Solitaire pipelayer holds position without anchors

Full speed ahead

Precision manoeuvring of ships ensures smooth running at port and can contain cataclysms at sea. E&T looks at the latest in maritime engineering.

As icy winds blasted down from the Arctic in the final weeks of 2010, the world's largest pipe-laying vessel, Solitaire, was ploughing through the Baltic Sea, leaving behind a large-bore gas pipeline. One of three pipe-layers working for the Nord Stream project, which is laying two gas pipelines to link Russia and Germany, she was able to continue through harsh weather thanks to a sophisticated dynamic positioning system. Should Solitaire be blown off her planned course at any time, ten thrusters instantly move her back into the target position.

Dynamic positioning (DP) systems are used extensively in the offshore industry and have allowed operators to drill ocean wells at depths of over 3,000m. They performed a vital role during last summer's Macondo oil spill, when a large number of rigs and support vessels needed to maintain position in close proximity over the leaking well.

Holding a vessel in position demands the highest level of seamanship, particularly in hostile waters such as the North Sea. So the design of automatic positioning systems has taxed the ingenuity of the control industry. To work effectively they have to take account of the erratic nature of position signals, as well as being able to distinguish cyclic rolling, heaving and pitching motions from the steady forces exerted by wind and tide.

The automated positioning system will receive signals from an anemometer, gyrocompass and a range of radio and acoustic position-location systems. While the wind speed and direction will generally be used in a feed-forward control loop, a computerised system is needed to process all this data and determine the power setting of the thrusters to maintain the desired position. The system has to be fine-tuned to take account of the particular characteristics of the vessel. To assist operators, the systems are normally provided with various control modes ranging from simple joystick control to full-DP.

Meeting the DP-3 rules

The failure of a positioning system can have catastrophic consequences, so the design is subject to strict rules and careful scrutinisation by classification societies. These rules look beyond the automated control system and its backup power supplies to the entire propulsion chain, including electric generators, switchboards and the thrusters themselves. The rules that apply to the most vulnerable vessels, such as drilling rigs, diver-support vessels, crane barges and pipelaying vessels, are termed DP-3, and call for a high-level of redundancy.

The DP-3 rules require a dual-redundant arrangement for the main computer control system, as well as an additional emergency computer with its own sensors and uninterruptible power supply, fitted in a separate compartment. Multiple sets of generators, switchboards and thrusters must also be installed in separate compartments, with thrusters arranged so that if one compartment is lost by flooding or fire, the vessel will retain 50 per cent of its positioning capability. The reliability of the overall system must be verified with failure modes and effects analysis (FMEA).

The Solitaire has been equipped to meet these requirements, and is certified to NMD Class 3/LR DP (AAA) standards. DP system supplier Kongsberg Maritime completed work on upgrading the positioning systems in April 2009, changing the previous equipment to its new K-Series technology.

Solitaire's operator, Allseas, says that the 300m converted bulk carrier's ship-shaped hull provides excellent workability. Since 1998 she has achieved a remarkable lay speed of over 9km a day, while operating the in-house developed Phoenix automatic welding system. She has laid numerous deepwater pipelines and holds a deepwater pipelay record of 2,775m.

Once the pipeline has been assembled on deck, it passes out over a curved 'stinger' and falls smoothly to the seabed in an 'S' configuration. The pipe is supported by tensioners with holding force of up to 1,050t. The positioning system needs to control ten azimuth thrusters to compensate for this force. The thrusters power a diesel generating plant capable of delivering 50,000MW.

Nord Stream entering Russia

Solitaire entered the Finnish Exclusive Economic Zone in November 2010, after laying a key section of the Nord Stream pipeline in Russian waters. At around the same time the Nord Stream consortium announced that it had completed more than half of the first of its two 1,224km gas pipelines through the Baltic Sea.

Three pipelaying vessels and a flotilla of support ships are employed on the overall project, with the first pipeline due to supply gas to the EU in late 2011. When both lines are completed in 2012, they will transport 55 billion cubic metres of natural gas per year – estimated to be enough to meet the needs of more than 26 million households. Pipe diameter is 1,153mm, and it has a maximum operating pressure of 220 bar.

While the final budget for the project has yet to be established, it is thought to be in the region of £7.4bn. Nord Stream's main shareholder is Gazprom (51 per cent) with partners Wintershall (15.5 per cent), E.ON Ruhrgas AG (15.5 per cent, Gasunie (9 per cent), and GDF SUEZ (9 per cent).

The Nord Stream will have enormous strategic importance in coming years, with start of production from the Shtokman Field. This field lies in Arctic waters off northern Russia, and is in the ten largest gas fields in the world, with initial reserves of 3.9 trillion cubic metres of gas and 56 million tonnes of gas condensate. Development will present an enormous challenge due to the field's remote location, 600km from land and 300m below the surface of ice-infested seas.

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