Motion Monitoring of a Moored Floating Platform During Installation Over a Subsea Template
Authors: Stahl B. , Nelson W.E. 
Published: December 2007

Summary
This paper describes a unique approach to the acquisition and analysis of dynamic motion response data for a large North Sea platform jacket during docking with a subsea drilling template. The motion-monitoring system, the resultant in-ocean measurement of dynamic response, and the capacity for onsite decision making through comparison of measured data with limiting operational criteria are examined. Comparisons are made between measured jacket responses and those predicted by model tests and analytical techniques, and guidance is provided for similar future designs and operations.

Introduction
Amoco's Northwest Hutton jacket was successfully installed and docked with its subsea drilling template in 144m of water in late Sept. 1981. The Northwest Hutton field is located approximately 160 km northeast of the Shetland Islands in the U.K. sector of the North Sea. The template was installed 2 years earlier as part of an early production system that permitted several production production system that permitted several production wells to be drilled with a semisubmersible while the jacket was being fabricated. Precision mating of the jacket to the template was required to facilitate tieback of these wells to the surface. The docking maneuver utilized a mooring system integrated into the jacket to provide a controlled horizontal approach to the template (Fig. 1). Following a conventional launch and upending operation, the jacket was ballasted to rest on the seafloor near the template. A winch and control package was installed and the anchor mooring system was deployed. The jacket was then deballasted a few meters off the seafloor and winched toward the template with the aid of a position-monitoring system consisting of TV cameras and acoustical, electrical, and mechanical sensors, and was finally positioned against two previously installed bumper piles. The jacket previously installed bumper piles. The jacket was securely latched onto each pile and fully ballasted to the seafloor to conclude the docking operation. A detailed presentation of the docking operation is given in Ref. 1. Position monitoring of the subsea drilling template is described in Ref. 2. Engineering of the docking operation began in 1979 with a series of model tests designed to evaluate the mating concept and to help select a viable mooring configuration. Subsequent model testing and computer analyses were performed in 1980 to assess the dynamic response of the moored jacket to various seastates. This information was used to design the bumper piles to withstand the impact loads during docking and to assess the limiting motion characteristics of the jacket to ensure a safe docking operation. Given the limiting motion characteristics of the jacket, a means for evaluating the platform motions onsite during the docking installation was needed to ensure that the operating limits would not be exceeded, rather than relying purely on the judgment of the installation superintendent. The motion- monitoring system described in the paper provided the onsite motion evaluation to be used as a decision-making aid. In addition, the motion data measured during the docking installation were used for subsequent comparison with motion responses of the jacket as predicted by model tests and computer simulations.
JPT
p. 1239