Boots – shoes – and leggings
Patent
1987-03-31
1989-08-15
Lall, Parshotam S.
Boots, shoes, and leggings
364559, 73 65, 340689, G08B 2300, G01M 112
Patent
active
048581372
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the determination of the stability of a floating structure, particularly when it is in service.
The stability of any floating structure such as a ship or a semi-submersible oil drilling rig, i.e. its resistance to capsizing, is obviously an important factor in its safety. It is therefore the practice for countries in whose waters such structures operate to require the structures to comply with certain regulations on stability.
In general, the stability of a floating structure is characterized by its stability arm. This is the difference in vertical height existing between the vertical center of gravity of the structure and its metacentric height as determined from simple hydrostatics. The Metacenter is that point on the structure's axis through which for small angles of inclination the line of action of the floating structure's buoyancy force normal to the water surface will act. This can be seen more clearly from FIGS. 1 and 2 of the drawings, which are described in more detail below.
In practice, because of the effects of mooring cable tensions, riser tensions, etc., the position of the Metacenter (M) as predicted from the hydrostatics of the structure will change. A stiffening of the structure's resistance to inclination will augment the height of the Metacenter and vice versa. The modified position of the Metacenter may be called the Protocenter (P).
The stability arm (GM) of the floating structure is conventionally measured by inclining the structure through a change in its weight distribution. If the moment of this weight distribution change about the floating structure's center of flotation is m, then from Naval Architectural Theory for small angles of induced inclination theta, the stability arm (GM) is calculated by: ##EQU1## D=displacement of the structure at the time the inclination occurs.
Conventionally, the moment generated (m) and the resulting angle of inclination (theta) will be devised to act in the roll or least stiff rotational axis of the floating structure.
One method which may be used to determine structure stability is to compare the structure's vertical center of gravity (VCG) with standard precalculated curves of maximum VCG. In order to make this comparison, it is necessary to determine an estimated service VCG.
As part of the standard procedure for determining VCG, an inclination test is carried out. A known heeling moment is applied, e.g. by moving a weight to a given position across the deck of the structure, and the resulting inclination is measured.
However, the inclination of the structure at any instant is affected by the wind and wave motion to which it is subjected. The conventional stability test, therefore, involves moving the structure to sheltered water close to the shore so that the effects of wave and wind action can be minimized. This incurs a commercial/operational penalty and the basic stability of the structure can, therefore, only be determined at relatively long intervals. Changes made to the structure or to the equipment and stores carried can lead to a change in stability in the period between tests.
It would clearly be desirable if the inclination of the structure in response to a given load, and hence the stability of the structure to capsizing could be determined while the structure remains at sea performing its normal duties.
According to the present invention, the method of measuring the inclination of a floating structure resulting from a change in weight distribution of the structure comprises:
(a) making a plurality of changes in the weight distribution of the structure, the changes being distributed about the center of flotation, taking signals at intervals over a period of time from two inclinometers so as to measure the inclination of the structure along two orthogonal non-vertical axes,
(b) feeding the signals to signal processing apparatus, which apparatus
(i) determines from the signals a value of the change of inclinations along each of the two orthogonal non-vertical axes,
(ii) calculates an average value of
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patent: 3329808 (1967-07-01), Fisher
patent: 3917937 (1975-11-01), Sten
patent: 4347574 (1982-08-01), Parsons
patent: 4549277 (1985-10-01), Brunson et al.
patent: 4647928 (1987-03-01), Casey et al.
Lall Parshotam S.
Mattson Brian M.
The British Petroleum Company p.l.c.
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