Non-Linear Wave Force Analysis of Perforated Marine Structures

ABSTRACT A computational system has been developed which can be used to calculate the interactions of waves with any configuration of perforated walls. The mathematical model includes the non-linear pressure flow relation at the perforated wall. Using these programmes a study has been made of nearly hollow cylindrical and double concentric perforated systems as employed in offshore designs. The behavior of these systems is discussed and compared with the behavior of planar harbor breakwaters, and with solid offshore designs. The main conclusions are that:In contrast to the behavior of planar breakwaters the wave forces on cylindrical jackets are minimized by reducing the absorption of wave energy and maximizing the wave flow through the system.Perforated protection walls around large solid structures do not reduce the total wave force, but may be added to an existing structure for other reasons without greatly increasing the forces.Reduction of the structure cross-section around the wave surface offers the best design solution to the wave force problem. 1. INTRODUCTION Considerable confusion exists in offshore engineering circles as to the purpose and mode of functioning of perforated Caissons in marine environments. There are various existing designs included in this category: long planar breakwaters with a perforated wall and a solid back-wall, examples of which exist at Baie Comeau Harbor and Chandler Harbor in Quebec, Canada; a large cylindrical oil tank surrounded by a larger radius perforated wall (Ekofisk tank) and the supporting jacket of various production oil rigs where the central solid, section (riser) is much smaller than the radius of the outer perforated wall. Such perforated walls are usually explained as wave energy absorbers, and indeed, this was the concept on which the original planar coastal breakwater was based e.g. (1) Jarlan. However, as we show, it is not necessarily advantageous to have wave energy absorption in a structure which stands entirely in the sea rather than at the side of it. In contrast to a breakwater on the edge of the sea which must necessarily incur force, a structure standing fully in the sea only suffers as much wave force as it impedes the incoming sea; if the waves can pass through there will be much less overall force than if the waves build up round it. This is the situation involved where a perforated wall is used as the support jacket of a platform. The advantage of such a jacket lies in the high stability of support it offers the platform topsides, but there would be a high wave force penalty from the largest longest waves involved in survival conditions if a structure with such a wide base at the wave line were designed as a solid cylinder. For the sea wavelengths causing the largest forces, the force on a solid cylinder increases approximately as the volume (i.e. radius squared) of the structure; this fact is used to advantage by multi-leg platform design compared with large central solid structure.