Anchor Manual

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vryhof anch

orma

nual

2000


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vryhof

anchor manual 2000

Copyright

Vryhof a nchors b.v., krimpe n a /d yssel, t he net herlan ds 1999.

No pa rt of this boo k may be reproduced in any form, by print , copy or in a ny other w ay w ithout

w ritt en pe rmission of vryhof.

Vryhof , Ste vin Mk3, Ste vpris, Ste vsha rk a nd Stevma nt a a re registered t rad e ma rks.

Vryhof reserves all intellectua l and industrial property rig hts such a s any a nd all of the ir paten t,

trad ema rk, design , manuf acturing, reproduction, use and sales rig hts thereto an d to an y article

disclosed t herein.

All informa tion in t his ma nua l is subject t o chang e w ithout prior no tice. Vryhof a nchors is not

lia ble an d/or responsible in any w ay f or th e informa tion provided in this manua l.

First e dit ion pub lished 1984. Print run 7,500 copies.

Second editio n pub lished 1990. Print run 7,500 copies.

Reprint secon d e dition print run 5,000 cop ies.

Third e dit ion pub lished 2000. Print run 2,500 copies.

ACCREDITED BY

THE DUTCH COUNCIL

FOR CERTIFICATION

Reg. No 24

DETNORSKEVERITASINDUSTRYB.V., THENETHERLANDS

ISO-9001CERTIFICATED FIRM

2

Vryhof anchors

p.o. box 105, 29 20 AC krimpen ad yssel, the netherlands

www.vryhof.com [email protected]


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A sto ne a nd som ething tha t loo ked like a rope. For millennia t his w a s the

typical anchor. Over the last 25 years of more recent history, vryhof has

brought the ar t to a more mature sta tus. They have grow n into a w orldlea der in engineering a nd m a nufa cturing of mo oring system s fo r all kinds

of f loat ing s t ructures . In doing so the company has secured numerous

anchor a nd ancilla ry eq uipment pat ents, a nd sha red its experience w ith

others.

The compa ny und ersta nd s th a t t he need s of t he industry ca n not b e sa tisfied

by the supply of sta nda rd ha rd-w a re only. Universa l a nd t a ilored solutions

roo te d in proven eng ineering sho uld be ba sed o n long practica l experience.

Vryhof ha s been a nd w ill be introducing new a nd o rig ina l a nchor desig ns

w ell into th e 21st cent ury. With th eir prod ucts, ad vice a nd this ma nua l, it

sha res this know ledg e w ith t hose w ho a re da ily fa ced w ith complex mooring

situations.

This ma nua l is intend ed a s a m ea ns of referen ce fo r all w ho p urcha se, use,

ma inta in, repair or a re in a ny w a y involved w ith a nchors. Thoug h w rit t en

from one anchor manufac turer s s tandpoint , the informat ion contained

herein is a pplica ble t o ma ny t ypes of a nchors. Tot a l ob jectivity is, o f cou rse,

impossible.

It is hoped this ma nua l w ill cont ribut e t o the w ork and success of a ll w ho

w ork w ith a nchors. They a re the o nly fixed reference po int fo r man y of the

floa ting structures on t he w orlds of ten t urbulent w a ters.

Introduction 3


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General

1


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Mooring system s

Moo ring system s ha ve been a round just a s long a s ma n ha s felt the nee d f or

a nchoring a vessel a t sea . These syste ms w ere used, a nd a re still used , on

ships a nd consisted of one or mo re lines conn ected to the bo w or stern o fthe ship. Generally the ships stayed moored for a short duration of t ime

(days).

When the explorat ion and production of oil and gas started offshore, a

need for more permanent mooring systems became apparent . Numerous

different m oo ring system s ha ve been d eveloped over the yea rs, of w hich a

sho rt selection is presented here.

Semi-submersible drilling rig - generally the semi-submersibles are

moo red using a n eight po int mo oring . Tw o mo oring lines com e to g ethe r at

ea ch of th e columns of t he semi-subm ersible.

CALM buo y- g enera lly the b uoy w ill be mo ored using fo ur or more moo r-

ing lines at eq ua lly spa ced a ng les. The mo oring lines gen era lly ha ve a cat e-

na ry sha pe. The vessel con nects to t he b uoy w ith a single line a nd is free t o

w eathervane around the buoy.

SALM buoy- these types of b uoys have a moo ring tha t consists of a sing le

moo ring l ine at ta ched to an a nchor point on t he seab ed, underneath t he

buo y. The a nchor po int ma y be g ravity b a sed or piled.

Turret mooring - th is type of mo oring is g ene rally used o n FPSOs and FSOs

in more ha rsh environ men ts. Multiple moo ring lines are used, w hich come

to g et he r at th e t urnt a ble b uilt int o t he FPSO or FSO. The FPSO or FSO is a bleto rota te a round the turret to obt a in an o pt ima l orienta t ion re la t ive to t he

preva iling w ea ther condit ions.

semi-sub mo orin g

typical tur ret m oor ing

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f ig . 1-01

catena ry system

f ig . 1-02

ta ut leg system

Spread mooring - generally used on FPSOs and FSOs in milder environ-

ment s. The mo oring lines are directly con nected to th e FPSO o r FSO a t bo th

the s tern a nd b ow of the vessel.

When o il and g a s explora t ion a nd production w a s conducted in sha llow to

deep w a ter, the most commo n moo ring line conf igura tion w a s the ca tena ry

moo ring line consist ing o f cha in o r w ire rope. For explora t ion a nd produc-

t ion in deep to ult ra-deep w a ter, the w eight of the mo oring line sta rts to

b e c o m e a l im i t in g f a c t o r in t h e d e s ig n o f t h e f lo a t e r . To o v e r-

come this problem new solutions w ere developed consist ing o f synthe tic

ropes in th e mo oring line (less w eigh t) a nd /or a ta ut leg mo oring syste m

(fig . 1-01 and f ig. 1-02).

The ma jor di f ference betw een a ca tena ry moo ring and a ta ut leg mo oring

is tha t w here the ca tena ry mo oring a rrives a t the seab ed ho rizon ta lly, the

ta ut leg mo oring arr ives a t t he seab ed a t a n a ngle . This mea ns tha t in a ta ut

leg moo ring the a nchor point ha s to be capa ble of resist ing bo th ho rizonta l

a nd vert ica l forces, w hile in a ca tena ry mooring t he a nchor po int is only sub-

jected to horizonta l fo rces. In a ca tena ry mo oring , most of t he resto ring fo r-

ces a re generated by the w eight of the mo oring line . In a ta ut leg moo ring ,

the resto ring fo rces a re gene rat ed b y the elast icity of the moo ring line.

An ad vant ag e of a t aut leg mooring over the cat enary moo ring is tha t the

foo tprint o f the t a ut leg moo ring is sma ller than t he fo otprint o f the ca te-

na ry mo oring , i.e . the moo ring ra dius of the t a ut leg mo oring w ill be smal-

ler tha n the m oo ring ra dius of a ca tena ry moo ring fo r a simila r applica t ion.

Mooring system s 6


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7

f ig . 1-03

fig . 1-04

Mooring components

A typica l moo ring system ca n b e d ivided in t hree different com pone nts, the

moo ring line, the connectors a nd t he a nchor point .

Mooring line

Chain

The m ost comm on prod uct used fo r moo ring lines is chain w hich is a vailab le

in d ifferent diam eters a nd g rad es. Tw o different desig ns of chain a re used

freq uent ly, stud link an d stud less cha in. The stud link chain is most com mo n-

ly used fo r moo ring s tha t h a ve to be reset n umerous t imes during their life-

time , fo r insta nce sem i-sub me rsibles, w hile stu d less link cha in is o ft en u sed

fo r perma ne nt mo o ring s (FPSOs, bu o ys, FSOs). A cha in mo o ring line ca n b e

terminat ed in either a com mon link or a n end link (fig . 1-03).

Wire rope

When compa red to chain , w ire rope ha s a low er w eight tha n chain , for the

sa me brea king loa d a nd a high er ela st icity. Commo n w ire ropes used in o ffs-

ho re mo oring lines are six stra nd a nd spiral stra nd . The w ire rope is te rmi-

na te d w ith a socket (fo r insta nce op en spelter, closed spelter, CR) fo r con -

nection to the other components in the mooring system. Generally wire

rope is more prone to da ma ge a nd corrosion t ha n cha in (fig . 1-04).

Synthetic fibre rope

A recent development is the use of synthetic fibre ropes as mooring line.

Typical ma te ria ls th a t can be used a re polyeste r an d hig h mo du lus po ly-

et hylene (Dyneem a ). The ma jor a dva nt a g e of synth et ic fibre ropes is th elig ht w eight of the ma teria l and the high elast icity. The synthet ic f ibre rope

is g enera lly terminat ed w ith a specia l spoo l and sha ckle f or connection to

the o ther com ponen ts in the m oo ring system .


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Mooring components

f ig . 1-05

fig . 1-06

fig . 1-07

fig . 1-08

Connectors

ShacklesThe sha ckle is a con necto r th a t is very com mo n in t he of fshore industry. It

con sists of a bo w , w hich is closed b y a pin. Ma ny diffe rent t ypes of sha ckles

a re a vaila ble, depend ing on the a pplica t ion. The shackle can be used in

bot h temporary and permanent m oorings (fig . 1-05).

Connecting link kenter typeThe conn ecting link kent er type is mo st com mo nly used fo r the con nection

of two pieces of chain mooring l ine , where the terminat ions of the two

pieces have t he sa me d imension s. The con necting link kenter t ype ha s th e

s ame o u t s ide l e n g t h a s a c h a in l in k o f t h e s ame d i ame t e r . G e n e ra l l y

co n n e c t in g lin k s k e n t e r t y p e a r e n o t u se d in p e r m a n e n t m o o r in g

system s, as they ha ve a shorter fa t igue life t ha n the cha in (fig . 1-06).

Connecting link pear shaped

The p ea r sha ped connecting link is similar t o th e conn ecting link kenter

type, except t ha t i t is used f or the conn ection o f t w o pieces of mo oring line

w ith terminat ions tha t ha ve different dimensions. Like the conn ecting link

kenter type, the pea r sha ped connecting links a re not used in perma nent

mo oring syste ms (fig . 1-07).

