With European gas import needs projected to grow by about 200bcm by the year 2015, Nord Steam will meet about 25percent of this additional requirement. After completion in 2012, the pipeline across the Baltic Sea will connect the world’s largest gas reserves, located in Russia, with the European gas pipeline network, securing natural gas supplies for the continent over the coming decades.
The Nord Stream system is being meticulously planned down to the smallest technical detail. This includes everything from the carrying-out of risk assessment studies, to the exact determination of the seabed route, to the selection of materials for the pipeline, to the technical design for a safe operation of the system.
The pipes for the pipeline will be manufactured from high-tensile steel in accordance with DNV Offshore Standard
OS-F101. The pipe wall thickness will vary from approximately 27mm to approximately 41mm in accordance with the dropping pressure along the route. Inside, the pipes will be covered with antifriction coating in order to increase performance.
On the outside they will be coated with an anti-corrosion layer. To give them extra weight and ensure their stability on the seabed, the steel pipes will be coated externally with concrete of a thickness of between 60mm and 110mm. Each pipe joint is 12m long.
To ensure that the pipeline can withstand the high internal pressure of over 200bar, independent checks and quality control will be carried out both by the pipe mills, by independent pipeline experts as well as by experts from Nord Stream AG. During manufacturing in the pipe mills, the pipes will be thoroughly inspected with non-destructive methods like ultrasonic testing, magnetic particle testing and radiographic testing to detect possible material defect which are not visible from outside. The mechanical properties of the steel material will also be tested continuously during the manufacturing of the pipes. That way, only pipes with the specified strength and material toughness will be used. Finally, each of the finished pipes will be subjected to a number of quality checks before they are released from the pipe mill for further use. As part of this process each pipe will be filled with water to a pressure substantially higher than the maximum operational pressure.
Metallic objects
Before engineering work starts, the seabed will be thoroughly surveyed along the whole pipeline route for potential obstacles. Side scan sonars will search for metallic objects in and on the seabed. Investigation of the soil, the water current, the sea temperatures and waves parameters are also part of the surveys and are pre-requisite for the engineering works. The engineers will then determine in detail where it is necessary to prepare the seabed before pipeline laying to avoid long free spans. Further, it will be decided where the pipeline must be buried and how obstacles or particularly sensitive areas can be avoided.
The steel pipes will be coated in specialised coating yards and stored on land before being shipped with pipe carrier vessels to the large
pipe-laying vessel. The pipe-laying vessels have a large storage capacity and are manned and equipped to lay pipes 24 hours a day if the weather conditions allow doing so. On board the pipe-laying vessel, the pipes will be inspected again for transport damages before they are released for welding. The pipes will be lined up and welded together. Upon completion, each weld will be inspected by ultrasonic testing to identify possible welding defects. Acceptable welds will be covered with an anti-corrosion coating. The pipeline will be then lowered from the vessel into the water via the ‘stinger’ which is attached in the stern of the
pipe-laying vessel and supports the pipeline. That way, stress on the pipeline during laying remains within controlled and acceptable limits.
In detail, the pipeline-laying sequence will be as follows:
o First, in a ‘double joint welding station’, two 12m pipe joints will be welded together to a 24m long ‘double joint’ with a welding pass both from the inside and the outside.
o After ‘double joint welding’ the automatic ultrasonic equipment will inspect the weld and if found acceptable, the double joint will then be released for introduction into the firing line. All double joint welds will be subjected to a very accurate non-destructive ultrasonic test using 30 or more ultrasonic probes to examine every millimetre of the weld area. This allows even the smallest welding defects to be detected and eliminated.
o The pre-fabricated 24m long double joint will be then conveyed into the central assembly line, known as the ‘firing line’. Here, the double joint will be connected to the pipeline end and welded together by semi-automatic machines. Welding of the numerous girth weld layers will take place in five to six welding stations in order to achieve a high productivity.
o The ‘firing line’ welds are also subject to an accurate automatic ultrasonic inspection using 50 ultrasonic probes to assure that only defect-free welds will be produced. After acceptance of the weld, it will be coated and the next 24m length can be lowered to the seabed. This will be done by pulling the pipe-laying vessel 24m forward while keeping the pipeline under tension over the stinger.
o Using this method, over 3km of pipeline can be laid per day. For most of the offshore route, the pipeline will rest on the seabed. In some areas like near the landfalls the pipeline needs to be buried and backfilled with sand in order to assure sufficient on-bottom stability. In some areas, eg areas with frequent ship traffic like fairways the pipeline may also need to be buried and backfilled in order to protect it from the risk of ship anchor impacts.
