Abstract
This paper presents recent investigation results and geotechnical considerations used to design a manmade gravel island in support of offshore oil production in the Beaufort Sea off the coast of Alaska. Winter geotechnical investigations were conducted in 1997, 1998, 2013, 2014, and 2015 along multiple subsea pipeline alignments and several proposed island locations and included finding a gravel resource, analyzing sea ice safety, drilling boreholes, performing cone penetrometer tests, recording subsea ground temperatures, and laboratory testing to measure engineering properties of the soils. The results were used to characterize the geologic setting and permafrost conditions, estimate thaw settlement of the gravel fill and underlying permafrost, compressibility and strength of seabed sediments, island slope stability, and shear resistance to global ice forces. The island site is located in 20 feet of water with a finish elevation 15 feet above sea level for a total fill thickness of 35 feet. Total island surface settlement outside of the well row is estimated at three feet in the long term. A soft marine organic deposit underlies the island and critical slope stability conditions will occur during construction when steep subsea perimeter slopes are possible. Fill placement above the water line integrates a 30-foot setback from the edge of submerged fill to maintain minimum factors of safety and improve constructability. After consolidation and soil strength gain, the island stability factor of safety increases and long term ice pack loading and local wave scour management become more critical. Based on island geometry, seabed soil properties, and global ice forces, the island has an expected safety factor of 3.0 against shear failure. The results of the geotechnical investigations allowed permitting efforts to proceed by confirming conventional and proven island construction materials and methods can be used with some additional engineering considerations.
