MaTS
r- 1
REPORT ON INVESTIGATION REGARDING 'ANCHORS AND ANCHOR TESTS'
ON BEHALF OF THE
NETHERLANDS INDUSTRIAL COUNCIL FOR OCEANOLOGY
IRO
DA1 Wig
MARIEN TECHNOLOGISCH SPEURWERK
Netherlands Marine Technological Research
d
Netherlands Industrial Council for Oceanology
P1983-2
By: Engineering Consultants Svasek BV - Rotterdam
Heer Bokelweg 145
March 1983
Table of Contents Page Preface 1 Introduction 1 Scheme of investigation 4 Technical aspects 6
Classification and certification 12
7. Conclusions 17
The Netherlands Industrial Council for Oceanology (I.R.0.) requested Svasek B.V. Engineering Consultants in 1982 to perform a study on anchors and mooring systems.
At present classification of modern anchors (such as
Bruce, Flipper Delta and Stevin) takes place according to the "Rules of Shipping", which are standardized and
commonly used by all classification bureaus.
Rules applied on offshore units originate from the Rules
of Shipping.
The I.R.O. feels that the Rules do not completely match
the capacities of modern anchors and has requested Svasek B.V. the following:
To investigate which parameters are the governing factors for the holding power of anchors and to study the standards for classification.
To investigate whether the introduction of a new anchor class is feasible and in which way this can be achieved.
2. INTRODUCTION
Over the last ten years an enormous evolution in the field of anchors and anchoring systems has taken place.
Until the early seventies there were hardly any new
developments in the field of anchor applications. However, due to the increasing sizes of dredging vessels and pipe
laying barges, and the growth of the offshore industry, the need for high holding anchors with minimal anchor weight arose.
The anchor world is familiar with a wide variety of anchor types and anchor applications, but anchoring based on the
principles of the traditional bow anchor is by far the
most applied.
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-Classification societies have played a very important role in the history of the anchors The demand that a ship must be able to fix its position temporarily has always been a
case of the utmost importance for insurance companies. The insurance companies were responsible for the setting up of classification societies which laid down rules for
classification in the "Rules of-Shipping", based on their
experience.
The anchor forms part of the ship's equipment which needs classification before a ship can be insured. This means the,anchor-eystem (anchors and chains) must meet minimum
requirements.
In the Rules of Shipping two anchor classes appear, both characterized by different relative holding capacities
(ratio of holding capacity and anchor weight, also called efficiency). These anchor classes give the frame work for anchor applications. Moreover the anchor classes are often used to indicate the holding capacity of the individual anchor makes for commercial purposes.
It is general practice to carry out anchor tests in order to determine the quality of anchors and mooring systems. In fact these anchor tests have contributed to the
gathering of knowledge and insight which are the basis of the present standards in the classification of anchors.
The increasing sizes in the offshore and dredging industry of past years has resulted in anchors able to provide high holding power being needed. Parallel to this it is tried
to keep the size and weight of the anchor limited.
Furthermore the offshore industry requires a high degree of reliability of anchored units whilst permitting
relatively small displacements, even under extreme weather
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3
The technology of anchor design at present is able to cope ,
-with these requi-rements,'but anchor classes that are used for anchors of seagoing vessels are not sufficiently tuned to mooring systems of offshore units.
There are three makes Of anchors, two of which are of
Dutch design, which 'cab: develop considerably more holding
power than required by the highest class in the shipping
rules, defined as "high holding power"..
3 . If classification societies would create a new class for
these types of:anChors, both the offshbreT.And anchor
The investigation of anchor standards and anchor behaviour was sub-divided as follows:
discussions with anchor users
discussions with classification societies inventory of anchor tests
review of literature
3.1 ANCHOR USERS
An important category involved with anchor applications is the category of anchor users. They are the ones that
determine what is expected from an anchor. From a series of informative discussions with users it has become clear what the characteristics of the types of anchor
applications are and what their opinion with respect to future developments is. Apart from the anchor users there were discussions with some anchor designers and scientists
of the faculty of shipbuilding from the Technical
University of Delft.
