damage to the outer skin since the loss of ballast is then confined to a limited number of cells perimeter wall 7 may be formed of a plurality of straight facets circular arcs or a combination of the two the latter two alternative embodiments being illustrated in fig4 and 5 respectively referring now to fig4 perimeter wall 7 is formed of arcuate wall sections 12 of equal radius which may vary given a particular application arcuate wall section 12c which forms the portions of the perimeter wall located at the corner of the overall hexagonal shape of the structure 1 subtends a greater arc than those arcuate sections 12 in the non corner positions so that lines drawn tangent to the perimeter of structure 1 form a substantially regular hexagonal pattern alternatively the perimeter wall 7 may be formed of a combination of planar wall sections 16 arcuate wall sections 18 located at each hexagonal corner of structure 1 and connecting wall segments 14 connecting segments 14 join the perimeter 7 to the interior structure formed by walls 8 in either of the embodiments shown in fig4 or fig5 the interior cellular arrangement coupled with the perimeter connecting structures are designed to provide an efficient system to effectively distribute environmental loading imposed by ice floes or other ice features so that the forces are.