Connecting link c type

Like t he con necting link kenter t ype, th e conn ecting link c type is used fo r

the connect ion of tw o pieces of moo ring line w ith termina t ions tha t ha vethe sa me d imensions. The ma jor difference betw een t he kenter type a nd

the c type is the w a y tha t t he connector is open ed a nd closed. This conne c-

to r is g enerally not used in perma nent moo ring s (fig . 1-08).

8


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Mooring components

Swivels

A sw ivel is used in a mo oring syste m, g ene rally of a te mpo rary type, t o relie-

ve the t w ist a nd to rque tha t b uilds up in th e mo oring line. The sw ivel isof ten placed a few l inks from the anchor point , a l though i t can also be

pla ced b etw een a section o f chain a nd a section o f w ire rope. There a re

ma ny dif ferent types of sw ivels a va ila ble , a l thoug h a disa dvanta ge of most

com mon sw ivels is tha t t hey ma y not function w hile und er loa d, w hich is

ca used by h ig h f riction inside t he t urning mechanism. A new developm ent is

sw ivels tha t a re ca pa ble of sw ivelling unde r loa d, d ue to specia l bea ring sur-

fa ces inside t he me cha nism (fig . 1-09).

Anchoring point

Dead weight

The d ea d w eigh t is prob a bly the o lde st a nchor in existe nce. The h olding

ca pacity is generat ed by the w eight of the ma teria l used a nd pa rt ly by the

fr ict ion betw een the d ea d w eight a nd t he sea bed. Common ma teria ls in use

tod a y for dea d w eight s are steel and concrete (fig . 1-10).

f ig . 1-10

f ig . 1-09

9


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Mooring components

f ig . 1-12

Drag embedment anchor

This is th e mo st po pular type o f a nchoring po int a vaila ble to da y. The d rag

embedm ent a nchor has been designed to penetrate in to the sea bed, e i therpa rt ly of fully. The ho lding ca pa city o f t he d rag embe dm ent a nchor is g ene-

rat ed b y the resista nce of the soil in fron t o f t he a nchor. The d rag emb ed-

ment a nchor is very w ell suited fo r resist ing la rge h orizon ta l loa ds, but n ot

for large vert ica l loa ds al though there are some drag embedm ent a nchors

avai lable on the market today that can res is t s ignif icant vert ical loads

(fig . 1-11).

Pile

The pile is a ho llow ste el pipe t ha t is insta lled int o t he seab ed by me a ns of a

piling ha mme r or vibra to r. The ho lding capa city of th e pile is g enera te d b y

the friction of the soil along the pile and lateral soil resist-ance. Generally

the pile ha s to b e insta lled a t g rea t d epth below seab ed to obt a in the requi-

red ho lding cap a city. The pile is ca pa ble of resisting bo th ho rizo nt a l a nd

vertica l loa ds (fig . 1-12).

f ig . 1-11

10


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Mooring components

Suction anchor

Like the pile, th e suction a nchor is a hollow steel pipe, altho ug h t he d ia me-

te r of t he pipe is much la rge r tha n th a t o f t he pile. The suction a nchor is fo r-ced in to the sea bed b y means of a pump connected to t he top o f the pipe ,

crea ting a pressure diff erence. When pressure inside th e pipe is low er tha n

outside, the pipe is sucked into the seabed. After installation the pump is

removed. The ho lding ca pa city o f t he suction a nchor is g enerat ed by th e

friction of th e soil along th e suction a nchor a nd lat era l soil resista nce. The

suction a nchor is ca pa ble of w ithsta nding b ot h horizonta l a nd vert ica l loa ds

(fig . 1-13).

Vertical load anchor

A new d evelopm ent is th e vert ica l loa d a nchor (VLA). The vertica l lo a d

a nchor is insta lled like a convent iona l dra g embe dm ent a nchor, but pe net-

rat es much deeper. When t he a nchor mo de is cha ng ed f rom th e insta lla t ion

mod e to the vert ical (normal) loa ding mo de, the a nchor ca n w ithsta nd b oth

horizonta l a nd vert ica l loa ds (fig . 1-14).

f ig . 1-13

fig . 1-14

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History of drag embedment anchors

Histo ry traces th e use of a nchors to China a s fa r ba ck as 2,000 BC, tho ug h it

is q uite proba ble tha t the y w ere used prior to t his. At th a t t ime the g eneraltend ency w a s to use la rge sto nes, ba skets of sto nes, ba g s of sa nd o r even

log s of w oo d load ed w ith lead w hich w ere then fa stened t o l ines. It w a s th is

w eight a s w ell a s a certain deg ree of frict ion on t he bo tt om w hich secured

a vessel in po sit ion.

With the introd uction o f iron into a nchor construction, t eeth or f lukes w erebuilt on the anchor, allowing penetrat ion into the seabed, thus offering

a dd ition a l sta bility. Yet t hese primitive a nchors w ere of po or construction

a nd of ten broke und er pressure. Curved a rms w ere introduced in 1813, a nd

fro m 1852, the so-called Adm ira lty Anchor w a s used fo r ships of th e Roya l

Navy. Another refinement in the 19th century was the elimination of the

st o c k, t h e cro ssp ie ce a t t h e t o p o f a n a n ch o r w h ich e n su re d t h a t t h e

posit ioning of the anchor would allow the flukes to penetrate the soil . A

sto ckless a nchor w a s invent ed in 1821 an d be ca me p op ular, prima rily a s a

result o f t he ea se of ha ndling a nd stow ing , qua lit ies st ill valued to da y.

A large number of anchor types has been designed and commercialised

over the yea rs. Som e ha ve prospered , ot hers no t. The mo st recent d esig ns

a re the results of va st experience and extensive test ing, a nd a re fa r more

eff icient t ha n t heir histo rica l prede cessors. A sho rt o verview of th e a nchors

in use to da y, is presente d o n the fo llow ing pa g es.

History of em bedm ent anchors 12


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Based upon certain charateristics such as fluke area, shank, stabilisers, it is

po ssible to classify the va riou s a nchor types. To a llow a rou g h com pa riso n o f

a ncho r t ype eff iciency, an indica tion (*) is provided fo r a 10 t a ncho r a s (HOLDINGCAPACITY = WEIGHT * EFFICIENCY).

Class A eff iciency rang e *33 to 55

slend er a nchors w ith ultra -penet rat ion.

Characteristics of anchor types

StevprisClass A

Stevshark

FFTS

13


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Class B eff iciency rang e *17 to 25

anchors w ith e lbow ed shank, a llow ing for improved penetra t ion .


Bru ce TS

Hook

Bruce SSClass B

14

h f h


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Class C eff iciency rang e *14 to 26

anchors with open crown hinge near the centre of gravity and relat ively

sho rt sha nk a nd st a bilisers or built-in sta bilisers.


Stevfix

Stevmud

Flippe r Delt a

StevinClass C

15

Ch i i f h


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Class D eff iciency ra ng e *8 to 15

a nchors w ith h ing e a nd sta bilisers a t t he rea r and rela t ively long sha nks a nd

stabilisers.


LWT

Moo rfa st - Sta to - Off drill

Boss

Danfor thClass D

16

Ch t i ti f h t


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Class E eff iciency ra ng e *8 to 11

a nchors w ith very sho rt, th ick sta bilisers; hing e a t t he rea r and a rela tively

sho rt, mo re or less sq ua re-sha ped sha nk.


Stokes

Snugstow

Weldhold

AC14Class E

17

Ch t i ti f h t 18


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Class F eff iciency ra ng e *4 to 6

a ncho rs w ith sq ua re sha nk, no sto ck sta bilisers. The sta bilising re sista nce is

built-in the crow n.


Beyers

Union

Spek

US Na vy Sto cklessClass F

18

Characteristics of anchor types 19


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Class G eff iciency ran g e *


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History of vryhof anchor designs

A brief chrono log ica l summa ry of the types of a nchors vryhof ha s desig ned

fo r use in the o ffshore a nd dredg ing industries:

1972 - The Stevin a nchor: The o rig inal desig n. The w ing w a s no t yetenlarged . The a nchor ha d a sq ua re sha nk. It is no long er ma nufa c-

tured.

1974 - The Hook anchor: originally designed for permanent moorings.This d esig n w a s surpa ssed in 1980 by t he Ste vpris d esig n a nd is no

long er manufa ctured.

1977 - The Stevin Mk3 a nchor: is the improved version of the orig ina lStevin a nchor. It w a s eq uipped w ith a n enlarged crow n a nd f luke

a rea a nd a strea mlined sha nk for mo re efficient penet rat ion. This

a nchor is st i ll ma nufa ctured a nd in use in off shore a nd dredg ing

a ctivities. It h a s a ll cla ssifica tio n societ ies a ppro va ls.

Hook

Stevin Mk3

20

History of vryhof anchor designs Stevfix21


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1978 - The Stevfix a nchor: this anchor w a s desig ned w ith specia l f luke

points fo r harde r soils a nd a la rger fluke a rea t ha n th e Stevin, butha s be en surpassed by t he Stevpris a nchor. It is no long er ma nuf a c-

tured.

1979 - The Stevmud a nchor: th e Ste vmud is essent ia lly the Ste vin a nchorw ith a considera bly enla rged fluke a rea. This a nchor t ype w a s a lso

surpassed by th e Stevpris a nchor a nd is no long er ma nufa ctured.

1980 - The intro d uct ion o f t he St evpris an d Ste vsha rk a ncho rs. TheStevpris anchor is a deep penetrat ing a nchor w ith a ploug h sha-

ped sha nk, surpa ssing t he perfo rma nce of a ll ea rlier de signs in th e

vryhof range, and incorporating the latest experience, research

a nd kno w ledg e of th e a nchor de signe r. The Ste vsha rk a nchor is a

specia lly re info rced Stevpris anchor, eq uipped w ith a serrat ed

sha nk and cutt er-teet h f or bet ter penet rat ion in ha rd soils, such a s

cora l types or san dsto ne. The fluke po int s are specia lly reinfo rced

to w ithsta nd high point loa ds.

Stevmud

Stevpris

21

History of vryhof anchor designs Stevshark Mk522


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1990 - The Stevpris Mk5 a n d Stevshark Mk5 w ere introd uced. The

improved versions of the original Stevpris and Stevshark anchors.Improvements have concentra ted o n tw o fea tures: hig her holding

ca pa city a nd ea sier han dling .