3.2 CLASSIFICATION SOCIETIES
The classification societies have drafted the anchor classes for the shipping rules. If extensions on the present anchor classes are required it is essential to check the present status of today's rules, what type of applications they refer to and how they were drafted. It is the ship's anchor from which work anchors and
offshore anchors (see section 4.1) for longterm anchoring and permanent mooring are derived. Ultimately a new type of anchor class will specifically meet the anchor
applications of the offshore industry. From the anchor study it shall become clear what the involvement of the classification societies is with respect to the
application of work anchors and offshore anchors.
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3.3 ANCHOR TESTS
In the past many tests with anchors have been performed not only for classification purposes but also for
fundamental research or research for special applications. An inventory of tests performed in the past has been made for as far as results were available. It must be noted that commercial aspects were mainly decisive in the handing over of anchor test results.
3.4 LITERATURE
Until 1970 the marine anchor was only studied by nautical experts. From 1970 onwards soil mechanical experts started to be interested in the behaviour of anchors and the way in which the holding power is developed. The traditional character, the recent developments, the simplicity of the anchor combined with the complex soil mechanical aspects are responsible for the fact that little literature is
available.
However it may be assumed that interesting and relevant information is being withheld to protect commercial
interests.
The greater part of the available literature concerning the traditional anchor, has been reviewed in this study.
4.1 GENERAL
The marine anchor prevents a floating construction from
9 displacement (except for a certain tolerance) due to wind,
current, waves or external mechanical forces (i.e. drilling forces) or a combination of these.
The following anchor types are at present being applied: the traditional anchor, based on the ship anchor being embedded by an external horizontal force.
the ballast anchor, which obtains its holding power due
to friction with the seabottom.
- the pile anchor, a steel pipe is driven into the bottom. A moveable head is mounted on this to which the anchor
line is fixed.
the suction anchor, which is forced into the bottom by creating a vaccuum under a large steel hull.
the jet anchor, a steel frame is sunk into the bottom by means of water jetting through holes in the frame.
the vibration anchor, based on the same principle as the jet anchor i.e. fluidising the surrounding soil.
Further discussion will be limited to the traditional
anchor.
In figure 1 the various parts of the traditional anchor are defined and the main anchor parameters are indicated.
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413/83.03.28
bottom
pulling direction
Fk Fr'sFk-shackle
load
Fs-force-soil on
shank-Ff-force-soil on
fluke-W -wheigt of anchor
steel cilinder
crown
definition of the various parts
of the traditional anchor
shank
Fffluke)
trimmin palm
pulling direction
shank
spacing flukes
fluke area
\ wing tip
trimming palm
FIGURE 1
The characteristics of mooring systems of floating drilling platforms are basically the same as those of the work anchor, however anchors are preferably used only once per location.
High standards account for:
holding power
displacement tolerance
reliability.
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The main characteristics of the sub-division of the traditional anchor are:
a. The ship anchor
The traditional bow anchors of ships serve for
temporary mooring, preferably in calm water: only used in exceptions
soil type unknown, anchor is supposed to perform in
any soil
simple handling, the anchor is randomly dropped - long distance for embedding and high forces for
breaking out the anchor are allowed
no great accuracy for positioning required.
b. The work anchor
Dredging and salvage vessels and pipe-laying barges use work anchors to fix their position or for repositioning
during operations:
frequent anchor cycles per location anchor continually in use
soil type is known, free choice of anchor type the anchor is properly presented to the bottom essential: short distance for digging in, no slip,
large holding power, low break-out force high accuracy for positioning.
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4.2 ANCHOR PARAMETERS AND HOLDING POWER
The information that is presented in the following four sections is mainly taken from the available literature
(see appendix Al).
For indication of the holding power capacity of an anchor, often the concept "efficiency" is used, which is the
maximum holding power divided by the anchor weight. Recent investigations have shown however, that the anchor weight is hardly relevant for holding power and mainly
determining the mechanical strength. Nevertheless historical development caused "efficiency" to be the commonly used anchor parameter, since it is a practical value and directly indicating the cost of an anchor.
