riser tube 20 consists of tube modules 46 preferably made of polyethylene which are interconnected by connectors 48 tension along the riser tube 20 is transferred via the mooring lines 52 which are guided substantially parallel to the riser tube 20 by means of mooring line guides 54 disposed on the middle stabilizing ring 50 of each connector 48 in the illustrated embodiment the riser tube has a diameter of 2 m and a wall thickness of 77 mm the mooring lines are preferably synthetic high strength cables e g made of dyneema u2122 fibres and may have a diameter of about 10 cm as shown in fig4 to 6 the connectors 48 comprise a circular cylindrical sleeve portion 56 which receives therein the end portions of the tube modules 46 to be connected together a flow restrictor in form of a perforated disk 58 best shown in fig5 and 6 is arranged in the middle of the sleeve portion 56 the flow restrictors 58 serve to control the flow velocity of the gas oil water mixture 16 caused by the expansion of gas as it rises along the riser tube the speed of ascension depends on the open cross section of the perforated disks 58 therefore the total area and the distribution of the holes in each flow restrictor 58 may be chosen in accordance with a predetermined flow pattern for instance the perforated disk 58 u2032 of fig6 will restrict the flow to a greater extent than that of fig5 due to the absence of the central opening 60 see fig5 the flow restrictors are configured in such a way that the resulting flow velocities do not.