The Aasgard is an oil, gas and condensate development offshore mid Norway in 300 m water depth. The field is developed with early oil production from a floating production, storge, and offloading vessel(Aasgard A), gas producton by semi submersible unit(Aasgard B), condensate export through a floating storage unit (Aasgard C) and gas export through a trunk pipeline4 to northern Germany as shown in Fig 1.(Fig. 1 is available in full paper.
Soil Conditions at the three locations are well documented through extensive site investigations comprising CPT and sampling borehole at altogether more than 680 locations. The sediments at Aasgard consist of low plasticity clay deposited close to the ice sheet which covered all of Scandinavia up to approximately 10 000 years ago. Due to variable depositional conditions and ice loading after deposition, the undrained shear strength of the clay have much greater variation than usually encountered in marine clays. The undrained shear strength typically increase from 5 to 25 kPa at sea bottom to 40 to 800 kPa at 9 m penetration with large variations in strength at all depths. There are also quite large variations in strength at locations no further than 100 m apart. The ranges in strength for Aasgard A, B and C locations are presented in Fig. 2.(Fig. 2 is available in full paper)
The undrarned shear strength at Aasgard also have significant variation with depth at each location No location are consistently stronger than then all other locations, but contain some low and some high strength area with each layer The upper bound strengths are consequently not defined as an envelope to the highest strength at all locations, but as the highest overall strength likely to be found at one location in the area The lower bound strengths are closer to an envelope of the lowest strengths identified as some locations have consistently low strengths and it is likely to find a location with strength close to the lower envelope. Other characteristics of the clayey soil conditions at Aasgard are a relatively high content of gravel, stones and boulders, low plasticity and low water content as shown in Table 1.(table 1 is available in full paper)
The number and size of boulders are estimated based on cone tip resistance and registration from drillers logs. Geophysical surveys have been carried out with instrumentation carried by ROV's providing high quality data. Sub seabed boulders have been difficult to identify, unless being very large (one to two metre in diameter), as the sediments consist of all grain sizes giving a chaotic reflection. Boulders at seabed have been identified down to sizes of 0.2 m diameter, and areas with larger seabed boulders have been avoided. The geophysical surveys have identified boulders of several meters in diameter at the sea bottom in other areas of the Aasgard field, supporting the design assumption of accounting for penetrating anchors past boulders up to approximately 0.5 m diameter and relocating anchors if even larger boulders are met.