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RATIONAL PROCEDURE FOR ADVANCED NON-LINEAR ANALYSIS OF FLOATING STRUCTURES

Objective


A computer procedure for the evaluation of the wave force Quadratic Transfer Functions has been developed. A computer programme to simulate the model basin conditions has been improved to include non linear effects. New methods for the statistical analysis of non linear systems have been identified and found suitable for several typical offshore structures. Guidelines for the dynamic analysis of large floating structures have been prepared.

Floating structures such as moored tankers, semisubmersibles and tension leg platforms (TLP) are characterized by nonlinear dynamic behaviour. The goal of this project is the development of rational procedures for both mathematical and physical modelling of large floating structures. This goal has been achieved by improvement of already existing computer programs based on the 3-dimensional potential theory (nonlinear hydrodynamic effects correctly formulated up to the second order have been included), by study and definition of the procedures relevant to model basin tests in order to simulate correctly all the required nonlinear effects and by definition of the probability density function (PDF) of nonlinear responses in order to get the extreme values required by the designer. On the basis of the identified methodologies a computer program has been developed working as a postprocessor of the dynamic simulation procedure. The final goal of the project was the definition of an optimized procedure to be followed in the design and/or verification of floating systems for which the effects of second order hydrodynamic forces cannot be neglected. Extensive series of model tests have been performed in order to check the theoretical results coming from the application of the new procedures and a good agreement has been found in most of the cases.

Both numerical and experimental methods have been developed for the analysis of floating offshore structures such as moored tankers, semisubmersibles and tension leg platforms.
Many improvements have been made in the evaluation of wave forces and in motion dynamic analysis as well as in the statistical analysis of nonlinear systems. A procedure for the determination of nonlinear hydrodynamic forces on floating structures has been incoporated into an existing computer program and procedures have been developed for the determination of second order hydrodynamic forces. Experience has also been gained with testing of floating structures in wave time series of very long duration. Tools for the analysis and verification of compliant offshore structures have been improved and expertise has been increased in the areas of the quadratic wave loading mechanism and of statistical aspects.
FLOATING STRUCTURES SUCH AS SEMISUBMERSIBLE VESSELS, TENSION LEG PLATFORMS (TLPS) AND MOORED TANKERS ARE CHARACTERIZED BY NON-LINEAR DYNAMIC BEHAVIOUR. TODAY'S EXISTING DESIGN PROCEDURES FOR THESE SYSTEMS REQUIRE A STRONG IMPROVEMENT IN - THE FLUID/STRUCTURE INTERACTION MODELS IN ORDER TO DESCRIBE CORRECTLY THE NON-LINEAR EFFECTS, THAT ARE NORMALLY CRITICAL DUE TO THEIR INFLUENCE ON LONG PERIOD DYNAMICS.
- THE METHODS ALLOWING ACCURATE STATISTICS OF RESPONSE VARIABLES TO BE GENERATED AND THE PREDICTION OF EXTREME VALUES WITHOUT THE NEED FOR VERY LONG SIMULATION TIMES.

DEVELOPMENT OF RATIONAL PROCEDURES FOR SELECTION OF BOTH COMPUTER AND PHYSICAL MODELLING CONDITIONS AND INTERPRETATION METHODS OF THE RELEVANT RESULTS CONSTITUTE THE AIM OF THE RESEARCH PROJECT.

TO ACHIEVE THIS, THE SPECIFIC PROJECT OBJECTIVES ARE:
- IMPROVEMENT OF A 2ND GENERATION OF COMPUTER PROGRAMMES RELEVANT TO THE DYNAMICS OF FLOATING STRUCTURES THAT INCLUDE ALL NON-LINEAR EFFECTS CORRECTLY FORMULATED UP TO THE 2ND ORDER.
- DEFINITION OF PROCEDURES FOR MODEL BASIN TESTS IN ORDER TO SIMULATE CORRECTLY ALL THE REQUIRED NON-LINEAR EFFECTS.
- DEFINITION OF THE PROBABILITY DISTRIBUTION FUNCTION OF MAXIMA AND FATIGUE DATA FOR NON-LINEAR RESPONSES.
- DEFINITION OF METHODS FOR PREDICTION/SELECTION OF THE OPTIMAL SIMULATION CONDITIONS IN ORDER TO ALLOW A RELIABLE STATISTICAL EXTRAPOLATION OF THE RESULTS GETTING OVER THE PROBLEM OF THE RESPONSE SPECTRUM VARIABILITY AS FUNCTION OF THE TIME SIMULATION.

MODEL BASIN TESTS ARE TO BE PERFORMED TO CONVALIDATE THE COMPUTER PROCEDURES DEVELOPED WITHIN THE PROJECT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

TECNOMARE SPA - SOCIETA PER LO SVILUPPO DELLE TECNOLOGIE MARINE
Address
San Marco 3584
30124 Venezia
Italy

Participants (3)

Dansk Hydraulisk Institut
Denmark
Address
5,Agern Allé
2970 Hørsholm
FINCANTIERI - CANTIERI NAVALI ITALIANI SPA
Italy
Address
Passeggio S. Andrea 6
34123 Trieste
REGISTRO ITALIANO NAVALE
Italy
Address
Via Del Viminale, 43
00184 Roma