Air gap and impact load prediction

DNV's new guidelines, OTG13 and OTG 14, put forth new recommendations for the estimation of air gap and horizontal wave impact loads. Here we describe a combined simulation and model test procedure to address the new DNV guidelines and provide input to the air gap analysis and horizontal wave impact loads.


Depending on the project phase air gap analysis can be carried out using frequency domain diffraction tools such as Diffrac, time domain simulations with aNySIM XMF and/or model tests in one of MARIN’s high-end facilities. aNySIM XMF can simulate the wave frequent and low frequent motion behaviour of one or multiple (floating) bodies with hydrodynamic interactions. If large nonlinear effects are expected air gap analysis can be carried out through model tests. Model test results can also be used to verify simulation results and provide input to simulations and determine applied correction factors (e.g. wave asymmetry factor, alpha).

If air gap exceedance is detected additional model tests with a large number of force panels in the front of and underneath the platform deck can be carried out to determine impact loads on the deck box of the floater.

In addition to model tests CFD simulations can also be carried out to determine impact loads. In this case, the number of simulations is typically restricted due to computational limits, however more detailed flow information is available, e.g. on the distribution of the impact load on the deck box.


Air gap analysis
Contour plots of the air gap around the structure for simulations results with and without DNV correction factors and identification of most critical sea states.

Impact load measurements
Measured impact loads, distribution of impact loads over deck area covered with force panels, 90% percentile load based on 2 parameter Weibull distribution. In addition to the impact load measurements high speed videos can be recorded of critical events and flow kinematics derived with CFD.


Contact person photo

Joop Helder

Senior Project Manager

Jule Scharnke

Senior Project Manager


Linear diffraction analysis
A first assessment of a floaters air gap can be evaluated using linear diffraction analysis combined with DNV (OTG13) correction factors

Time domain simulations
If air gap exceedance is detected in Phase 1, time domain simulations can be used to combine the coupling between wave frequent and low frequent motions as well as the OTG13 correction factors.

Air gap model tests
If air gap exceedance is confirmed in Phase 2, model tests can be carried out to determine asymmetry factors and/or verify simulation results.

Model tests and/or CFD
Simulations to determine impact loads If air gap exceedance is detected in Phase 2 and/or Phase 3, impact loads on the platform deck need to be determined according to DNV’s OTG14.
shortcrest effect


BreaKin JIP 2016-2018
Increased the understanding of the scale effects involved in wave-in-deck model tests and provided a between link wave kinematics and impact loads.

ComMotion JIP 2015-2018
Developed fast and efficient CFD method for predicting wave loads on moving and deforming structures in extreme waves.

ShorTCresT JIP 2011-2013
Accounted for short crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver a concreted (empirical) design methodology.

CresT JIP 2008-2010
Developed models for realistic extreme waves and a design methodology for the loading and response of floating platforms.