International Dredging Review

International Dredging Review

In 1997 the hopper dredge Nautilus was working on a port development project in Finland’s port of Kokkola when live ammunition began appearing in the draghead. The contractor Terramar Oy a Boskalis subsidiary contacted the Finnish Maritime Administration who learned from the Finnish Defense Forces that Kokkola had been used through the years as a dumping ground for ammunition: 12000 tons of it according to the records. This dumping had taken place from the end of World War II until the early 1970’s.

Investigations showed that the ammunition had been dumped outside the normal sailing routes and at the desired depth but not at the desired location which had been planned 50 km away. The Defense Forces estimated that 8000 tons of ammunition had reached its correct destination but that the exact location of another 4000 tons was not known since the records were inaccurate especially at the beginning of the dumping operations.

Included were rifle ammunition artillery and mortar shells anti-tank weaponry and 40 500-kg bombs of which two were found outside the channel leaving 38 bombs unaccounted for. These discoveries and the fact that Kokkola had been bombed during the war creating the possibility of unknown ordnance being present caused officials to judge the area extremely dangerous. Dredging was stopped.

Terramare Oy with specialists from Boskalis started brainstorm sessions to find a safe solution to the problem. Joining in the discussion were the Finnish Defense Forces Research Center and national Health and Safety authorities. While one group of engineers worked on proposals to remove the material containing the ordnance another began an investigation into quantities and locations. With this latter in mind Heinrich Hirdes GmbH a German Boskalis subsidiary was requested to do a ferromagnetic survey of the area.

The resulting map showed an astonishing outcome. Around 1700 suspect items more than 37 mm in cross-section in an area of 100 hectares (250 acres) were identified. Some were likely to be nets machinery and old tools mot most were undoubtedly dangerous. The solution had to be safe and economically feasible. Using divers to do the work would take years.

“A method had to be found such that the dredging work would meet all normal safety standards for personnel” said Paavo Kolari Terramare project manager. “It soon became clear that the only solution would involve remote controlled operation” he said.

An island was created protected by bunds to place the ordnance-contaminated material. This would be covered with uncontaminated material when the work was complete and would be off limits with no connection to the shore. It was foreseen that it could serve as a base for energy generating windmills.

For dredging purposes it was decided that the material within a circle of 10 meters of each object larger than 37 mm would be assumed to be contaminated. To remove this the clamshell dredge Kahmari was rebuilt and re-engineered to be remotely controlled. This had to be done in a way that no time delays would occur between the real action and the remote control station giving the operator the feel of being actually aboard the dredge. Television cameras would be trained on all operating parts of the dredge.

“We went so far as to transmit the sound of the engines” said Kolari. “However the operators preferred the sounds of our Finnish Broadcasting Service and most of the time didn’t make much use of this facility”

The operation was made technically even more complicated by a separate system of cameras that had to be installed (also remotely controlled) to enable representatives of the Finnish Defense Forces to follow the process. Their on-line guidance and advice would make the work safer. When dredging started no one was allowed within 300 meters which the Finnish Defense specialists declared would be safe even in the event of detonation of a 500-kg air bomb. All personnel would be transferred to a separate pontoon at the 300-meter distance. This meant that all functions including the crane and all its components winches and release systems for barges would be remotely controllable.

Bucket position was determined by real time kinematic on the fly differential global positioning system (RTK-OTF-DGPS.) With sensors measuring wire length boom and pontoon angle the exact xyx coordinates of the bucket were known in real time and shown to the operator together with the video display. While the images were recorded on tape for future study the dredge locations – bucket by bucket- were recorded in another computer program and shown to the operator. This enabled the operator and the client to be sure the full surface of the contaminated area had been dredged. The radio was designed in a way that the signals would not interfere with each other. When the self-propelled split hopper barge was full it was released from the dredge.

The captain was on a tugboat towed behind the barge. This required the skill to guide the barge on which he was not present taking into account the effects of the tugboat. If something were to fail on the barge such as a rudder or engine the tug would be able to take over and tow the barge to a safe place to await instructions. But the reliable working barge sailed into the bund-protected discharge area and opened up. This opening was backed up by a second diesel-hydraulic system. After discharging according to plan the barge sailed to a wash installation elsewhere in the protected area to ensure that the sticky clay soil was fully removed.

After various suspected areas were dredged in this way a second ferromagnetic survey was done to verify that the area was “ferro clean.” Then the manned dredges Meri-Pekka and Koura II came in to dredge the area to the desired depth and to formally finish the job. But even with a clean bill of health the rule of “safety first” mandated that these dredges were protected by 12 mm steel plates and bulletproof windows.

“It was obvious that the success of the project did not depend only on the technology but even more on the skill bravery and commitment of the teams” said Kolari. “Over the summer of 1998 we did a lot of testing in which the full crew participated. This gave us the opportunity to change and redesign procedures and technical parts. We also rehearsed safety procedures in cooperation with the client the Finnish Defense Forces and Safety authorities.”

The Kokkola project proved that even in the traditional and dull domain of dredging here is always a need for innovation and cooperation sometimes under considerable pressure. These are essential components of any company’s arsenal. The Boskalis Westminster Dredging Group is incorporating these techniques into its operating methods for use in other places around the world where dredging could pose a threat to people nearby.