Downhole tools and methods of setting in a wellbore

1. A downhole tool disposed in a drillstring, the downhole tool comprising:

a bypass valve unit comprising at least one bypass port, wherein the bypass port is configured to control fluid flow from a central bore of the drillstring to a wellbore annulus;

an activation sub having a frangible obstruction disposed in a central bore thereof and configured to prevent fluid flow to an anchor downhole during a circulation mode,

wherein the frangible obstruction is configured to break at a specified fluid pressure in the central bore to allow fluid communication to below; and

a first bleed port located in an outer wall of the bypass valve unit and configured to manage fluid pressure in the central bore after the at least one bypass port is closed.

2. The downhole tool of claim 1 , wherein the frangible obstruction is configured to break at between about 2,500 and 3,000 psi.

3. The downhole tool of claim 1 , further comprising a measurement-while-drilling tool disposed in the drillstring.

4. The downhole tool of claim 1 , wherein the at least one bypass port is closed prior to breaking the frangible obstruction.

5. The downhole tool of claim 1 , wherein fluid in the circulation mode circulates at a flow rate of about 200 gallons per minute.

6. The downhole tool of claim 5 , wherein the bypass port is configured to cycle in response to an increase in the flow rate.

7. The downhole tool of claim 1 , wherein the frangible obstruction is configured to break in response to a pressure increase.

8. The downhole tool of claim 1 , wherein the activation sub is disposed downhole of the bypass valve unit.

9. The downhole tool of claim 1 , wherein the bypass valve is configured to cycle in response to a circulating fluid flow rate increase.

10. The downhole tool of claim 1 , wherein the frangible obstruction comprises a burst disc.

11. The downhole tool of claim 1 , wherein the frangible obstruction comprises a shear out plug.

12. A method of setting an anchor in a wellbore, the method comprising:

circulating a fluid from a central bore of a drillstring through a bypass port in a bypass valve unit and into a wellbore annulus;

disposing a frangible obstruction in a central bore of a drillstring downhole of the bypass valve unit, wherein the frangible obstruction is configured to prevent the fluid from flowing down the central bore of the drillstring;

closing the bypass port, wherein fluid from the central bore is prevented from flowing into the wellbore annulus; and

increasing the fluid pressure in the central bore of the drillstring and breaking the frangible obstruction, wherein fluid is allowed to travel down the central bore past the frangible obstruction to set the anchor; and

controlling the increased fluid pressure in the central bore after closing the bypass port with a first bleed port.

13. The method of claim 12 , further comprising increasing the fluid pressure in the central bore of the drillstring to set the anchor in the wellbore.

14. The method of claim 12 , further comprising running the downhole tool in the wellbore with a measurement-while-drilling tool.

15. The method of claim 12 , further comprising increasing a flow rate of the circulating fluid in the central bore to slide a piston of the bypass valve unit and close the bypass port.

16. The method of claim 12 , further comprising breaking the frangible obstruction at between about 2,500 and 3,000 psi.

17. The method of claim 12 , wherein the frangible obstruction comprises a burst disc.

18. The method of claim 12 , wherein the frangible obstruction comprises a shear out plug.