Hydraulic and earth engineering – Marine structure or fabrication thereof – Floatable to site and supported by marine floor
Reexamination Certificate
2001-07-09
2003-11-11
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Floatable to site and supported by marine floor
C405S203000, C405S195100, C114S265000
Reexamination Certificate
active
06644893
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an offshore platform for a hydrocarbon production facility, and more particularly to a rigid jacket of the offshore platform which has a quadrapod structure.
BACKGROUND OF THE INVENTION
Offshore production of hydrocarbons is typically enabled by placement of a platform at a desired offshore site. The platform comprises a jacket and a deck supported by the jacket, which houses a hydrocarbon production facility. The height of the upright jacket is greater than the depth of the water at the offshore site of the platform so that when the base of the jacket is positioned on the sea floor, the top of the jacket extends above the sea surface. The deck is mounted atop the jacket so that the deck is likewise positioned above the sea surface. Most offshore platforms are bottom-founded structures, which are fixed to the sea floor by attaching pilings driven into the sea floor to the base of the jacket.
There are a number of criteria, which must be considered in designing the jacket of a bottom-founded offshore platform. In particular, the jacket must be designed with sufficient strength to resist collapse or bending during periods of high loads or stress. For example, the jacket incurs relatively high loads or stress during initial placement of the jacket at the offshore site and during prolonged continuous in-place operation of the offshore platform in the sea environment, where the jacket is exposed to a broad range of wave forces. Waves are characterized by their height and period. Very large or tall waves are long period waves because of the physical limits on the steepness of the waves, while small or short waves are short period waves. For example, a classical Gulf of Mexico hurricane wave is 75 feet high and has a 12 second period. Smaller waves have correspondingly shorter heights and shorter periods.
Bottom-founded jackets are characterized as either “rigid” or “compliant”. A rigid jacket is substantially stiffer along its length than a compliant jacket. A rigid jacket is designed and constructed with sufficient strength to prevent the platform from being pushed over by very large waves. The natural period of sway for a rigid jacket is usually less than 3 seconds and the natural period of whipping is significantly shorter. The short natural period of the jacket avoids a resonant sway response to more numerous small waves, which could damage the platform through fatigue.
A compliant jacket, such as disclosed in U.S. Pat. No. 5,480,265, is not strong enough to directly resist large waves. Instead, a compliant jacket relies on substantial inertia and a very long sway period on the order of about 45 seconds to prevent the platform from being pushed over by very large waves. Thus, even very large waves with 12 to 16 second periods pass through the jacket too quickly for the jacket to respond to the wave. However, the fundamental requirement of a long sway period limits the utility of compliant jackets to relatively deep water on the order of about 1000 feet or more. The critical fatigue issue for a compliant jacket having a long sway period is the whipping period, which causes the jacket undue fatigue if it is too great. Therefore, it is necessary to design and construct a compliant jacket having a sufficiently long sway period, yet also having a whipping period within acceptable limits, to avoid failure of the jacket structure from wave induced fatigue. Although compliant jackets can frequently be constructed at a lower cost than rigid jackets due to reduced material requirements, design of compliant jackets can oftentimes be more complex than rigid jackets due to the difficulty in achieving both an optimal sway period and an optimal whipping period.
As noted above, the jacket is subjected to relatively high loads and stresses both during placement of the jacket and during in-place operation of the offshore platform. These loads and stresses have different characteristics, which must be considered in the design of the jacket. Most jackets are fabricated at onshore locations. After fabrication, the jacket is loaded onto a transport barge with the jacket lying on its side in a horizontal orientation. The jacket is transported atop the transport barge to a desired offshore site for placement. Placement of the horizontally oriented jacket on the sea floor is effected by one of two methods, either lifting or launching. The lifting method employs a heavy-lift vessel positioned alongside the transport barge, which engages the horizontal jacket, raises the jacket off the deck of the barge, reorients the jacket to an upright vertical position, and sets the jacket down in a vertical orientation on the sea floor at the desired offshore site. The launching method tilts the deck of the transport barge so that the horizontal jacket, which has a plurality of interior buoyancy compartments, slides laterally under the force of gravity along the deck into the sea and floats in a substantially horizontal orientation on the sea surface. The jacket interior is then flooded in a controlled manner, upending the jacket to a vertical orientation. A derrick barge positions the jacket over the desired offshore site and the jacket interior is flooded further, setting the jacket down on the sea floor.
A jacket, which is placed by the launch method, is preferably provided with a plurality of launch runners to facilitate sliding the jacket off of the barge. The launch runners protrude from the external framing of the jacket and are typically fortified by a separate launch box, which is added to the framing of the jacket and strengthens the jacket structure against the severe loads and stresses encountered during launch of the jacket.
There are any number of rigid jacket configurations known in the prior art. Nevertheless, it is commonly recognized that the most efficient configuration of a rigid jacket is a tripod configuration having three convergent legs. By most efficient configuration, it is meant that the tripod configuration achieves the greatest strength with the lowest material requirements, at least with respect to in-place operation of the platform. However, a rigid jacket having the tripod configuration lacks two parallel legs, to which launch runners can be mounted for launching the jacket. Therefore, placement of a rigid tripod jacket must be performed by the lift method. This undesirably limits the maximum design weight of the jacket to the capacity of the available heavy-lift vessel, which in turn limits the water depth and topsides load capacity for which the jacket can be designed.
The present invention recognizes a need for a launchable jacket having the desired in-place performance characteristics of a rigid jacket, yet having substantially reduced material requirements relative to known launchable rigid jacket designs. Accordingly, it is an object of the present invention to provide a launchable rigid jacket, which has sufficient strength to resist damage or failure caused by launching the jacket from a barge at a desired offshore site. It is another object of the present invention to provide such a launchable rigid jacket, which has sufficient strength to resist damage or failure caused by wave forces during in-place operation of the jacket at the offshore site. It is still another object of the present invention to provide such a launchable rigid jacket, which has reduced material requirements for its construction. These objects and others are accomplished in accordance with the invention described hereafter.
SUMMARY OF THE INVENTION
The present invention is a launchable rigid jacket of an offshore platform. The jacket is characterized by a base, a top and a height extending from the base to the top. The jacket has a structure including an exterior framing and a plurality of cross-sectional and intermediate frame sections. The exterior framing comprises four outside legs, each extending at least a majority of the height of the jacket. A first pair of the four outside legs is more closely spaced apart and a second pair of the four ou
Ebel Jack E.
Marathon Oil Company
Shackelford Heather
Singh Sunil
LandOfFree
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