Including soil mechanical aspects in the search for anchor parameters it becomes clear that efficiency never will be an over-all anchor parameter, to indicate the holding
capacity of an anchor.
Peuch, et al (1978, lit.13), have analysed the kinematical behaviour of the anchor, in which a number of stages are
distinguished.
The opening of the flukes is stimulated by the presence of "trimming palms" (fig.1), which place the flukes in the digging position, after the anchor has reached the bottom. The penetration is the stage from the start of embedding until the flukes are completely buried. This is determined by the fluke-shank angle, where a relative small value
for oC works in favour of penetration.
The burial itself is also dependent upon the fluke-shank
angle (:( , and for every soil type there is an optimum
value, for instance:
0
0. sand - 32
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An anchor has a high stability when effective provisions (stabilisers), such as steel cylinders or wing-tips
(fig.1), prevent the anchor from overturning .
Furthermore a large value for fluke spacing D, relative to fluke area, increases stability, as well as the symmetry
of the anchor.
Finally, the holding power is greatly dependent on the
soil characteristics such as angle of internal friction and cohesion. The anchor itself contributes mainly only by the size of the fluke area.
4.3 THE ANCHOR CHAIN
Habib (1982, lit.6) discussed the influence of the
anchoring line.
The anchor cannot be seen separately from what it is 4
pulled by, either chain or cable or a combination of both (fig.2).
Due to friction with the soil, part of the "deck load" is
directly transferred by the chain, so reduced force is transferred to the soil by the anchor itself ("shackle
load").
Fast burial is achieved by horizontal pulling and a heavy chain will favour this, but also contributes to holding
power.
The buried part of both chain and cable will have a
certain uplift while the anchor is pulled. The anchor will stop burying when the uplift balances the burial force of the burial force of the anchor.
Habib concludes that the thinner the line, the more deeply the anchor penetrates and the greater the tension in the
C- 4, f p-lift"
W
weight burial fore Fvn
V. V V' V IF 'W
,,deck (oad"
forces on anchor and in chain
FIGURE
21chain or cable,'
shackle toad F bottom
-a
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4.4 SOIL TYPES
Throughout a great part of the studied literature it
appears that soil mechanics plays an important role in
anchoring.
Due to the specific behaviour of anchors in various anchoring grounds, a number of different soil types for anchoring is distinguished by the users of anchors.
In soft mud and soft clay the anchor will be buried deeply, while holding Power increases only slowly with
increasing burial depth.
In order to reach a significant burial depth, a considerable horizontal distance is covered. Break-out forces are generally high.
In hard clay a higher holding power is reached than in soft clay, due to larger cohesion, however increase of holding power with burial depth is limited.
Loosely packed sand in general is good anchoring soil, the anchor penetrates normally 1 to 2 times the shank length, and horizontal displacement is small. The break-out force
is relatively small, if the anchor is pulled by the crown.
(fig.1).
Soft rock is an anchoring ground frequently met by
cutter-dredgers. The anchor hardly penetrates but merely hooks into small outcrops where a large holding power may be build up. However, the fluke-shank angle needs to be
small to prevent the anchor from overturning and
overtopping.
Finally anchoring in many grounds quite frequently occurs. Marl is known as difficult anchoring soil and comparable with soft rock.
Anchor behaviour in various soil types has been
investigated by Ruts (lit.14), Peuch et al (lit.13) and Saurwalt (lit.18 and 19).
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4.5 REVIEW ANCHOR TYPES
In the figures 3 and 4 the most applied anchor types are presented. The original anchor from which all others are derived is the traditional stock-anchor. Anchor makes like Pool, A.C. 14, Danforth an Stato, have found wide
application in shipping. Furthermore the three modern anchors Flipper Delta, Stevin and Bruce are shown. These three makes generally perform a lot better than the ones mentioned before, and for which the system of anchor classes has been set up by the classification societies.
4.6 MATHEMATICAL MODELS
Several times efforts have been made to set up
mathematical models in order to compute the holding power
of an anchor.
A few of these models are given in the literature: Ruts (1981, lit.14) and Saurwalt (1976, lit 26).