In shallow waters between 5m and 15m deep, inaccessible to the large pipe-laying vessels, smaller vessels with less draught will be used employing the same procedures as large pipe-laying vessels.
After completion of the construction works, the pipeline will be filled with water and will be pressure tested for at least 24hrs to a pressure which is higher than the maximum future gas pressure in the pipeline. This will be done as a final proof to demonstrate that the pipeline has sufficient strength to transport the gas at high pressures and that it is 100percent leak tight before taking it into operation. After this final pressure test, the pipeline will be emptied and all water will be removed before the first gas is introduced.
A service platform will be an integral part of the pipeline system, serving to support the commissioning, to provide system maintenance, to improve the flexibility of operation and to function as a safety precaution in case any of the pipelines are damaged.
The platform will be located approximately at the midpoint of the pipeline in the Swedish Exclusive Economic Zone about 48km east of the small island of Gotska Sandön and 68km north-east of the main island of Gotland. When an observer is more than about 31km away from the platform even the tip of the platform vent stack will not be visible, because it will be ‘below the horizon’ due to the curvature of the earth.
During commissioning, the presence of the platform means that ‘intelligent pigs’ will only have to travel half the distance along the pipeline, greatly reducing the wear on the ‘pigs’ and thereby improving sweep efficiency. This also reduces the risk of fluids bypassing the ‘pig’ during commissioning.
The service platform features a pipeline cross-over manifold. In the event of a pipeline leak or other operational problems, it would only be necessary to isolate half a pipeline (600km long) rather than a whole pipeline (1200km long). In general, it is common practice on long pipelines to provide periodic isolation points (valves) to reduce risks during pipeline commissioning and operations. The service platform provides these isolation valves at the approximate mid-point of the pipelines. Thus, if there is pipeline leak, the ability to isolate half a pipeline rather than a whole pipeline means that the gas lost through the leak is halved. In addition, the interruption of gas supply is reduced while the pipeline problem is being resolved.
When construction work has been finished, the pre-commissioning and commissioning of the pipeline has been conducted and the final approval from the authorities has been given, the pipeline will be filled with gas and commercial use can start.
To start transportation, the pressurised and measured gas will be taken at the intake point of the Nord Stream system in Vyborg, Russia. At the same time, an equal volume of gas will be taken off at the delivery point of Nord Stream at the receiving terminal in Lubmin, near Greifswald, Germany. To safeguard that this flow regime works properly, the pipeline must be filled with a volume of cushion gas. In the case of no transportation, eg after a shut down, the pressure of this cushion gas will result in a value below 170bar, which is the minimum design pressure of the entire system. This pressure must not be exceeded even after pressure equilibrium after a certain period of time. To make sure that this volume of cushion gas will not exceed the maximum value, a computer based online supervisory system will be adopted to balance the intake and outtake volumes in a proper way.
The system will be remotely controlled by satellite from a dispatching centre where specialists will observe the system 24hours a day throughout the year. In case of emergency they are able to directly interact with the safety features like the isolation valves.
Maintenance procedures will be worked out and conducted by special Nord Stream staff to keep the system on the same high reliability level during its entire lifetime as it is at the beginning of operation. Amongst others, ‘pig’ runs with ‘intelligent pigs’ will be done regularly to examine for corrosion or external impacts. The ‘pigs’ are fitted with high-resolution ultrasonic sensors that can detect even the smallest irregularities. Necessary remedial measures can then be derived from such results and operational safety can be ensured.
Nord Stream is a natural gas pipeline that will link Russia and the European Union via the Baltic Sea. Gas import of the European Union, 336bcm in 2005, is projected to grow by 200bcm to 536 per year in 2015 (Source: Global Insight, 2007). Connecting the world's biggest gas reserves with the European gas pipeline network, Nord Stream will meet about 25percent of that additional requirement. The project will be an important contribution to long-term security of supply and a test of the energy partnership between the European Union and Russia.
Nord Stream AG plans to have the first of two parallel pipelines operational in spring 2011. Each line is approximately 1220km long, providing a transport capacity of some 27.5bcm per annum. Full capacity of about 55bcm a year will be reached in the second phase when operation of the second line starts.o
For further information, visit www.nord-stream.com
