have spring operated retractable hooks 154 to engage in the female lock down stabs 88 the roof structure consists of the side wall 94 silo roof 96 and porch roof 98 the side wall 94 is made in sections that are designed to fit around the extreme outer edge of the silo including the porch support these sections are latched to the stiffening ring 84 at the top of the upper silo by removable lock stabs this is done in the moon pool after the porch support 70 is attached the latching mechanism facilitates removal for sea floor clearing or replacement if a side wall section is damaged by snagging fig1 and 20 show the side wall stab in connections the bottom surface of side wall 94 is provided with a hollow stab element 182 which has pivotable locks 184 when bullet shaped retainer 188 is removed the locks can retract with the retainer in place as shown in fig1 the locks cannot be retracted stab 182 is inserted in female element 190 of stiffening ring 84 with retainer 188 removed with the stab 182 in place as shown in fig1 retainer 188 is lowered into place by a guide line guided by guide 189 this locks the side wall in place on the silo the side wall sections are sufficiently non rigid to deform under the full load conditions imposed in the event of a troll board snag and are replaceable using diverless techniques the side walls form a forty five degree angle with the roof to avoid snags the side.