West Seno Drilling and Production Riser Systems
Authors: Utt Michael E. , Emerson Eric , Yu Jim 
Published: October 2008

Abstract
West Seno TLP-A is the first application of several design features, including:Fixed (i.e. non-stroking) tensioners for the production risersA drilling / production riser design in which 90% of the length uses threaded & coupled connectionsTender-supported drilling operations on a tension-leg platform The paper will describe these and other innovations in the riser systems, which are used for both drilling and production.

Application:
The West Seno TLP-A platform will support 28 top tensioned well risers in a 4 × 7 pattern. The design of the platform includes dry trees, accommodates tender assisted drilling, and supports a workover rig.
The mild oceanographic and meteorological environment of the Makassar Strait makes it feasible to tension the TLP's risers with a "fixed' tensioner. That is, tension is applied to the top of the riser, then the position is fixed relative to the deck of the TLP, and riser tension varies with the tide level and the lateral position of the TLP. In locations where this design is appropriate, the savings can be significant.

Significance
The design of riser systems for floating production systems in mild oceanographic and meteorological environments, such as the Makassar Strait, can take advantage of the environment by using innovations such as "fixed" tensioners and threaded and coupled tubular connections. The savings from more conventional designs for more severe environments can be significant.

Well Design
A typical well for the West Seno development includes:3-1/2-inch production tubing7-inch production liner to TD9-5/8-inch liner, with polished-bore receptacle (PBR)9-5/8-inch tieback, ending in the PBR13-3/8-inch casing (run with the drilling/production riser)20-inch drilling liner30-inch (nominal) driven conductor, with 27 - 3/4-inch ID at mudline13-3/8-inch drilling/production riser

Riser Design
The design of the risers follows from the design of the wells. The design internal pressure for the riser and for the tieback is 6,316 psi. That requirement is met by using 13-3/8-inch, 68 pound/foot, grade P-110 casing for most of the length of the riser.
The Makassar Strait is characterized by a large (about 10 Sverdrup) persistent North-to-South through flow current. If one were to consider only waves, the Makassar Strait might be thought of as a lake. Considering the currents, it is better thought of as a river. The flow through the Makassar Strait is an order of magnitude larger than the flow of the Mississippi River.
The persistent currents are often large enough to cause significant vortex-induced vibration (VIV) of drilling or production risers deployed in the Makassar Strait.
Figure 1 shows profiles of the estimated 1-year, 10- year and 100-year return period currents for the West Seno site. Maximum surface currents exceed 3 knots. The water depth (horizontal scale in the figure) is about 3300 feet at the TLP-A site.
Figure 1 Seno Extreme Currents (Available in full paper)
Other than the design internal pressure, every design feature of the West Seno drilling/production risers is controlled by fatigue, and most of the driving energy for fatigue comes from vortex-induced vibration.