Observation and Analysis of Earthquake Response of a Coupled Pile Offshore Platform
Authors: Ueda Shigeru , Shiraishi Satoru 
Published: October 2008

This paper describes about the earthquake observation on a deepwater platform and the neighboring ground. Seven sensors are set in the ground and on the platform. Three sensors (vertical 1, horizontal 2) are set five meters under the sea bed near the platform, three sensors (vertical l, horizontal 2) are set on the platform and the remaining one is set opposite to the aforesaid sensor on the platform. Ten records of earthquake motions have been obtained since March 1978 and those are analyzed. Also vibration characteristics of the platform are analyzed. And the results computation by use the one degree of freedom model and the multi degree of freedom model, are compared with the results of observation. Then, such items as added mass coefficient of pile, damping factor of the platform, the mat-hermetical model of earthquake response, analysis and the modeling of the platform are examined in detail.

Introduction

Coupled pile structures are often constructed for deepwater terminals. This type of structures are subjected to load from waves, winds, earthquakes and ships berthing and mooring. In the seismic zone, load from earthquakes is a principal load to determine the structural dimension of those structures supporting heavy superstructure. Against, there have been many deepwater platforms constructed in the world, there is no earthquake records observed on the deepwater platform.

Therefore nobody could evaluate and discuss the earthquake resistant design of those deepwater terminals by use of the actual earthquake records. Then, the investigation was done at the presently operating platform in Kashima, Japan. The platform is 200,000 DWT oil terminal of a coupled pile structure. The structure is supported with ten vertical piles and eight battered piles. (Figure 1) Three seismometers are set in the ground and four seismometers are set on the platform. The, earthquake records have been obtained.

Those records are analyzed and dynamic response characteristics of the platform is examined. Frequency spectrums and transfer functions are computed from the records.

On the other hand, computation programs of earthquake response analysis were made by use of both one degree of freedom model and multi degree of freedom model. For the earthquake response analysis, it is important to determine correctly, the values of such items as added mass of piles and damping factor of the structure. Those values are examined in detail in accordance with the analysis of the earthquake records and the earthquake response/ analysis.

Furthermore, in the analysis of this report, working stress in the member is limited within elastic region, because of the difficulty to obtain the strong earthquake records which may cause to induce plastic deformation in the structure members. The difficulty was predicted in advance, however, the earthquake response analysis in elastic region is still important to examine the vibration characteristics of the platform in operational condition. For instance, API recommendation prescribes that the safety of the platform shall be examined both for strength level earthquakes and ductility level earthquakes and the stress of members should be lower than the allowable stress for strength level earthquakes.