SEA-LEGS, A Stable Ocean Platform Concept
Authors: Glosten L.R. 
Published: October 2008

The paper traces the history of development and reports on recent model test programs. In SEA-LEGS the working superstructure is supported on three tubular hulls in a manner which permits changing the configuration so that the supporting hulls are vertical while on station but horizontal when travelling. The concept promises to extend to very large platforms the proven combination of motion stability and ready mobility exemplified by FLIP, with which the author has been associated.

The paper traces the history of development and reports on recent model test programs.

Introduction

At the outset, in reporting on this new stable platform concept, SEA-LEGS, it should be pointed out that its development owes much to the author's relationship with FLIP, the floating instrument platform which has been operated very successfully in oceanographic research by Scripps Institution of Oceanography for the past seven years. The author's participation in the final design and construction of FLIP has been previously discussed [1]. Moreover, during her operating life he has been retained as a consultant by Scripps and has had the opportunity to study performance closely and to participate in modification and improvement programs. This experience has been invaluable in the development of the present concept.

Because of the acknowledged influence of FLIP on the design of SEALEGS, a review of her basic characteristics is in order. For those interested, more complete discussions have been published [7]. The vessel is shown operating on station in Fig. 1. Essentially she is a. long, slender, basically cylindrical hull which in the stable operating configuration draws some 300 feet. In this attitude she resembles, and behaves like, a very large hydrometer and the term "hydrometer hull" is a useful brief description. The form of the underwater portion of the hull was governed by the basic objective that she remain as motionless as practicable at sea under any wave conditions that would normally be anticipated.

Ocean waves are essentially surface phenomena. The motion and disturbance which are so apparent at the surface, and the resulting pressure variations, die out exponentially with descent below the surface until at a depth equal to the wave length they have virtually disappeared. Heaving, or vertical oscillation, is caused by varying pressures on the bottom, or more precisely on the exposed horizontal areas of the vessel's form. If the bottom can be placed at sufficient depth, the pressure variations can be kept to very low values. This is the case with FLIP. Moreover, her water plane area is small enough so that the natural period of heave (27 seconds) is large with respect to that of the passing wave components containing significant energy and the large amplitudes associated with synchronism do not occur.

There is a further refinement to the FLIP hull form which contributes to its vertical motion stability. The lower half-length of the hull is 20 feet in diameter while at the waterline this dimension is reduced to 12.5 feet. A conical section joins the two cylinders. It can be shown, and this will be discussed in more detail later in this paper, that such a "shoulder" can take advantage of the decay in wave-induced pressure variations so as to completely cancel out heaving forces at one particular wave period.