Deepwater drilling using a passive compensator

The simulation highlights several interesting features of the compensator, and three are of particular interest. The first simulation graph highlights a rapid fluctuation in the load variation while drilling on a very hard formation. This effect had been observed while drilling on hard formation in the North Sea. It was commonly referred to as bit bounce and was usually ascribed to the rotation of the drill bit. In this simulation there are no extraneous disturbance inputs such as the turning of the drill bit. Indeed, these fluctuations may be likened to the contact instability, which is described in the literature and can simply be said to arise due to local resonance. This local resonance is triggered by the non-linear nature of the compensator stick-slip or Stribeck friction. There are several ways of dealing with the problem such as modifying the compensator seal friction to avoid the Stribeck effect or modifying the compensator behavior or impedance using an active sub-system. A more conventional way is to modify the drill string stiffness to avoid this condition. This could be achieved by including one or more bumper sub-type devices at suitable points along the drill string. However, the bumper sub characteristics might have to be altered depending on the changing circumstances or by using a bumper sub designed for the task. The simulation illustrates that the compensator load variation is mainly a function of the seal friction. Flow friction used to be a problem in earlier compensators, but is no longer. The size of the air bank is important mainly during high heave conditions but can be reduced during moderate to low heave as typically practiced by drillers to conserve high-pressure air. Present seal technology is good and may not readily be improved in a practical format. There are various other possibilities, for example rotating the piston rods may not be practical, but adding dither might help, as could using an active sub-system with purpose-designed algorithms to reduce the friction effect. The second graph clearly illustrates that the conventional method for measuring load and deriving the load variation by using a sensor at the dead-line anchor differs significantly from the actual variation in the drill string. This may not be entirely surprising as the wire rope system linking the crown and traveling blocks will have multiple resonance frequencies, which will influence the measurement. This does illustrate the need to pick a better measurement point, such as using a load cell in the direct load path - for example - below the traveling block. This simulation study illustrates and quantifies some of the problems associated with the present passive compensators and points to measures which may lead to improvements in their performance.