## SEABOTTOM INSTABILITY AROUND SMALL STRUCTURES IN MARINE ENVIRONMENT

**Od** 1990-01-01
**do** 1993-12-31

## Dane projektu

### Całkowity koszt:

EUR 1 680 792
### Wkład UE:

EUR 588 277
### Kraj koordynujący:

Italy
### System finansowania:

DEM - Demonstration contracts
## Cel

Aim of this project is to deepen some aspects of the stability of the sea floor when eroded by the action of the currents in order to :

- identificate the way of behaviour of the flows near the sea bottom and thus also of submarine systems and small structures

- study the correlation etween environment, fluid, the perturbations induced by the structures and the sediment movement

- define the engineering parameters to adopt when designing submarine systems envolved with these phenomena.

The results of the proposed research will allow the engineer to develop a better design philosophy in order to reduce the effect of the scouring and consequently to improve the reliability of any immersed structure.

The project has resulted in a practical methodology for the estimation of the scouring expected around submarine offshore structures of small dimensions exposed to current and waves. The methodology is based on the definition of the scour extension and depth for a series of reference structures and the assessment of scale factors that should allow to estimate the scour expected for most of the actual small submarine offshore structures of interest in the offshore engineering.

These scale factors account for the geometric and environmental differences between the reference and the actual structures, thus allowing to extrapolate the scour information, available for the reference structures to the actual structures and natural conditions.

The definition of the scouring characteristics for the reference conditions have been obtained mostly form laboratory simulations, while the definition of the scale factors has resulted from the comparison of the laboratory results, the informationobtained from a field tests performed with a structure of realistic dimenions exposed to natural conditions, and the results of numerical simulations of the scour evolution.

The numerical model developed during the project is based on a k-e turbulence closure model for the simulation of the flow field around three-dimensional structures, coupled with a three-dimensional model of sediment transport and a model for the bottom evolution. It has been verified against the laboratory data with satisfactory results and then used to integrate and extend the information get from the laboratory.

The choice of structure shapes to be considered has been limited to rather simple geometries, chosen in such a way to account for most types of structures actually used in the offshore engineering. As concern the flow conditions, unidirectional flows or waves superimposed and in the same direction as the steady current have been mostly considered, although a series of investigation has been performed also with reversing, tidal flow. Only sandy bottoms have been considered, thus excluding either the effect of cohesion on scouring and the investigation of the effect of different sediment characteristics on scour evolution. To our best knowledge, this is the first project of its kind on the subject, and this accounts for some of the above limitations imposed to the research and also for the somewhat not conclusive nature of the results achieved.

Finally the project has also allowed to appreciate the importance of some aspects on scour evolution which has not been possible address completely within the limits of the project itself and could be usefully considered in possible future research activities. As examples, these include the analysis of the asymptotic behaviour of the scouring with the increase of the incident flow velocity, the relationship between flow conditions and the time evolution of scouring and the effect of waves on scour evolution.

The project aims to define the scour occurring around submerged three dimensional structures of small dimensions under the effect of either steady current and waves superimposed to the current. This goal is pursued with the help of laboratory tests, in field investigation and numerical modelling. The final results has been organized in form of formulas and diagrams allowing to estimate the scouring expected in most situation of practical interest in the offshore engineering of Small Submarine Offshore Structures. This should enable the engineer to take into account the potential scour in the design phase and plan the appropriate protective and preventive actions.

The project has been divided into four phases.

Phase 1 : The available information on fluid dynamics and scour around submerged structures have been reviewed and the further activities have been planned.

Phase 2 : An extensive series of laboratory tests have been performed to investigate the fluid dynamics and scouring around three-dimensional structures or simplified shape in steady current and waves superimposed to current conditions.

Phase 3 : An in field experiment has been performed to verify the scour in real world conditions around a submerged structure of a simplified shape but realistic dimensions.

Phase 4 : The information collected in previous phases are bought together and supplemented with the results of numerical simulations.

The results obtained are organized in forma of diagrams and formulas to provide the engineer with a practical method to estimate potential scouring around three dimensional submerged structures.

## Koordynator

Viale Alcide De Gasperi 16

20097 SAN DONATO MILANESE

Italy

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