Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2000-03-23
2002-04-23
Will, Thomas B. (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S211000, C405S223100, C405S224400, C166S367000, C114S264000, C441S133000
Reexamination Certificate
active
06375391
ABSTRACT:
INTRODUCTION
This application claims priority to Norwegian patent application Ser. No. 1999.1470, filed Mar. 25, 1999 and Norwegian patent application Ser. No. 2000.0831, filed Feb. 18, 2000. This application concerns a frame for stabilizing risers on a petroleum production vessel, preferably for production risers with “dry” wellheads, i.e. with Christmas trees arranged on the deck of a freely floating platform. The petroleum production considered takes place at very large sea depths, very likely more than 1200-1600 meters.
APPROACH TO THE PROBLEM
The invention is in one embodiment adapted for use in sea areas with the estimated wave height H
max
being in the order of amplitudes between 5 and 10 m, thus considerably less than the H
max
in the order of 25-30 m required for areas in the North Sea, conditions requiring considerably larger dimensioned and consequently heavier, more expensive vessels. A considerable problem in petroleum production at sea is to guide risers through the splash zone and the upper current- and wave-affected zone below the sea surface. In this zone, large tensions, tension variations, bending moments, wave actions and accelerations occur on the risers and their connection points, for example Christmas trees.
PRIOR ART
The prior art is described in the patent specifications GB 2 147 549, U.S. Pat. No. 5,558,467, and WO 95/28316. A similar shallow water construction which is not a vessel, and which cannot be applied in deep water, is described in GB 2 139 570.
“Spar” Buoy
A deep semisubmersible construction called a “Spar” buoy, may be adapted for production drilling, petroleum production or storing of petroleum fluids at sea. Such a design can consist of one single, heavily ballasted, column of very deep draught, having a relatively large buoyancy volume arranged at a high level in the column, at or below the water surface, and having a column through the splash zone and a work deck above water. The lower ballasted part can comprise a framework. Such a column stabilized construction design has little heave or vertical movement but its large draught can entail that even small angular movements, as measured in degrees, still entails considerable horizontal accelerations near the top and the lower end of the construction. Such a deep column-stabilized design has an advantage in that it encloses the risers in the critical area from the splash zone at the sea surface and down to a depth more than 100 meters so that wave and current forces do not reach the upper part of the risers. Such a design has economic disadvantages in that the deep draught requires heavier plate dimensioning to resist the higher water pressure. Heavier dimensioned steel plates entails higher weight and price. The deep draught of the assembled operative platform requires assembly at deep draught in deep water near the field, meaning higher lifting and assembly costs.
WO 99/10230 “Buoyant substructure for offshore platform” describes a buoyant substructure floating vertically standing in the sea (e.g. as an offshore platform) comprising at least three separate columns being interconnected. At least one of the columns is arranged to be ballasted by the end which is arranged to have deep draught, where the columns are interconnected by short beams.
NO 174 920 “Flexible marine platform with surface production wells is described as a platform consisting of a rigid construction carrying a deck, pontoons fixed to the lower part of the rigid construction and a flexible construction constituted by columns fixed by their upper ends to the rigid construction and to the pontoons, and by their lower ends to a foundation arranged at the seabed, whereby the columns are in tension. Guide plates are illustrated, but no buoyancy elements on the risers.
Tension Leg Platforms
Another solution for production platforms is tension leg platforms, so-called TLP's. Tension leg platforms are anchored via vertical tension legs or tethers anchored to the sea bed. The risers of such a tension leg platform may be guided by guide plates described in PCT publication WO 97/29944 and published Norwegian patent application NO 1998.3337, so that the risers get a parallel and small relative vertical movement relative to the platform deck and relative to each other. The tension legs are usually anchored with pile suction anchors, being vacuum-sucked down into the sediments in the sea bed, or gravity based structures on the seabed. At least two problems occur with such an anchoring solution:
a) At the large depths which may be in question: more than 1200-1600 meters, the seabed sediments may consist of less compacted unconsolidated organic mud, fine silt and clay particles with low density, low shear resistance and high water content, as distinct from glacially worked compacted clay/sand-containing sediments which constitute an essential part of the sea bed in the North Sea and the Norwegian Sea.
b) The sea bed can contain petroleum fractions forming so-called “hydrates” being kept in a partial frozen phase at shallow depths below the sea bed and is presumed to be deposited from escaping petroleum fluids from deeper geological layers at higher temperatures. These hydrates are unstable and can pass to the gas/liquid phase if they are supplied with heat. In a sedimentary basin, deeper geological layers usually have a higher temperature than the surface layers. Petroleum production entails a heat transfer from the upwardly flowing/rising petroleum fluids in top of the well, to and may result in an unwanted fluidization of hydrates in layers close to the seabed. Thus, there is a risk of gas formation at the suction anchors and a risk for sudden loss of tension in a tension leg.
In deeper waters the separation between the risers must be large in order to avoid collision during hydrodynamic drag. This separation usually requires a larger and thus heavier tension leg platform.
Semisubmersible Platform
A third solution is ordinary column stabilized or semisubmersible constructions in the form of platforms. An essential problem with semisubmersible constructions with an open moonpool is that the production risers will hang freely movable and unstabilized through the splash zone and the upper water masses. This is difficult if there are buoyancy members on the risers, and especially if the buoyancy members are arranged in the splash zone, because, as mentioned above, problems with wave forces laterally and vertically on the buoyancy members and the risers occur.
SHORT SUMMARY OF THE INVENTION
A solution to the above mentioned problems is given according to one embodiment of the invention as defined in the patent claims enclosed: A system for use in petroleum production at sea, includes a guide frame for one or more riser pipes on a semisubmersible production vessel with one or more main buoyancy member arranged separately on at least one riser to carry the main part of the riser's weight. Each riser is arranged for separately carrying a Christmas tree on top, near the deck of the vessel. The guide frame comprises vertical main elements arranged to extend vertically downwards from the deck, through the splash zone and through the upper, more wave- and current-influenced zone of the sea, down to a depth of about 50-150 meters below the sea surface, where drag forces are less pronounced. The novel features by the invention is as follows:
The guide frame has horizontal guide plates comprising vertically open cells formed of a horizontally arranged framework, preferably of beams, with lateral stabilization devices for guiding the risers' and the main buoyancy members' vertical relative movement and restricting the horizontal relative movement with respect to the guide frame.
The guide plates are arranged in at least two levels on the guide frame: (1) a lower guide plate is arranged at the lower ends of the vertical main elements of the guide frame, and (2) another guide plate is arranged at or just below the splash zone.
One main buoyancy member is arranged for being held on the riser preferably in level with, and guid
Børseth Knut
Often Ola
Mayo Tara L.
PGS Offshore Technology AS
Rothwell Figg Ernst & Manbeck
Will Thomas B.
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