Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
1998-06-02
2001-10-09
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S224000, C114S264000
Reexamination Certificate
active
06299384
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of offshore structures. More specifically, the present invention is an earthquake-compliant offshore platform jacket structure for use in regions of strong seismic activity.
BACKGROUND OF THE INVENTION
Offshore oil and gas production is often conducted from platforns secured to the ocean bottom. One important design constraint for such platforms is that there be no substantial dynamic amplification of the platform's response to storms (a combination of wind, waves, and currents). Typically, most of a storm's wave energy falls within the 9 to 16 second period range. Consequently, to minimize the impact of storms on offshore structures, designers have engineered two types of structures, Steel Piled Jackets (SPJs) and Compliant Towers (CTs). These concepts differ mainly in the way they avoid wave energy. The SPJ, which is a stiff battered (i.e., having sloped sides) structure, uses its 2 to 4 second natural period to remain substantially below the period of storm wave energy. On the other hand, the CT, a flexible unbattered structure, uses its 20-30 second natural period to stay substantially above the storm wave energy. Generally, SPJs are economically viable structures in water depths less than approximately 1,000 feet, whereas CTs are economically viable structures in water depths greater than approximately 1,000 feet.
In addition to storm loads, some platforms may also be subjected to earthquakes. As can be seen from
FIG. 1
, most earthquake (seismic) energy falls within the 0 to 2 second range. A conventional SPJ will likely have a natural period of 2 to 4 seconds. While this period is well distanced from the typical 9 to 16 second wave period of storm energy, it is close enough to the 0 to 2 second excitation period of earthquake energy so as to be heavily influenced. Typically, a SPJ, because its low natural period is close to the earthquake energy, will have a large response in a seismic event, and thus most members will be designed primarily by the earthquake loads. In areas where earthquake loads are strong, the resulting structure tends to be a relatively heavy, inefficient SPJ. A CT, on the other hand, would have little dynamic response during an earthquake, but would be a very expensive concept for developing compliancy in the water depths where an SPJ is typically the preferred alternative.
Therefore, it would be desirable to have a more efficient offshore structure which can be used in areas subjected to strong seismic activity. The present invention satisfies this need.
SUMMARY OF THE INVENTION
The present invention is directed to a new concept, the Earthquake-Compliant Jacket (ECJ), having a configuration which effectively “tunes” the structure to avoid the majority of both storm wave and earthquake energy. In one embodiment, the invention is an earthquake-compliant offshore platform jacket structure for use in an earthquake-prone body of water, comprising: a substantially vertical space-frame structure extending upwardly from the floor of the body of water to a point located above the surface of the body of water; foundation means for attaching the space-frame structure to the floor of the body of water; and a deck structure attached to the upper end of the space-frame structure. The space-frame structure, the foundation means, and the deck structure are designed such that the natural vibrational period of the space-frame structure is greater than the primary excitation period of earthquake energy and less than the primary wave period of storm energy in the body of water. Preferably, the space-frame structure is substantially vertical and has a substantially uniform horizontal cross section. In a preferred embodiment, the space frame structure has a square cross section. The foundation means, in a preferred embodiment, comprises a plurality of skirt piles, attached to the lower end of the substantially vertical space-frame structure, which extend downwardly into the floor of the body of water. The natural vibrational period of the space-frame structure is preferably between about 4 seconds and about 8 seconds.
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Glasscock M. Sidney
Jones Josh E.
Steele Kenneth M.
Bagnell David
ExxonMobil Upstream Research Co.
Morgan Kelly A.
Singh Sunil
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