Patent Application
20100190393
The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. Aspects of the invention relates to an assembly of turret and disconnectable buoy, in which the turret at its lower end and the buoy at its upper end are provided with mating coupling provisions.
In offshore oil production, use is made of so called FPSO's (Floating Production, Storage and Offloading). FPSO's are ships which are permanently or semi-permanently anchored offshore at an oilfield for the purpose of receiving well fluids from the oil reservoir, separating the oil from the water and gas and storing the oil on board for later transfer to another vessel. Generally, the FPSO's are anchored by means of single point mooring systems of the internal or external turret type.
In some areas of the world, weather conditions can deteriorate so badly, such as due to hurricanes, that it is required to enable the FPSO to disconnect from its anchoring system and sail temporarily to safer waters. Likewise, in iceberg infested waters it is sometimes necessary to disconnect the FPSO to avoid damage to the vessel.
In these instances use is made of an disconnectable turret mooring system, which can be of the internal or the external type (in the one case the turret is fitted within the confines of the vessel hull, in the other case the turret is fitted forward of the bow of the vessel), comprising a disconnectable buoy, wherein the turret at its lower end and the buoy at its upper end are provided with mating coupling provisions.
In the design of a disconnectable turret particular care needs to be given to the interface between the lower end of the turret and the upper end of the disconnectable buoy.
Ideally this interface is preloaded in such a way as to avoid that a gap is created between the turret and the buoy under external anchoring and wave load conditions. Since generally the turret and the buoy are cylindrical or conical structures with a large diameter, it is also important to interlock these structures in such a way that both act as a single structure when connected. Another objective is to achieve a good fit between the structures at their interface, without resorting to tight and hence expensive machining tolerances.
The assemblies according to the state of the art provide a buoy with a protruding conical upper end which has to be received in a correspondingly shaped conical lower recess of the turret. These known assemblies suffer from the significant drawback that whereas the conical interface eases fit up of the two structures, the conical recess must be fabricated extremely heavy and sturdy to resist the radial loads which follow from the two parts being connected. Very significant preload is required to assure that the buoy and turret do not separate under external loads arising from anchoring and waves. If such preload cannot be achieved sufficiently, locking means, which connect the disconnectable buoy to the turret, are subject to high cyclic load variations following from these anchoring and wave loads and hence may fail prematurely.
This Summary and the Abstract herein are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.
An assembly of a turret and a disconnectable buoy are provided, in which the turret at its lower end and the buoy at its upper end are provided with mating coupling provisions, wherein the coupling provisions comprise an annular projection protruding from one of the turret and buoy towards the other of the turret and buoy and a correspondingly shaped annular recess on the other of the turret and buoy for receiving the annular projection, wherein the annular projection has a base which is wider than a top thereof, whereas the annular recess has a top which is wider than a base thereof.
The annular projection and annular recess are machined to a tolerance such that if they are engaged one to another, under an axial compression force, one of these parts deforms more than the other in a radial direction such as to become compliant with the other part in a form-fit. By applying a nominal preload to this interface, both parts of the assembly now work as one single part, there being no tendency for the engaged parts to separate.
The tolerances of machining are selected such that the diameter growth of the weakest part, typically the lower end of the turret structure, under a compression force, is limited to such a percentage of tangential elongation that it avoids achieving the yield stress of the material (steel) being employed in the parts. Once this stress level is reached, a further increase of the compressive force merely increases local stresses in the radial section of the parts and increases the contact stresses between these parts. This is a very stiff load path, with a locking device which connects the disconnectable buoy to the turret forming part of a much lesser stiff load path. Hence these locking devices are not subject to any significant cyclic loads.
Since a very high internal contact stress is achieved over the tapering surfaces of the projection and recess, these parts can also act as a sealing device against the seawater to allow the turret to be pumped dry after a connect operation. An additional sealing member (e.g. a soft rubber or mild steel ring) may be fitted between the annular projection and annular recess.
Aspects of the invention will be elucidated while referring to the drawing, in which:
Referring to
Referring to
It is possible too that the annular projection is part of the turret and that the annular recess is provided in the buoy. Further it is conceivable to provide more than one annular projection with corresponding annular recess; in such a case it would be possible that each of the turret and buoy comprises at least one annular projection and at least one annular recess.
It is noted that the provision of an annular projection and annular recess does not prevent that, in accordance with the state of the art, the lower end of the turret 2 and upper end of the buoy have corresponding tapering shapes (as indicated schematically in
Finally
The present application is a national stage filing of International patent application Serial No. PCT/EP2007/057319, filed Jul. 16, 2007, and published as WO 2009/010089 in English.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/57319 | 7/16/2007 | WO | 00 | 4/14/2010 |
