None
Not Applicable
1. Field of the Invention
This invention relates to floating vessels. More particularly, it relates to platform-type semisubmersible vessels
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A semi-submersible is a particular type of floating vessel that is supported primarily on large pontoon-like structures submerged below the sea surface. The operating decks are typically elevated about 100 or more feet above the pontoons on steel columns. This design has the advantage of submerging most of the area of the components in contact with the sea and minimizing loading from waves and wind. Semisubmersibles can operate in a wide range of water depths, including deep water. They are usually anchored with six to twelve anchors tethered by chains and/or wire cables, which are computer controlled to maintain stationkeeping. Semi-submersibles (or “semis”) can be used for drilling, workover operations, and production platforms, depending on the equipment with which they are equipped. When fitted with a drilling package, they may be called semisubmersible drilling rigs.
As shown in
Certain semi-submersibles of the prior art have columns that are rectangular (four faces per column) and are positioned such that the column faces (sides) are set at 45 degrees with respect to the longitudinal axis of the adjoining pontoon.
The column shape of the present invention is similar to that of the “EXMAR OPTI-EX” semi-submersible illustrated in
The present invention relates to the shape of each of the columns on a semi submersible. In a 4-column semi-submersible according to the invention, each of the four columns has five faces, four of the five faces are perpendicular to at least one adjacent face. The fifth face is the outermost face of the column. The angle between the fifth face and the longitudinal axis of an adjacent pontoon is preferably about 45 degrees.
The column configuration of the invention is a variant of the existing deep draft semi submersible configuration used for the dry tree semi submersible vessels Independence Hub and Thunder Hawk which operate in the Gulf of Mexico.
The invention may best be understood by reference to certain illustrative embodiments which are shown in the drawing figures.
Referring now to
As may be seen in
In the embodiment illustrated in
The ratio of the length of the diagonal that is parallel to the non-orthogonal face to the distance from the mid-point of the non-orthogonal face to the opposite corner is preferably greater than or equal to 4/3. Stated another way, the geometric center of a column's cross section should be inboard of the center of a circle which circumscribes the cross section. This configuration is illustrated graphically in
The 5-sided column illustrated in
A semi-submersible equipped with the column form disclosed herein has at least two advantages:
A column form according to the present invention may provide an advantage in the vortex-induced motion (VIM) response of the vessel.
VIM amplitudes are the highest at a current heading along the platform diagonal. VIM amplitudes are minimal when the current is perpendicular to the column face (four-faced column). A key parameter in VIM response is the projected width of the column onto a plane perpendicular to the current. Each column has the same projected width for a semi submersible having columns of rectangular cross section. A configuration according to the present invention has two distinct widths for each current heading (different from perpendicular). The different widths result in a configuration where two of the columns are in full lock-in and two columns are not in full lock-in, and thus a reduction in VIM amplitudes may be achieved.
If the aspect ratio of the column (along the diagonal) is approximately 3:4, only a 6% increase in column width is necessary to maintain the buoyancy contribution of the column (as compared to a conventional column of rectangular cross section).
A semisubmersible vessel according to the invention may comprise a plurality of pontoons arranged generally in the form of a regular polygon having n sides and a column at each vertex of the polygonal form, each column having a generally five-sided transverse cross section with at least three sides disposed at a right angle to at least one adjacent side and two adjacent sides having pontoons attached thereto disposed at an angle of 360/n degrees to one another. It will be appreciated by those skilled in the art that angle α in the cross-sectional view of a 5-sided semi-submersible column shown in
Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.
Number | Name | Date | Kind |
---|---|---|---|
4226555 | Bourne et al. | Oct 1980 | A |
4913238 | Danazcko et al. | Apr 1990 | A |
6447208 | Huang et al. | Sep 2002 | B1 |
7140317 | Wybro et al. | Nov 2006 | B2 |
7240633 | Barlow | Jul 2007 | B2 |
Entry |
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http://wiki.seasteading.org/index.php/Semi-submersible; Jan. 7, 2014. |
Elements of Ocean Engineering (section Compliant Structures, Tension Leg Platform, p. 54); Society of Naval Architects and Marine Engineers, 1997. |
“Exmar Offshore Floating Production—OPTI-EX,” Obtained from http://www.exmaroffshore.com/floating—prod—optiex.html, Date retrieved Feb. 21, 2014, 2 pages. |
Number | Date | Country | |
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20120285361 A1 | Nov 2012 | US |