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Top tension riser systems developed for ATP’s Telemark Hub

Posted on 11 May 2010

ATP Oil and Gas’ Telemark Hub in the Gulf of Mexico is being developed with a new deep draft floating production system, a triangular design with columns that provides the motion characteristics of a spar but with the wave transparency of a semisubmersible. The top tension riser system was developed for drilling risers and production risers with dry trees and includes standard riser joints, specialty joints and an innovative riser tensioning system. “The robust mooring of the Telemark Hub is suitable for dry trees,” said David L Garrett with Stress Engineering Services.

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 tensioner module is supported by the hull rather than the deck, and the same tensioner is used for all risers, with nominal tension value adjusted to the design value of the particular riser. The system is 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 riser tensioner module is supported by the TTR frame. The riser tensioning system supports the risers from a tensioner deck that is supported vertically on the four cylinder rods and laterally by two guide posts. 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.

“Titan has slots for six dry tree risers,” Mr Garrett said. “Three risers are planned for the initial development.”

Riser design followed 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 producing zones using the dry tree risers have a maximum shut-in tubing pressure (SITP) of 8,000 psi. The BOP, tree and other equipment are rated for 10,000-psi service, Mr Garrett noted. The production risers were designed for a minimum service life of 20 years while drilling risers were designed for a minimum service life of five years. The drilling risers will be pulled after the initial wells are drilled and sent for inspection and storage until they are needed again.

Configuration

Specialty joints include a tapered stress joint with a wellhead connector at the bottom, a keel joint where the riser passes through a guide at the keel, a centralizer joint inside the TTR frame, and a tensioner joint at the top to connect to the riser tensioning system. The keel joint and centralizer joint have two identical segments joined by a flange in the middle.

The standard joints for the 21-in. drilling riser are X-80 line pipe with welded-on connectors. The joints for the 10 ¾-in. outer production risers are 95-ksi casing with threaded connectors as is the 7 5/8-in. inner production riser. The threaded connectors are specially designed for riser service, including modifications to enhance fatigue performance, which were established by testing.

“There is a substantial margin for fatigue,” Mr Garrett emphasized.

Helical strakes are attached to the standard riser joints over the upper 1,000 ft to suppress VIV.

Performance

The primary loading on the risers is from motions on the hull. “Heave is negligible in a 10-year sea state,” Mr Garrett said, “and small in a 100-year sea state with motion characteristics similar to a spar.”

The stroke range is only about half of the riser’s 26.5-ft stroke limit, Mr Garrett noted. 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 was checked for all cases following API RP 2RD, and the maximum stress for all operating and extreme cases easily meets the recommended practices’ limits, Mr Garrett noted. 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.

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