1996 - Int rod uction o f th e Stevma nta VLA (Vert ica l Lo a d Ancho r). Ba sedon industry demand for an anchor that could withstand vert ical

loa ds, the Ste vma nt a VLA w a s developed . The Ste vma nt a VLA is a

new desig n in w hich a tra dit iona lly rig id shan k has been repla ced

by a system of w ires conn ected to a pla te. The a nchor is desig ned to

a c c e p t v e r t i ca l (o r n o r m a l ) l o a d s a n d i s i n st a l le d a s a

convent ional drag embedment anchor with a horizontal load to

the m udline to ob ta in the d eepest pene tra t ion po ssible. By cha n-

g ing the point o f pulling a t t he a nchor, vert ica l (or no rmal) loa ding

of th e fluke is ob ta ined t hus mob ilising t he ma ximum p ossible soil

resista nce. As a VLA is de eply embe dd ed a nd a lw a ys loa de d in a

direc t ion normal to the f luke , the load can be appl ied in any

direction. Consequently the anchor is ideal for taut-leg mooring

systems.

Stevmanta

22


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Theory

2

Introduction 24


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Theory

Anchor design used to be based on prac t ical experience of the anchor

ma nufa cturer only. Now a da ys, science has becom e a ma jor fa cto r in thedesig n process, complement ing the experience of the a nchor m a nufa cturer.

Based on te st results , bo th in the la bo rat ory and in the f ield, a much bette r

understa nding o f a nchor beha viour has been a chieved.

The perfo rmance of a n a nchor is influenced by ma ny different pa rame ters,

of w hich the fol low ing a re only a few : f luke a rea a nd d esign , shank design ,soil con dition s, loa d cond ition s, type of m oo ring line.

This chapt er present s a sho rt overview of ho w th ese para met ers influence

th e performa nce of t he a nchor. It is by no mea ns com plete, but it w ill g ive a

bet t er understa nding o f ho w a n opt ima l anchor design can be a chieved. In

the la st pa rt of this cha pter, a few relevant test results are presente d.

Introduction

Anchor holding capacity 25


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g p y

f ig . 2-02

fig . 2-01

fig . 2-03

fig . 2-04

Criteria for an chor holding capacity

The ho lding capa city of a n a nchor is g overned by th e fo llow ing pa rame ters:

The f luke area , w hich is limited by t he strengt h o f t he a nchor desig n. The pene tra t ion o f t he a nchor. The pene tra t ion o f the a nchor is g overnedby the soil type (deep penet rat ion in very sof t cla y an d shallow penet ra-

t ion in sand), the anchor type (design), the type of mooring line that is

used (cha in o r w ire rope) and the a pplied loa d.

An increase in fluke area or an increase in the penetrat ion depth of the

a nchor results in a hig her ho lding ca pa city.

In the fo llow ing pa rag raphs, the influences on t he a nchor penetra t ion a re

furt her clarified .

Streamlining of the anchor

A strea mlined a nchor is very impo rta nt fo r opt ima l penetrat ion in t he soil.As can be seen in f ig. 2-01a n d f ig. 2-02, an a nchor w hich ha s protruding

pa rts w ill encoun te r much mo re soil resista nce an d conseq uent ly w ill no t

penetrate a s deep a s a more st reamlined a nchor w ith t he sa me f luke a rea .

Shank shape

A sq ua re sha nk, w hich is com mo n fo r most olde r type single sha nk an cho rs,

w ill ca use penet rat ion resist-a nce due to the fa ct t ha t t he soil ca n no t pa ss

ea sily pa st the sha nk. A clod of soil w ill form und erneat h t he shan k, eff ecti-

vely increasing the resistance of the soil (fi g. 2-03). Bevelling the shank

a llow s dee per penetra t ion.When t he single sha nk is repla ced b y a t w in

sha nk constructio n (fo r inst a nce Stevpris, FFTS), usua lly t w o th in pa ra llel

steel plates, the soil ca n mo re ea sily pa ss throug h a nd pa st t he shank, an d

conseq uent ly the tw in shank anchor ca n penetra te deeper (fi g. 2-04).

Anchor holding capacity 26


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Mooring line

An a nchor conn ected to a w ire rope mo oring line w ill penet rat e deeper

tha n the same a nchor connected to a cha in moo ring line(f ig. 2-05 and f ig.

2-06). This is caused by th e h ighe r lat era l resista nce (pene tra tion resista nce)

a long th e cha in mo oring line. This eff ect is no ticea ble in a ll soil con dition s,

but especia lly in very sof t cla y w here very deep pen etra t ion can b e o bt a i-

ned. The ho lding ca pa city o f a cha in mo oring line, due to frict ion in a nd on

the seab ed, is la rger tha n the ho lding capa city of a w ire rope mo oring line.

When a n a nchor rea ches its ultima te ho lding ca pa city, i.e. it w ill not resist

any higher loads, at shallow penetrat ion a wedge shaped piece of soil ( in

front a nd a bo ve the a nchor) w ill fail. The ho lding capa city of the a nchor can

then b e described a s a combinat ion o f t he fo llow ing para meters (fi g. 2-07

and f ig. 2-08):

The w eight of th e a nchor (A). The w eight of th e soil in the f a ilure w ed g e (B). The f riction of th e soil in the f a ilure w ed g e a lon g fra cture lines (C). Friction be tw een fluke surfa ce a nd soil (fluke a rea ) (D). The b ea ring ca pa city o f shan k and mo oring line (E). The f riction of th e m oo ring line in a nd on th e soil (E).

g p y

f ig . 2-05

fig . 2-06

fig . 2-07

A

B

C

E

D

f ig . 2-08

Criteria for good anchor design 27 Scale influence


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Anchor pa ramet ers ca n be sca led from ge om etrica lly proportiona l anchors

using th e sca le rules in t able A .

There a re several a tt ribut es of a n a nchor w hich a re crucial in a ssuring its

effe ctive perfo rman ce:

The a nchor must off er a hig h ho lding ca pa city; a result o f t he f luke a reaa nd shank desig n in com binat ion w ith penet rat ion a nd soil type.

The d esig n o f t he a nchor should b e such t ha t t he a nchor is ca pa ble ofbeing used successfully in practically all soil conditions encountered over

the w orld, ran g ing from very sof t cla y to sand , cora ls a nd calca renites.

The f luke/sha nk a ng le of th e a nchor sho uld be ea sily a djusta ble, a llow ingthe a nchor to be q uickly de ployed in d ifferent soil cond it ions.

The d esig n must be so conceived a nd produced th a t t he high loa ds com-

mo n in practice ca n be resisted a nd t ha t t he a nchor ca n be ea sily hand led,insta lled, retrieved a nd sto red.

Th e p e n e t r a t io n o f a n a n ch o r de p e n ds u p o n i t s sh a p e a n d de s ig n .Obstructing pa rts on the a nchor should b e a voided a s much as possible.

The sta bility o f a n a nchor en coura g es its penetra t ion a nd, conseq uently,its holding capacity. Efficient stabilisers are an integral part of a good

a nchor d esig n.

The sha nk must permit pa ssa g e o f t he soil. The surfa ce a rea of a n a nchor fluke is limited by t he req uired structura l

streng th of t he an chor.

The a nchor d esig n m ust ha ve opt ima l mecha nica l streng th to fulfil req ui-remen ts an d stipulat ion s of th e cla ssifica tion societies.

The a nchor should be d esig ned to ensure an opt imum b etw een structur-a l st rength of the a nchor a nd holding capa city.

The a nchor should be strea mlined fo r low penet rat ion resista nce.

g g

table A

Model Reality Related

to Weight

Length L n W 1/3

Fluke area A n2

W2/3

W eight W n3 W

Penetration P n W 1/3

M om ent M n4 W 4/3

M om ent of inertia I n4 W 4/3

Section M odulus S n3 W

Bending stress M /S n4/n3=n W 1/3

Shear strength F/A n3/n2=n W 1/3

Aspects of soil in anchor design 28


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g

Aspects of soil mechanics in anchor design

Until th e ninet een seventies a nchor de sig n w a s larg ely an e mpirical process.

The re w a s no t m uch science involved , mo re use of e xperience. It is no t e a sy,

fo r insta nce, to ca lcula te the Ult ima te Holding Ca pa city (UHC) of a n a nchor

fro m t he com mo nly kno w n soil mecha nics fo rmulas. The m a in problem is

the predict ion o f t he volume o f soil mo bilised by t he a nchor. To a la rge d eg -

ree, it is th is volume w hich de te rmines th e UHC. Deta iled u nd ersta nd ing o f

so i l ch a r a c t e r i st i cs a n d b e h a v io u r i s e sse n t i a l in t h e a n c h o r d e si g n

process and of increasing bene fit in ha ndling a t sea . It is this understa ndingw hich is the h a llma rk of a com peten t a nchor de sig ner an d b uilder.

For anchor design and installat ion, the availability of good soil data is of

utm ost importa nce as the soil is of g rea t influence on a nchor beh a viou r. The

fo llow ing a re influenced by t he soil cond it ions encount ered:

Anchor type - som e a nchors a re mo re suited fo r sof t soil cond ition s (sof tclay), w hile o th ers a re mo re suited fo r ha rd soils (sa nd a nd ha rd cla ys), a lt-

houg h there are a numb er of a nchor types on the m a rket tha t a re suited f or

mo st soil cond ition s encoun te red.

Holding capacity- in ha rd soil like sa nd a nd ha rd cla y, the ma ximum a tt a i-

na ble ult ima te ho lding capa city w ith a certa in a nchor type a nd size is hig-

her tha n the a t t a ina ble ult ima te ho lding capa city in very sof t cla y.

Penetration and drag - in very sof t cla y the a nchor w ill penetrat e d eeper

than in harder soil l ike sand. As a consequence, the drag length of the

a nchor w ill a lso b e long er in very sof t cla y tha n in hard soil.

