This paper focus mainly on the approach to geotechnical design and comparison of penetration resistance with measured resistance for 20 suction anchors installed at the Njord Field in the North Sea during the summer of 1997. The measurements were performed using a dedicated data acquisition system which produced detailed real-tune information on a computer display during the installation.
Also included are comparison of penetration resistance with measured resistance for some of the 16 suction anchors installed at the Visund Field in the North Sea during the autumn of 1997.
Both the Njord Field and the Visund Field are operated by Norsk Hydro. The soil investigations, laboratory testing and reporting, were performed by the Norwegian Geotechnical Institute (NGI) for both locations. For the Njord Field, NGI acted as a consultant to Norsk Hydro for verification of the geotechnical design. The geotechnical design for the Visund suction anchors was performed directly by NGI as a sub-contract to the anchor supplier W O E Anchor Contracting AS).
The Njord Field is located at Haltenbanken in Block 640717 about 150 km north of Knstiansund in Norway, see Figure 1. The development consists of a Floating Production Unit (FPU) and a Floating Storage Unit (FSU) both catenary moored with a total of 20 suction anchors at a water depth of more than 330 meters. The field layout is shown in Figure 2.
The Visund Field is located at Block 3418 about 150 km west of Floro in Norway, see Figure 3. The development consists of a Floating Production Unit (FPU) catenary moored with a total of 16 suction anchors at a water depth of approximately 335 meters. The field layout is shown in Figure 4.
There is currently a tendency to perform project schedules with drastically reduced execution time. When the anchor design starts only indicative locations may exist, and the final anchor locations may end up several hundred meters away.
This is normally not a problem for homogenous sod conditions, however, the geotechnical design of suction anchors in such a situation may be quite challenging if the sod conditions vary significantly across the field. A rational approach to sod investigation and geotechnical design therefore requires some consideration. This would apply for all type of anchors designed to be installed without proof loading to the ultimate holding capacity.
The Njord Field has the largest variation in the soil conditions for these two fields and this paper will focus on the approach used at Njord together with comparison of the measured penetration resistance versus the predicted penetration resistance for the Visund Field.
At project initiation only general knowledge existed about the sod conditions at the Njord Field. About 10 m of the upper soil profile consisted of a soft, normally consolidated clay underlain by a very hard overconsolidated clay not likely to be penetrated. Numerous plough-marks on the seabed showed that the upper soft clay layer had been scoured by drifting icebergs
Initially, detailed bathymetry map was produced and based upon this map