Top Tensioned Riser Systems for a New Floater Type
Authors: Garrett David L. , Shivers Robert M. , Frazer George Ross , Trent David , Massaglia Jacky 
Published: July 2010

The Telemark Hub is being developed with a new design deep-draft floating production system [1]. This paper describes development of top-tensioned drilling risers and production risers with dry trees. The riser system includes standard riser joints, specialty joints and an innovative riser tensioning system. For the drilling and production phases, up to three risers may be used:a 21 in. low pressure (5,000 psi) riser,a 14 in. intermediate pressure (7,500 psi) riser, anda 10-3/4 in. high pressure (10,000 psi) riser. The single barrier drilling risers have a surface blow out preventer (BOP) 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 18-3/4?? blind/shear rams and is operated from the surface.

The dual casing production risers have a 10-3/4 in. outer riser and a 7-5/8 in. inner riser.

The ram style hydro-pneumatic riser tensioning system is designed for a nominal tension of 1,200 kips and a stroke of 26.5 ft. The tensioner module is supported by the hull rather than the deck. The same tensioner is used for all risers, with the nominal tension value adjusted to the design value for the particular riser. The system was designed for post-Katrina hurricane criteria, including robustness checks for the 1,000 year return period hurricane.

Introduction

The Telemark Hub is in Mississippi Canyon block 941 in approximately 4,000 ft water depth. Titan is moored with 12 taut polyester mooring lines as illustrated in Figure 1. A top-tensioned riser (TTR) support frame is located near the top of column. The riser tensioner module, illustrated in Figure 2, 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 is supported 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 the riser is connected to an 18-3/4 in. wellhead profile with a hydraulically actuated connector. Titan has slots for six dry tree risers; three risers are planned for the initial development.

Global performance of the system was analyzed using coupled analysis [2]. The hull, the mooring, and the risers were included in a single model. Results from the global model also served as a check of global motions and mooring analysis performed by others. The global model provides loading for detailed design of the riser and riser tensioning system components.

Riser design requirements follow API RP 2RD [3]. System robustness was assessed for post-Katrina hurricane conditions including the 1,000 year return period hurricane [4, 5]. Site specific storm and current conditions were established for design. Environmental loading conditions for both strength and fatigue included storms, vortex induced vibrations (VIV) due to current acting on the risers, and vortex induced hull motions (VIM) due to current acting on the hull. Engineering criticality assessments (ECA) were performed to provide guidance for inspection during fabrication and to establish that the inspection interval exceeds the service life.

The producing zones planned to be produced 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. Production risers were designed for a minimum service life of 20 years. Drilling risers were designed for a minimum service life of 5 years. The drilling risers will be pulled after the initial wells are drilled and sent in for inspection and storage until they are needed again.