The pipeline has been laid in three sections defined by the changing wall thicknesses required for each, as shown in Table 1.

The first tie-in, between Zones 1 and 2, off the coast of Finland, was completed earlier this year at the 297 km point (measuring from Vyborg in Russia). The second of these 17-day operations, off the Swedish island of Gotland, is due for completion by mid-September.

Each of the two tie-ins was carried out in an underwater welding habitat and was remotely controlled from Technip’s newly-built diving-support vessel the Skandi Arctic. The specialised subsea equipment used to perform the operations was provided by the pipeline repair system (PRS) pool of pipeline operators, administered by Statoil. Two of the 150 tonne capacity pipe-handling frames, the welding habitat, and the diamond cutter came from this pooled resource, while a third of the PRS’ pipe-handling frames had to be specially upgraded to cope with the weight and diameter of the pipe involved.

The welding habitat supplied by the PRS creates a dry zone on the seabed, where divers can work without diving equipment, to set up the automatic welding machine, the operation of which was completely controlled from the DSV. The vessel also transported and operated all of the equipment necessary to move, lift, cut, and weld the pipeline sections together.

The construction work for the tie-ins was undertaken by teams of divers, who lived for the duration of the operation in a 24-man hyperbaric dive system on the Skandi Arctic. Divers were lowered in three-man capacity diving bells to the seabed for the work, which took place at a pressure of 11 bar; this pressure was maintained in their living quarters for the whole period of the operations. In each case, the pressure-reduction process at the end of the work took a further five-and-a-half days.

Diver safety

One of the most important aspects of an operation of this type is diver safety. While the hyperbaric chambers and associated regimes are a fairly familiar aspect of offshore operations, away from the North Sea there has not until now been a facility available to which divers could be evacuated in case of an emergency on the DSV.

When Nord Stream was planning this ambitious pipeline construction operation, it decided that no hyperbaric work would take place without diver safety being assured: it was realised that a shore-based unit would be required to which divers could be taken in an hyperbaric lifeboat, as those facilities that exist in the North Sea were too distant to be usable.

Nord Stream’s diving contractor, Technip, engaged the Italian specialist Drass Technologie Sottomarine, of Livorno, to design and deliver what is now known as the hyperbaric rescue unit (HRU). In this, up to 18 divers can live in safety and relative comfort for the period needed for them to decompress. Their medical conditions can be monitored externally, and advice and medical support can be provided – fresh food, and other supplies can be passed in through a pressure lock, and the unit is equipped with shower and lavatory facilities.

Bearing in mind the extreme life-saving importance of its purpose, the HRU is equipped with multiple back-up systems for power and gas supply, and for monitoring all the vital aspects needed to ensure uninterrupted operation. The HRU is on 24/7 standby at the small Swedish port of Hargs Hamn, near Stockholm and roughly equidistant from the tie-in sites on the two pipelines, and is able to be brought to a state of complete readiness very rapidly.

A final aspect of safety as it concerns Nord Stream and Technip’s complex subsea operations is the provision of the MT Achilles as a ‘guard’ vessel. This ocean-going tug is on standby nearby the DSV throughout its time on site, in order to ensure no other vessels encroach into the area of operations, as well as to be able to tow the hyperbaric lifeboat to shore if it needs to be used. The importance of using the MT Achilles has already been proved, as she was required to pull a drifting freighter out of the way of the Skandi Arctic while the DSV was at anchor during the first tie-in operation.