Abstract: An alarm system comprising at least one signal line (2, 4) for indicating damage to and/or an attempt to interferewith the line. The line is particularly suitable for incorporation in a net-structure particularly intended for use as a barrier net for denying foreign objects, such as under-water vessels, frogmen and the like access to water passageways, channels etc. To this end the signal line includes suitably flexible tubing (2, 4) which contains a coloured indicating or marking agent and is connected to sensors (8-11) which are constructed to cause an alarm signal to be given when there is a change in pressure and/or in the flow conditions in the tubing. The indicating agent in the tubing--suitably a coloured liquid--is placed under a pressure of such magnitude that in the event of perforating damage, such as a fracture or rupture, to the tubing the agent will exit therefrom and indicate visually the location of the fracture signalled by one of the sensors (8-11).

Abstract: A barrier arrangement for blocking a water passage or channel against the admission of a foreign or unfamiliar object, such as underwater vessels, frogmen and the like. The arrangement comprises a net (1) of considerable tensile strength intended to be stretched across the water passage. The bottom edge of the net (1) is anchored to the sea-bed and the net can be activated from a passive state, in which it may lie on the sea-bed, to an active barrier state. To this end, the upper edge of the net (1) is connected to an inflatable and deflatable lifting hose (3) which when evacuated is essentially flat and which when inflated is able to lift the upper edge of the net towards the surface of the water, so as to stretch the net between the sea-bed and the surface of the water. The invention also relates to a barrier net and to an anchoring means for use with the arrangement.

Abstract: An inflatable boom raising hose is sized such that the hose, upon inflation, raises the boom from the sea bed. The hose carries internally a chain which acts as a weight to drive the hose and boom to the sea bed, which takes up tensile forces and which prevents folds being formed in the boom raising hose as it sinks to the sea bed and which insures air passage therethrough, full evacuation of the air therefrom and complete collapse of the boom raising hose during evacuation of air therefrom at one or both ends thereof when the hose is deflated.