The following cases of offshore measurements of damping are known to me :

Atkins Research and development : Forties field platform FB, wave force investigation, final report, volume 1, Epsom, UK, december 1980. The structural damping in the first two sway modes has been derived from the response data and gives values of 4.2% critical and 2.2% of critical respectively.  

I Langen, N Spitsøe, J Heavner, T Thuestad: "Valhall QP EMP Project, Structural System Identification. Progress report 1.1" (1983). They examined waves with (Hs Tz)-values of : (7.6 9.7), (4.0 8.3), (2.5 6.5). They indicate a damping of 1-2% for the first two natural frequencies.  

R L Bruce: "Ekofisk 2/4-H Preliminary Study of Platform Behaviour" (1983). Analysed the storm of 29.12.1981 with Hs = 6.5m and Tz = 8.4 s. Equivalent damping for the first 3 modes where close to 2%. Higher modes had closer to 1%.  

W E Gundy, T D Scharton, R L Thomas: "Damping measurements on an offshore platform", OTC 3863, Houston (1980). They analysed forced vibrations on 2/4-D. The lowest vibration modes have a damping of 1- 3% of critical. Higher vibration modes have a damping of 2 - 3% of critical.  

Ruhl and Berdahl: "Forced vibration tests on a deepwater platform", OTC 3514 (1979). Analysed the South Pass 62C platform in the Mexico Gulf. The first mode had about 1.6% of critical. The first torsion mode had about 1.2%.  

R L Bruce and I Langen: "Ekofisk 2/4-H Structural System Identification". They analysed 5 storms with Hs between 2.4m and 5.2m. The lowest modes gave between 2.1% and 2.6 % damping. Higher modes between 0.7% and 1.4%.  

T C Thuestad and F G Nielsen : Submarine impact with the Oseberg jacket, Journal of offshore mechanics and arctic engineering, vol. 114/1, February 1992. This is also presented at OMAE 1990. They demonstrate that the collision between the Oseberg jacket and the submarine in 1988 the damping was in the order of 0.5-3% of critical. The damping decreased when increasing the amplitude (the dent) of the member. The damping was non-linear of Coulomb type.  

Fugro McClelland : Magnus Structural monitoring system, review of response at platform natural frequencies, Glasgow, November 1991. They reviewed three see states with (Hs Tz) - values of : (1.9 5.4), (7.6 9.8) and (5.1 8.5). For the two lowest sea states they got 1.8% damping.  

R M Webb and R B Corr : Full scale measurements at Magnus, undated (received by me in 1993). Give at a structural damping of about 1%.  

M Kenley : Measurements of overall loading on steel jackets, Verbal presentation at Seminar on Full scale measurements of offshore structures, Statoil, Stavanger, 23.4.1992. Measurements at Tern give about 1.5-2.0% damping.

D Kanakuran and A Johansen: Analysis of instrumentation data from Draupner E jacket - structural response and loading, Sintef report STF22F96746, Trondheim, 1996 and D Karunakaran, M Bærheim and B J Leira : Measured and simulated dynamic response of a jacket platform, Proceedings OMAE, volume II, page 157-164, Yokohama, 1997 presnts the same. They concludes with 1-2% measured damping for the vibration modes in the X-direction and 2-4% for modes in the Y-direction, where there is a bridge connection to adjacent platform. In the conclusions on page 3 a damping of 1-2% is used in the x-direction. At page 10 the values is said to be 1-1,5% and in the table at page 21 values of 0,6-1,4% damping is found. The presentation of the results in this document is presented rather confusing... There is no clear connection between sea state and damping. Two sea states of 8.1m and 8.5m copmpared with a sea state of 11.7m have in average the same damping values.  


 The measurements give the following results :





















There is a large scatter. An average is about 1.8%. From my point of view a slightly conservative value should be applied. For jacket structures a damping value of 1.5 % is reasonable for low aea states and 2% for extreme sea states. A damping of 1.5% is at present used both by Norwegian and UK designers for the Norwegian Continental Shelf.


Status 2.9.97 by Arne Kvitrud

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