Effect of Short-crested Waves on the Dynamic Responses of Truss SPAR Platforms

ABSTRACT
Short-crested wave in real sea condition is defined as the linear
summation of a series of long-crested waves propagated to different
directions, where the magnitude and directions are randomly generated.
Also, it is complex and three dimensional. In this paper, an
experimental and numerical study on the truss spar responses subjected
to long and short-crested waves are presented. The experimental study
on a typical truss spar model subjected to long and short-crested waves
were performed in the wave tank of Offshore Laboratory in Universiti
Teknologi PETRONAS. In the study, physical motions in surge, heave
and pitch of the model with four linear spring mooring lines restrained
were measured. Also, a numerical MATLAB code incorporating the
short-crestedness of the waves to predict the dynamic analysis of the
truss spar was developed. The results were obtained in terms of
Response Amplitude Operator (RAO) for surge, heave and pitch
motions using both methods. The predicted RAO were compared with
the measured RAO and they agreed fairly well. Also, it was observed
that the responses for short-crested waves were much lower compared
to the responses for long-crested waves.
KEY WORDS: Short-crested waves; model test; truss spar; dynamic
responses; response amplitude operator.