6 PROJECT DESCRIPTION
6.4 Construction activities
Construction activities in Danish waters include pipe-lay and seabed intervention works, see Figure 6-14.
Figure 6-14 NSP2 construction activities in the Danish sector.
The pipeline installation phase in Danish waters is expected to last approximately 125 days in total for the two pipelines, and the installation is assumed to be sequential, meaning that one pipeline will be installed at a time in Danish waters. Construction activities in Danish waters are scheduled in the course of Q2 2019. It is noted that the schedule may be subject to change during project development.
Pipe-lay
Pipeline installation will be carried out by pipe-lay vessels adopting the conventional S-lay tech-nique. This method is named after the profile of the pipe as it moves across the bow or stern of the pipe-lay vessel and onto the seafloor, as it forms an elongated “S” (see Figure 6-15). The individual pipe joints will be delivered to the pipe-lay vessel, where they will be assembled into a continuous pipeline and lowered to the seabed.
Both pipelines will be constructed in specific sections for subsequent interconnection. Abandonment and recovery operations involve the leaving and later retrieval of the pipeline somewhere along the route. Abandonment of the pipeline may become necessary if weather conditions make positioning difficult or cause too much movement within the system.
Figure 6-15 The S-lay pipe-lay vessel and survey support vessels.
It is anticipated that a DP vessel will be used for pipe-lay in the Danish section of the route.
A DP vessel is kept in position by horizontal thrusters that constantly counteract forces acting on the vessel from the pipeline, waves, currents and wind. A computerised system automatically op-erates the thrusters when required. The average lay rate is expected to be in the order of 3 km/day for a DP pipe-lay vessel, depending on weather conditions, water depth and pipe wall thickness.
An alternative to a DP pipe-lay vessel is an anchored pipe-lay vessel, which is kept in position by up to 12 anchors, each weighing up to 25 t. Independent anchor handling tugs will manoeuvre the anchors, which are directly connected to, and controlled by, a series of cables and winches. The tugs will place the anchors on the seabed at predetermined positions around the pipe-lay vessel to move the pipe-lay vessel forward and ensure tension can be maintained on the pipeline during laying. A typical anchor pattern is shown in Figure 6-16.
Figure 6-16 Anchoring patterns on the seabed as an anchored pipe-lay vessel moves forward.
Pipe-lay operations will require establishment of exclusion zones around pipe-lay and supporting vessels to ensure safe construction. During construction of NSP, the exclusion zone for the DP vessel Solitaire was defined as a 2,000 m (approximately 1 nm) radius centred around the vessel. Ship traffic will be requested to avoid restriction zones. Exclusion zones are to be agreed on with the national maritime authorities.
Seabed intervention works
The offshore installation of the pipelines potentially requires additional stabilisation and/or protec-tion against hydrodynamic loading in some areas, which can be achieved either by trenching the pipeline into the seabed or with rock placement. Such intervention works include:
• Pre-lay rock berm installation at pre-determined locations on the seabed prior to pipe-lay;
• Post-lay rock berm installation over the pipeline at pre-determined locations on the seabed following pipe-lay;
• Post-lay pipeline trenching by lowering the pipeline below seabed level following pipeline instal-lation using a subsea pipeline plough.
An overview of the proposed pipeline route as well as the locations and types of potential seabed intervention works to be carried out in Danish waters are presented in Figure 6-17.
Figure 6-17 Potential intervention works in Danish waters.
The extent of the intervention works and volumes of rock needed for or sediments originating from the intervention works are shown in Table 6-4 and Table 6-5, respectively.
Table 6-4 Sections for post-lay trenching or rock placement in Danish waters (per line).
Section – Intervention work Line A and Line B
From KP To KP Length (km)
Section 1 - NSP crossing
Spot rock placement 3.1 3.3 0.2
Section 2 – Pipeline stability measure
Post-lay trenching 89.0 90.0 1.0
Section 3 – Pipeline stability measure and cable crossings
Spot rock placement
91.0 93.0 2.0
Section 4 – Pipeline stability measure
Post-lay trenching 94.0 103.0 9.0
Section 5 – Pipeline stability measure
Post-lay trenching 122.5 125 2.5
Section 6 – Pipeline stability measure
Spot rock placement 133.0 134.4 1.4
Section 7 – Pipeline stability measure
Spot rock placement 138.0 139.0 1.0
Section 8 – Pipeline stability measure
Spot rock placement 141.3 148.0 6.7
Section 9 – Pipeline stability measure
Post-lay trenching 157.5 159.5 2.0
Total 26
A summary of the possible volumes of trenching and rock placement is provided in Table 6-5.
