Objective
The aim of the project is to obtain comprehensive and reliable full scale measurements of riser response in the real marine environment. The test equipment and the test programme have been designed so that tests of increasing degrees of complexity can be carried out. Each such test will be performed a number of times under the same or similar environmental conditions, so that a high degree of confidence in the data can be achieved. The test rig has been designed so that it can impose both harmonic and non harmonic top motions in any chosen direction on the top of the riser and to vary the riser tensioning from one test to the other. This will facilitate the examination of the validity of existing calculation procedures for riser response, and the investigation of hydroelastic vibrations due to the vortex shedding. The twin riser tests are designed mainly to allow assessment of interference effects between two closely spaced risers.
The main results got during the period were:
- Final commissioning of the system. It was demonstrated the reliability of the testing station and of the subsystems: the mechanical equipment, the sensors installed in the riser and on the platform, the data acquisition system and the various links with the sensors, the hydraulic system and the onshore control station worked in a satisfactory way.
- It was demonstrated the operability of the offshore testing station from the control station located onshore (Marina di Ravenna).
- It was demonstrated the reliability of the expert system that supervises the running of the test sessions.
- Further development of the post-processing programs, of the environmental programs, expert programs, theoretical riser dynamic prediction programs.
- Starting of data acquisition campaign in single riser configuration in a systematic way.
The main problems found during the period were :
- Longer commissioning than expected due to the reinstallation ofthe riser after the breaking of the riser during a test on July 92.
- Debugging of the environmental programs. After the first data acquisition from the real environment it was evident the need to introduce filtering routines in order to treat the spurious spikes shown in the time histories of the wave meter and of the current meters.
- Improving of the program that performs the directional analysis of the sea.
- Debugging of the expert system that supervises the test sessions.
- We found big problems in the program that with an ARMA (Auto Regressive Mean Averages) routine try to predict in real time the impulsive surge motion of a TLP in the environment detected by the sensors. We had even problems with the mechanical equipment that was trying to achieve the motion dictated from the above mentioned program. Therefore we decided to add to the list of tests a new category named MTFTM (Multifrequency Top Motion) where the top motion is not a sinusoid but a superimposition of sinusoids. This new tests substitute the previous ones for which it was impossible to collect significative data. The system was able to do these tests without problems.
The basic objective is to install two instrumented risers on an already existing structure, a fixed platform in the Adriatic sea, in water depth about 70 meters. Such a structure would act as a support to the equipment required to carry out the tests.
The Mechanical Equipments installed on the platform are :
- A-frame structure;
- Monorail, hydraulic engine, trolley;
- Surface riser spacer, rise tensioning system;
- Circular railway on which all the above mentioned equipments are resting. It is possible to change the orientation of the A-frame and all the supported equipments on the circular railway.
- Bottom connector (installed at - 70 m. respect the mean sea level);
- Bottom riser spacer (installed at - 70 m. respect the mean sea level);
The Instrumentation System :
- Instrumented risers pod (Riser sections equipped with strain gauges);
- Linear and angular accelerometers, axial load cells;
- Force sleeves to evaluate hydrodynamic loads on the riser;
- Environmental sensors (Wave meters, current meters, wind meters);
The Data Acquisition System:
- MicroVax3400 offshore installed to collect the signals from the installed sensors, to control the mechanical equipment, to send the data to the onshore computer through the radio-link;
- Vax Station 3100 installed in the onshore control room. It is the interface used by the operator to run experiments. In the hard disk of this computer are stored the data collected by the offshore computer and transferred through the radio link.
The data acquired during the tests are processed with a set of computer programs developed during the project. The results are compared against theoretical results predicted by a suite of computer programs in frequency domain and time domain developed during the project to describe the dynamical behaviour of steel tensioned riser.
On the basis of such comparison, verification of the theoretical calculation procedures will follow with a view to etablishing if these available proceduresare adequate and for effecting modifications of existing calculation procedures or for developing new ones, where appropriate.
Programme(s)
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DEM - Demonstration contractsCoordinator
20097 S. Donato Milanese (MILANO)
Italy