Identifying buried mines and munitions for the Nord Stream pipeline project in the Baltic Sea was an assignment in which Marin Mätteknik (MMT) could not afford to make any errors. The seabed survey specialist had to develop special technology for the project, according to contract manager Olof Nilsson.
“The Baltic is the most mined sea in the world,” he says. “Around 85,000 mines were laid during the First and Second World Wars, of which only half have been recovered. A lot of munitions have also been dumped in this sea. Conventional methods were not sufficient for this task there was no commercially available equipment which could detect buried metal objects of small enough size.”
MMT’s search for buried mines on the proposed route of the Nord Stream pipelines in the Baltic Sea was undertaken with an array of gradiometers attached to an ROV.
Working together with Nord Stream and UK company Innovatum, MMT developed a surveying tool consisting of an array of 12 gradiometers capable of detecting metal objects as small as 10 cm (4 in.) buried up to 2 m (6.5 ft) below the seabed.
The 7-m (23-ft) wide array was attached to a work-class ROV. Data was transmitted to the mother vessel in real time and recorded. The operation has been very successful, detecting numerous suspicious targets, Nilsson says. The results will enable the pipeline to be re-routed to avoid these locations.
The survey was largely carried out in 2007 and will be completed this year. MMT’s scope covered the whole 1,220-km (758-mi) route of the two 48-in. (1.2-m) gas pipelines, which comprise the Nord Stream system. The survey made use of 3D seabed models developed with data the company had collected in a previous survey.
Another feature of the Baltic is its numerous shipwrecks, some dating back several hundred years. These tend to remain intact due to the brackish waters and reduced currents in this enclosed sea, says Nilsson.
Another MMT pipeline project in this region is Gassco’s 380-km (236-mi) Skanled. Last year the company survey led the route in Swedish and Danish waters, and gearing up for a detailed geophysical survey this summer, with planned ROV inspections of the seabed.
Other proposed pipeline projects it has participated in include the 120-km (74.5-mi) Baltic Connector Pipeline between Finland and Estonia, and the 200-km (124-mi) IGI line between Greece and Italy.
ROTV speeds up inspection
The company, which is certified to the ISO 9001 quality assurance standard, has made a breakthrough into pipeline inspection. This year it will start work on a long-term contract with DONG to carry out an annual inspection of 770 km (478 mi) of pipeline in the Danish sector.
For this work it will use a remotely operated tow vessel (ROTV). The steerable tow fish, purchased early this year, will carry out acoustic mapping using side-scan sonar; logging anomalies will indicate potential problems such as free spans or fishing tackle snagged on the pipeline. The main advantage in using a tow fish is that it can perform the work four times as fast as a conventional ROV, Nilsson says. This will enable the survey to be carried out in weeks rather than months.
For the DONG work, the tow fish will be deployed from MMT’s Franklin survey vessel. It also can be deployed from third-party vessels.
MMT, based in Gothenburg, employs a full suite of seabed mapping and survey techniques, including:
- Multibeam echo sounders to produce 3D models of the seabed
- Side-scan sonar to map seabed conditions and produce surface geology charts
- Shallow seismic to map subsurface geological conditions to produce models of the sediment layers
- Seabed sampling to map the sediment properties
- Underwater video, recorded by diver or ROV.
In recent years, wind farm surveys have provided a growing source of work. MMT has undertaken seabed mapping for projects involving a total of 1,300 wind turbines, including the world’s largest wind farm installed to date, the 80-turbine Horns Rev off Denmark.
The growing number of subsea power cable installations also has brought the company regular work, including projects connecting Sweden to other Baltic countries, Denmark and Norway, and Malaysia and Borneo. This summer MMT is due to perform a survey for a power cable connecting the Scottish mainland to the Shetland Isles.
The company has performed additionally several hydrographic surveys for the UK Hydrographic Office and Maritime and Coastguard Agency.
To cope with the large workload, MMT last year arranged a long-term charter on the Pollux survey vessel. Its own fleet is headed by the Franklin, a 56-m (184-ft) long vessel acquired in 2005, which is equipped for survey operations in water depths of 2,000 m (6,561 ft). Other vessels are the 26 m (85 ft) -long Triad, and the small Ping boat, used for near-shore and harbor work.