Riser Recoil Analysis
During marine riser operations it is sometimes necessary to perform an Emergency Quick Disconnect (EQD) to release the Lower Marine Riser Package (LMRP) from the BOP.
This is particularly important for a Dynamically Positioned (DP) vessel which can experience an accidental loss of control or power and the vessel either drifts or drives off location with the riser still connected. Under this condition an EQD would be performed to protect the wellhead and ensure full integrity of the well barriers in the BOP. With the speed of this operation there is no time to replace heavy drilling mud in the riser with seawater or to reduce tension for a controlled disconnect.
The EQD occurs under full tension with whatever contents are in the riser. For deep water drilling the overpull tension required to support the riser full of heavy mud can be very high and during EQD the tension is released very quickly causing the riser to recoil. At the same time the column of heavy drilling mud in the riser collapses as it falls out the bottom of the riser through the LMRP. This creates a very high vertical acceleration of the riser equipment.
What are the effects of riser recoil?
The effect of the riser recoil can be very damaging at surface to the tensioning equipment. If the riser vertical acceleration is too high during EQD then the Telescopic Joint (TJ) can fully collapse. Furthermore, the high acceleration can cause disengagement of the TJ lower barrel with the tensioner ring (i.e. jump through) or the tensioner lines can slacken and jump off the sheaves. If the tension is provided by Direct Acting Tensioners (DAT) then high riser acceleration can collapse the hydraulic cylinders. All these issues relating to high tension can cause damage to the equipment at surface. However, if the riser tension is too low then damage can also occur to the subsea equipment. If the speed of recoil of the LMRP is too low, then the LMRP can actually contact the BOP under vessel downward heave motion.
Why is Riser Recoil Analysis important?
Riser Recoil Analysis is performed to determine the optimum range of riser tension which can be safely accommodated to prevent damage throughout the system during EQD. AS Mosley generates a detailed global model of the marine riser and tensioner system including a representation of the collapsing mud column. Anti-recoil flow control valves are also incorporated with specific closure control algorithms for EQD. By correct specification of riser tension range the analysis will:
- Prevent full collapse of the telescopic joint which would cause impact at the end stops.
- Prevent disengagement of the lower barrel of the telescopic joint with the tensioner ring (i.e. jump through).
- Avoid slack tensioner lines and prevent them jumping off the sheaves.
- Avoid Collapse of Direct Acting Tensioner (DAT) hydraulic cylinders causing impact at the end stops.
- Prevent contact of the LMRP with the BOP under vessel downward heave motion during EQD.
LMRP lift-off at different points through wave cycle
Riser recoil simulation
Determination of the optimum system tension range through global analysis will help protect both personnel and equipment and minimise disruptions to operations.
For more information on Riser Recoil Analysis contact AS Mosley directly.