Motor-Structure Interaction and API 546 Compliance

Dynamic analysis was required for a motor and its 18.6 m (61 ft) × 4.3 m (14 ft) × 5.2 m (17 ft) (length × width × height) reinforced concrete table top support. Motor power rating was 17.2 MW (23 300 hp), weight 388 kN (43.6 ton) and nominal operating speed 1800 rpm. Governing document was API 546. Provision for motor-structure interaction was the key element of the finite element model; motor, not structure, responses were at issue. Motor rotor journal displacement was well below the API 546 limit, even though a dominant natural frequency was within the normal operating range. Though low modes were governed by columns, the dominant mode in the operating range was governed by strong-axis flexure of the 1070 mm (42 in) thick table top. The higher mode was self-restraining so the resonance condition was well controlled. The motor-structure system was judged compliant with API 546.