Patent Application
20080175672
This invention relates in general to offshore risers for oil and gas wells, and in particular to a dog-type drilling riser wherein the dogs are arranged in multiple tiers at different elevations.
Drilling risers are commonly used for offshore oil and gas well drilling operations. A drilling riser is made up of segments or joints that are secured to each other and lowered into the sea from the drilling vessel. Different types of connectors between the riser segments are used. One type employs bolts for bolting flanges of the mating connectors to each other. Another type has a pin member on one end that inserts into a box member of the next riser segment. Dogs are spaced around the box, each dog being radially movable into engagement with a grooved profile on the pin. Normally, a screw for each dog causes the radial movement when rotated.
Operators are drilling wells in increasingly deeper waters. Deeper water places more tensile loading on the riser segments and their connectors. Also, operators are now proposing to drill with the main blowout preventer on the drilling vessel, rather than at the subsea wellhead as in the prior art. Locating the blowout preventer at the surface requires the drilling riser to be able to withstand higher pressure than in the prior art, which further increases the loading on the connectors between the riser segments.
One solution with a dog-type riser would be to utilize more dogs. However, because a certain amount of supporting metal is required for each dog, in the prior art adding more dogs requires a greater diameter for the drilling riser. Operators prefer to have a slender drilling riser to reduce the expense and the weight of the drilling riser.
The riser of this invention is made up of a plurality of tubular members, each having a box on one end and a pin on the opposite end for connection to the other tubular members. The box has first and second sets of windows, each set of windows being spaced circumferentially about the box. The first set of windows is located at a closer axial distance to an end of the box than the second set. A dog or locking element is located in each of the windows. Each locking element is radially movable inward and outward for engaging and disengaging the pin of one of the tubular members.
In the preferred embodiment, the first windows are staggered around the circumference of the box relative to the second windows. That is, when viewed in sectional planes perpendicular to the riser axis, each of the second windows would appear between two of the first windows. Preferably, the circumferential distance between the windows of the first set is uniform and is the same as the circumferential distance between the windows of the second set. The diameter of the box at the first set of windows is thus the same as at the second set of windows.
In the preferred embodiment, the means for moving the locking elements radially inward comprises a rotatable screw, which is operatively coupled to each of the locking elements. When the screw is rotated, the locking element will move radially.
Each pin has two separate circumferentially extending grooved profiles, one above the other. Each grooved profile extends circumferentially around the pin. The grooved profiles are spaced apart from each other so that one set of locking elements will engage one grooved profile while the other set engages the other.
In the preferred embodiment, the connections are axially preloaded. This is handled by providing each pin with a flange that contacts the rim of the box. The locking elements have ramp surfaces that are angled and positioned so as to provide an axial preload between the flange and the rim when the locking elements engage the grooved profiles.
Box 15 has a greater wall thickness and greater inner diameter than pipe 13. The inner diameter of box 15 is configured to closely receive the pin 17 of the next upper riser segment 11, as illustrated in
Box 15 has an upper tier or set of windows 19. Windows 19 extend through the side wall of box 15 and are elongated in a circumferential direction in this example. Each window 19 has a greater circumferential width than axial height. Each of the windows 19 is located the same distance from rim 20. Also, the circumferential distance between each window 19 is uniform. In this example, there are four upper tier windows 19, each having its center-point 90 degrees from the center-point of an adjacent window 19.
Box 15 also has a second tier or set of windows 21 located below upper tier windows 19. Lower tier windows 21 are preferably identical to upper tier windows 19, but are spaced a farther distance from box rim 20 than first windows 19. Second tier windows 21 are also uniformly spaced, and the circumferential spacing is the same as between upper windows 19. The diameter of box 15 at upper tier windows 19 is the same as at lower tier windows 21. There is the same number of lower tier windows 21 as upper tier windows 19. Upper and lower tier windows 19, 21 are staggered relative to each other, with the center-point of each lower tier window 21 being on a vertical line that extends equally between two of the upper tier windows 19.
An upper connector 23 is mounted to each upper tier window 19. A lower connector 25 is mounted to each lower tier window 21. As shown in
A dog or locking element 37 has a recess on its outer end that rotatably receives engaging end 35. Locking element 37 preferably has a width and height slightly less than one of the windows 21. Locking element 37 moves radially relative to axis 14 (
In the example of
Referring again to
Referring to
In the alternate embodiment of
To connect the riser segments 11, a lower one of the riser segments 11 will be suspended at a riser make-up floor on a drilling rig (not shown). An upper riser segment 11 is lowered downward, and its pin 17 stabs into box 15 of the lower riser segment 11. The operator rotates screws 29 (
The invention has significant advantages. Placing the dogs in two tiers and staggering them relative to each other allows more connectors for a given riser diameter than could otherwise be employed. The additional connectors do not require an increase in box diameter. The two tiers of dogs can exert preload forces on the connection to provide a high-strength, small-diameter riser.
While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, the dogs could be moved radially by other means than a rotatable screw. In one other configuration, an axially movable cam ring could engage cam surfaces formed on the outer ends of the dogs so that moving the ring axially would cause radial movement of the dogs.
