Carlos Magluta Civil Eng. Dept. COPPE-UFRJ

　Luiz F. L. Rosa Structure Dept. FAU-UFRJ

　Ney Roitman Civil Eng. Dept. COPPE-UFRJ

　Paula F. Viero Eng. Graphics Dept. EE-UFRJ

　Elton J. B. Ribeiro Cenpes PETROBRAS

　

This work reports on a set of static and dynamic experimental tests carried out to study the influence of some parameters on the bending stiffness and the damping ratio of a flexible riser.

It has been designed and constructed, at the Laboratory of Structures of the Civil Engineering Department of COPPE/UFRJ (Federal University of Rio de Janeiro), an experimental setup to allow the tests, which were carried out on a riser specimen 4" in internal diameter. One of the specimen ends was fixed and the other one was kept free. Different levels of internal pressure and axial loading were applied to the riser specimen. Using loading and displacement transducers, the signals were acquired and, by processing and analyzing these signals, the experimental estimation of the bending stiffness and the damping ratio of the riser specimen was performed.

The results suggest that internal pressure and axial loading do not influence the bending stiffness of the riser. With regard to its damping ratio, it increases for both increasing internal pressure and decreasing axial loading.

　

During last years, a kind of pipe composed by some steel and plastic layers has been widely used in floating offshore petroleum production systems. This pipe, which presents low bending stiffness and high dynamic complacency, is named flexible riser.

This work is part of a design developed with PETROBRAS(Brazil State Oil Company). It presents several static and dynamic experimental tests, which goal is to investigate the influence of some parameters on the bending stiffness and the equivalent viscous damping of a flexible riser specimen. These two properties strongly affect the dynamic behaviour of the riser and they are not well known due to its composite layered cross section. Dynamic displacement amplitude, exciting frequency, internal pressure and axial loading were the parameters varied during the tests.

It has been designed and constructed at the Laboratory of Structures of COPPE / UFRJ an experimental setup to make the tests, performed on a flexible riser specimen 4 meters long and 4" in internal diameter, feasible. The specimen upper end was fixed and its lower end was kept free. Several levels of internal pressure and axial loading were applied to the specimen. Using displacement and loading transducers, some data were acquired to allow the estimation of bending stiffness and equivalent viscous damping ratio of the riser.

The experimental setup is presented, the tests are described, the methodology used for estimating the dynamic properties is explained and, finally, the influence of the varied parameters is discussed.

　

The experimental setup was designed and constructed to allow variation of the displacement imposed to the riser (and, by consequence, its curvature), exciting frequency, axial loading and internal pressure.

Figure 1 shows schematically the experimental setup and the loading system with a controlled motor, that was used for imposing displacement to the specimen free end, while is other end was fixed to a steel frame. Photo 1 provides a general view of the setup, showing axial loading applied through masses fixed to the riser free end.

Two load-cells were designed, constructed and calibrated at the Laboratory of Structures to measure the axial loading applied to the specimen. A system using an electric potentiometer was also developed to measure the position of the riser.

Figure 2 shows schematically the developed test control system, which is composed by a motor, an encoder used for measuring the motor rotation, an inverter, a personal computer with A/D and D/A board, and a transducer to measure the position of the riser free end. This system has two control cycles. The first one is used to control the velocity and the angular position of the motor through the inverter. The second one controls the riser free end position, acting on cycle no.1 through commands no.1, 2 and 3, as shown in Figure 2. A G¹ language computer program was developed to acquire data measured by the transducers. This program has the functions of: (i) calibrating the gain of the sensors; (ii) allowing the data acquisition of the static deformed configurations; (iii)controlling the motion of the riser free end with constant velocity; (iv) controlling the motion of the riser free end according to a harmonic function, where the central position, the amplitude and the exciting loading are chosen by the user(in this option the acquisition of the signals of all transducers is also performed); (v) allowing graphic visualisation of the signals of the transducers.