Seismic Risk Analysis for Offshore Platforms In the Gulf of Alaska
Authors: Wiggins John H. , Hasselman T.K. , Chrostowski Jon D. 
Published: October 2008

Six different techniques for examining the seismicity of the Gulf of Alaska region have been considered. Two of these methods were combined to predict probable earthquake intensity and maps of ground motion characteristics.

The modeling of offshore platforms and defining possible damage modes is considered next and the risks encountered by such structures located in the Gulf of Alaska seismic environment are outlined. Finally, the capacity of offshore platforms to resist intensities of the level set under new earthquake codes is examined.

EARTHQUAKE DESIGN RATIONALE

Earthquake engineering comprises three disciplines in the scientific community. The first deals with the seismic environment in which principally seismologists and geologists work. From the knowledge of the seismic environment, one can estimate ground shaking, structural response and the failure of various structural elements and components. The latter two areas are the concern of the structural engineer and specialists in engineering mechanics.

All of these disciplines and the knowledge inherent within them, have varying degrees of uncertainty. By combining all of the disciplines with the uncertainties inherent within each, one can estimate the seismic risk (uncertainty of loss)"of a particular structural design located at a particular geographical position.

Figure 1 illustrates the specific steps that must be treated in an earthquake engineering analysis. We shall first discuss the "proneness" of an area to earthquake activities covered by the number one (1). By combining the seismicity inputs with the soil-structure models (number two (2)), modes of vibration and estimates of damage can be computed in probabilistic terms. Taking the next step regarding failure (number three (3)) and carrying the study through the risk analysis stages (number four (4)) completes the exercise.

Until recently, earthquake design codes, as well as almost all codes and standards, have been developed with the unspoken "hope" that absolute safety would result. We now realize that some risk is involved with every standard or code used in design practice. Earthquake codes currently being developed for the National Bureau of Standards by more than 70 national experts are being developed with a clear expectation of risk (chance of loss) in mind.

SEISMICITY

The mechanism of earthquake action in the Gulf of Alaska is now generally agreed to be caused by a layer of roving plates which are moving relative to one another (Figure 2). The Pacific Plate is being forced northwesterly in relation to the American Plate. The area of interest is located in the vicinity of the junction of the Pacific and American Plates.

There are six basic methods or rationales which have been developed in order to make best first estimates about future seismic motions (see Table 1). Method 1 is deterministic in its approach. Maximum credible earthquakes are postulated to occur on known fault lines which intersect the earth's surface. Usually an earthquake magnitude and distance from source to site" is postulated by an expert.