Downhole apparatus for oscillating a tubular string

A method for oscillating a tubular string in a wellbore, the method comprising:

• (i) flowing a fluid through a flow path of the tubular string, the tubular string comprising a downhole apparatus, the downhole apparatus defining the flow path therethrough and comprising:
• a housing;
• a first mandrel disposed within the housing and moveable between a first extended position and a first retracted position for increasing and decreasing a length of the apparatus, wherein the first mandrel is biased toward the first retracted position by a first biasing member;
• and
• a second mandrel disposed within the housing and moveable between a second extended position and a second retracted position for increasing and decreasing the length of the apparatus, wherein the second mandrel is biased toward the second retracted position by a second biasing member;
• (ii) creating a single pressure pulse in the fluid flowing through the flow path and receiving the pressure pulse in a cavity interposed between the first and second mandrels;
• and
• (iii) moving the first mandrel toward the first extended position in a first axial direction and moving the second mandrel toward the second extended position in a second axial direction, opposite the first axial direction, under an action of the pressure pulse in the cavity.

A drill string comprising a downhole apparatus, the apparatus defining a flow path therethrough and comprising:

• a housing;
• a first mandrel disposed within the housing and moveable between a first extended position and a first retracted position for increasing and decreasing a length of the apparatus, wherein the first mandrel is biased toward the first retracted position by a first biasing member;
• a second mandrel disposed within the housing and moveable between a second extended position and a second retracted position for increasing and decreasing the length of the apparatus, wherein the second mandrel is biased toward the second retracted position by a second biasing member;
• and
• a cavity interposed between the first and second mandrels, the cavity forming part of the flow path,
• wherein the first mandrel is configured to move toward the first extended position in a first axial direction and the second mandrel is configured to move toward the second extended position in a second axial direction, opposite the first axial direction, under an action of a single pressure pulse in the cavity.