Compliant offshore structures have complex nonlinear dynamics. Subharmonic resonances can co-exist with small fundamental motions. To prevent a simulation missing an entire subharmonic peak, a comprehensive set of initial conditions must be explored. Chaotic nonperiodic motions can also arise in deterministic problems. These are extremely sensitive to starting conditions and require a statistical description. Chaotic responses and their adjacent subharmonics of arbitrarily high order mean that the duration of digital, analogue and laboratory simulations must be chosen with considerable care. Resonances of articulated mooring towers, encountered in tank tests, are used to illustrate these general concepts.

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