Det Norske Veritas - Germanischer Lloyd

DNVGL  GEOTECHNICAL DESIGN AND INSTALLATION OF SUCTION ANCHORS IN CLAY
 N DNV-RP-E303

Annotation

Scope of application

The design code applies to the geotechnical design and installation of suction anchors in normally consolidated clay for taut, semi-taut and catenary mooring systems. The design code is applicable to anchors for both temporary and permanent mooring.

The design code provides procedures for determination of the anchor resistance and (by reference) the characteristic load required by the code.

The code makes use of a relatively detailed resistance analysis. If a less detailed resistance analysis is applied, the designer should be aware of the limitations of the method and make sure that the effects of any simplifications are conservative in comparison with the results from the more advanced methods.

With reference to /17/ a number of existing 3D finite element methods meet the analysis requirements of this code. It was also reported in /17/ that the plane limit equilibrium method used in the calibration of this code, as well as a quasi 3D finite element model, where the 3D effects are accounted for by side shear on a 2D model, generally show good agreement with the 3D finite element analyses. Also a plastic limit analysis using a function fitted to approximate upper bound results gave good results.

This should thus open for different choices with respect to analytical methods, which will meet the analysis requirements in this code.

The partial safety factors for use in combination with this design code are calibrated on the basis of structural reliability analyses. The scope of the calibration /10/ covers conventional cylindrical suction anchors, with closed vents on the top cover, in normally consolidated clay, subject to extreme line tensions representative of a semisubmersible operating in 1,000m water depth in the Gulf of Mexico and at Haltenbanken, offshore of Norway, and a moored ship in 2,000m water depth at Haltenbanken. Two normally consolidated (NC) clay profiles have been included, one of them with a top layer with a constant undrained shear strength, whose thickness is determined by a requirement of strength continuity at the intersection with the underlying NC clay. The coefficient of variation (CoV) of the soil strength is set to 12 and 15% in the NC clay and to 20% in the top clay.