Retrieval fo rces - w hen a n a nchor is insta lled in very sof t cla y, th e req uired

retrieva l forces w ill be hig her t ha n in ha rd soil like sa nd . For exa mple, in

very sof t cla y the req uired retrieval fo rce o f a n a nchor can b e eq ua l to 80%-

90% of th e insta lla tion loa d w hile in ha rd soil (sa nd ) th e retrieval fo rce

migh t o nly be 20%-30% of th e insta lla tion loa d.

Soil classif ication 29 Undrained Shear Strength (kPa)


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Consistency ASTM BS

of Clay D-2488 CP-2004

Very soft 0 - 13 0 - 20

Soft 13 - 25 20 - 40

Firm 25 - 50 40 - 75

Stiff 50 - 100 75 - 150

Very stiff 100 - 200 150 - 300

H ard 200 - 400 300 - 600

Very hard > 400 > 600

table B

Soil streng th is g enera lly expressed in te rms of th e shea r streng th pa ram e-

te rs o f t he soil. The soil type is cla ssified ma inly by g ra in size d ist ribu t ion.

Grain size Soil description

< - 2 m Cla y

2 - 6 m Fine Silt

6 - 20 m Med ium Silt

20 - 60 m Co a rse Silt

60 - 200 m Fine Sa nd

200 - 600 m Med ium Sa nd

0.6 - 2 mm Co a rse Sa nd

2 - 6 mm Fine Gra vel

6 - 20 mm Med ium Gra vel

20 - 60 mm Co a rse Gra vel

60 - 200 mm Co b b les

> - 200 mm Bo uld ers

In g eneral, the soil types encountered in a nchor d esig n a re sa nd a nd cla y

(Gra in diam et er fro m 0.1 m t o 2 mm). How ever, mo oring loca tion s con sis-

ting of soils w ith g rain sizes ab ove 2 mm, such a s g ravel, cob bles, bo ulders,

rock a nd such, also occur. Clay t ype soils a re g ene rally chara cte rised by t he

undrained shear st rength , the submerged unit w eight , the w a ter content

a nd the plast icity pa ram ete rs. The con sisten cy o f clays is relat ed to the

und ra ined she a r stre ng th . How ever, Ame rican (ASTM) a nd British (BS) sta n-

da rds do no t use ide nt ica l values. The und rained shea r streng th va lues Su

ca n be derived in the lab ora to ry from unconf ined unconsolida ted tests (UU)

(t able B).

Soil classification 30 Su UCT SPT CPTkPa kPa N MPa


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On s i te the va lues can be es t imated f rom the resu l t s o f the S tandard

Pen et ra t ion Test (SPT) o r Con e Pe ne t rom et er Test (CPT). An a ppro xima t e

relat ion b etw een shea r st rength and the t est values a re show n in table C.

The mecha nica l resista nce o f sa nd y soils is predo mina nt ly cha racterised by

the submerged unit weight and the angle of in ternal fr ic t ion , . These

pa rame ters are estab lished in t he lab ora to ry. An a pproxim-a te correla t ion

be tw een the ang le a nd t he rela t ive de nsity of f ine to med ium sa nd is g ive

in table D. The und rained shea r streng th o f cla yey soil can a lso b e estima te d

ba sed o n ma nual tests.

In sof t cla y the t hum b w ill ea sily penet rat e several inches, ind ica ting a nundra ined shea r streng th sma ller tha n 25 kPa .

In firm (med ium) clay th e th umb w ill pene tra te several inches w ith mo de -rate e f fort , indica t ing a n undrained shea r st rength betw een 25 kPa a nd

50 kPa .

Stiff c lay will be easily indented with the thumb but penetrat ion willrequire great e f fort , indica t ing a n undrained shea r st rength betw een 50

kPa a nd 100 kPa .

Very stiff cla y is ea sily ind ent ed w ith the t hum bna il, ind ica ting a n und rai-ned shea r streng th b etw een 100 kPa a nd 200 kPa .

Ha rd cla y is inde nted w ith d iff iculty w ith the thumb na il, indica t ing a nundra ined shea r streng th larg er tha n 200 kPa.

The rock streng th can g enera lly be de scribe d by its compressive streng th

(table E).

A classificat ion system for soil based on the carbonate content and grain

size of the soil (Clark and Wa lker), is sho w n o n t he la ste pa g e o f this cha pter.

kPa kPa N MPa

0 - 13 0 - 25 0 - 2 0.0 - 0.2

13 - 25 25 - 50 2 - 4 0.2 - 0.4

25 - 50 50 - 100 4 - 8 0.4 - 0.7

50 - 100 100 - 200 6 - 15 0.7 - 1.5

100 - 200 200 - 400 15 - 30 1.5 - 3.0

> 200 > 400 >-30 >3.0

table C

Descriptive Relative Angle SPT CPT

term Density N MPa

Very loose < 0.15 < 30 0- 4 0 - 5Loose 0.15 - 0.35 30 - 32 4 - 10 5 - 10

M edium dense 0.35 - 0.65 32 - 35 10 - 30 10 - 15

D ense 0.65 - 0.85 35 - 38 30 - 50 15 - 20

Very dense > 0.85 > 38 > 50 > 20

Descriptive term Compressivestrength qu [MPa]

Very w eak < 1.25

W eak 1.25 5

M oderately w eak 5 12.5

M oderately strong 12.5 50

Strong 50 100

Very strong 100 200Extrem ely strong > 200

table D

table E

Fluke/shank angle 31


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The pene tra t ion o f a n a nchor into a certa in soil type is great ly influenced by

th e selected fluke/sha nk a ng le. For h inging a nchor t ypes (Ste vin, Da nf orth

et c.) th e fluke/sha nk an g le is th e a ng le betw een t he a nchor sha ckle, the

hinge a nd th e fluke tip. The me th od fo r mea suring th e fluke/sha nk an g le

fo r fixed sha nk a ncho rs (Ste vpris, FFTS, et c.) is no t w ell def ined . Of te n it is

the a ngle betw een the a nchor sha ckle , the rear of the f luke a nd t he f luke

tip, but no t a ll a nchor ma nufa cturers use the sa me d efinit ion.

The recom men de d fluke/sha nk a ng les fo r diff erent soil con dition s are pre-sente d in table F.

Some modern anchors, like the Stevpris Mk5, have an additional interme-

dia te fluke/sha nk a ng le o f 41o , w hich ca n be used in intermed ia te o r more

com plex soil cond it ions. For insta nce a t a locat ion w here the a nchor ha s to

pass through a la yer of sof t cla y befo re penetra t ing in to a la yer of sa nd.If a n a nchor is used w ith a n incorrect fluke/sha nk a ng le, it w ill neg a tively

influence perfo rma nce. This is th e ca se f or a ll a nchor t ypes.

In ha rd soil, a n a nchor w ith a fluke/sha nk an g le of 320 w ill g ive th e high est

ho lding po w er. If a n a nchor is used w ith th e fluke/sha nk an g le set a t 500, the

a nchor w ill fa il to penet rat e into the seab ed a nd w ill beg in to t rip, fa ll a side

a nd s lide a long the seabed (Fig . 2-9 and 2-10).

f ig . 2-09

fig . 2-10

Soil type Approximatefluke/shank angle

Very soft clay 50

M edium clay 32

H ard clay and sand 32

table F

Fluke/shank angle 32


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If a n a nchor is used in very sof t clay (mud) w ith t he fluke/sha nk

ang le set a t 32o, the a nchor w ill penetrate in to t he sea bed, how ever the

penet rat ion w ill be less tha n w hen a f luke/sha nk ang le o f 50o is used.

Consequen t ly the h olding capa city w ill be low er w hen t he f luke/sha nk

a ng le is set a t 32o, and the drag leng th longer (Fig . 2-11).

f ig . 2-11

mud ang le

sand a ngle

Fluke area 33


33/168

Because the fluke area of an anchor is of great influence on the holding

capacity, i t can be useful to compare the fluke area of different anchor

types tha t a re avai la ble on the ma rket t od a y. In g eneral, it can b e sta ted

tha t tw o a nchors of t he sa me w eight but o f d if ferent type (for insta nce a

Ste vin a nchor an d a Ste vpris Mk5 a nchor), do n ot necessa rily ha ve the sa me

fluke area. Consequent ly , two anchors of the same weight but di f ferent

type, w ill have d ifferent holding ca pa cit ies.

Som e exa mples:

Fig . 2-12show s a Stevpris Mk5 anchor and a Moo rfa st a nchor, bot h of iden-

t ica l weight . It d emonstrat es tha t in spite o f b eing the same w eight , the

fluke area s diff er substa nt ially. The ultima te h olding ca pa city o f t he Stevpris

Mk5 a nchor is 4 to 8.5 t imes higher tha n tha t o f t he sa me w eight Mo orfa st

anchor.

Fig . 2-13illustra te s th e d ifference in fluke a rea of th e Ste vpris Mk5 a nchor

in com pa rison w ith t he Bruce FFTS Mk4 a nchor, bo th of w hich ha ve ident i-

ca l w eight .

f ig . 2-13

fig . 2-12

Strength o f an anchor design 34


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Anchors should be d esig ned t o w ithsta nd t he loa ds applied o n them in the

diff erent loa ding situa tion s. Typica l loa ding situa tion s a nd a rea s of specia l

at tent ion fo r anchors a re:

During the proo f loa ding o f t he a nchors in the f a cto ry, af ter constructionha s been completed . On b a sis of the proof loa d results , the cla ssifica t ion

societ ies issue th e a pprova l certifica te .

While em bedded in the seabed

Depending on the soil cond it ions, different loa ding situa tions ca n o ccuron t he a nchor. In sa nds and cla ys, the loa d t ends to b e spread eq ua lly overth e a nchor, w hich g enera lly present s no prob lems. Ret rieva l is also very

simple, w ith out excessive loa ds pla ced on th e a nchor.

In very hard soils, the a nchor ha s to be a ble to w ithsta nd t he load w ithonly one o r tw o of the fluke t ips buried in t he soil, a s penet rat ion in very

ha rd soil cond ition s is g ene rally sha llow . In very sof t c lays (mud) penetrat ion of the anchor is uncomplicated.

How ever, recovery of th e a nchor ca n cause hig h loa ds, som et imes excee-

ding the loa d t ha t w a s used t o insta ll the a nchor.

Sidew a rds fo rces on t he t op of (sha llow ) buried a nchors ca n b e so extre-me th a t no a nchor is ca pa ble of resist ing t hem.