Mathematical models are, due to numerous soil parameters and characteristics, difficult to set up and calibrate. A large number of assumptions and simplifications must be made and in the resulting equations a number of unknown
STOCK ANCHOR STOCKLESS ANCHOR POOL ANCHOR A.c.14 ANCHOR DANFORTH ANCHOR HALL ANCHOR L.W,T. ANCHOR STATO ANCHOR
FIGURE 3(
STEVIN ANCHOR
"FLIPPER" DELTA ANCHOR
BRUCE ANCHOR
There is a distinct difference between classification and certification. From discussions with the classification societies it has become apparent what the difference is
and in which case one of both is applied. This will be discussed in the following two sections.
5.1 CLASSIFICATION
A classification society is an institute that verifies on the basis of regulations and standards, whether a newly designed construction or installation is of such quality that insurance companies will accept to agree on
insurance.
The classification society reviews independently and in fact stands between the insurer and the insured.
The regulations and standards are combined and laid down in so called "Rules",handbooks used as reference
-manuals by the "surveyors", which are the people from the classification societies that ultimately make the
decisions on classification.
The surveyors are in effect the ones that decide if a certain part of a construction or a piece of equipment is
classified or not.
The classification societies are organized in the
I.A.C.S., the International Association of Classification Societies. The IACS is represented in the IMO,
International Maritime Organization, the nautical division of the United Nations.
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The main classification societies are: Lloyds Register of Shipping
American Bureaux of Shipping (ABS)
Veritas
Det Norske Veritas (DNV)
Germanischer Lloyd
Under normal circumstances a ship does not use its anchors. The bow anchors are used when the ship is
cargo-handled offshore or has to wait offshore, or when the ship seeks for shelter in a bay during a storm. This type of anchoring is called "temporary mooring".
The regulations for anchors in shipping are given in the part of the Rules called "Rules for Shipping". A test is carried out on an anchor to determine if that anchor meets the requirement for a certain application.
Most classification societies require only one test for a certain type of anchor, and if the anchor proves to meet the standards, a certificate is made up for further
application.
In the test the new anchor is compared with a traditional anchor with known capacities. The test regulations require that tests are carried out in three different soil types, for instance sand, soft clay and soft rock, using two anchor weights with maximum ratio of 1:10.
In the Rules for Shipping the following classes exist: standard anchors, covering all traditional anchors high holding power anchors (HHP), in which class all modern anchors such as Stevin, Flipper Delta, Bruce and
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The HHP-anchor is defined as being able to have three times the maximum holding power capacity, with the same anchor weight, of a traditional anchor.
For the application of anchor systems the "equipment number" of the vessel is established on the basis of formulas given in the Rules of Shipping. This equipment number prescribes weights for a standard anchor and
chain. When a HHP anchor is used instead, a 25% reduction of the prescribed weight is allowed.
For dredging vessels and pipelaying barges, the
operational anchoring system is not classified, because work anchors are not directly involved in safety aspects.
Nevertheless, it is common use only to equip the vessel with classified anchors.
For the long term positioning of floating offshore units the regulations for mooring systems are poorly detailed as yet, because of the enormous recent developments and wide variety in the design. The Rules for Shipping are scarcely applicable, and therefore at present for offshore units the method of "certifying" is applied, which is explained
in the next section.
5.2 CERTIFICATION
The mooring system of a semi-submersible is indicated as "permanent positional mooring" by the classification societies. When a semi-submersible is certified, which covers the mooring system as well, a "Certificate of Fitness" is issued for the total unit, resulting from a study carried out by a "Certifying Authority".
By the appointment of governments, classification societies can be a Certifying Authority.
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Reviewing all anchor tests performed, it appears that only few test results, are considered to be objective by all parties involved, i.e. designers, manufacturers, clients and classification societies.
Many practical aspects make interpretation and comparison of the test results difficult. Some of these aspects are: homogeneity of the soil, anchors shall be of the original type and of the exact same weight, method of the
registration of parameters and measured values, and the degree to which the different tests may be assumed to be
identical.
