then observed and the maximum amplitude of the movement of the carriage 20 and consequently of the connecting end 22 is deduced therefrom in the example shown in fig2 in the case where for example seven rows r 1 to r 7 of rods 28 have disappeared while the other rows are intact it is deduced therefrom that the maximum excursion or maximum amplitude of the carriage 20 on the accommodating support 18 is 140 mm reference will now be made to fig4 which illustrates a second alternative of the invention according to which the frangible elements which are also formed by rods are no longer secured to the accommodating support 18 but to the on bottom pipeline with the aim of facilitating the description of this alternative the similar elements of the measuring device which were illustrated in the previous figures and which have the same functions have the same reference assigned with a prime mark u201c u2032 u201d therefore fig4 shows an on bottom pipeline 12 u2032 portion engaged in a longitudinal sleeve 30 u2032 this longitudinal sleeve is kept in a fixed position on the on bottom pipeline 12 u2032 via clamps 60 furthermore four lines 52 u2032 54 u2032 56 u2032 58 u2032 of rods that are respectively diametrically opposite in twos are screwed into the thickness of the longitudinal sleeve 30 u2032 the accommodating support is made up of a ring 18 u2032 provided with a border which surrounds it in a secured manner this ring 18 u2032 is anchored on the sea bed by partially burying said border.