Development of an Automatic Reentry Positioning System

ABSTRACT This paper describes the concept, design, analysis and sea trials of the reentry system, and particularly its positioning system. Its development was conducted aiming? for a new reentry positioning system which enables accurate and rapid positioning with no assistance from either divers or guidelines. Thrusters, hydrophones and a TV camera are fitted £0 the bottom end of a reentry riser string, and remotely controlled by a computer unit on the vessel. The dynamic characteristics of the reentry system were predicted by means of a computer simulation. An offshore pilot test was conducted in the Seto Inland Sea of Japan in 1984, using the semi-submergible offshore rig "HAKURYU 11", and the technical-viability and effectiveness of the system were confirmed. INTRODUCTION Up to now, for normal deployment, a wellhead assembly is run on the completion riser, located above a wellhead by controlling a vessel's position, and then installed on the wellhead. Guidelines and divers often assist the positioning of the wellhead assembly. However, in a deep sea where no assistance from either divers or guidelines is expected, it is difficult to locate the wellhead assembly accurately by controlling the vessel's position. Because there is flexure of the riser string caused by currents, and the position of the wellhead can not be accurately detected by the vessel's sonar system over a long distance. Furthermore, it is also difficult to conduct quick positioning because vessel's movement is reduced by flexure of riser string. The reentry system described in this paper has been developed to solve this problem, by employing a system which has a position detecting system and thrusters at the bottom of the riser string. DESIGN CONCEPT Fig. 1 shows the structure of the reentry system. The thruster unit, which has hydrophones, thrusters and a TV camera, is connected to the bottom end of the reentry riser string, and a wellhead assembly is fitted to the thruster unit with its running tools. After the wellhead assembly is submerged at the distance about 2 m above the wellhead, lateral position control begins. Lateral position control is conducted automatically by a computer-control system on the vessel. This system processes the position data of the wellhead (transponders) relative to the thruster unit which is detected by the hydrophones, and remotely controls the thrusters. Consequently, the thruster unit and the wellhead assembly are positioned right over the wellhead. Vertical position control is conducted independently from the lateral positioning by lowering and lifting the riser string using the draw works on the vessel. SYSTEM REQUIREMENTS The requirements made for the reentry system are as follows:The reentry riser should have hydraulic control lines and tubing pipes.The thruster unit should be able to travel laterally within the radius of 5 m from its initial position, when it is submerged 300 m below the sea surface.Tolerance for the lateral position control of the thruster unit should be less than 50 cm.