In this study the data of the main test series have been collected as far as they were made available, and the
It shall be noted that possibly more relevant tests have been carried out, however no more information is
available.
Moreover, in most operations with heavy anchors in the dredging and offshore industry, the anchor behaviour and forces in anchor lines are registrated. This information generally is not published and remains confidential. The results of the above mentioned test are briefly
documented in the (Dutch) original report of this study: "Anker en Ankerproeven" (lit.30), released by the I.R.O. results have been reviewed. A listing of the tests under consideration is given below:
Nr. Initiator Location Year
1 Dep. of Public Works, The Easterscheldt 1973
Netherlands
2 Klaren/Smit-Tak 1974/1975
3 Netherlands Offshore Co. 1975
4 Vrijhof Anchors Cuxhaven 1976
5 Statiol - Norway Norwegian 1977
Trench
6 Vrijhof Anchors Hollands Diep 1978
7 US Navy Indian Island 1979
8 US Navy San Diego 1979
9 US Navy Guan 1980
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The reporting and results of the tests have been studied
in order to investigate:
the methodology of anchor testing
the feasibility of establishing a new anchor class based on available test results.
The report of the test series carried out by Statiol in the Norwegian Trench contains detailed information about the method of testing, equipment, timing, etc.
Furthermore it appears that the efficiencies of the modern anchors, such as Stevin and Flipper Delta are in the order of 15 to 25. Efficiencies of Baldt and Stato anchors,
which are classified as "high holding power" anchors as well, usually do not exceed 8. This means that modern
anchors generally have efficiencies two to three times
greater as the other HHP-anchors.
From reviewing reports of anchor tests it again becomes
clear that for indication of holding power capacities, efficiency is commonly used as the general anchor
parameter. This is not justified, because efficiency suggests, that the holding capacity of a certain anchor having a certain wheight, has the same holding capacity in
different soil types. Moreover, it suggests that the same value for efficiency is valid for various anchor weights
of the same anchor make (1 ton, 10 tons, 40 tons), which
In order to give answers to the questions brought forward in the preface of this report, a study has been carried out comprising the following:
the study of available literature
- discussions with users of anchors and classification
societies
the study of previously carried out anchor tests. The following conclusions are drawn:
Anchor parameters
The study of anchor tests determined that anchor tests are carried out mainly in order to determine the
"efficiency" (ratio max. holding power-anchorweight), which is generally used to indicate the holding power capacity of an anchor.
Users of anchors in the dredging and pipe-laying industry commonly use efficiency as an anchor
parameter, although they realise this parameter only partially indicates the holding power capacities.
Classification Societies do not use the conception efficiency for anchor classification or in anchor
applications.
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d. From studying the relevant literature it appears that the holding power of a certain anchor largely depends on the type of soil, i.e. the soil parameters.
Penetration and burial of an anchor is determined by the fluke-shank angle and depends on the soil type. The anchor itself only contributes to the holding power by its fluke area. The anchor weight is not important for the holding power, but mainly related to the
mechanical strength of the anchor.
It becomes evident that the efficiency of an anchor is also very much dependent on the soil type.
Standards for Classification
The discussion with the Classification Societies and the studying of the regulations revealed the following:
e. The regulations for anchors in shipping are given in the "Rules for Shipping"
A test shall be carried out to determine if an anchor meets the requirements. A new anchor is compared with
a known anchor in three different soil types, using two anchor weights of each type (maximum weight ratio
1:10).
In the Rules of Shipping the following classes are
known:
standard anchors
high holding Dower anchors, defined by being able to develop three times the holding power of a standard anchor of the same weight.
f. In dredging and pipe laying classification is not
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For the classification of mobilc offshore units in
principle the regulations for shipping are made use of,
although they are only partial applicable because of the different requirements in anchoring.
The Norwegian Maritime Directorate (NMD) has recently set up national authority requirements. The
requirements for anchors are still very concise. The entire mooring system is tested on site under
supervision of the Classification Society.
Feasibility of New Anchorclass
An extension on the existing anchor classes for shipping is not feasible, because extremely high holding power is undesirable for the anchoring of ships. The shape of high holding power anchors is
usually not desirable for using them as bow anchors on
ships.
