In 1988 ACEL, the municipal gas company of the town of Lecco, commissioned a turnkey project involving the installation of three gas lines respectively 2.2, 3.9 and 4 7 Km in length, connecting the towns of Olcio, Dervio, Bellano and Varenna on the left shore of the lake. The customer needed an off-shore, instead of the classical on-shore, linkup, owing to the particular nature of the lake itself and its surroundings.
Lake Como is, infact, a typical glacial lake with great, craggy mountains forming its shoreline. Consequently, the solution of laying the linesin rock was discarded by the customer for environmental and economic reasons. The particular morphology, environmental constraints, and inordinate number of caverns and recesses, as well as the difficulties involved in procuring suitable equipment in the area, however, largely influenced the techniques adopted to lay the lines.
Lake Como is a very well-known Italian tourist center whose shores are dotted by numerous towns and villages connected to each other by a large fleet of hovercrafts and ferries Moreover, its shores are generally extremely craggy and rocky and drop precipitously below the water level to depths which, for our pipelines, exceed 300 meters.
One of the characteristics of the possible landing sites is that they are for the most part formed by alluvional cones of the torrents issuing from the many valleys high above the lake. These cones are made up of masses of unstable gravel and depend for their existence on the flooding of the torrents.
Therefore, the selection of the landing sites and the routes to reach them turned out to be one of the most important aspects of the project All the operations that we shall now undertake to describe took place over the period from November through December 1988.
The draw backs of the system within which the installation had to be planned may be summarized as follows
exceptionally great depths;
difficulties in obtaining equipment with power ratings of not more than 400 HP
mooring problems resulting from steepness of the rocky shoreline,
the danger of landslides in the areas where the alluvial cones were found,
the discovery of certain obstructive aspects in the ecological environment that made it absolutely imperative to recover all the material used in carrying out the project.
The classic solution of using barge to lay the pipe, equipped with prefabricated pontoons put together on the spot was, therefore, rejected, owing to the difficulties that would have to be faced in guaranteeing suitable anchorage, incontrolling the handling of the anchors themselves without adequate means, in not obstructing the navigation on the lake, and in operating the enormous stinger and/or tensioners required to guarantee the correct of stress in the line.
The problem was solved by prefabricating the strings on land, assembling them in the water to their planned length, and then moving them to their final destination using the surface-fow method.
