6 PROJECT DESCRIPTION
6.4 Construction activities and status
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 115 days for the NSP2 route with the NSP2 route V1 or 125 days for the proposed NSP2 route with the NSP2 route V2 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 in Danish waters is expected to be undertaken in 2020. 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 then be assembled into a continuous pipeline string on-board the pipe-lay vessel 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.
Generally, pipe-lay can be carried out using a DP or anchored pipe-lay vessel. Based on the project plan with construction in Q1/Q2 2020, a DP vessel will be used for pipe-lay in the Danish section of the route. However, in case an anchored lay vessel is to be used, it is also included in the EIA.
A DP vessel is kept in position by horizontal thrusters that constantly counteract forces acting on the vessel from the pipeline, waves, current and wind. For the larger DP vessels, the average pipe-lay rate is expected to be in the order of 3 km/day for DP pipe-pipe-lay vessels, depending on weather conditions, water depth and pipe wall thickness.
In the event that an anchored pipe-lay vessel is used for pipe-lay, the anchors will interact with the seabed and may cause localised seabed disturbance. The pipe-lay vessel 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 pipe-lay. A typical anchor pattern is shown in Figure 6-16.
Figure 6-16 Anchoring patterns on the seabed as the 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 the with national maritime authorities.
Seabed intervention works
The pipelines potentially require pre- or post-lay intervention works in some areas. These inter-vention works may be required for pipeline stabilisation or for integrity reasons. The interinter-vention works may consist of trenching the pipeline into the seabed, or accurate placement of rock. Po-tential intervention works are summarised as follows:
• 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 in-stallation 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. These show the current estimates but are subject to adjustments as part of the detailed design of the pipelines.
Table 6-4 Sections for post-lay trenching or rock placement in Danish waters (per line).
Section – Intervention works Each Line A and Line B
From KP To KP Length (km)
Section 1 – South of Bornholm, post-lay trenching or spot rock placement
129 (NSP2 route with V1)
146 (NSP2 route with V2) 133 (NSP2 route with V1)
150 (NSP2 route with V2) 4 Section 2 – South of Bornholm, spot
rock placement at crossing with NSP pipelines
137 (NSP2 route with V1)
155 (NSP2 route with V2) 137 (NSP2 route with V1)
155 (NSP2 route with V2) <1
A summary of the possible volumes of trenching and rock placement is provided in Table 6-5.
Numbers are approximate and subject to final optimisation.
Table 6-5 Possible sediment and/or rock volumes (conservative approach) for each NSP2 pipeline in Dan-ish waters (numbers are volumes per line).
Section / Intervention works Approx. volume
(m3)*
Rock placement
- South of Bornholm, NSP pipeline crossing
- South of Bornholm, stabilisation 30,000
21,440 Post-lay trenching
- South of Bornholm, stabilisation 24,600
* Quantities are approximate and subject to final optimisation.
Once the pipelines are on the seabed, dependent on the seabed conditions, the pipeline may be-come 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 unconsolidated rock fragments graded in size to locally re-shape the seabed, thereby providing support and cover for sections of the pipeline to ensure its long-term integrity.
Rock placement will be carried out using material extracted from quarries on land. The types of rock placement works that are envisaged for seabed intervention include gravel supports (pre-lay and post-lay) and gravel cover (post-lay) in discrete locations.
To prepare the seabed for pipe-lay, the entire route is surveyed beforehand. Pre-lay gravel berms will then be strategically placed in order to support the pipeline in areas of uneven seabed (causing unacceptable pipeline free spans), to serve as basement structures at tie-in and 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 inter-vention works for the reduction of anticipated freespans and as a pipeline stability measure.
Rock placement activities include gravel works in which coarse crushed rock material is placed 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 is shown in Figure 6-20. The final shape/dimensions and position of the berms will be developed as part of the detailed pipeline design.
Figure 6-20 Rock berm; typical dimensions are shown in metres.
6.4.2.2 Post-lay trenching
In sections where additional stabilisation of the pipelines might be required, the possible approach is post-lay trenching, with a trench depth of approximately one pipeline diameter. Post-lay trench-ing will be carried out ustrench-ing 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, and the mother vessel or a separate tow vessel will pull the plough along the seabed, laying the pipeline into the ploughed trench as the plough advances.
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 (existing and planned) and the two existing NSP 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. An example of a cable crossing configuration is shown in Figure 6-22. The crossing configuration shown comprises a combination of flexible and rigid mattresses.
Figure 6-22 Layout of a typical cable crossing (cable is shown as a black dotted line).
There were no pipeline crossings on the NSP project. Pipeline crossings will be similar to that un-dertaken in Finnish waters for the crossing of NSP2 Line A over the NSP pipelines.
The typical crossing of pipelines is shown in Figure 6-23.
Figure 6-23 Typical design for crossing of pipelines.
Construction status
Permits have been granted in Germany, Sweden, Finland and Russia, and construction work is ongoing in these jurisdictions. The Allseas DP pipe-lay vessels Solitaire and Pioneering Spirit are performing offshore pipe-lay in Swedish, Finnish and Russian waters.
Almost 2,500 km of pipes have been delivered from the pipe mills to the coating plants. Concrete-weight-coating of the pipes has been completed at two of the three coating sites: Kotka, Finland and Volzhsky, Russia. The coating plant in Mukran, Germany will coat the additional line-pipes required for SE route.
Construction at the German landfall site commenced in February 2018 and is progressing as planned. The two offshore pipelines were pulled-in from the offshore lay vessel through microtun-nels to the gas receiving area in early August 2018 and as of the end of December 2018, both pipelines have been installed to 16.5 km from the Danish/German EEZ border. In Sweden the two pipelines in Sweden have been partially installed. One pipeline is being installed from Finland head-ing south and the second is behead-ing installed from a point 6 km from the Danish/Swedish EEZ border towards Finland. Over 500 km of pipeline have been laid in Swedish waters. The munitions clear-ance and prelay rock placement are completed in Finland and the first line has been installed. The second pipeline will be laid during the summer 2019. In Russia landfall construction is progressing as planned and the nearshore dredging and onshore trenching works are nearing completion to allow the two offshore pipelines to be pulled-in during summer 2019.