The dynamic response of offshore drilling platforms during earthquakes and heavy sea conditions becomes increasingly important as water depth increases. Predicting the structural response of a platform depends upon the characteristics of the excitation and the dynamic properties of the platform. Recorded data on the behavior of full scale structuresduring earthquakes and heavy sea conditions is scarce. However, analytical methods have been developed recently which can be used to silll1late the dynamic response of deep water platforms. This paper describes an experimental program whose major objectives are (a) collection of data describing the characteristics of earthquake and wave excitations and the resulting structural response and (b) validation of analytical methods for predicting the dynamic response of platforms.

The Earthquake/Wave Instrumentation System was designed and installed on Platform Hope off carpinteria, California. The system measures and records accelerations, wave height profiles, and the accelerations of the platform during these events. This paper describes the innovations in the system that were required to meet the concepts and objectives of the experimental program.

Early results from operation of the system are reported. This includes analysis of ambient vibration tests which yield data on the fundamental natural frequency of the platform. Wave data from a moderately large storm is reported.

Basic System Concepts

There are three basic concepts which led to the development of the Earthquake/Wave Instrumentation System:

  • the need for design tools which would provide better estimates of the dynamic behavior of platforms during earthquakesand heavy sea conditions,

  • the need for strong-motion instrumentation on an offshore platform to supplement the onshore network, and

  • the need to delay large data handling expenditures until the value of the data could be determined.

These concepts formed the guidelines for decisions on the eventual system design and hardware.

Dynamic Behavior of Platforms

Offshore platforms, whose heights are less than 300 feet, can usually be designed using static methods only. These platforms exhibit essentially static behavior, since their natural frequencies are sufficiently separated from the frequency band associated with high-density wave energy. Platforms can be economically designed in the 300 foot to 600 foot range that are relatively stiff and have adequate frequency separation. However, above 600 feet the lowest frequencies approach the frequency band of maximum wave energy. This implies the structure will exhibit dynamic behavior and that it should be designed using methods which considers its dynamic properties and response characteristics. All deep water platforms in areas of high earthquake activity should be designed using dynamic methods.

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