Visser & Smit Hanab, sister company of VBMS, was commissioned by the U.T.E. Emisario Submarino Baix Llobregat joint venture to construct an outfall involving the pulling into the sea of a steel pipeline by means of the so-called bottom pull method. The pipe has a diameter of 2,435 mm and is coated both internally and externally. Moreover, a reinforced concrete weight coat was applied to the outside of the pipe in order to provide strength and stability. The total length of the pipe measures 3,560 metres. The Long Sea Outfall in Llobregat is one of the largest outfalls in the Mediterranean. The outfall has a length of 3.4 kilometres and a diameter of 3 metres. The flow rate is 5 cubic metres per second. The outfall forms part of an enormous sewage treatment plant which is used to treat the sewage water coming from the two million inhabitants that live to the south of Barcelona. As the coastal area to both the north and south of Barcelona has suffered from high levels of pollution over the years, there was urgent need for such an outfall.
On shore, south of Barcelona, the joint venture had already laid out the working area for the construction and launching of the outfall pipe. The coated pipes from Euro Pipe were delivered in lengths of 9 metres each; on site, a reinforced concrete weight coat was applied. Next, the pipes were welded together in strings of 144 metres. Finally, a total of 25 strings were temporarily stored on concrete beams that were constructed at right angles on the launch way. The first string was fitted with a special pulling head, designed by the Technical Bureau of Visser & Smit Hanab. The launch way consisted of a railway with carts, the so-called boogies, that continued into a cofferdam with 15 rollers that can each carry a weight of 80 tons.
Visser & Smit Hanab decided, together with associate company Stemat, to convert a crane pontoon “Dina M” into a pulling pontoon. To this end, the pontoon had to be extended by 10 metres and fitted with a heavy pulling eye with a pulling force of 600 tons. On board, four Capstan winches with a pulling force of 125 tons and three reels with heavy pennants were installed.
After mobilisation in Rotterdam, the “Dina M” was towed to the port of Barcelona. Here, preparations for the pulling of the strings commenced. To do this, four 10-ton anchors had to be positioned at a distance of 5.5 kilometres from the coast, after which they had to be pulled in at a holding force of 125 tons. The total pulling force needed for the pulling of the last string was calculated at 500 tons. The pulling wires were paid out up to 300 metres below the beach of Llobregat, the starting position of the first pipe pull. Also, a heavy pennant of 1,100 metres was put out to sea and connected to the pulling head of the first string and the pulling eye aboard the “Dina M”.
To start off, the joint venture pulled the first and second string into the cofferdam, after which Visser & Smit Hanab undertook the pulling of the remaining 23 strings. In this way, one string was pulled per day, which gave the joint venture plenty of time to roll over the next string, weld two strings together, test the weld and then coat it and fill it up with reinforced concrete.
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