Cellular Tendon - Enabling Technology for Ultra-Deep Water TLPs
Authors: Yu Zhuoming Jim 
Published: June 2014

Abstract
The cellular tendon concept described in this paper is an enabling technology proposed for Tension Leg Platforms (TLP) to meet the industry's demands for producing oil and gas in ultra-deep waters beyond 1500 m. The technical merits of the concept for ultra-deepwater field development have been demonstrated in this paper using TLPs with representative large, medium or small payloads in water depths between 1,500 and 3,000 meters. The technical readiness is enhanced by using mature industry products for tendon components, and by adapting existing industry practices to construct and install the Cellular Tendons. The Cellular Tendon has advantages over conventional tendons in technical robustness and economics of the project, as well as enabling/enhancing local fabrication content.
The intellectual property of this design concept is protected under a pending global patent.

Background
Historically, TLPs are ideal platforms for deep water drilling and oil and gas production with dry trees worldwide. Currently there are over a dozen TLPs installed in regions including the GOM, Southeast Asia, and West Africa. Several TLPs are planned for installation in the GOM, North Sea and Brazil in the near future. The application of TLPs with a conventional tendon system has reached technical and economical limit for water depths beyond 1,500 meters. For a TLP in ultra-deep water, the technical and commercial practicality is constrained by the feasibility of the tendon main body and it meeting stiffness and collapse requirements. Also, the cost of conventional tendon installation in certain regions is sometimes prohibitive for the commercial viability of the project.

Design of the Cellular Tendon System
The Cellular Tendon concept has the same main components as a conventional tendon. A conventional tendon consists of three major parts: a tendon top segment (TTS) interfaces with the platform, a tendon bottom segment (TBS) connects to the tendon foundation at the seafloor, and a main body that links the two through the water column. The Cellular Tendon innovation resides in the design of the main body. Instead of one pipe, it consists of multiple metallic tubules. It also has an upper transition unit to interface with the TTS and a bottom transition unit to interface with the TBS. The top and bottom interfaces—TTS and TBS--are kept the same as in conventional tendons. The same tension-monitoring units in the conventional tendons can be used in the Cellular Tendons as well.
Figure 1 is the elevation view of the TLP moored to seabed by the tendons. The tendons are connected to the tendon porch through the top interface and the foundation through the bottom interface.
Figure 2 is the schematic of the Cellular Tendon system. Unlike the single carbon steel pipe used as the main body in a convention tendon design, the main body consists of multiple metallic tubular strings. Each individual string is composed of pipes butt welded at their ends. All the strings are arranged in parallel and assembled on-shore. The pipe material can be carbon steel or aluminum. Conventional carbon steel pipes have been used in many existing TLP tendons are considered mature industry practice and are preferred to be used first in the field development. The number of strings in the main body is designed to meet the stiffness requirements, strength requirements, fatigue requirements, and to be practical for fabrication and installation. The outer diameter and/or wall thickness of the pipes can vary along the string. In general, the OD of the pipes is smaller and /or the pipe wall thickness is increased for those segments at greater water depths.