A deep-draft spar platform is regarded as a competitive alternative structure for deepwater oil field development. The natural periods of classical spar platforms are relatively long, 160s for surge, 60s for pitch and 30s for heave, due to small waterplane area compared to submerged volume. Hence spar platforms are not usually excited vertically. However, the heave motion is largely amplified at resonance due to small damping. It may happen when they are exposed to swell. Particularly, when the pitch natural period is approximately double the heave natural period and the magnitude of heave motion reaches a certain threshold, large unstable pitch motions are induced. In reality, however, it is normally assumed that the nonlinear unstable motions could be prevented by mooring lines. In this paper, the nonlinear behavior of spar platform with mooring lines is analyzed by perturbation techniques. The method of multiple scales is used to determine the periodic response of two modes and stability in the case with and without mooring lines. Also model tests are carried out in a wave tank with a scaled model to verify the analysis. The model is set under the condition of internal resonance in which the pitch natural period of the model is twice the heave period. The motions of spar platform with and without mooring lines are measured and compared. In experiments, the model exhibits large-amplitude pitch response triggered by heave response at heave resonance regardless mooring lines.
Spar technology has been utilized for offshore structures such as research vessels, communication relay stations, and storage and offloading platforms. Recently its application has extended to deepdraft platforms for deepwater production (Halkyard,1996; Glanville,1991). The shape of spar platforms is usually a long hollow cylinder with large diameter, which is normally moored by means of conventional spread chains.
