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Reliability assessment of drag embedment anchors in sand and the effect of idealized anchor geometry

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Abstract

In this study, the reliability of drag embedment anchors in the sand was assessed and the effect of anchor geometrical idealization on reliability indices was investigated as an inherited characteristic of analytical approaches. The anchor holding capacity was obtained by performing a series of iterative limit state analyses and a probabilistic model was developed for the selected anchor families. The tensions of the mooring lines connected to a semisubmersible platform were obtained by performing a series of time-domain dynamic mooring analyses using the OrcaFlex software. The uncertainties in environmental loads, metocean variables, and stress distribution along the catenary mooring lines were incorporated into the line tensions through the response surfaces. An iterative procedure was performed by adopting the first-order reliability method (FORM) to calculate the comparative failure probabilities in sand and clay. The study showed significant dependence of the anchoring system reliability on geometrical configuration of anchors, the seabed soil properties, and the environmental loads. It was observed that the implementation of the reliability-based design into the existing in-filed trial procedures could significantly improve the efficiency and cost-effectiveness of the design practice.

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Abbreviations

As :

area of shank.

d :

nominal chain diameter.

D :

pad-eye embedment depth.

df :

fluke thickness.

ds :

average depth of the shank.

dua :

the absolute displacement of the anchor.

dus :

soil wedge displacement.

dusa :

displacement of the soil relative to the anchor.

dw :

wave direction.

dwc :

current direction relative to wave.

dww :

wind direction relative to wave.

En :

normal circumference parameter.

En :

tangential circumference parameter.

f :

form factor (Neubecker and Randolph, 1996a).

F :

friction force.

Ff :

the fluke force.

Ffb :

the force on the back of the fluke.

Fs :

the shank force.

h :

back edge of the fluke.

H :

depth of fluke tips.

Hs :

significant wave height.

Lf :

fluke length.

Lf :

caisson length (Silva-González et al., 2013)

Ls :

shank length.

Nq :

standard bearing capacity factor.

Nqs :

shank bearing factor.

pF :

probability of failure.

pFa :

annual probability of failure.

q :

bearing pressure.

Q :

normal soil reaction on chain segment.

\( \overline{\mathrm{Q}} \) :

average bearing resistance per unit length of chain over embedment depth.

R :

anchor capacity at mudline.

R :

soil reaction.

Ra :

anchor capacity at pad-eye.

Rd :

design anchor capacity at mudline.

Rd,a :

design resistances at the pad-eye.

ri :

distance between point i and anchor shackle.

s :

length of chain.

SF :

side friction.

T :

line tension.

Ta :

line tension at the pad-eye.

Td :

design line tension at mudline.

Td,a :

design tensions at the pad-eye.

Tdyn,max :

mean maximum dynamic line tension.

Tdyn,max-C :

characteristic mean maximum dynamic tension.

Tmean :

mean line tension.

Tmean-C :

characteristic mean line tension.

To :

Chain tension at mudline.

Tp :

spectral peak period.

T* :

normalized tension.

∆t :

extreme sea state duration

U10

wind velocity.

Uc :

surface current velocity direction.

w :

chain self-weight per unit length.

Wa :

anchor dry weight.

Ws :

the mobilized soil mass.

xa :

anchor horizontal displacement.

X :

absolute displacement of point i.

x* :

horizontal distance normalised by D

∆x :

absolute penetration increment of the origin Y

absolute displacement of point i

∆y :

absolute penetration increment of the origin z

depth below mudline

z* :

depth normalised by D

β :

inclination of fluke

β :

reliability index

βannual :

annual reliability index

∅′ :

soil friction angle

∅p :

sand peak friction angle

γ′ :

effective unit weight of soil

γdyn :

partial safety factor on dynamic line tension

γmean :

partial safety factor on mean line tension

λ :

failure wedge angle

λ :

mean annual rate of extreme sea states

ηa :

anchor efficiency

μ :

chain-soil friction coefficient

Θ :

vector of environmental variables

θ :

line tension angle

θa :

line tension angle at the pad-eye

θi :

polar coordinate angle of point i

θfs :

fluke-shank angle

θo :

line tension angle at mudline

∆θ :

rotation increment of the origin

ψ :

dilation angle

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Acknowledgments

The authors gratefully acknowledge the financial support of this research by Memorial University of Newfoundland through VP start-up fund and school of graduate studies (SGS). The technical advice of Mr. Mohammad Javad Moharrami is also kindly acknowledged.

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Authors and Affiliations

  1. Civil Engineering Department, Faculty of Engineering and Applied Science, Memorial University of Newfoundland, A1B 3X5, St. John’s, NL, Canada

    Amin Aslkhalili & Hodjat Shiri

  2. Process Engineering Department, Faculty of Engineering and Applied Science, Memorial University of Newfoundland, A1B 3X5, St. John’s, NL, Canada

    Sohrab Zendehboudi

Authors
  1. Amin Aslkhalili
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  2. Hodjat Shiri
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  3. Sohrab Zendehboudi
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Corresponding author

Correspondence to Hodjat Shiri.

Appendix 1

Appendix 1

The user interface of the developed VBA macro is shown below. The macro receives the input parameters related to the anchor configuration, seabed soil parameters, and anchor kinematic parameters as the input values. Then using the adopted limit state solution the holding capacity and key outputs are calculated.

Fig. 17
figure 17

User interface of developed VBA macro for calculation of anchor holding capacity

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Aslkhalili, A., Shiri, H. & Zendehboudi, S. Reliability assessment of drag embedment anchors in sand and the effect of idealized anchor geometry. Saf. Extreme Environ. 1, 39–57 (2019). https://doi.org/10.1007/s42797-019-00006-5

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  • DOI: https://doi.org/10.1007/s42797-019-00006-5

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