA in Russia. From the PTA, NSP2 will run to the edge of the Baltic Sea in an underground configuration and thereafter will continue underground further into the nearshore area. After several kilometres in the sea, the pipelines will emerge from the seabed and continue lying unburied, on top of the seabed, to the Finland border.
The two pipelines will be spaced approximately 20 m apart in the onshore area and approximately 100 m apart in the offshore area. On the inland side of the PTA, NSP2 will be connected to an upstream pipeline system. The key elements of NSP2 at the Russian landfall are:
• Workers camp, PTA and laydown areas (temporary development footprint of approximately 42 ha);
• PTA (permanent facility of approximately 6.1 ha);
• Conventional open-cut constructed pipeline section extending approximately 3,800 m towards the shoreline from the PTA and requiring a working corridor of 85 m;
• Construction of a causeway and cofferdam, which transitions into a trench that extends approximately 3.3 km offshore;
• Construction traffic from Ust Luga Port (approximately 40,000 heavy vehicle movements);
• Duration of construction (approximately 2 years);
• Pre-commissioning of onshore facilities;
• Simultaneous construction of upstream compressor station and feeder lines;
• Nearshore dredging and backfilling (linear extent of approximately 3 km);
• Shore pull (pipeline pull from offshore lay vessel to shore).
A causeway and cofferdam are required because vessel-based dredgers operate to a minimum water depth of 2.5-3 m; therefore land-based trenching equipment is deployed in the very shallow nearshore area. The key elements of the causeway and cofferdam are as follows:
• Causeway dimensions (from shoreline): approximately 300-500 m length x 22 m wide x 4 m high (above sea level).
• Cofferdam (created in centre of causeway): 10 m trench width with 6 m wide road on either side of the sheet piled walls of cofferdam.
• Sheet piles: buried depth 12-15 m (20 m high sheets).
• Causeway wave/surge protection: rock (sourced from inland quarries) is to be used on the outer margins of causeway to protect against wave action.
• Causeway core: imported fill and/or excavated sand from the cofferdam (if suitable).
• Construction duration: approximately 21 days.
• Trench spoil volume: approximately 20,000 m3 (500 m x 10 m x 4 m).
• Piling method: vibro-piling.
• Working hours: daylight only.
• Construction method: causeway construction, sheet piling and excavation of cofferdam to happen simultaneously as the causeway pushes out from shoreline.
• Reinstatement: the causeway is to be progressively removed after pipe-laying. Causeway material is to be reused as backfill if suitable, otherwise removed from site.
Typical construction activities for the onshore pipeline section, comprise:
• Relocation of red list plant species and any animals prior to stripping;
• Vegetation stripping and grubbing (tree roots removal);
• Topsoil stripping and storage;
• Grading and subsoil storage;
• Installation of temporary drainage works;
• Placement of geotextile and gravel for temporary access roads;
• Phased trench excavation;
• Dewatering;
• Pipeline stringing (welded sections placed parallel to trench);
• Placement of bedding material in trench;
• Placement of welded pipeline sections in trench using side booms;
• Phased backfilling and compaction;
• Pre commissioning;
• Construction of permanent access road;
• Removal of construction equipment and materials;
• Technical reinstatement (grading and profiling of site), including installation of permanent drainage system;
• Reinstatement of groundwater hydrological characteristics as required;
• Biological reinstatement including topsoil cover and seeding.
The various construction activities are depicted in Figure 6-20 below.
Figure 6-20 Construction of onshore facilities in Russia.
Pipes and equipment that are required for the onshore sections of the project will be delivered by road. The project may require the construction of some new temporary access roads for this purpose. Areas will also be needed for a number of temporary facilities throughout the various construction phases, such as pipe, equipment, materials and soil storage areas, and catering and sanitary facilities for workers. These areas will be rehabilitated following completion of the construction works.
Construction work will be confined to a narrow strip of land approximately 85 m wide with the option of narrower working corridors (where feasible for safe construction) in the sensitive forest section. Red list plant species will be relocated prior to vegetation clearing, and the topsoil layer will be removed by excavators and stored on site for subsequent reinstatement after completion of the pipeline construction activities.
Once the temporary and service roads are available, sections of 12 m pipe will be aligned along the route in preparation for the welding operations. The handling and lifting of these pipe joints will be carried out by mobile crane, side-boom tractors or excavators.
The pipeline trench is typically dug by excavators equipped with suitably profiled buckets. Once the trench is completed, the prefabricated pipelines will be lowered into the trench by means of side-boom tractors (see Figure 6-21).
Figure 6-21 Onshore pipeline trench excavation (left) and the pipeline being lowered into the trench.
After completion of the pipeline installation, the trench will be backfilled and compacted with the previously stored soil, up to the original ground level. In areas where a high ground water level is encountered, concrete weights may be laid over the installed pipelines to overcome the buoyancy effect of the water. The top layer of soil removed at the start of the construction activities will then be reinstated. Grass will be planted to finally reinstate the pipeline working corridor, but no trees will be allowed to grow over the pipelines.
Nearshore dredging (trench excavation)
In the nearshore section of the pipeline route, from the shore out to a water depth of approximately 12 m (a distance of approximately 3.3 km), dredging will be performed to excavate a trench into which the pipelines will be placed and subsequently covered. The excavation of the trench in the shore approach section will be executed by the following equipment:
• Backhoe dredger;
• Trailing suction hopper dredger.
