Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Effects of Filtered Slow Drift Motion on Deepwater Platform Global Performance
Da Li1,3, Cong Yi1, Yongjun Chen 2
CNOOC Research Institute, Beijing, China
2
DMAR Engineering, Inc., Houston, Texas, USA
3
College of Shipbuilding Engineering, Harbin Engineering University, Harbin, Heilongjiang, China
1(Emmerhoflf, 1996).

ABSTRACTDuring the coupled time domain dynamic simulation, it’s been
considered more accurate to use filtered slow drift motion rather than thetotal motion of each time step for wave phase shift calculation.
Butterworth filter is one of the most widely used low pass filter due toits “smoothest” frequency responses and simplest transfer function.
Based on a deepwater semisubmersible production platform, sensitivity
studies, including with and without filtering and different cut-off
frequencies, are conducted to investigate the effects of using filtered slow
drift motion for wave phase shift calculation in the dynamic simulationon the platform global performance.

In the time domain simulation, if wave phase shift calculation is basedon the slow drift motion (low frequency position), instead of the total
motion (instantaneous position), then double counting of quadratic loads
can be minimized or avoided.
In this paper, Butterworth low pass filter is employed to perform "realtime" filtering of platform motions while the simulation is in progress.
The total motions calculated at previous time step(s) are filtered toremove the instantaneous wave frequency part, and only the remaining
low frequency part is used as input for current time step’s diffraction
loads calculation. The cutoff period which separates low frequency andwave frequency usually takes a value between 30 to 40 seconds.

KEY WORDS: Slow Drift Motion, Global Performance, Butterworth
Filter

SIGNAL FILTERINGINTRODUCTION

A signal generally consists of various frequency components. The
frequency contents and their powers are normally derived throughoperations such as the Fast Fourier Transform (FFT). Filtering is themodification of a measured or calculated signal using an algorithm
and/or logic to remove the undesirable characteristics of the signal before
it is used for other purposes. In the algorithm, approximating
polynomials are typically used to define the transfer functions. Higherorder polynomials increase roll-off rates and bring the filter closer to theideal response, but also make the filter more complex in design andimplementation (Shenoi, 2005).

For offshore deepwater platform, its wave force database is commonly
calculated based on small wave amplitude assumption. Wave phaseangle is a function of both time and position. In time domain simulations,the platform’s position is affected by large waves, and in turn the wave
forces on the platform are also affected. This leads to nonlinear responses
in waves. However, the quadratic part of the nonlinear behavior isalready accounted for in the quadratic transfer function (QTF) data.
Therefore, there are risks of double counting the quadratic loading on
platform - one is the application of the linear diffraction analysis loads
using the instantaneous platform position, and the other is the use of QTF
data.

Different types of filters allow different frequency bands to pass(passband) or to stop (stopband). Fig. 1 illustrates the most common
types of filters: low-pass filter allows low frequencies to pass andattenuates high frequencies; high-pass filter allows high frequencies topass and attenuates low frequencies; and band-pass filter only allows
frequencies within a specified frequency band to pass. The frequency
bands are normally defined by cutoff frequencies.

The instantaneous motion of moored floating platforms in deep water
typically comprises two categories: wave frequency motion which isresponse to wave loads; and low frequency motion such as slow drift
motion due to wind, wave and current. Slow drift motion is relatively
steady with oscillating period much longer than the dominant period in
the wave-spectrum of interest, due to the relatively large mass of theplatform and the weak restoring forces of the mooring system

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