Site Location Survey for the Heritage Platform

Abstract

ABSTRACT This paper describes a geophysical and geotechnical site survey for the Heritage Platform to assess jacket levelness and water depth. Data were obtained from deep-tow seismic and cone penetrometer equipment which were positioned within the survey area through the use of an on-bottom network of medium frequency acoustic transponders. This allowed the high degree of seafloor mapping accuracy to be carried through to jacket placement. Upon set-down, the jacket was found to be well within the required levelness tolerance and in a water depth within one foot of the design specifications. This application demonstrates the accuracy with which the methods described in the paper can characterize the seafloor for engineering applications. INTRODUCTION The Heritage Platform was one of two jackets installed in 1989 as part of the Santa Ynez Unit Expansion Project. Harmony was installed in the Summer of that year in approximately 1200 feet of water. Heritage was installed in the Fall in about 1075 feet of water. The near surface soils in the vicinity of the Heritage location consist of a thin clay layer at the surface with thickness varying between 0 and 20 feet. This clay layer is commonly characterized as being seismically transparent since it is faintly recorded on high resolution sub-bottom profiler records. Immediately below the clay is a stratum of dense sand. This uppermost sand stratum extends to a depth of about 90 feet. Below that penetration, layers of stiff clay and silty clay, ten to twenty feet thick, alternate with other sand strata several tens of feet thick (1). The top surface of the uppermost sand layer is cut by numerous channels formed by ancient turbidity flows. The thickness of the surficial clay varies with the presence or absence of these channels; being thin over high areas such as channel levees or interchannel highs and thicker where channels are present. The seafloor and the top surface of the sand slope to the southwest with a gradient of about 1 to 2 percent. The bottom framing of the Heritage jacket was designed to match the slope of the seafloor as determined from regional bathymetry and sub-bottom profiler data obtained in the early 1980's. The mudmats were sized to penetrate through the soft surficial clay and bear on top of the dense sand just below. These early surveys indicated that the thickness of the clay and the seafloor slope were fairly constant over the area selected for jacket placement. However, a detailed review of data from soil borings performed in 1984 and drop cores from a pipeline survey in 1988 indicated that the clay thickness was considerably more variable than originally thought. More variability in the clay layer thickness implied a more irregular bearing sand surface. This suggested the possibility that the jacket could exceed the levelness tolerance by as much as 1.4 degrees upon set-down on the seafloor. Correcting levelness after set-down would have been difficult because of the size of the jacket and stiffness of the sand layer.