underside of the barge 24 the dock 20 is then reflooded the barge level tried and the platform 2 then floated out of the dock 20 firmly attached to the barge 24 the platform is then towed to the installation site as will be appreciated the counterweight 16 counters the mass of the tower 8 during transportation to the installation site thereby increasing the stability of the barge 24 it will also be appreciated that when installed the counterweight will contribute to the anchoring of the structure to the water bed as shown in fig6 once the barge 24 is in the desired installation position for example over a pre drilled well head 38 the base 4 is lowered towards the seabed 30 using the jacks 28 prior to installation of the platform a positioning means 32 having guides 34 is positioned on the water bed 30 as shown in fig6 the positioning means may simply be a large mass of concrete with suitable guide means 34 and which may itself have been installed by the method of the invention during lowering a guide 40 on the base 4 will engage in the guides 34 provided on the positioning means 32 so as to accurately position the base 4 on the water bed 30 the exact position of the guide 40 on the base 4 is pre determined by measurement of the position of the guides 34 of the positioning means 32 relative to the well head 38 once the skirts 6 penetrate the seabed 30 the weight of the base 4 is transferred to the seabed 30 and the base 4 is firmly secured allowing the strand jacks 28 to be.