During a nchor han dling

Ca re should be t a ken during the ha ndling of the a nchors, as the load sexerted by t he w inches, vessels a nd chain can som et imes exceed th e struc-

t u r a l st r e n g t h o f t h e a n ch o r a n d c a u se d a m a g e . An ch o r d e s ig n e r s

a tt empt to desig n the a nchors fo r these high loa ds, how ever this is not

always possible due to variat ions in the magnitude of the loads during

handling operations.

Strength of an anchor design 35


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Large forces can be exerted on the anchor when high winch power isused, th e a nchor is ca ug ht o n the a nchor rack or ca ug ht b ehind the stern

ro ller o f t he AHV.

The u se o f a n impro per a ncho r/cha ser comb ina t ion. When a cha ser is usedtha t is either too sma ll or to o larg e, the chaser could ja m o n the sha nk of

the a nchor and cause da mag e .

The streng th of th e Ste vpris a nchor is no w mo re closely exa mined in the

light of the rema rks ma de b efore .

Strength of the shank

The prisma tic sha pe o f t he Stevpris a nchor no t o nly ensures opt imal pene t-

rat ion of the soil but also guarantees maximum strength. Although the

Stevpris design also has limitations, it is one of the better designs to with-

stand s ideward forces on the shank, a frequent occurrence in prac t ice .When using a n a nchor in very sof t cla y (mud ), the b ending mo ment on the

sha nk is low during t he insta lla t ion a nd w hen t he a ncho r is in t he soil.

How ever, during the b reaking out of the a nchor, h igh bending m oments

could b e introduced in the sha nk due t o t he high retrieval fo rces req uired

in very soft clay. In extremely sticky soils, the breaking out force of the

a nchor can rise t o 80% or 90% of a pplied a nchor loa d; in certa in insta nces,

it can even exceed 100%. To reduce t hese fo rces the brea king ou t procedu-

re is undertaken a t low speed to a llow t ime for the a nchor to break out .

Strength o f an anchor design 36


36/168

Strength o f the f luke

The strengt h o f t he f luke and especia lly the fluke po ints of a n a nchor a re

very importa nt w hen w orking in extrem ely ha rd soils such as cora l, limesto -

ne a nd ot her rock types. It is po ssible in such insta nces th a t t he t ot a l ho lding

ca pa city of the a nchor w ill ha ve to b e susta ined by the fluke points alone.

This mea ns the structure must be strong enoug h t o w ithsta nd extreme b en-

ding fo rces. Loa ding in no rmal soil cond it ions is not a problem due to the

fa ct t ha t t he loa d is eq ua lly spread over the fluke.

In f ig. 2-14, the different fo rce po ints a re show n f or varying soil cond it ions.

The location on the fluke w here the proof loa d is a pplied, is a lso indica ted .

Strength in extremely hard soils

In very ha rd soils such as ca lca renite, coral a nd limesto ne, a n a nchor w ill not

penet rat e very deeply. Conseq uently the loa d a pplied t o t he a nchor ha s tobe h eld by th e fluke t ips of the a nchor a nd a sma ll port ion o f t he f luke. This

mea ns tha t extremely high loa ds w ill be a pplied t o t he fluke t ips, com pa red

to no rma l soil con dition s such as sa nd a nd clay.

For use in very hard soil condit ions, vryhof has designed the Stevshark

a nchor, a mo dified version o f th e Stevpris ancho r. To crea te th e Stevsha rk,

the S tevpr is anchor has been s t reng thened, consequent ly a S tevshark

anchor having the same outs ide dimensions and holding capaci ty as a

Ste vpris a ncho r w ill be h ea vier.

Streng th calcula t ions of t he Stevsha rk design ha ve been ma de to g ua rant ee

suff icient streng th in the fluke point s. The Stevsha rk anchor is de signe d to

w ithsta nd t he a pplica t ion of the m a in pa rt of t he loa d o n just i ts f luke t ips.

f ig . 2-14

rock

proof loadclay sand

Strength of an anchor design 37


37/168

To prom ot e penet rat ion, t he Stevsha rk a nchor has a serra ted sha nk and ca n

be provided w ith cutter po ints on t he fluke t ips. Balla st w eight ca n a lso b e

added inside the hollow flukes of the anchor, up to 35% of the anchor

w eight . This is impo rta nt w hen w orking in very ha rd soil, w here the a nchor

w eight pressing on the f luke t ips promo tes penetra t ion , i .e . increa sed

be a ring pressure.

Anchor loads and safety factors 4000

3000

3895

Tot al d yna mic

38


38/168

The loa ds in a moo ring system a re ca used by t he w ind, w a ves a nd current

ac t ing on t he f loat er. Depending on the loca t ion of the f loa ter in the w orld,

different met ocean cond it ions w ill prevail. In t he t a ble below , som e extre-

me met ocean cond it ions a re presented fo r different a reas.

The loa ds ind uced in the m oo ring syste m can b e divide d into q ua si-sta tic

loa ds and to ta l dyna mic loa ds. The q ua si sta t ic loa d is the loa d d ue to the

sw ell, w ind, current a nd the freq uency of the system . For q ua si-sta t ic loa ds,

the system s tend to mo ve a t a low freq uency, genera lly w ith a period of 140to 200 secon d s.

On to p of th is q ua si-sta tic loa d t here a re the individ-ua l w a ve fo rces ca using

a high f requen cy mo tion. The high freq uency motion causes dyna mic shock

loa ds w ith a period of 10 to 14 second s due t o t he rolling of the vessel and

the mo vements of t he a nchor lines through the w a ter. The q ua si-sta t ic loa dplus the individua l w a ve fo rces is ca lled the to ta l dyna mic loa d. G enerally

the q ua si-sta t ic loa ds w ill be eq ua l to 50% to 90% of the to ta l dynam ic loa d.

See Fig . 2-15 fo r a n exa mple of t he difference betw een the q ua si-sta t ic loa d

and the to t a l dynamic loa d .

f ig . 2-15

3000

2000

1000

08300 8400 8500 8600 8700 8800 9800

Qua si stat ic

2342

LoadinkN

Time in seco nd s

Location Waveheight Wave period Windspeed Current m s m/s m /s

Ca mpo s Ba sin 8 10 12 - 15 25 1

G ulf o f Mexico 11 14 44 - 48 1

No rt hern No rt h Sea 15 - 16 15 - 17 38 - 39 0.9 1.2

Po rcupine Ba sin 16 - 18 16 - 20 39 - 41 1.0 1.5

Vo rine Ba sin 14 - 15 16 - 17 37 - 39 1.0 1.5

West o f Af rica 4 - 6 10 - 16 20 1

West o f Shetla nds 15 - 17 16 - 19 39 - 41 1.0 3.0

Anchor loads and safety factors 39 Permanent Quasi-static Total dynamicmooring load load


39/168

The q ua si-sta t ic a nd to ta l dynam ic loa ds are g enerally ca lcula ted fo r the

inta ct a nd d a ma g ed loa d condit ion. The inta ct loa d condit ion is the cond i-

t ion in w hich a ll the m oo ring lines are inta ct . The d a ma g ed loa d cond it ions

is the cond it ion in w hich o ne o f t he mo oring lines has broken.

From the q ua si-sta t ic loa d a nd t he to ta l dyna mic loa d, the req uired holding

ca pa city of t he a nchor ca n be ca lculat ed . This is ca lled th e ultima te h olding

capaci ty (UHC) for drag embedment anchors and the ul t imate pull-out

cap a cit y (UPC) f or VLAs. The req uired h o lding cap a cit y is calcula te d bya pplying th e fa cto rs of sa fe ty specified by t he cla ssificat ion societ ies.

In t he t ables G and H, the f a ctors of saf e ty a re presented fo r the di f ferent

loa d cond it ions for d rag embe dm ent a nchors (fo r insta nce t he Stevpris Mk5

a nchor), a ccord ing t o API RP 2SK. The f a cto rs of sa fe ty used b y th e ma jor

cla ssifica t ion societies a re g en era lly simila r to t ho se g iven in API RP 2SK (2nded ition , 1996).

For VLAs, t he recent ly used fa ctors of sa fe ty sug g ested by ABS, a re presen-

ted in table I.

The f a cto rs of sa fet y fo r VLAs are higher t ha n t he f a cto rs of sa fet y requiredfo r drag emb edm ent a nchors, due to the d iffe rence in fa ilure mecha nisms.

When a drag embedment anchor reaches its ult imate holding capacity, i t

w ill cont inuously drag t hroug h the soil w itho ut g enera ting a dd it iona l hol-

ding ca pa city, i.e. th e loa d w ill sta y eq ua l to th e UHC. When a VLA exceed s

its ultima te p ullout ca pa city, it w ill slow ly be p ulled o ut o f t he soil.

Intact load condition 1.8 1.5

D am aged condition 1.2 1.0

Temporary Quasi-static Total dynamic

mooring load load

Intact load condition 1.0 0.8

D am aged condition N ot required N ot required

VLA Total dynamic

load

Intact load condition 2.0

D am aged condition 1.5

table G

table H

table I

Anchor behaviour in the soil 40


40/168

Drag embedment anchors

Dra g embedm ent a nchors are g enerally insta lled b y applying a loa d eq ual

to the m a ximum inta ct loa d. The a nchor w ill then ha ve penetrat ed t o a cer-

ta in de pth, but w ill st i ll be capa ble of further penet rat ion b eca use the ult i-

ma te h olding capa city ha s not been reached. The a nchor w ill a lso h a ve tra-

velled a certa in ho rizon ta l dista nce, ca lled the dra g leng th. After insta lla -

t ion th e a nchor is ca pa ble of resist ing loa ds equa l to the insta lla t ion loa d

w itho ut furthe r penet rat ion a nd d rag . When the insta lla t ion loa d is excee-

ded , the a nchor w ill cont inue to penetra te a nd d rag until the soil is ca pa bleof providing sufficient resista nce o r the ult imat e ho lding ca pa city ha s been

reached.

How ever, there are certa in effects w hich allow the a nchor to w ithsta nd f or-

ces larg er than the insta lla t ion loa d w ithout further penetrat ion a nd d rag .

