The traditional way to secure the on site stability of jacket structures is by piling the foundation to the sea floor. The process of piling is both expensive and time consuming, and as an alternative it has been proposed to use bucket shaped concrete foundations. The principle of this is that large, heavy pods, made up from steel and concrete, and shaped like inverted buckets should penetrate the soil, much like the skirts of a condeep, and thus provide a stable, reliable basis for the jacket structure (fig. 1) with four pods, each of the the main legs will be solidly resting on and anchored to a concrete block. The structure is to be installed in one piece. The idea is convenient and appealing. However, there are potential hazards. One essential requirement is that such a structure should be safely and correctly installed m a controlled manner at a prescribed location by a conventional crane vessel. once the structure has made contact with the sea bed, the possibilities for adjusting the position or inclination quickly becomes marginal. The dynamics of the jacket during installation is discussed at some length below, with particular attention given to the critical phase when the sea bed is approached.
One interesting feature of a submerged structure is that is added mass is a function of the distance to the sea floor. This sensitivity of the added mass on the separation distance is particularly strong in the present case, where the pods can be geometrically represented by cut off cylinders (figs. 2 and 3).
