The top tension riser system is developed for drilling risers and production risers with dry trees.
The single-barrier drilling risers have a surface BOP (SBOP) and a subsea isolation device (SID) to provide emergency shut-off in case of a breach or failure of the drilling riser. The SID consists of two 18 ¾-in. blind/shear rams and is operated from the surface.
The hull or the deck can support the tensioner module. The same tensioner can be used for all risers, with nominal tension value adjusted to the design value of the particular riser. The system should be designed for post-Katrina hurricane criteria, including robustness checks for the 1,000-year-return period hurricane.
A top-tensioned riser (TTR) support frame is located near the top of the column. The TTR frame supports the riser tensioner module. The riser tensioning system supports the risers from a tensioner deck that is supported vertically on the four cylinder rods and laterally by two guideposts. The risers extend through the TTR frame and are laterally supported near the keel by a guide tube. The bottom of the riser is connected to an 18 ¾-in. wellhead with a hydraulically actuated connector.
Riser design should follow API RP 2RD. Environmental loading conditions for strength and fatigue included storms, vortex-induced vibrations (VIV) due to the currents acting on the riser, and vortex-induced hull motions (VIM) due to currents acting on the hull.
The primary loading on the risers is from motions on the hull.
For operating conditions, stroke generally is about 1 ft. In the 1,000-year hurricane robustness case, the tensioner may bump the down stop, producing additional tension in the riser.
Riser stress will be checked for all cases following API RP 2RD, and the maximum stress for all operating and extreme cases easily meets the recommended practices’ limits. Fatigue damage from all sources, including storms, VIV and VIM was calculated, and, for a 20-year service life, the calculated fatigue life should be greater than 200 years. The calculated fatigue life of each of the risers exceeds 1,000 years.