The se a re:

The set-up and consolidation e ff ect

Set -up a nd consolida tio n ma inly occur in claye y so ils. The p ene tra t ing

a nchor disturb s th e soil a nd th e soil te mpo rarily loses streng th . With time,

th e d isturb ed clay recon solida te s to its initial shea r streng th , this ta kes fro m

a f ew hours up to 1 mont h, depend ing o n the soil type. Beca use not a ll the

soil a round th e a nchor is disturb ed , the set-up eff ect fa ctor is less th a n th esen sitivity inde x indicat es. The d istu rba nce ma inly red uces th e soil resista n-

ce parallel to the fluke. On reloading, the parallel soil resistance gains

streng th, i t ta kes a la rger loa d t o mo ve the a nchor a g a in. Eq uilibrium dicta -

tes tha t a lso t he no rmal loa d, i .e . the b ea ring soil resista nce to the f luke,

increa ses; con seq uent ly th e loa d a t t he shackle increa ses a lso w ith th e set -

up fa ctor. Observat ions on a nchors fo r drilling rig s a nd th eo retica l con side-

rat ions for a 3 to 4 w eek consolida tion t ime d emo nstra te a typica l set-up

effec t fa cto r = 1.5.

Anchor behaviour in the soil 41 01.2

actor


41/168

The rate effect

An increa sed rat e o f loa ding increa ses the soil resista nce, con seq uent ly th e

a nchor ho lding capa city increa ses. This must be t a ken int o a ccou nt w ith

respect t o t ot a l dyna mic loa ds. For a nchor beha viour the ra te ef fect fa cto r

indicates how much higher the dynamic high frequency load may be wit-

hout ca using extra m ovement of the a nchor once insta lled a t th e insta lla -

tion loa d. The ra te of loa ding influences pore p ressure variat ions, viscou s

inter-g ran ular f orces a nd inertia f orces. Typica l ra te eff ect f a cto rs a re 1.1 to

1.3 for t ot a l dyna mic loa ds, see Fig. 2-16 w here t he ra te eff ect is present ed

fo r tw o different soil cond it ions (Su = 10 kPa a nd Su = 50 kPa).

Using the ra te e f f ect a nd set-up fa ctors, the b ehaviour of the a nchor a f ter

insta lla tion can b e predicte d mo re accura te ly.

Vertical Load AnchorsA VLA is insta lled just like a con vention a l dra g emb ed men t a nchor. During

insta lla t ion (pull-in m od e) the loa d a rrives a t a n a ng le o f a pproxima tely 45

to 500 to t he fluke. Aft er trig g ering t he a nchor to t he norma l loa d po sit ion,

th e loa d a lw a ys a rrives perpend icular to th e fluke. This cha ng e in loa d d irec-

tion g enerat es 2.5 to 3 t imes mo re holding ca pa city in relat ion t o t he insta l-

lat ion loa d. This mea ns tha t o nce the req uired UPC of th e VLA is know n, th ereq uired insta lla tion loa d f or t he VLA is a lso kno w n, be ing 33% to 40% of

the required UPC.

As a VLA is dee ply embe dd ed a nd a lw a ys loa ded in a direction no rmal to

the f luke, the loa d can b e a pplied in a ny direction. Conseq uently the a nchor

is idea l for ta ut-leg moo ring system s, w here g enerally the loa d a ng le varies

from 25 to 450.

f ig . 2-16

1.1

1

0.9

0.8 0 200 400 600 800 1000

Time f a cto r St

Rateeffectf

Su= 10 kPa Su= 50 kPa

Proof loads anchors 42


42/168

Proof loads for high holding pow er anchors

The proo f loa d a cco rding t o Cla ssifica tio n Societ ies rules is a pplied a t 1/3rd

of the fluke length and is carried out immediately on fabricat ion of the

a nchor. It is ob ta ined by placing the a nchor in a test yoke in w hich a hydrau-

lic cylinder applies the test loads, controlled by a calibrated manometer

(fig . 2-17). The vryhof a nchor t ypes ha ve been a pproved by t he f ollow ing

Cla ssificat ion Societies:

The Ame rican Burea u o f Shipping

Burea u Verita s Det No rske Verita s Ge rma nischer Lloyd Lloyd s Reg iste r o f Shipping Reg istro Ita lia no Na vale USSR Reg ist er o f Shipp ing

Nippo n Ka iji Kyo kai Norw eg ia n Ma rit ime Directo rat e

In th e ea rly da ys there w ere no specific regulat ions rega rding the holding

pow er a nd streng th of mo oring a nchors. The rules w hich d id exist w ere

of ten f ollow ed reg a rdless of the t ype of vessel.

Som e a nchors w ere a pproved a s high h olding pow er an chors. This so-cal-

led HHP approva l w a s ob ta ined a ft er ca rrying out f ield t ests in various types

of soil in w hich it ha d t o be show n tha t a n a nchor provided a holding po w er

of a t least tw ice tha t o f a sta nda rd sto ckless a nchor. If a n HHP anchor w a s

requested b y the o w ner, the a nchor ha s proof tested in strict a ccorda nce

w ith t he rules, not hing mo re. See table Jfo r som e exam ples of HHP a nchor

proof loa ds. A more d eta iled overview of HHP a nchor proo f loa ds is g iven in

the product da ta sect ion .

f ig . 2-17

Anchor Proof Load Anchor

weight factor weight

1 t 26 t 26 x

5 t 79 t 15 x

7 t 99 t 14 x

10 t 119 t 12 x

15 t 155 t 10 x

20 t 187 t 9 x

table J

Proof loads anchors 43

29 t Danforth

Proo floa d HHP an chors, UHC= 250 t.


43/168

The use of t he specified p roo f loa ds for HHP an cho rs ca n lead to situa tions

w here different types of a nchors w ith th e sa me holding capa city are proof

loa ded at di f ferent load s, see f ig. 2-18. From t his fig ure it can b e conclud ed

that the proof load of the anchors should preferably be re lated to the

brea k-loa d of the mo oring line o n t he vessel.

Nowadays the rules and regulat ions are far more rigid, and the require-

men ts ha ve been substa nt ially increa sed . There a re now specia l rules fo r

mob ile o ffshore units a nd permanen tly moo red structures .

If a nchors need m ob ile o ffshore units cert if ica t ion, t he f ollow ing properties

ma y be required:

Proo f load of the a nchors at 50% of the brea king loa d o f t he cha in .

Submission of a streng th ca lcula t ion o f the a nchor t o the cla ssifica t ionsociety prior to commencing anchor production: this includes determi-ning the m echanica l st rength of the a nchor a s w ell as proving tha t t he

a pplied ma teria l can w ithsta nd t he proo fload .

A sta tem ent o f do cument ed ho lding po w er from t he a nchor supplier. Subm itta l of a Qua lity Assura nce/Qua lity Con trol Ma nua l.

In f ig. 2-19, a m oo ring system is show n in w hich a ll of the compo nent s a re

ba la nced. The streng th o f t he mo oring line, holding capa city of the a nchor

a nd streng th o f th e an chor a re all in the correct proportion a nd comply w ith

th e rules.

f ig . 2-18

0 50 100 150 200 250

10 t Stevin Mk3

4.5 t Stevshark Mk5

4 t Stevpris Mk5

Proofloa d in t

f ig . 2-19

0 10 20 30 40 50 60 70 80 90 100

Breakload chain

Ultimate holdingcapacity anchor

Damaged load floater

Proofload chain

Pretensionload anchor

Intact load floater

Proofload anchor

Balan ced m oo ring system API RP 2SK

Loa d in %

Quality control 44


44/168

ACCREDITED BY

THE DUTCH COUNCIL

FOR CERTIFICATION

Reg. No 24

DETNORSKEVERITASINDUSTRYB.V., THENETHERLANDS

ISO-9001CERTIFICATED FIRM

The a pplica t ion o f m ore a dva nced a nd com plex technolog y in a nchor

construction ha s broug ht a bo ut req uirements fo r a system a tic a pproa ch to

q ua lity. Init ia ted by va rious a utho rit ies they a re cont inuously refined a nd

fo llow ed up by opera t ing compa nies such a s vryhof a nchor. Like ot her

com pa nies, vryhof ha s become increasing ly aw a re of the vita l importa nce

of managerial aspects and their influence on the total quality-assurance

a nd cont rol system .

D e s i g n a n d f a b r i c a t i o n o f a n c h o r s f o r p e r m a n e n t m o o r i n g s a r e i n

a cco rda nce w ith t he q ua lity req uirement s of th e Rules NS/ISO 9001 a s

described in our Quality Assurance Manual. Vryhof anchors obtained the

ISO 9001 certificate No. QSC 3189 issued by Det Norske Veritas for Design,

Ma nufa cture of a nchors, and Sales of a nchors a nd mo oring compo nent s .

Quali ty control is maintained throughout product ion . A compilat ion ofcertifica te s is present ed t o a client upo n com pletion o f a project.

I d i

Anchor tests 45


45/168

Introduction

In a dd ition t o pra ctica l experience of users a nd a ssociat es, a nchor tests a re

one o f t he mo st re liab le mea ns of forecast ing a nchor perfo rmance a nd t hus

ma king a proper choice of a nchor type a nd size.

Examining anchor tes ts that have been carr ied out in the past , certa in

con clusions ca n be m a de :

Man y tests w ere underta ken in w hich t he results w ere recorded a ccurat ely.

Deta iled repo rts, how ever, ha ve not b een very com mon .

Anchor tes ts of the past are not a lways easy to in terpret or comparebecause of different soil a nd a nchor t ypes.

Test results ha ve not a lw a ys be en interpret ed ind epen de nt ly. The mo re te sts results are strictly com pa red to pra ctica l results, the be tt er

one can f oreca st th e holding pow er and g eneral beha viour in practice.

Vryhof is in the perfec t s i tuat ion of having detai led tes t data avai lable

to g ether w ith extensive practica l da ta ob ta ined d uring insta lla t ion an d use

of a nchors on projects on site .

Research in to anchor behaviour and the u l t imate ho lding capac i ty o f

a nchors is of ten carried o ut b y test ing a mo del an chor, prefera bly fo llow edby a fu ll-scale te st in th e f ield. The o pt ima l a nchor t est con sists of mo de l te sts

w ith 10 kg a nchors, fo llow ed by f ull-sca le tests w ith 1 t a nd 10 t a nchors. The

a nchors should b e pulled until the ult ima te holding ca pa city is reached.

