The Casing Cage Concept For Deepwater Structures (1993)

ABSTRACT This paper introduces the casing cage concept and discusses the feasibility of using a casing cage to provide lateral support to the well system casings in compliant towers and deepwater jackets. The casing cage provides a solution to the problem of separating the structural functions of the platform from the well system. INTRODUCTION What is a Casing Cage? The casing cage is a structure which provides lateral support to the well system casings (or conductors) [1]. The casing cage is framed nearly independently of a Compliant Piled Tower (CPT) or other deepwater jacket structure, and is located within an open area in the main structure of the platform. It is attached rigidly to the platform only at the top end, with all other attachments being slip guides that limit the horizontal movement of the casing cage, but allow vertical movement of the casing cage relative to the surrounding structure. Figure 1 shows the casing cage constructed in two sections for a 2500 ft. CPT platform. The casing cage consists of four legs with casing guide panels welded to the casing cage legs at intervals selected to provide adequate casing stability. The upper two casing guide panels have a different configuration from the others because these panels must withstand greater casing wave loading. The casing cage legs rest in guides at each CPT horizontal framing elevation. These guides allow transfer of lateral loads only. The entire casing cage is suspended from the CPT with struts located near the top of each section. The casing cage shown in Figure 1 is slightly buoyant for in-place conditions. Thus, the struts extend from the cage to the lower CPT horizontal level in order to maintain tension in the struts. Additional design refinements may suggest that a neutrally buoyant or ballasted casing cage configuration may be desired. Figure 2 shows the location of the casing cage in the upper CPT section. Why Use a Casing Cage? The casing cage solves a significant problem associated with conventional casing (or conductor) support systems. A conventional casing Support system is framed continuously to the CPT or deep water jacket. Previous detailed CPT analyses showed that casing support members attracted significant loads for launch and in-place conditions. The result was that the size of the casing support members had to be increased to meet allowable stress criteria. In the past, typical design practice was to ignore the secondary casing support members (i.e., not include them in analytical computer models). This practice appeared acceptable for shallow water structures. However, detailed deepwater CPT analyses showed that the casing support members attracted significant loads resulting in inefficient member sizes. The need to solve this problem led to the development of the casing cage. Figure 3a shows a CPT horizontal elevation with conventional casing support members directly framed to the platform. Figure 3b shows the same horizontal elevation when a casing cage is used. The slip guides for the casing cage legs are evident in Figure 3b. Note in Figure 3b that CPT horizontal elevations and casing cage guide framing elevations are never in the same plane.