Ships – Floating platform
Reexamination Certificate
2002-04-02
2003-07-08
Basinger, Sherman (Department: 3617)
Ships
Floating platform
C114S266000, C114S267000
Reexamination Certificate
active
06588358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to floating platforms that serve as landings for vessels, and more particularly to a moored floating platform having a configuration of pontoons and bridge assemblies that reduces platform motions caused by waves and wave phenomena, thereby allowing a more nearly level platform to and from the vessel for passengers and stevedores.
2. Background Art
Floats used for vessel landings are constructed either as walkways, supported at intervals by buoyant cells, or barge-type hulls where the deck of the barge is the landing area. Landings constructed as walkways are typically simple surfaces supported at regular intervals by floatation cells that may be Styrofoam or sealed chambers. The walkways are segmented and connected at joints. Each walkway segment is fitted with one or more buoyant cell. The walkway lands on top of the buoyant cell(s). The landing, acting over the length of the walkway with the buoyant supports, does not act as a structural monolith, where moment and shear loads at one end are transferred to the opposite end. In this sense these landings may be categorized as articulated, such that their components are connected by a series of shear loaded joints which do not transfer moment.
Barge-type hulls act as monolithic structures where moment and shear are transferred through the structure, and loads applied at one extreme end influence the reaction at the other extreme end. These are single structures with internal subdivisions to form tanks and internal boundaries. Variations on this design may include a series of individual pontoons linked together to form a single but composite structure. This type of composite structure links the individual pontoons closely together so that they tie together to form a single barge-like structure. The characteristic of these landing floats is that a continuous buoyant volume is developed along the length of the float body. These floats have the appearance of a barge where the immersed dimensional envelope defines the buoyant volume.
These floating bodies, when acted upon by waves, rock, roll and oscillate on the water surface, generally with six degrees of freedom, namely roll, pitch, yaw, heave, surge and sway. Waves force these bodies to experience motions on the basis of periodicity and wave length. And yet each floating body also has its own natural oscillation period. When the forced wave period and the natural period of the floating body coincide, wave induced loads on the floating body reach maximum amplitudes.
In wave mechanics, the period and wave length are directly related. Short wave lengths have short periods and high frequencies. The effect that a wave form has upon a floating body can also be expressed in terms of the wave length, since period and length are related. Waves that are extremely short, compared to the length of the floating body, will have little effect upon the body. This is due to the fact that the body is being acted upon by a number of waves simultaneously along its length and their net effect is to cancel each other out. The longer the wave length the less the phase differences between competing waves along the length of the body. Long wave lengths, compared to the length of the body, will have a greater effect upon motions.
The wave form exerts a force upon the floating body that is both buoyant and inertial. The buoyant force is created because the wave form elevates or depresses the water profile around the floating body thereby altering the buoyant loads along the body's length. A floating body that encounters a wave crest at the front of the body will experience an upwards load that tends to lift the front relative to the rear. The wave creates a “trimming moment” that tends to trim the front up and the rear down. As the wave creates passes under the body, that trimming moment will eventually cause the rear to trim up and the front down.
In this way the wave form can be seen to create a rocking motion, or pitch, along the length the body. As the wave form passes along the length of the body, it also alters the net elevation of the water level so that the wave tends to lift the body. In this way the wave form can be seen to create an up and down motion, or heave, on the body.
The inertial force is created by the water particles themselves as they move in an orbital path, pushing against the body's surfaces. When the wave front is acting against the front of the floating body, the wave particles are also pushing against the front as they move in their orbit. The push action of the water particles tends to force the front of the body backwards. As the wave passes under the body, the push action of the water particles in the trough of the wave tend to push the body forward. In this way the wave form can be seen to cause a fore and aft motion, or surge, in the floating body. If the wave front acts slightly askew to the alignment of the floating body, the wave particles tend to force the front to one side and then back to the other side as the wave passes under the body. In this way the wave form can be seen to cause a twisting motion, or yaw, in the floating body.
The wave front that strikes the floating body on the side exerts an effect similar to a head on encounter. The wave front buoyant force lifts one side of the body then the other side as the front moves under the body. This creates a rolling motion in the body. The side inertial force of the wave particles tends to push the body sideways, thus creating a swaying motion. As the wave front moves under the body, it also alters the net elevation of the water level so that the wave tends to lift the body, thus creating heave. Where the side wave hits slightly askew, the body tends to yaw.
All floating bodies have natural periods whereby they oscillate in uniform harmonic motion to all six degrees of freedom. Waves force a floating body to experience these motions and the body tends to oscillate at its natural frequency. Wave frequencies that are out of phase with the body's natural period tend to have less effect upon the body's motion than those frequencies that are closer to the body's natural response frequencies.
Motion dampening of a floating body is traditionally of three types. The first is to design a body's natural period to be significantly displaced from the peak period of the design wave form. The second is to develop systems internal to the floating body that respond with counter moments to the buoyant forces that the wave front exerts upon the body. The third is to utilize keels to resist rolling motions.
Each approach has limitations when applied to moored floating platforms. These limitations are a consequence of economic and mass limitations. In simple terms, floating moored vessel landings need to be compact, inexpensive and resilient to vessel impacts. As a consequence, moored vessel landings are traditionally designed as simple cubic structures, described as barges tied off to piles.
The natural period of rolling or pitching of a floating body is dependent upon the distribution of its mass. Basically, when the body rolls or pitches, it describes an arc of rotation about a center, generally located near the center of gravity. The period of the rotation is assumed to be harmonic. According to the laws of simple harmonic motion, or SHM, the period of oscillation is a function of how the mass is distributed about the focal point of the rotation. In a compound body in rotation, the distribution of mass can be assumed to be located at a point from the mass center called the “radius of gyration,” or gyradius. The gyradius is the point located from the motion center where the entire mass of the body appears to be located. The position of the gyradius is solved by dividing the mass moment of inertia of the body by its mass and taking the square root of the quotient.
Increasing the mass moment of inertia, and consequently the SHM period, involves increasing the dimensional and cubic measurements of the floating body.
The second method o
Basinger Sherman
Johnson Larry D.
Johnson & Stainbrook LLP
Stainbrook Craig M.
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