Session: 01-03-01 Hydrodynamics

Paper Number: 79068

79068 - Validation of Measurement Techniques Used for Slamming 

Model test campaigns conducted recent years indicate large wave impact loads on vertical surfaces above the waterline for various types of moored floating structures. In 2013, a model test campaign to investigate column slamming on a 1:55 scale model of the Heidrun platform was conducted. Heidrun is a large concrete Tension Leg Platform (TLP) with column diameters of 31 m. The main objective of the test campaign was to estimate the characteristic slamming loads, defined as the q-annual extreme 3-hour slamming load level of 10^-2 for the Ultimate Limit State (ULS) and 10^-4 for the Accidental Limit State (ALS), see Vestbøstad et al (2017).

 

The basis for the estimate of characteristic loads was slamming pressure measured on a grid of slamming panels mounted in a section on the columns of Heidrun. This section has 42 force transducers mounted in a 7x8 configuration to cover 80 degrees of the cylinder circumference over a height of 24 meters. The force sensors used in the test have a very high natural frequency and the foundation for the sensors was made very compact and stiff to reduce structural dynamic effects. The sampling frequency used was 19.2 kHz. The same slamming section has also been used in an experimental investigation on a rigid column to assess slamming loads from long and short crested breaking waves, see Økland et al (2020).

 

In order to validate the measurements of slamming pressure, the slamming section used in the above tests was dropped in a small water basin at different configurations to resemble impacts from waves with varying front steepness and velocity of wave front.

In Lian et al (2015), most of the results from the drop tests were presented and compared to theoretical solutions. It was concluded that the peak slamming coefficient is close to the expected theoretical value, but for small impact angles the load measured during the drop becomes significantly lower than theoretical observations and previous tests results, e.g. by Campbell and Weynberg (1980). Possible sources of errors in the tests may be retardation/change of velocity during impact or effect of 3D flow in drop.

 

In the work presented in this paper these findings have been investigated further. An alternative drop test using a cylinder with smaller diameter, and neglectable effect from 3D flow, has been carried out.  Numerical simulations of the drop tests have been conducted and compared to experimental results.

 

In November 2019, the drop test with the Heidrun slamming section was repeated but with a heavier section to reduce the effect of retardation. 

In the present paper results from drop tests and numerical calculations are compared and findings are reported.

 

References

 

Campbell, I. M. C. and Weynberg, P. A. (1980). ”Measurement of Parameters Affecting Slamming” Final report, Rep. No. 440, Technology Reports Centre No. OT-R-8042. Southampton University, Wolfson Unit for Marine Technology.

 

Lian G, Økland O.D. and Vestbøstad T.M. (2015). "Impact Loads from Drop Test of a Circular Section with 42 Force Transducers" OMAE2015-41491, Proc of the 34th Int Conference on Ocean, Offshore and Artic Engineering,  May 31 - June 5, 2015, St. John’s, Newfoundland, Canada

 

Vestbøstad T.M., Økland O.D., Lian G and Stavang T.P. (2017). "Column Slamming Loads on a TLP from Steep and Breaking Waves", OMAE2017-61786, Proc of the 36th Int Conference on Ocean, Offshore and Artic Engineering, June 25-30, 2017, Trondheim, Norway.

 

Økland O.D., Lian G and Vestbøstad T.M. 2020). "Experimental Investigation of Slamming Loads on Vertical Column Exposed to Short and Long Crested Breaking Waves", Proc of the 39th Int Conference on Ocean, Offshore and Artic Engineering, June 28- July 3, 2020, Fort Lauderdale FL, USA. To be published.

 

Note: This paper was approved and reviewed for the 2020 conference (ASME Paper Number: OMAE2020‐18289), but was not published as the planned conference was cancelled due to COVID 19.

Presenting Author: Ole David Økland Sintef Ocean

Authors:

Ole David Økland Sintef Ocean
Gunnar Lian Equinor ASA
Tone Vestbøstad Equinor ASA