Seismic Evaluation of Platform Cranes

ABSTRACT Design of platforms in potentially seismic regions requires a thorough seismic analysis of both the platform and the equipment mounted to it. Typically, seismic design of most platform equipment is done by estimating an equipment?s natural period, and using this period to determine a design seismic acceleration to apply to the equipment and its mounting structure. Most deck equipment has extremely short natural periods due to its compactness and rigidity, and this method of seismic analysis is quite adequate. Typically, only drilling derricks are analyzed in more detail. For typical offshore platform cranes, often the period of the crane is estimated by lumping the mass of the crane and any lifted load at the top of the pedestal and computing the corresponding natural period. Due to the flexibility of the long crane booms typically found offshore, this may not result in a conservative design procedure. The natural period of the boom may be in a region of higher seismic energy pedestal/lumped mass. Estimates of the crane natural period should be based on the worst natural period (as compared to the input seismic spectrum). Formulae are included in the paper to allow estimation of the natural period of the crane major components. If any of the crane components? natural periods are in a region of high seismic energy, a thorough seismic analysis of the crane should be performed to ascertain seismic stress levels. This was done for a design example included in the paper. The original boom design was overstressed by the seismic event due to the boom lateral response. The boom design was revised to reduce its response to the seismic event. INTRODUCTION Seismic design is required for offshore platforms in many parts of the world. Typically such seismic design results in 1). Platform structure sufficient to resist the design seismic avent, and 2) a design criteria for any equipment mounted on the platform. The seismic motion transferred from the platform to attached equipment varies with location in the platform this is due to the response of the platform to the earthquake. Typically, a response spectrum is developed for each deck level in the. Platform where equipment design criteria is required equipment and platform attachments are designed using this deck response. The response of the deck equipment to the seismic event depends on the equipment?s natural period. If the equipment?s natural period is in a region of high energy from the deck response, then the equipment response to the seismic event will be substantial. Conversely, if the equipment?s natural period is far removed from any significant deck response period, the equipment will exhibit no natural frequency excitation to the seismic event, and will only be subjected to the "rigid body" translational motion of the deck. Most deck-mounted equipment has an extremely short natural period due to its compactness and high stiffness.