Ultra-Deep TLPs

The use of TLPs has traditionally been considered to be depth limited based on tendon stiffness requirements. If the TLP heave/pitch/roll periods become longer that 4-5 seconds, the system is susceptible to direct wave energy at resonance, leading to motions and fatigue difficulties. As the tendons become longer, they need to be larger in cross section to maintain the same stiffness. At some depth, the weight of the tendons becomes excessive both in terms of lost payload and in terms of cost. The use of composite materials instead of steel for tendons shows promise in terms of weight savings, but has not solved the excessive cost issue, nor been fully accepted on an equivalent reliability basis.

A dynamic oscillation suppression system based on classic vibration absorber principals has been developed, tested and patented by Atlantia. Work is progressing to incorporate this oscillation suppression system into an ultra-deepwater TLP. The oscillation suppression system allows the TLP to operate safely and effectively with longer heave/pitch/roll periods, permitting the use of much lighter tendons. This will enable the use of steel tendons at water depths of 2400-3000 meters, and may also enhance the use (and lower the cost) of composite material tendons to even greater depths.

A new four column triangular hull shape for an ultra-deepwater SeaStar has been developed, and is being prepared for testing at the Offshore Technology Research Center in May 2005. In a parallel effort, a more traditional square four column TLP is also being developed to use the oscillation suppression system for applications with higher well counts.