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Well Control on a floating rig nnn

slip joint and riser in the moon pool on a semi-submersible drilling rig

Well Control on a floating rig can present special problems that are not encountered on land or fixed bottom rigs. These problems can be summarised as follows.

Kick Detection can be more difficult due to the motion of the vessel. Mud surges in a flow line or fluid movement in mud pits can make the detection of small inflows into the well very difficult to detect.

Fracture Gradients. Fracture gradients are a function of overburden and formation fluid pressures. As water depth increases the effective overburden pressure, as measured from the rig floor decreases, therefore the pressure required to fracture the formation will also decrease with water depth. Increased water depths reduces the pressure differential between the mud weight required to control formation pressure and the pressure that will cause losses to the formations.

A Land Rig. Casing is set 6000 ft from RKB. A 12 ppg mud is required to give a suitable overbalance at 8000 ft. The fracture of the rock is 0.79 psi/ft. If a kick was to occur, the maximum pressure without losses (MAASP) on the choke gauge at surface would be 996 psi.

Land Rig, MAASP

A Floating Rig (shown below) in 900 ft. of water, with the same shoe depth and fracture gradient, the 12 ppg mud would give a maximum surface choke pressure of 624 psi.

Floating Rig, MAASP

In 2.970 ft of water, (shown below) with the same shoe depth and fracture gradient. It would not be possible to circulate 12 ppg mud without losses. A 12 ppg mud would give zero tolerance, as it almost equals the pressure required to fracture the formations

Floating Rig, MAASP 2