J-Tube Design for Flexible Umbilicals

ABSTRACT The design of J-tubes with one or more bends under functional loads is dictated by a single force TL, the cable tension at the bottom of the J-tube. This facilitates the analysis of J-tubes using standard beam theory or, for complex shapes, a simple computer model. J-tubes experience both pull-in and pull-out loads in practice, and the latter govern J-tube design because the force TLis maximized. 1. INTRODUCTION Flexible umbilicals comprising power, signal and fluid lines have found increasing use offshore in the control of satellite wells and subsea isolation valves. They are frequently brought into Topsides through steel J-tubes pre-installed in the jacket. Previous papers [1] have predicted pull-in loads for single bend J-tubes, but addressed the installation of a steel flowline rather than a flexible umbilical. The load required to pull an umbilical through a multiple bend J-tube is appraised in the present work; however, the stress analysis of the J-tube itself is the primary concern. A design method is developed, critical loadings other than pull-in are identified, and an optimum J-tube support configuration is highlighted. Throughout, it is assumed that the umbilical behaves essentially as a cable. A limiting flexural stiffness for the umbilical, beyond which the approach is not applicable, is estimated. 2. J-TUBE OPERATIONS J-tubes are subject to both pull-in and pull-out loads, as indicated in Fig. 1. The primary function of the J-tube is to enable the umbilical to be installed by pulling it up from seabed through the J-tube. This is performed using a winch located above the J-tube, which draws a cable attached to the nose of the umbilical. Prior to installing the umbilical, the interior of the J-tube is generally protected by a diaphragm or plug at each end. 'l'& prevents free inflow of water into the J-tube and retards corrosion. Depending on its design, the lower seal can be removed by pulling outward, or inward through the J-tube. One form of proprietary rubber diaphragm is shown in Fig. 2. During fabrication, a messenger line is passed through the J-tube and attached to the diaphragm. In-situ, the umbilical can be connected to outer side of the diaphragm just prior to pull-in. Winching in the messenger line from Topsides first breaks the diaphragm, and then pulls in the umbilical. Umbilical installation methods vary, depending on the Operator. Another commonly used method involves the J-tube messenger line being pulled out of the J-tube by a winch on the lay vessel. The cable is then connected to the umbilical nose on the vessel deck to enable umbilical pull-in. A defective umbilical would be removed by pulling it out of the bottom of the J-tube. Additionally, the umbilical maybe pulled out in-service due to accidental snagging from anchors or trawling. To summarize, the following operations should be addressed in J-tube design:umbilical pull-inbottom seal pull-in/pull-outumbilical/cable pull-out