Two new methods are proposed to predict airgap demand. Airgap demand is the maximum expected increase in the water surface elevation caused incident waves interacting with an offshore structure. The first new method enables inclusion of some second-order effects, though it is based on only first-order diffraction results. The method is simple enough to be practical for use as a hand-calculation in the early stages of design. Two existing methods of predicting airgap demand based on first-order diffraction are also briefly presented and results from the three methods are compared with model test results. All three methods yield results superior to those based on conventional post-processing of first-order diffraction results, and comparable to optimal post-processing of second-order diffraction results. A second new method is also presented; it combines extreme value theory with statistical regression to predict extreme airgap events using model test data. Estimates of extreme airgap events based on this method are found to be more reliable than estimates based on extreme observations from a single model test. This second new method is suitable for use in the final stages of design.

1.
WAMIT 5.3, 1999. WAMIT: User Manual Versions 5.4, 5.4PC, 5.3S. Dept. of Ocean Engineering, M.I.T.
2.
Winterstein
,
S. R.
, and
Sweetman
,
B.
,
2001
,
Air Gap Response of Floating Structures: Statistical Predictions vs Observed Behavior
.
ASME J. Offshore Mech. Arct. Eng.
,
123
, pp.
118
123
.
3.
Sweetman
,
B.
, and
Winterstein
,
S. R.
,
2003
,
Non-Gaussian Air Gap Response Models for Floating Structures
.
ASCE Journal of Engineering Mechanics
,
129
(
3
), pp.
302
309
.
4.
Teigen, P., and Trulsen, K., 2001. Numerical Investigation of Non-linear Wave Effects around Multiple Cylinders. In: Proc. Intl. Offshore Polar Eng. ISOPE.
5.
Manuel
,
L.
,
Sweetman
,
B.
, and
Winterstein
,
S. R.
,
2001
,
Analytical Predictions of the Air Gap Response of Floating Structures
.
ASME J. Offshore Mech. Arct. Eng.
,
123
, pp.
112
117
.
6.
Manuel, L., and Winterstein, S. R., 2000. Reliability Based Predictions of a Design Air Gap for Floating Offshore Structures. In: Proc. 8th ASCE Conf. on Probablistic Mechanics and Structural Reliability. ASCE.
7.
Sweetman, B., Winterstein, S. R., Meling, T. S., and Birknes, J., 2001. Airgap Prediction: Use of Second-Order Diffraction and Multicolumn Models. In: Proc., ISOPE 2001-IL-13, available from www.rms-group.org. International Society of Offshore and Polar Engineers.
8.
Fokk, T., 1995. Veslefrikk B Air Gap Model Tests. Tech. Rep. 512167.00.01, MARINTEK Trondheim, Norway.
9.
Sarpkaya, T., and Issacson, M., 1981. Mechanics of Wave Forces on Offshore Structures. Van Nostrand Reinhold Company.
10.
Hasselmann, K., Barnett, T. P., Bouws, E., H. Carlson, H., Cartwright, D. E., Enke, K., Ewing, J. A., Gienapp, H., Hasselmann, D. E., Kruseman, P., Meerburg, A., Mller, P., Olbers, D. J., Richter, K., Sell, W., and Walden, H., 1973. Measurements of Wind-Wave Growth and Swell Decay during the Joint North Sea Wave Project (JONSWAP). Deutschen Hydrographischen Zeitschrift, 12.
11.
Clough, R. W., and Penzien, J., 1975. Dynamics of Structures. McGraw-Hill.
12.
Cartwright
,
D.
, and
Longuet-Higgins
,
M. S.
,
1956
,
The Statistical Distribution of the Maxima of a Random Function
.
Proc. R. Soc. London, Ser. A
,
A237
, pp.
212
232
.
13.
Davenport
,
A.
,
1964
,
Note on the Distribution of the Largest Value of a Random Function with Application to Gust Loading
.
Proc. of the Institution of Civil Engineers
,
28
, pp.
187
196
.
14.
Benjamin, J. R., and Cornell, C. A., 1970. Probability, Statistics and Decision for Civil Engineers. McGraw-Hill.
15.
Gumbel, E. J., 1958. Statistics of Extremes. Columbia University Press, New York.
16.
Sweetman, B., 2001. Airgap Analysis of Floating Structures subject to Random Seas: Prediction of Extremes using Diffraction Analysis versus Model Test Results. Ph.D. thesis, Stanford University.
17.
Madsen, H. O., Krenk, S., and Lind, N. C., 1986. Methods of Structural Safety. Prentice-Hall, Inc., New Jersey.
18.
Winterstein
,
S.
,
1988
,
Nonlinear vibration models for extremes and fatigue
.
J. Eng. Mech. Div.
,
114
, pp.
1772
1790
.
19.
Kac
,
M.
, and
Siegert
,
A. J. F.
,
1947
,
On the theory of noise in radio receivers with square law detectors
. J. Appl. Phys., 383–400.
20.
Næss
,
A.
,
1986
,
The Statistical Distribution of Second-Order Slowly-Varying Forces and Motions
.
Appl. Ocean. Res.
,
8
, pp.
110
118
.
21.
Næss, A., 1992. Prediction of extremes related to the second-order, sum-frequency response of a TLP. In: Proc. 2nd Intl. Offshore Polar Eng. ISOPE.
22.
Winterstein
,
S. R.
,
Ude
,
T. C.
, and
Marthinsen
,
T.
,
1994
,
Volterra Models of Ocean Structures: Extreme and Fatigue Reliability
.
J. Eng. Mech.
,
120
, pp.
1369
1385
.
23.
Longuet-Higgins
,
M. S.
,
1952
,
On the Statistical Distribution of the Heights of Sea Waves
.
Journal of Marine Research
,
11
, pp.
245
266
.
24.
Tayfun
,
M. A.
,
1980
,
Narrow-Band Nonlinear Sea Waves
.
J. Geophys. Res.
,
85
, pp.
1548
1552
.
25.
Tayfun
,
M. A.
,
1986
,
On narrow-band representation of ocean waves: 1. Theory
.
J. Geophys. Res.
,
91
, pp.
7743
7752
.
26.
Tayfun
,
M. A.
,
1986
,
On narrow-band representation of ocean waves: 2. Simulations
.
J. Geophys. Res.
,
91
, pp.
7753
7759
.
27.
Tayfun
,
M. A.
, and
Lo
,
J.-M.
,
1989
,
Envelope, phase and narrow-band models of sea waves
.
J. Waterw., Port, Coastal, Ocean Eng.
,
115
, pp.
594
613
.
28.
Jha, A. K., and Winterstein, S. R., 2000. Nonlinear Random Ocean Waves: Prediction and Comparison with Data. In: Proc. of the 19th International Conference on Offshore Mechanics and Arctic Engineering.
29.
Sweetman
,
B.
,
Winterstein
,
S. R.
, and
Cornell
,
C. A.
,
2002
,
Air Gap Response of Floating Structures: First and Second-Order Transfer Functions from System Identification
.
Appl. Ocean. Res.
,
24
,
2
, pp.
107
118
.
30.
Chakrabarti, S. K., 1987. Hydrodynamics of Offshore Structures. Springer-Verlag.
31.
WAMIT 4.0, 1995. WAMIT: A radiation-Diffraction Panel Program for Wave-Body Interactions-User’s Manual. Dept. of Ocean Engineering, M.I.T.
You do not currently have access to this content.