Implements or apparatus for applying pushing or pulling force – Device or member for contacting and guiding moving cable – Including rotatable – cable contacting – pulley wheel element
Reexamination Certificate
2000-07-14
2001-10-02
Marcelo, Emmanuel M. (Department: 3653)
Implements or apparatus for applying pushing or pulling force
Device or member for contacting and guiding moving cable
Including rotatable, cable contacting, pulley wheel element
C254S386000, C166S355000
Reexamination Certificate
active
06296232
ABSTRACT:
The present invention relates to a riser-tensioner for exerting a tensile force from a drilling vessel or drilling platform upon a riser, comprising a riser ring which is for fastening attachably to the top end of the riser, and one or more cables extending by way of cable pulleys from said vessel or platform to said riser ring, said cable pulleys being under such influence of tensioning means that a tensioning force can be applied to the cable.
PRIOR ART
During the drilling of a borehole by means of a drill string from a drilling vessel or drilling platform, it is customary to construct below said vessel or said platform a riser which extends to the seabed. The riser encloses the drill string and makes it possible for the drilling fluid to be conveyed from the borehole by way of the radial opening between the outside wall of the drill string and the inside wall of the riser in the direction of the drilling vessel or drilling platform. The drilling fluid contains a great variety of information on the circumstances at the bottom of the well, and the analysis of such information is necessary for achieving the optimum drilling process.
A riser is constructed from riser parts extending from the drilling vessel or drilling platform in the direction of the seabed. The riser is kept under control from the drilling vessel or drilling platform by the fact that a tensile force is exerted upon a riser ring fixed at the top end of the riser. This riser ring is generally connected by means of cables to the drilling vessel or drilling platform, and a tensioning force can be applied to the cables. For this purpose it is known to design the drilling vessel or drilling platform with cable pulleys which are placed on either side of a tensioning cylinder, the cables which extend from the riser ring being conveyed by way of the cable pulleys to a fastening on the platform. The presence of the tensioning cylinders ensures that length variations of the cables, which are caused, inter alia, by movements of the drilling vessel or drilling platform relative to the top side of the riser (read: relative to the seabed), are absorbed. The device by means of which the riser ring is connected to the vessel or platform is also called a riser-tensioner.
A riser-tensioner of the type mentioned in the preamble is known from U.S. Pat. No. 3,897,045. The tensioning means of this known riser-tensioner are formed by cylinders which are provided with cable blocks on either side. These cylinders are placed essentially above the drilling floor. From the blocks placed at the bottom side of the cylinder, cables fastened to the riser ring extend on either side of the riser.
Since drilling is carried out at increasingly great depths by means of drilling vessels and drilling platforms, the risers used are becoming increasingly long, and consequently increasingly heavy. This means that the forces which have to be transmitted by the cables and the cable pulleys are also becoming increasingly great, and thus also the dimensions of the cables and the cable pulleys themselves.
A first major disadvantage of the known riser-tensioner is that, on account of the large cable pulleys which are necessary in the tensioners, an ever-increasing amount of space is needed for the riser-tensioners on board drilling vessels or drilling platforms.
Owing to the movements of the vessel or the platform relative to the riser, the cable will be bent on the various cable pulleys while it is under high tension, with the result that the cables are exposed to great fatigue stresses. From time to time (for example, once a year) the cables on the riser-tensioner must be replaced. Since the cables extend from the vessel or the island and end on the riser ring, the cable must be removed from the riser ring and new cable supplied from the platform to the riser ring. This is a job which cannot be carried out without risks for the workers involved.
Another major disadvantage of the known riser-tensioners is therefore that the replacement of cables in riser-tensioners can be carried out only with great difficulty.
It is the object of the present invention to provide a riser-tensioner which can be provided with relatively small cable pulleys and in which the cable of the riser-tensioner can be replaced simply and rapidly, without the cable having to be removed from the riser ring.
That object is achieved in the present invention by the fact that the riser ring is provided with riser ring sheaves, and that the cables are guided in an outgoing reeving from the vessel or the platform to said riser ring sheaves and are guided back by way of said riser ring sheaves in a return reeving to said vessel or platform. The advantage of the riser-tensioner according to the present invention is that the cable which is supplied from the drilling vessel or drilling platform to the riser ring does not end at the riser ring, but is guided back by way of a return reeving to the drilling vessel or drilling platform. That means that for replacement of the cable in the riser-tensioner the cable can be coiled up at one side and supplied at the other side, without connection or disconnection of the riser ring being necessary.
British Patent Application 2,170,240 discloses a riser-tensioner which comprises a collar, in the case of which a cable is guided from a drilling vessel or drilling platform by way of a first reeving from the drilling vessel or drilling platform to the collar, and by a second reeving from the collar back to the drilling vessel or drilling platform. However, the purpose of this known riser-tensioner is to permit disconnection of the riser safely and quickly when the movements of the drilling vessel or drilling platform relative to the seabed become too violent. This is the case, for example, when there is strong wind which causes high wave action.
When the riser is disconnected at the bottom side, the riser-tensioner according to GB 2,170,240 ensures that the riser is drawn up a certain distance, but cannot overshoot. A stop fixed on the outside wall of the riser will help to move the collar which is fixed around the riser. Owing to the construction of the known riser-tensioner, a downward force is subsequently exerted upon the collar by the cables of the riser-tensioner. That force is transmitted to the riser itself by way of stops mounted on the riser itself.
In normal use the cables of the riser-tensioner run substantially horizontally from the drilling vessel or drilling platform in the direction of the collar. Vertical movements of the riser relative to the drilling vessel or drilling platform are absorbed by the cables fastened to the riser ring at the top side of the riser. During use, the functioning of the riser-tensioner therefore corresponds entirely to that of the riser-tensioner discussed above with reference to U.S. Pat. No. 3,897,045. The riser-tensioner according to GB 2,170,240 also has the same disadvantages during use.
A further disadvantage of the riser-tensioners according to the prior art is that the cable in the riser-tensioner is bent alternately to the left and to the right during winding of the successive pulleys. This alternate bending increases the fatigue to which the cable is exposed.
A further object of the riser-tensioner according to the present invention is to position the cable pulleys in such a way that the cable placed around them can be bent in only one direction.
It is advantageous here for the cable pulleys on said vessel or said platform to be positioned in such a way that the cable is bent in only one direction while it is being placed around the cable pulleys. The advantage of this measure is that the cable is no longer bent in various directions relative to its longitudinal axis. That means that the fatigue stress on the cable—and thus the chance of giving way—is considerably reduced.
It is advantageous here for the riser-tensioner to be fastened below the drilling floor. In this case it is advantageous for the tensioning means of the riser-tensioner to be formed by cylinders, and for the cylinders to be placed in such
Huisman Special Lifting Equipment B.V.
Marcelo Emmanuel M.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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