It is o bviou s th a t f ull-scale t esting o f a ncho rs can be expensive. La rge AHVs,

strong w inches a nd strong mo oring lines a re req uired, w hich are not a lw a ys

available. For example, a 5 t Stevpris Mk5 anchor, deployed in sand, is

ca pa ble of sto pping a mo dern AHV a t its full bo lla rd pull.

Anchor tests 46

acity A G B

CT ti 10 t St i Mk5 h t it lti t h ldi it i d


46/168

f ig . 2-20 Drag

HoldingCap C

D

E

F

Testing a 10 t Ste vpris Mk5 a nchor t o its ultima te ho lding ca pa city in sa nd

w ould require a horizont a l pulling ca pa city of a pproxima tely 600 t .

If a nchor tests a re to be compa rab le, the test ing prog ram should prefera bly

meet , as a m inimum, th e fo llow ing criteria :

An a ccura te a nd sophistica te d me a suring syste m sho uld be used. The a nchors sho uld be t ested up t o t heir ultimat e ho lding capa city. Drag and penetrat ion of the a nchor should b e recorded during test ing .

The a nchor should be held und er tension w ith a blocked w inch for 15

minutes, to investiga te a ny drop in ho lding ca pa city.

Reading test curves

The be ha viour of a n a nchor during ten sioning can b e a ccurat ely interpreted

from the ho lding ca pa city versus drag curve. Sample t est curves are presented

in Fig . 2-20. Properly interpreted performance curves can explain a lotab out anchor beha viour.

Curve A is very ste ep a nd represent s a strea mlined a nchor in very stiff soil. Curve B is a no rma l curve fo r a nchors in sa nd a nd med ium clay. Curve C is a curve of a n unsta ble a nchor. This can b e caused b y a w rong

fluke/sha nk an g le set ting , a short sta biliser or a fluke tha t is to o long .

Curve D is a n orma l curve f or a n a nchor in very sof t clay. Curve E is a n a nchor w ith a 32o fluke/sha nk a ng le in very so ft cla y. Curve F represent s a n a ncho r tha t is t urning cont inuo usly. This can b e

caused b y the a bsence of sta bilisers, a t oo larg e fluke/sha nk an g le or a

low efficiency a nchor a t continuous dra g .

Curve G represents a n a nchor pene tra t ing in a la yer of st if f c la y overla inby very sof t cla y.

Curves A B D E a nd G show a very sta ble rising line w hich indica te s th a t

Anchor tests 47 150

100

Sand8 m sof t clay

on rocka

cityint


47/168

Curves A, B, D, E a nd G show a very sta ble rising line, w hich indica te s th a t

th e a nchor builds up its holding ca pa city con sta ntly until the ultima te ho lding

capa city ha s been reached, a f ter w hich the anchor show s cont inuous drag .

The o t her curves a re larg ely self-explan a to ry.

Test results

Vryhof s extensive da ta ba se of t est results w ith d ifferent a ncho r typ es, sizes

and soil condit ions, has been frequently used in anchor design. Data has

been ob ta ined from practice, sca le mo dels a nd f rom th ird pa rt ies. The d a ta

ha s been interpreted and a f terw a rds incorporated in the ult imate holding

ca pa city, dra g a nd penetra t ion g raphs of the Stevin Mk3 a nd Stevpris Mk5

a nchor as w ell a s in th e ultima te pu ll-out ca pa city g raph o f t he Stevma nta VLA.

Norwegian Contractors (1984)

In 1984 Norw eg ia n Con tra cto rs ca rried out te sts at Dig ernessund et, Stord ,Norw a y. The purpose of the se te sts w a s to det ermine the correct a nchor type

a nd size fo r the mo oring system of the Gullfa ks A plat fo rm during the construc-

tion of the plat fo rm at Dig ernessundet . Altho ugh the construction w ould to ok

pla ce a t o ne loca tion, it w a s know tha t t hree different t ypes of soil cond itions

w ould be encountered: sa nd, sof t mud a nd a n 8 m mud layer on rock. After the

initia l tria ls the Stevpris anchor w a s selected fo r furth er testing.

The 3 t Stevpris anchor tha t w a s used fo r the t ests at a 3.30 pulling a ng le,

produced a ma ximum h olding capa city o f 150 t in the sa nd, 102 t in the very sof t

clay a nd 150 t in the la yer of m ud o n rock. As th e mo oring syste m req uired

a survival load of 1500 t, a 65 t Stevpris (mud location), 40 t Stevpris (sand

loca tion ) a nd 60 t Ste vsha rk (mud on ro ck loca tion) w ere selecte d f or th e fina l

mooring. Fig . 2-21sho w s th e test results o f t he 3 t Stevpris a nchor, w hile f ig. 2-22

show s the result o f t he ten sioning o f th e final an chors w ith a loa d o f 820 t .

f ig . 2-21

100

50

25

0 10 20 30 40Holdingcap

soft clay

Drag in meters

f ig . 2-22

800

700

600

500

400

300

200

100

0

20 40 60 80

Full scale G ullfa ks A anch ors

Holdingcapacityint

Drag in meters

A B* C

A = 40 t Stevpris in sand

B = 60 t Stevshark in mud on rock

C = 65 t Stevpris in mud

Survival load = 1500 ton

* Final pretension loa d on site

Anchor tests 48 700600

500

7 2

7-3

7 4

Large scale anchor test jip - 7 &2 t

d

in

kips

Large scale an chor tests in the Gulf o f Mexico


48/168

f ig . 2-23

400

300

200

100

0 50 100 150 200 250 300 350 400 450 500

Drag dista nce in feet

7-2 7-47-1

2-2

2-1

HorizontalloadLarge scale an chor tests in the Gulf o f Mexico

In 1990, tests w ere perfo rmed w ith 2 t a nd 7 t Ste vpris Mk5 a nchors, a s pa rt

of a n a nchor t est Joint Ind ustry Project (JIP). The a nchors w ere t ested using

a w ire rope f orerunner.

The 2 t Stevpris anchor w a s tested up t o its ultima te ho lding ca pa city o f 107 t

(235 kips). Due to insuff icient pulling capa city, th e 7 t Ste vpris an cho r co uld n ot

be pulled up to its ultima te holding ca pa city. Based o n th e results of tests, the

ultima te ho lding capa city of the 7 t Stevpris a nchor w a s ca lcula ted to be larger

tha n 338 t (745 kips) (f ig. 2-23).

Uplift

Anchor tests 49


49/168

Uplift

Stevpris a nchors a re w ell ca pa ble of resisting uplift loa ds w hen t hey a re deep-

ly embed ded . Anchors in sa nd a nd f irm to ha rd cla ys do not penetrat e very

deeply and only take small uplift loads. Stevpris anchors installed in very soft

cla y and mud penet rat e dee ply, a t ypica l penet rat ion f or a 15 t a nchor is 15 to

25 met ers. Due to the inverse cat ena ry in t he soil, the a nchor line a rrives a t t he

a nchor sha ckle a t a n a ngle of 20o to 30o w ith t he mud line. Once the a nchor is

insta lled, a loa d ma king a n a ngle up to 20o w ith th e horizont a l a t mud line w ill

not cha ng e the loa ding d irection a t t he a nchor! A Stevpris a nchor ha s been

tested in th e Gulf of Mexico w ith g rad ua lly increa sing pull a ng le (fi g. 2-24).

The m a ximum resista nce w a s ob ta ined fo r 18o uplift a t mud line.

f ig . 2-24

35 000

30 000

25 000

20 000

15 000

10 000

5 000

0

60

50

40

30

20

10

0

0 50 100 150 200 250 300

= dyn load

= pull angle

Line

anglevsmudine

Line lengt h pulled in feet

Lineloadinlbs

18

Cyclic eff ect fa ctor

Anchor tests 50


50/168

Cyclic eff ect fa ctor

The lo a ding a t t he a ncho r is cyclic. Exxo n perf o rmed cyclic te sts on a ncho rs

reported by Dunna vent a nd Kw a n, 1993. Altho ug h t he m a ximum cyclic loa d

w a s less tha n the init ial insta lla t ion load , the sta t ic loa d a pplied a ft er the

cycling phase revealed 25 to 50% larger anchor resistance than the initial

installat ion load (fi g. 2-25). This eff ect is expla ined b y furt her pen et rat ion

of the anchor. Applying this knowledge to the anchors, the stat ic anchor

resistance after some storm loading improves by the cyclic effect factor of

1.25 t o 1.5.

f ig . 2-25

0.15

0.1

0.0

0 50 100 150 200 250 300 350

Time in secon ds

Ancho

rresistanceinkN

Initial sta tic ca pa city

Cycling

Increased capacity

vs initial stat ic

Anchor tests 51

Tests w ith Stevmanta anchors


51/168

f ig . 2-26

200

150

100

50

0

0 5 10 15 20 25 30 35


Block w inch

Change mode

Lineloadin%

Tests ha ve bee n perf orme d in the G ulf of Mexico a nd of fshore Bra zil. The

Stevma nta a nchor being pulled in w ith a loa d eq ua l to F, accepted a vert ica l

loa d to the a nchor of up to 2 t imes F! Among st t he ma ny tests the a nchor

rela xa t ion w a s mea sured. The a nchor w ith a f luke a rea o f 0.13 m 2 w a s

pulled in a t 0o pull a ng le (fig . 2-26), then loa ded vert ica lly to a loa d eq ua l 1.6

times the m a ximum insta lla t ion loa d. At t his loa d t he w inch w a s blocked.

Anchor tests 52

This permitt ed t he mo nitoring of the loa d w ith t ime (fi g. 2-27)a s w h a t


52/168

f ig . 2-27

200

150

100

50

0

20.00 22.00 0.00 2.0 0 4.00 6.00 8.00

Time in secon ds

Lineloadin%

Chang e from

pull-in to normal mode

Block w inch

w ould b e expected in real circumsta nces a t a consta nt loa ded a nchor line.

