Geotechnical design and installation of suction anchors in clay
|Publication Date:||1 April 2017|
Scope of application
The design code applies to the geotechnical design and installation of suction anchors in normally consolidated clay for taut, semitaut 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.
It is generally recognised that the robustness of the code will increase as the scope of the calibration becomes broader, and in the future it may be necessary to broaden the scope to include other design situations than those covered by the present code calibration. For example, the current edition of this design code assumes that the governing loads lead to undrained conditions in the clay. If drained conditions need to be considered, this has to be evaluated on a casebycase basis. A general description of the calibration procedure adopted is given in /11/.
The specified material factor γm on anchor resistance for use in combination with the design code presented herein is based on the results of the reliability analysis and code calibration reported in /10/. An epistemic uncertainty in the soil strength with a coefficient of variation of about 15% is accounted for and comes in addition to a coefficient of variation (CoV) for the natural variability of the soil strength of up to 15% (20% in the top clay). It is noted that the quantification of the epistemic uncertainty is difficult. Therefore, efforts to improve the techniques for soil investigation and the methods of interpretation of the results from field and laboratory investigations are necessary to keep the epistemic uncertainty as low as possible.
The requirement to the material factor γm is given with the assumptions
- that the anchor resistance is calculated using the mean cyclic shear strength of the clay as the characteristic value
- that the submerged weight W' of the anchor is included in the characteristic anchor resistance RC upon which the material factor is applied, and
- that the partial material factor is used together with the partial safety factors specified in this code.
The installation accuracy in terms of outofverticality (tilt) and misorientation has been accounted for in the reliability analyses by assuming expectations of no tilt and no misorientation in conjunction with standard deviations of σ = 3º for tilt and σ = 3º for misorientation. The installation has thus been assumed to be carried out with this accuracy. However, in practice one must design the anchor for the installation tolerance specified in the design basis, and after installation of the anchor one must verify that this tolerance has been met.
For an explanation of symbols and terms used in this recommended practice, see [1.6]. Additional symbols and terms may also be defined in the text.