The Application of Knee Brace Structure in the Design of Tension Leg Platform

Since its first use in offshore project for drilling and production platform in 1984, more than two dozens of Tension Leg Platform (TLP) structures have been installed in oil and gas fields. The trend for using TLP structure in the exploration of deepwater oil fields has also gained momentum in recent years. The major driving force behind this is because the TLP structure has relatively smaller heave motion, thus it can be used for dry tree application in deepwater. Knee brace has found applications in the design of TLP structures. These vertically diagonal structure members are mainly used to support heavy vertical loads when the column spaces are large. The applications of knee braces have been used for two different stages: the integration stage and the in-place stage. For large topsides, the integration of the platform is usually performed in two or more separate lifting. The knee brace is used to temporarily support the pieces of the topsides before they are integrated together. For conventional type of TLP structures, the top tensioned vertical risers (TTRs) are placed in the middle of the platform to stay away from the mooring system and other subsea systems. The large column-to-column space and heavy TTR loads create some technical and economic challenges for the design of deck structures. To effectively take these vertical loads, knee brace is used to cut down the span of the deck supports. The challenge for this application is how to effectively share the vertical loads while at the same time not to induce much global dynamic action on the knee brace structure. This paper will review the application of knee braces in TLP projects, study the impact of the knee braces to the global action of the hull and topsides structures, and discuss the results of the analysis through a project example. The study will concentrate on the global load distribution through the knee braces, and the interaction between the knee braces and the hull structure under the global load action. Analysis results will be presented and the impact of the knee braces to the global structure will be discussed. These results will show that with proper configuration layout, the knee brace can be designed mainly to take the vertical static load, thus minimize the participation of the dynamic effect. The Author will also discuss the pros and cons of the knee brace structure in TLP application and some key issues which deserve attention during the application.