Active control for installation of underwater structures
KEISUKE WATANABE1, HIDEYUKI SUZUKI2, and KOICHIRO YOSHIDA2
1 Department of Marine Design and Engineering, Tokai University, 3-20-1 Orido, Shimizu, Shizuoka 424-8610, Japan
2Department of Environmental and Ocean Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract： The trend towards deepwater development requires a new approach to underwater installation of offshore structures. The present method using crane vessels has some drawbacks in operations at more than 2000m depth. The natural period of the coupled system of the rigged structure and the crane vessel becomes longer, so that it is no longer possible to manipulate the cranes to achieve the desired positioning accuracy. This paper examines the application of an active control technique for underwater installations as one of the solutions to the present problems. An active control technique also has the advantage that it can deal with the structural flexibility which allows the structure to be large and light-weight. This structural flexibility imposes problems of suppressing the elastic responses and securing the stability of the control system. In this paper, an H∞ controller combined with a low authority control/high authority control (LAC/HAC) feedback controller is designed for cases where structural flexibility cannot be ignored. A robust model-following controller is examined for cases where the structure can be treated as rigid. In order to confirm the control algorithm and verify the possibility of the active control installation method, basin tests are executed using two types of neutrally buoyant
flexible models with ultrasound ranging systems and thrusters.
Key words： H∞ control, flexible structure, underwater docking
Address Correspondence to; K. Watanabe
Received for publication on Feb. 13, 1997; accepted on Oct. 16, 1998
Introduction
A new approach to installing underwater structures will be needed in the near future for deepwater developments whose target water depth is more than 2000m. The present method using cranes on installation vessels1 has some limitations for installation operations in such deep water. The dynamic behavior of the coupled system of the rigged structure and the crane vessel is more complicated, and the natural period of this pendulum becomes longer, so that the position-keeping of the vessel and the manipulation of the crane to achieve the desired positioning accuracy will become much more difficult. Hence, an innovative method which can improve on the present method is desired.
In this paper, an installation method using an active control technique2 is discussed as one possible and effective option instead of the present method. The point of this method is that the structure becomes“intelligent”and is self-propelled, so that the underwater operation is executed automatically using positioning sensors and actuators. The expected advantages of this method are as follows. The installation operation is scarcely affected by sea conditions because the connections to the facilities at sea are kept to a minimum. The installation accuracy no longer depends on the target water depth to ensure that the positioning system is settled near the target point. This method also has the potential to be applied to such activities as relocation or dismantling of underwater structures, and underwater mating of partitioned structures, which may lead to possibilities of recycling or the automatic construction of offshore structures.
Actively controlled installation methods have another advantage in that structural flexibility is allowed if it is taken into account in the control strategy. This means that this method can reduce the number of extra strong members which are needed only for the installation operation. Therefore it will become the rational installation technique to use in circumstances when offshore structures are required to be made as light-weight as possible for the sake of cost reduction, so that they are liable to become flexible structures. In addition, if the structural response is controlled and additional structural strength is not required, a neutrally buoyant structure will be possible, which is, at least structurally,
