Structural Design Of An Instrumented Mast To Be Emplaced On Cobb Seamount In The Northeastern Pacific Ocean
Authors: Peterson P.L. , Haelsig Richard T. , Forssen D. Ken 
Published: October 2008

A feasibility study is summarized which analyzed and compared two diverse configurations being considered for a mast which will provide a stable platform for engineering and scientific measurements in a unique deep ocean environment associated with a seamount in the Pacific Ocean. The representative configurations are a guyed spar and a trussed tripod. Factors included in the preliminary design analysis were loadings due to the maximum design wave, static deflections, the stability and large deflection characteristics of the structure due to statically applied wave forces, guy reactions, and the dynamic or modal Characteristics.

INTRODUCTION

The Cobb Seamount is a unique submarine mountain situated in the Northeastern Pacific Ocean approximately 270 miles from the coast of Washington. As shown in Figure 1, it is separated from the continental margin by an 8,500 foot deep flat plain known as the Cascadia Basin. It is one of about 100 prominent undersea mountains in this region. However, the Cobb mount is unique in that it has the only summit known to extend well within the region penetrated by light of the sun. The Seamount, called a guyot because of its flat top, is an extinct volcano of a symmetrical, terraced configuration with a centrally located pinnacle. Figure 2 illustrates the detailed bathymetry of the Seamount.

The mountain is indented with three important terraces which in ascending order occur at approximately 3000 feet, 600 feet, and 270 feet. The central pinnacle rises from the relatively broad 270 foot pedestal at a steep 45° slope to within 120 feet of the surface. The summit is relatively flat in the form of an oval with a total area of about 80 acres. The pinnacle is distinctly ribbed consolidated basalt and is probably the volcanic neck of the mountain which submerged for the last time below the rising seas of the Holocene Period, about ten thousand years ago.

Abundant benthic and pelagic activity has been noted on and about the summit down to the "twilight" 270 foot terrace. Initial bottom faunistic surveys have indicated that the Cobb Seamount community is similar to sublittoral communities in Puget Sound, but with some subtle differences. The fish population is extensive with an apparent prevalence of red snapper and vermilion rockfish. An abundance of bird life has been noted in the area of the pinnacle, indicating that the seamount is probably a prime fish feeding ground. The bioenvironmental system supported in this isolated region is scientifically significant. The Seamount presents the definitely unique environment of a characteristic North Pacific deep ocean water column coupled with a remote nonshoaling bottom located in the photic zone.

The water mass around Cobb Seamount is dominantly Sub-Arctic water mixed with 10 to 20 percent Equatorial Pacific water. Hydrographic observations of currents and water structure around the mountain made by the University of Washington have revealed no gross changes or anomalies which could be attributed to its presence. There is some indication of a pile-up of water on the upcurrent side of the pinnacle out to about the extent of the 600 foot terrace.