Historically, DP units have not been permitted to carry out these functions in shallow water because the critical well components can be overloaded very quickly, leaving insufficient time to activate the emergency disconnect system. Today, advanced tethering technology greatly reduces the consequent bending to the critical well components and provides a means of safely operating DP drilling units in shallow water.
TVO’s Hercules high-strength BOP system is designed to resolve these concerns by delivering sufficient strength and stiffness to resist loads caused by rig movement. By shifting the weak point to the riser system above the BOP, the system protects the well system below the mudline from overloading in the event of a drift-off, ensuring the BOP is never unstable and that the capacities of the wellhead and casings are never exceeded. The system functions even if the lower marine riser package (LMRP) fails to disconnect.
Once the components of the system are connected, the system is passive. If a drift-off occurs, any load increase transfers to the tensioners and suction piles, which hold the BOP in position and ensure well integrity is not compromised.
Case study
An international operator recently applied this technology in the Gulf of Mexico (GoM) for a well intervention program for four exploration wells that were drilled in the 1990s.
Before work began on the site, riser analysis was performed to prove the concept, and BOP drill-through engineering analysis was carried out to prove the rig could support the system.
A geotechnical study also was done to assess the suction pile anchoring system’s capacity to hold the floater in place.
Modeled results showed that an untethered system reached a weak point at a vessel offset of 28.1 ft (8.5 m) in only 83 seconds. With the tethered BOP (TBOP) system installed, it took 171 seconds to reach a weak point at an offset of 114 ft (35 m). These results gave the operator confidence that the high-strength tethering system would protect the well as designed.
The next step was to ensure the appropriate equipment could be secured and develop well-specific operating guidelines to define safe operating limits for the DP drillship that would carry out the work.
The system installed comprised four dyneema tethering systems designed for an operational load of 200 metric tons with a 300-metric-ton survival load anchored to the seabed by 9.5-ft suction piles fitted with 4-ft wings to increase their lateral loading capacity. Because the shallow-water area of the GoM where these wells are located is a major pipeline corridor, anchor placement had to be extremely precise to provide the required support while avoiding existing infrastructure.