Volumes are approximate and subject to final optimisation.
Table 6-5 Possible sediment and/or rock volumes for intervention works in Danish waters (per line).
(Conservative approach - quantities are approximate and subject to final optimisation).
Activity and section Approx. volume
(m3) Rock placement
- Section 1 - NSP pipeline crossing
- Section 3 Stabilisation and cable crossings - Sections 6-8 - Stabilisation
30,000 48,700 77,800 Post-lay trenching*
- Sections 2, 4, 5 and 9 – Stabilisation 89,900
* Alternatively, these sections potentially could be stabilised with spot rock placement (to be agreed with authorities).
Once the pipelines are on the seabed, dependent on the seabed conditions, the pipeline may become naturally embedded. Examples of how NSP appears on the seabed are shown in Figure 6-18.
Exposed on seabed Naturally embedded
Covered by rock Trenched
Figure 6-18 Examples of how NSP appears on the seabed.
6.4.2.1 Rock placement
Rock placement is the use of crushed rock fragments graded in size to locally re-shape the seabed,
The types of rock placement works that are envisaged for seabed intervention include supports (pre-lay) and cover and/or additional support (post-lay) in discrete locations.
To prepare the seabed for pipe-lay, the entire route is surveyed beforehand. Gravel berms will then be strategically placed in order to support the pipeline in areas of high seabed relief, to serve as basement structures at pipeline crossing areas and to stabilise the pipelines, where required. Rock placement is only envisaged in Denmark for preparation of crossings of infrastructure and where necessary as pre-lay and post-lay intervention works for the reduction of anticipated freespans and as a pipeline stability measure.
Rock material is placed on the seabed in a controlled manner by a fall pipe (see Figure 6-19).
Figure 6-19 Rock placement on the seabed through a fall-pipe.
The geometry of each gravel support is engineered according to seabed conditions, bathymetry in the surroundings, currents, etc. A typical geometry of the rock berms that would be placed on the NSP2 pipelines for stability in the Rønne Banke area is shown in Figure 6-20. The final shape/di-mensions and position of the berms will be developed as part of the detailed pipeline design.
Figure 6-20 Rock berm; dimensions are shown in metres.
The positions of the rock berms will be designed by Nord Stream 2 AG taking rock placement tol-erance into account. This will ensure that rock berms do not overlap with the designated habitat types in the Natura 2000 area.
6.4.2.2 Post-lay trenching
Post-lay trenching will be carried out using a pipeline plough (see Figure 6-21) deployed onto the pipeline from a mother vessel located above the pipeline. The pipeline will then be lifted by hydraulic grippers into the plough and supported on rollers at the front and rear ends of the plough. The rollers will be equipped with load cells to control the loading onto the pipeline during trenching. A tow wire and control umbilical will be connected to the plough from the mother vessel, which will pull the plough along the seabed, laying the pipeline into the ploughed trench as the plough ad-vances.
Typically, the mother vessel is capable of pulling the plough independently, although assistance from another vessel may occasionally be required, depending on the overall tow force generated.
Figure 6-21 Post-lay trenching. Typical pipeline plough in operation on the seabed.
The excavated material displaced from the plough trench (also known as “spoil heaps”) will be left on the seabed immediately adjacent to the pipeline. Partial natural backfilling will occur over time due to currents close to the seabed.
Due to the nature of post-lay trenching operations, seabed soils will be present on the pipeline plough when it is recovered on board the plough support vessel. Accordingly, it is proposed that an expert lead from the Danish Navy be mobilised to the plough support vessel for the duration of the post-lay plough operations in order to check for any chemical munitions that may have come into contact with the trenched pipeline section.
Crossings of infrastructure (cables and pipelines)
The proposed NSP2 route crosses power and communication cables and the two existing Nord Stream pipelines. As successfully done for NSP, it is envisaged to develop specific crossing designs for each cable crossing, typically consisting of concrete mattresses, which will be agreed with the cable owners. There were no pipeline crossings for the NSP project; a typical pipeline crossing design according to normal industry practice, e.g. in the North Sea, shall be developed and agreed upon.
The typical crossing of pipelines is shown in Figure 6-22.
Figure 6-22 Typical crossing of pipelines.