Dredge volumes vary between the open cut base case and micro-tunnel shore crossing option. A cofferdam is required for the open cut method, and dredge volumes are approximately 205,000 m3. Conversely, for the micro-tunnel option, as a dredged channel is required for the pipe-laying vessel, some 475,000 m3 of spoilis required to be dredged. A conservative approach has been adopted regarding sediment plume modelling for the impact assessment in Chapter 10 – Assessment of environmental impacts, and dredge volumes are based on the micro-tunnel option rather than the open cut base case scenario as this represents the ‘worst case’ with respect to dredging duration, maximum sediment concentrations and, therefore, potential impacts.
Pipeline installation
The planned methodology for the installation of the pipelines at the landfall is a shore pull technique. This typically involves the synchronised operation of a lay barge anchored close to the shoreline and a winch installed onshore. After the pipeline offshore trench is excavated to the required depth, the winch is installed and its wire is laid from the winch out along the bottom of the trench to the anticipated position of the lay barge.
Figure 6-22 Typical shallow water lay barge (with anchor tug and pipe transportation vessel).
The lay barge (Figure 6-22) is positioned as close as possible to the shoreline (subject to its operating draft), and the previously installed pull-in wire is retrieved and connected to the end of the pipeline that is being assembled on board the lay barge.
The trench requires backfilling after the pipelines are laid in the pre-dredged trench. For this purpose, the soil which had previously been dredged and temporarily stored is used as backfilling material.
In the shallow water section, close to the shoreline, the excavators used for dredging operations are also used for backfilling activities. In deeper water, backfilling operations are performed by a splitter hopper barge, which transports the soil from the storage area and dumps it into the trench.
Landfall Germany 6.7.2
The pipeline route in the German sector has a total length of approximately 83 km. In the section with water depth less than 17.5 m, the pipelines will be laid into a pre-dredged trench.
The key elements of NSP2 at the German landfall are:
• PTA working and laydown areas (temporary development footprint of approximately 8.2 ha);
• PTA (permanent facility of approximately 5.6 ha);
• Twin micro-tunnels of 700 m with entry portals within the PTA worksite and exit portals offshore;
• Duration of construction (approximately 2 years);
• Pre-commissioning of onshore facilities;
• Pre-commissioning equipment for offshore pipelines;
• Simultaneous construction of downstream GRS and feeder lines;
• Nearshore dredging and backfilling (linear extent of approximately 49 km);
• Shore pull (pipeline pull between offshore lay vessel and shore).
Pipeline installation
The burial depth of the pipelines varies along the pipeline route. The burial depth ranges between 0 m and 1.55 m in accordance with local safety requirements. Where shipping lanes are crossed in the shallow nearshore area inside the Greifswalder Bodden, the burial depth will be increased to take into account a possible deepening of the shipping lanes.
In order to minimise the underwater excavation works, and thus the environmental impacts, the selected trench profile has been adjusted to a width and a burial depth that is ALARP for safe construction and operation. In the sections in which both pipelines are laid in one single trench, the bed width of the trench will be 8.5 m in its straight sections.
Onshore activities
The 800 m long landfall section of the pipelines is defined as the section between the seaward end of the twin micro-tunnels and the PTA. On the seaward side of the coastline, the pipelines will be located inside the trench, followed by two individual 700 m long micro-tunnels. Inside the micro-tunnels, the pipelines cross under the coastline, the beach, other pipelines, a road and a railway. Finally, the pipelines end in a construction trench at the PTA. In this section, the pipelines cover an elevation of 4.5 m.
The construction of start shafts for the micro-tunnels will commence at the landward side within the PTA construction site. The tunnelling equipment will be installed and set at the start shafts.
When the tunnelling operations are completed, the tunnelling equipment and machinery will be dismantled and removed from the tunnels and then from the start shafts. Subsequently, the tunnel boring machines at the seaward tunnel ends will be excavated and recovered. Thereafter, the tunnel ends will be prepared for the shore pull-in of the pipelines.
In parallel to the tunnelling operations, the common pipeline trench within the Greifswalder Bodden will be dredged. The pre-lay trenching will continue across the Boddenrandschwelle and along the eastern slope of the Boddenrandschwelle.
The common pipeline trench will be backfilled and the seabed surface reinstated as pipe-laying progresses.
After the second-generation lay barge has completed the pipe-laying operations at KP 55, it will be relocated and set up at the seaward end of the tunnels, in order to facilitate the shore pull-ins of the two pipelines through the tunnels.
6.8 Pre-commissioning and commissioning
Following construction and prior to operations, pre-commissioning and commissioning activities are undertaken.
Pre-commissioning refers to a series of activities carried out before the introduction of natural gas into the pipelines. Pre-commissioning serves to confirm the mechanical integrity of the pipelines and ensures they are ready for safe operational use with natural gas.
Commissioning activities include the filling of the pipelines with natural gas prior to operation.
Pre-commissioning – offshore pipeline sections 6.8.1
After installation, the NSP2 pipelines will undergo a series of activities to prepare the pipeline system for use. These activities include cleaning, gauging and testing/leak detection.
The offshore pipeline pre-commissioning concept for NSP2 will be completed after receipt of the pipe-laying bids and finalisation of the pipe-laying scenario.
NSP2 plans a ‘dry pre-commissioning’ concept where the offshore pipelines will not be flooded and there will be no hydro-test or hyperbaric tie-ins as performed for NSP. DNV (certifying authority) has agreed to a conditional concession to the DNV design code OS-F101. Should the concept not be accepted by the national permitting authority, the back-up would be a ‘wet pre-commissioning’ solution, i.e. each section of the pipelines would be pressure tested with seawater which would be discharged in Russia outside the Kurgalsky nature reserve. Therefore, two options are under investigation.
These are:
• Option 1: Dry pre-commissioning without pressure testing using alternative testing methods and without hyperbaric weld tie-ins (HWTIs).
• Option 2: Standard wet pre-commissioning operations as done for NSP. For this option, HWTIs are required.