Method of riser deployment on a subsea wellhead
The riser assembly is plugged adjacent its bottom end either as it is made up or after full makeup or anywhere in between. A heavy fluid is either added to the riser string as it is made up or delivered into the string after makeup to displace seawater or other fluids that may have been in the riser as it is assembled. The floating rig can be secured over the subsea well head and the riser now full of heavy fluid is less prone to be displaced by wave action and preferably simply dangles straight down so that the connection to the subsea wellhead can be simply accomplished. The effect of wave action is minimized or eliminated when deciding placement of the floating rig with respect to the wellhead. Repositioning of the rig takes less effort and is needed less frequently.
The field of this invention is riser placement on a subsea wellhead and more specifically when using a floating rig.
BACKGROUND OF THE INVENTIONFloating rigs are connected to subsea wellheads through a string called a riser that spans the distance between the rig floor and the wellhead located just above the sea floor. Floating rigs are normally positioned and secured to hold a selected position while still able to compensate for the rise and fall of the surface from wave action. Before or after a rig is positioned the riser assembly is made up in sections on the rig floor and run in. If the riser is made up before, the currents can be used to guide the rig closer to the subsea BOP. The water depths for some floating rigs can be in the thousands of meters and the wave action at the surface has a component below the surface that can deflect the riser string as it is made up and lowered toward the wellhead. The skewing effect is well known and in the past the position of the floating rig with respect to the wellhead is adjusted to take advantage of the expected lateral forces from wave action so that when the riser is fully assembled, its lower end will be in close proximity to the wellhead so that the two can be stabbed in, or simply connected with divers or remotely operated vehicles (ROVs). The problem lies in the inherent unpredictability of the tidal forces both in strength and in direction. Repositioning of the floating rig is time consuming and may require the riser string to be pulled back onto the rig before it can be repositioned.
The present invention is directed at a technique to minimize the effect of wave action as opposed to the past techniques of simply trying to compensate for its predicted force and direction. Those and other aspects of the present invention will be more readily understood by those skilled in the art from a review of the description of the preferred embodiment and the drawings as well as the claims below that define the full scope of the invention.
SUMMARY OF THE INVENTIONThe riser assembly is plugged adjacent its bottom end either as it is made up or after full makeup or anywhere in between. A heavy fluid is either added to the riser string as it is made up or delivered into the string after makeup to displace seawater or other fluids that may have been in the riser as it is assembled. The floating rig can be secured over the subsea well head and the riser now full of heavy fluid is less prone to be displaced by wave action and preferably simply dangles straight down so that the connection to the subsea wellhead can be simply accomplished. The effect of wave action is minimized or eliminated when deciding placement of the floating rig with respect to the wellhead. Repositioning of the rig takes less effort and is needed less frequently.
In the end, the method allows placement of the rig 10 over the wellhead 12 with little or no concern over lateral deflection of the string 14 away from the wellhead 12 due to current force. The need to guess in advance the deflection effects of currents, which can be highly variable, is greatly reduced or eliminated. The rig 10 is unlikely to need repositioning during the procedure of running the riser 14 and connecting it to the wellhead.
The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.
Claims
1. A method of connecting a riser to a subsea wellhead comprising:
- assembling the riser;
- installing a plug in said riser:
- adding a heavy material in the riser above the plug;
- lowering said riser toward the wellhead; and
- connecting the riser to the wellhead.
2. The method of claim 1, comprising:
- making said riser resist deflection from current forces due to said heavy material.
3. The method of claim 2, comprising:
- positioning a rig closer to being over the wellhead due to reduced riser deflection from current when holding said heavy material.
4. The method of claim 1, comprising:
- installing and setting said plug as said riser is assembled at the surface;
- filling in said heavy material as additional riser sections are added at the surface.
5. The method of claim 1, comprising:
- installing said plug after said riser is fully assembled and lowered;
- displacing seawater in said riser with said heavy material.
6. The method of claim 5, comprising:
- sealing off said riser with said plug after said displacing.
7. The method of claim 1, comprising:
- using a liquid as said heavy material.
8. The method of claim 7, comprising:
- including solids in said liquid.
9. The method of claim 1, comprising:
- using a gel as said heavy material.
10. The method of claim 1, comprising:
- removing said plug after said connecting.
11. The method of claim 10, comprising:
- bringing the plug up the riser with flow from the well.
12. The method of claim 10, comprising:
- drilling or milling the plug to remove it.
13. The method of claim 1, comprising:
- said installing further comprises using an inflatable plug, a mechanically set plug, non-elastomer plug or a wiper plug landing on a seat.
Type: Application
Filed: May 26, 2006
Publication Date: Nov 29, 2007
Inventors: Larry T. Palmer (Spring, TX), David Dolyniuk (Tomball, TX)
Application Number: 11/442,092