Experimental Studies of Scale Model Drag Embedment Anchors

92 Pages·2015·5.87 MB·English

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Civil and Environmental Engineering - Theses College of Engineering and Computer Science
5-2013
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Tiffany Z. Monaco
Syracuse University
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Monaco, Tiffany Z., "Experimental Studies of Scale Model Drag Embedment Anchors (DEAs) Subjected to
Impulse Forces" (2013). Civil and Environmental Engineering - Theses. 4.
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ABSTRACT

The goal of this research is to increase the understanding of the performance of drag
embedment anchors (DEAs) under a dynamic impulse force to inform the design of
mooring systems subject to impulses. The research methodology included scale-model
testing, data analysis, and comparison with other performance predicting methods. The
investigation was conducted on a 1/6th scale model Danforth-style drag embedment
anchor in cohesionless soil subjected to an in-plane impulse force. For all impulse forces
less than 0.8 seconds, the DEA consistently held up to 200% of the maximum quasi-static
holding capacity. For very short impulses, less than 0.3 seconds, the DEA held up to
600% of the maximum static holding capacity. The DEA did not continue to penetrate
along a predicted anchor trajectory when subjected to impulse forces. Instead, it
exhibited a rotational motion. Based on the results of this investigation, it is believed that
the inertial forces of the soil and anchor, which oppose the acceleration of the impulse,
develop a moment. This causes the anchor to rotate. The resulting change in anchor
orientation alters the balance of forces restraining the anchor and, under certain
conditions, the anchor may become unstable and fail under the impulse force. Further
field testing and computer modeling of the proposed inertial force method of dynamic
equilibrium is recommended to predict its reliability under expected impulse loading.
EXPERIMENTAL STUDIES OF SCALE MODEL DRAG EMBEDMENT
ANCHORS (DEAs) SUBJECTED TO IMPULSE FORCES

by
Tiffany Z. Monaco, P.E.

B.S., U.S. Naval Academy, 1993
M.S., Syracuse University, 2013

A Thesis
Submitted in partial fulfillment of the requirementsfor the degree of
Master of Science in Civil-Geotechnical Engineering

Syracuse University
May 2013
Copyright © Tiffany Monaco, May 2013
All Rights Reserved
iv
ACKNOWLEDGEMENTS:

I am grateful beyond words for the support of so many who have encouraged me to
complete my Masters Degree. To Dr. Clemence, Dr. Bhatia, and Dr. Kriebel, thank you
for understanding and accommodating my family and work obligations that I juggled
throughout. I am grateful to Mr. Gerritt Lang at the Naval Facilities Engineering Service
Center (NFESC) for support of this research project, and to the U.S. Naval Academy
Hydromechanics Laboratory Staff for their expertise in research support.

Thank you to my parents, Joan and Tony Zallnick, for doing what great parents do, and
then some. To my children, Sam, Abby and Peter, who may be the only kids their ages to
have run MatLab, thanks for your patience and “help.” And, to my husband, A.J., thanks
for saving a space on the wall and having confidence in me that I’d finish.
v
TABLE OF CONTENTS
Abstract ................................................................................................................................ i
Title Page ............................................................................................................................ ii
Copyright page ................................................................................................................... iii
Acknowledgements ............................................................................................................ iv
Table of Contents ................................................................................................................ v
List of Figures .................................................................................................................... vi
List of Tables .................................................................................................................... vii
1 Introduction ...................................................................................................................... 1
1.1 Background ............................................................................................................................ 1
1.2 Objective ................................................................................................................................ 5
1.3 Significance of Proposed Research Program ......................................................................... 6
2 Literature Review ........................................................................................................... 10
3 Test Media, Equipment, Instrumentation ....................................................................... 20
3.1 Test Media ............................................................................................................................ 20
3.2 Equipment and Testing Apparatus ....................................................................................... 23
3.2 Instrumentation and Data Collection ................................................................................... 28
4 Procedures and Results .................................................................................................. 30
4.1 Procedures ............................................................................................................................ 30
4.2 Results .................................................................................................................................. 32
5 Discussion of Results ..................................................................................................... 38
5.1 Summary .............................................................................................................................. 38
5.2 Observations ......................................................................................................................... 42
5.2 Recommendations for Further Analysis ............................................................................... 44
6 Development of Analytical Model ................................................................................. 46
6.1 Static Equilibrium Model ..................................................................................................... 46
6.2 Inertial Force Method of Dynamic Equilibrium .................................................................. 48
7 Conclusion & Recommendations for Further Research ................................................ 53
7.1 Summary .............................................................................................................................. 53
7.2 Future Work ......................................................................................................................... 54
Vita .................................................................................................................................... 58
Appendices ........................................................................................................................ 59
vi
LIST OF FIGURES

FIGURE 1.1 COMPONENTS OF DRAG EMBEDMENT ANCHOR MOORING (ROCKER, 1985) ................................. 1
FIGURE 1.2 ANCHOR EMBEDMENT (ROCKER, 1985) ........................................................................................ 4
FIGURE 1.3 PHOTO OF PORTABLE PORT SECURITY BARRIER (TRUSTON TECHNOLOGIES, 2011) .................... 8
FIGURE 1.4 MODEL OF IMPULSE FORCE DUE TO VESSEL IMPACT (SEELIG, NIXON & MCCOY, 2007) ............. 9