Two of the main classification societies, Det Norske Veritas and Germanischer Lloyd, have a positive
attitude towards new standardized regulations for mooring systems of mobile offshore units, but
initiatives on this matter have to come from others.
For the establishing of a new anchor class, detailed propositions must be made to maritime inspection
services and the classification societies, although the realisation of the new class is uncertain and a lot of
time may be involved.
The standards for a new anchor class incorporated in the Rules for Permanent Positional Mooring could be set up using all results of previously carried out anchor
tests.
The often confidential character of the test results should therefore be taken away.
Appendix Al List of Literature American Bureau of Shipping Bruce Anchor Ltd. Buckle A.K. Buckle A.K. Habercom G. Habib Klaren P.J. Klaren P.J. - 20 - 413./83.03.28
Rules for building and classing
steel vessels
leeszaal centrale bibliotheek, 1973
The Bruce Anchor Design,
WL 21515, 1973
Anchoring hardware selection and survey J. Soc. Underwater Techn.
2(76)2 juni
Anchoring and Mooring equipments on ships Trans. of the Royal Inst. of Nay. Arch. 1974 pg. 223-235
Marine anchors, two bibliographies NTIS PS-77/0832/2wo, PS-77/0832/4wo
1977
Anchors and model tests of high capacity anchorings
Investigation on behaviour and
holding power of anchors (in Dutch)
Weg en Waterbouw 31, 1971 no.2 TH 219 A, 1971
Anchors in tandem
9. Klaren P.J.
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The behaviour of anchors for offshore-industry vessels Norske Sivil Ingeniorers Forening
Fortoynings/Forankringsproblemer Offshore 25-28 nov.1977
Anker Advies Bureau, Nieuwerkerk a/d Ijssel, 1977
10. Koster J. Digging in of anchors into the bottom of the North Sea
Delft Hydraulic Lab publ. no. 129,
1974
11. Lloyd's register Rules and regulations for the
con-of shipping struction and classification of steel ships, 1976
12. Mariott M. Subsea anchoring systems
J. Soc. for Underwater Techn. WL 10 C, March 4 11-13, 1977
13. Puech A, et al Behaviour of anchors in different
soil conditions
Offshore Techn. Conf. mei 78 no.
3204, 1978
14. Ruts Anchors (in Dutch)
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Author of 15 until 26: K.J. Saurwalt
Movements and equilibrium of anchors holding on an impervious
planar sea bed
Schip en Werf. 38e jaargang no. 25, 1971
Movements and stability of anchors holding on an impervious inclined uneven sea bed
Schip en Werf. 38e jaargang no. 9, 1972
Stocked anchors holding on an impervious sea bed Schip en Werf. 39e jaargang no. 26, 1972
Anchors penetrating and holding on a soft planar sea bed Schip en Werf. 40e jaargang no. 16, 1973
Anchors digging in and holding in a soft planar sea bed Schip en Werf. 40e jaargang no. 25, 1974
Exploratory anchor experiments
Schip en Werf. 41e jaargang no. 26, 1974
Application of the developed theories in respect to model and full scale anchor tests
Schip en Werf. 42e jaargang no. 1, 1975
The evolution of the anchor (in Dutch) Schip en Werf. 41e jaargang no. 7, 1974
On the holdingpower of ship's anchors Dissertation 12 juni 1975
The determination of the holding power of anchors, using indicative relationships (in Dutch)
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For the testing of anchors, cooperation is necessary
(in Dutch)
Schip en Werf. 42e jaargang no. 24, 1975
A new Dutch anchor-model (in Dutch)
Schip en Werf. 43e jaargang no. 25-SB-7612, december 1976
Vrijhof anchors,
Anchor Manual, 1982
Y. Yamamoto
A basic study on the stability of anchors, 1977
Anker Adviesbureau
Brochure: Flipper Delta Anchor, 1982
Svasek B.V.
Ankers en Ankerproeven (in Dutch), publication by the Netherlands Industrial Council for Oceanology, 1982