FIGURE 2.1 ANCHOR CAPACITY CURVES (NCEL, 1987) ............................................................................... 11
FIGURE 2.2 ANCHOR FAILURE SURFACES (NEUBECKER & RANDOLPH, 1996) ............................................... 14
FIGURE 2.3 FORCES ACTING ON ANCHOR UNDER EQUILIBRIUM (NEUBECKER & RANDOLPH, 1996) ............ 15
FIGURE 2.4 FREE-BODY DIAGRAM (NEUBECKER & RANDOLPH, 1996) ......................................................... 15
FIGURE 2.5 THREE-DIMENSIONAL FAILURE WEDGE (NEUBECKER & RANDOLPH, 1996) ................................ 16
FIGURE 2.6 STRAIN-RATE ADJUSTMENT OF FRICTION ANGLE FOR SOIL TYPE (ROCKER, 1985) .................... 18
FIGURE 2.7 HOLDING CAPACITY FACTORS FOR PLATE ANCHORS IN COHESIONLESS SOIL (ROCKER, 1985) . 19

FIGURE 3.1 PHOTO OF 1/6TH SCALE MODEL OF 200-LB DANFORTH STYLE ANCHOR ..................................... 20
FIGURE 3.2 INITIAL TEST SET-UP SKETCH ..................................................................................................... 24
FIGURE 3.3 PHOTO OF TEST SET-UP (USING SONIC DISPLACEMENT DEVICE) ................................................. 25
FIGURE 3.4 PHOTO OF TEST SET-UP (USING STRING POTENTIOMETER) .......................................................... 27
FIGURE 3.5 CALIBRATION OF OMNI LOAD CELL WITH POWER AMPLIFICATION ............................................ 28

FIGURE 4.1 SUMMARY OF TEST RESULTS (PEAK FORCE AND IMPULSES) ...................................................... 35
FIGURE 4.2 TEST RESULTS FOR IMPULSE TEST #62 ........................................................................................ 37

FIGURE 5.1 SAMPLE DISPLACEMENT PLOT .................................................................................................... 38
FIGURE 5.2 PEAK FORCE AS A PERCENT OF MAX. HOLDING CAPACITY VS. TIME TO PEAK ........................... 39
FIGURE 5.3 IMPULSE FORCE OF TEST #68 ...................................................................................................... 40
FIGURE 5.4 RELATIONSHIPS BETWEEN IMPULSE, MAX FORCE, AND TIME TO MAX FORCE ........................... 42
FIGURE 5.5 IMAGES OF INITIAL AND FINAL ANCHOR PLACEMENT ................................................................. 43

FIGURE 6.1 SIMPLIFIED STATIC FREE-BODY DIAGRAM ................................................................................. 47
FIGURE 6.2 SIMPLIFIED IMPULSE-MOMENTUM DIAGRAM .............................................................................. 50
vii
LIST OF TABLES

TABLE 1.1 RECOMMENDED FACTORS OF SAFETY FOR DRAG EMBEDMENT ANCHORS (ROCKER, 1985) .......... 2
TABLE 1.2 VARIABLES AFFECTING ANCHOR PERFORMANCE .......................................................................... 5

TABLE 2.1 AVERAGE ANCHOR EFFICIENCIES FOR SMALL ANCHORS (ROCKER, 1985) .................................. 13

TABLE 3.1 ANCHOR PROPERTIES .................................................................................................................... 21
TABLE 3.2 SPRING PROPERTIES ...................................................................................................................... 26

TABLE 4.1 ESTIMATING STATIC HOLDING CAPACITY OF SCALE ANCHOR ..................................................... 31
TABLE 4.2 NUMBER OF TESTING CONFIGURATIONS ....................................................................................... 33
TABLE 4.3 PARTIAL SUMMARY OF TEST RESULTS ......................................................................................... 33
TABLE 4.4 MAXIMUM, MINIMUM AND STANDARD DEVIATION OF RECORDED PEAK FORCES ....................... 35
1
1 INTRODUCTION
1.1 Background

Drag Embedment Anchors (DEAs) are a specific type of anchor used in offshore moorings.
They are predominately used for temporary moorings for marine vessels or other moveable
offshore structures. They embed in the upper seabed floor, can be removed relatively easily,
placed in another location, and have a high holding capacity-to-weight ratio. There are various
configurations of anchor geometry, typically consisting of shank (the body), and fixed or
moveable fluke(s). DEAs may or may not have a stock for rolling stability. Figure 1.1 below
identifies the relevant components of the DEA and mooring system.

Figure 1.1 Components of Drag Embedment Anchor Mooring (Rocker, 1985)
DEAs are designed to hold in place up to a pre-determined capacity under quasi-static loading.
However, there is no generally accepted predictor of their reliability and performance under
dynamic loads. When used in applications subject to impulse or cyclic forces, a design factor of
safety can be adjusted depending on the acceptable condition of the mooring, as shown in Table
1.1. This information is based on the American Petroleum Institute’s (API’s) recommended
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