The results show tha t t he ho lding capa city of the a nchor do es not cha ng e

significa nt ly du ring con tinuo us loa ding , as th e ob served decrea se in ten sion

w a s du e to mo vement of the w inch. The subsequent pulling a t 7:00 AM

show ed tha t f or o nly a sma ll movement, t he f ull pla te ca pa city (2 x insta lla -

t ion loa d) could b e rea ched . Cont inuous pulling ca used the a nchor t o loo se

resista nce a nd b reak out .

To dem onstrat e t ha t the fea ture o f these a nchors is not only a vert ica l resis-

Anchor tests 53


53/168

ta nce , the a nchor w a s insta lled w ith a horizonta l pull, the mo de chang ed t o

the n orma l (vert ica l) mod e a nd the a nchor subsequen tly pulled w ith a n

uplift a ng le of 30o (fig . 2-28). The b eh a vio ur is simila r to th e ea rlier vert ica l

pull te st. How ever, fo r the 30o pull a ngle the a nchor did not break out but

mo ved slow ly a long th e pulling d irection th roug h th e soil. The g rap hs clear-

ly show this eff ect a nd t ha t the a nchor ca n be used f or substan tial horizon-

ta l loa ds.

f ig . 2-28

200

150

100

50

0

0 5 10 15 20 25 30 35 40


Chang e from

pull-in to norma l modeLineloadin%

Soil table 54


54/168

Increasing lithif ication

Increasi

ng

grain

size

of

particulate

deposits

0.002m

m

0.063mm

2mm

60mm

Carbonatesilt

Carbonatesand

Carbonategravel

Carbonat

eclay

Siliceouscarbonate

Siliceouscarbonate

silt

sand

Mixedcarbonateand

non-carbonategravel

Calcareou

sclay

Calcareoussilicasilt

Calcareoussilicasand

Clay

Silicasilt

Silicasand

Silicagravel

Calcilutite

Calcisiltite(ca

rb.

Calcarenite(carb.

Calcirudite(carb.

(carb.Caly

stone)

Siltstone)

Sandstone)

Conglom.OrBreccia

Conglomeratic

Clayeycalc

ilutute

Siliceouscalcisiltite

Siliceouscalcarenit

e

calcirudite

Calcareous

Calcareaous

claystone

Calcareoussiltstone

Calcareoussandstone

conglomerate

Conglomerateor

Claysto

ne

Siltstone

Sandstone

breccia

Fine-grainedlimestone

Detritallimestone

Conglomerat

limestone

Fine-gra

ined

Fine-grainedsiliceous

Siliceousdetrital

Conglomerate

agrillaceouslimestone

limestone

limestone

limestone

Calcareous

Calcareousc

laystone

Calcareoussiltstone

Calcareoussandstone

conglomerate

Conglomerateof

Claysto

ne

Siltstone

Sandstone

Breccia

Crystallinelimestoneormarble

Conventionalmetamorph

icnomenclatureappliesin

thissection

Appro x. Ro ck Very w ea k Wea k to mo d era t ely w ea k Mo d era te ly st ro ng to st ro ng St ro ng t o ext emelyst reng t h st ro ng

Cement a t io n o f Very w ea k t o f irmly Well cement ed so il (w ell cemented )so il cemented so il ro ck

Tota l carbo nate content %

90

50

10

90

50

10

90

50

10

50


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Practice

3

Practice

Introduction 56


56/168

Altho ugh theo retica l know ledg e of a nchors is essential fo r go od a nchor d esign

a nd selection , the practica l issues are just a s importa nt . The h a nd ling of a n

a nchor a nd t he selection a nd use of support eq uipment is of eq ua l impo rtance.

Anchor ha nd ling is a critica lly impo rta nt a nd of te n complica te d p rocess. It is

influenced b y such fa cto rs a s the w eight a nd shape o f th e a nchor, the na ture

of the soil, the depth of the w a ter, the w eat her cond it ions, the a vailab le

hand ling eq uipment a nd the t ype and w eight of moo ring line . It is fo r these

rea son s th a t a nchor ha nd ling is a subject w hich req uires ca reful con sidera -

t ion. Witho ut proper anchor han dling , optimal performa nce of a n a nchor is

no t p ossible.

In t he process of ha ndling a nchors, various types of support eq uipment a re

necessa ry or beneficia l. An a nchor ma nua l w ould b e incom plete w itho ut

con sidera tion o f th ese a uxilia ry items, th e reasons for their use, their op e-

rat ion and the a dvanta ges and d raw ba cks involved.

This cha pte r gives a n overview of th e recom men de d pro ced ures th a t should

be fol lowed for anchor handling and the types and use of the support

e q u ip m e n t d u r in g t h e h a n d l in g o p e r a t io n s . Th e f o l lo w in g h a n d l in gprocedures a re by no mea ns com plete, but they d o g ive som e sug g estions

w hich ca n be applied to each anchor handling procedure and a da pted fo r

specific circum st a nces a nd loca tio ns.

Som e of th e to pics covered in th is cha pte r are:

requirements for a soil survey, connection of the anchor to the mooringline, chasers, ha nd ling th e Ste vpris a nchor, ha nd ling th e Ste vma nt a a nchor,

th e Ste vten sione r, a ncho r ha nd ling /supp ly vessels.

Soil survey 57Typical contents survey report

Cone penetration resistance.

Sleeve friction.For the dimension ing of dra g e mb edm ent a nchors, the a vaila bility of site-spe-

f l d F d f d b d h


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Pore pressure.

SPT values.

G ranulom etry and percentage fines.W et and dry densities.

W ater content.

Drained and undrained triaxal tests.

Undrained shear strength, also rem oulded.

Unconfined com pression tests.

Plasticity lim its.

Specific gravity.

CaCO 3 content.

Shell grading.

Angularity and porosity.

Com pressibility.

Cem entation.

Norm alised rock hardness test (point load test).

RQ D index, rock quality designation.

table K

cific soil da ta is impo rta nt. For a dvice on specifying d rag emb ed ment a nchor

type /size a nd calculat ing expected be ha viou r, th e site -specific soil da ta sho uld

be compa red w ith soil da ta of previous drag embed ment a nchor (test) sites.

The soil survey requireme nt f or th e de sign o f d rag emb ed men t a nchors usua lly

consists of on ly sha llow bo reholes, w hile in a nchor pile d esig n d eep b oreho les

are required. For suction anchor design therefore a more extensive soil

investig a tion is g enera lly req uired w hen compa red to drag embed ment a nchors.

When choo sing b etw een a nchor pile, suction a nchor a nd d rag embe dm ent

a nchor the financia l implica tions of the soil survey sho uld be ta ken into a ccoun t.

A typical soil survey for drag embedment anchor design requires a survey

depth of tw ice the lengt h of the f luke in sa nd a nd 8 t imes the f luke length

in very soft clay. In most cases a depth of 8 to 10 meters is sufficient ,

a ltho ug h in very sof t cla y a reconn a issa nce d epth of 20 to 30 met ers should

be considered. For optimal drag embedment anchor dimensioning, each

a nchor locat ion should idea lly be surveyed . The soil investig a tion ca n con-

sist of boreholes, vibrocores, cone penetrat ion tests or a combination of

these. Cone penetration tests including sleeve friction are preferred, but

they should b e a ccom pa nied by a t least on e vibrocore or sa mple boreho leper site t o o bt a in a description o f t he soil. Depending upo n the type o f sur-

vey performed and the soi l condit ions encountered, the survey report

should present the test results obt a ined on site a nd in the la bo rat ory includ-

ing the po ints as show n in table K.

It is possible to dimension the dra g embe dm ent a nchors based o n limitedsoil info rma tion (for insta nce few er bo reho les). The lack of soil da ta ca n be

com pensat ed b y cho osing a con servat ive (larg er) a nchor size.

Pile or anchor 58

The choice bet w een pi les and a nchors is on ly po ssible f or perm a nen t

t Pil t d i t t h h d tit t b

Description Pile Suction Anchor

pile

Soil survey - - +

P t +


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systems. Piles a re not a g oo d investmen t w hen a n a nchored e ntity must b e

moved . But t he choice is of ten ma de f or piles on e mo tiona l g rounds; a pile

do es not dra g ! How ever, a nchors tha t a re properly pre-tensioned on site

w ill a lso no t d rag .

While it is a psycho log ically loa de d subject, experience ha s sho w n t ha t t he

cho ice b etw een a nchor a nd pi le is merely a ma t t er of econo mics. The

required pi le weight for a sys tem is equal to the required weight of a

Stevpris anchor. Piles cost about 40% of equivalent capability anchors.

How ever, th e insta lla tion costs fo r piles a re much hig her. Piles req uire a fo l-

low er a nd a pile ha mm er. The insta lla tion sprea d fo r piles is much mo re sig-

nifica nt ; a crane b a rge w ith support spread versus the t w o a nchor ha ndling

vessels. The w ea th er do w nt ime fo r a sprea d involving a cran e vessel is much

long er tha n w hen AHVs a re used. To a llow dra g of the a nchors during pre-

tensioning, extra cha in leng th is req uired. Som etimes the preten sion loa d

fo r piles is much less th a n f o r a ncho rs. The survey w o rk f o r an chors is g en er-

a lly much simpler tha n fo r piles. When a ba ndo ning a f ield, a nchor remova l

is much chea per th a n remo val of insta lled piles. The cho ice b et w een piles

a nd a nchors stro ng ly de pend s upo n t he circumsta nces. The table L ca n he lp

in est ima ting t he costs fo r the tw o a lterna tives.

Suction piles a re an a lterna tive fo r drag embe dm ent a nchors and piles, a lso

fo r MODU a pplica t ions. The a dva nta ge is the a ccurat e po sit ioning o f the

suction piles. The d isa dva nt a g e is th e cost o f t he pile itself a nd th e cost of

th e insta lla tion . Also m a ny soil types do no t a llow suction pile a pplicat ion s,

w herea s dra g em bed ment a nchors ca n be used in any soil type.

Procurem ent + - -

Installation spread - - +

Installation tim e - - +

Pile ham m er - + +

Follow er - + +

Pum p unit + - +

Pretensioning + - -

Extra chain + + -

Rest value pile/anchor - + +

Rem oval of anchor point - + +

RO V + - +

+ less expensive - m ore expensive

table L

Introduction

In soil such as sa nd a nd med ium to ha rd cla y a n a nchor w ith a fluke/sha nk

Setting the f luke/shank angle 59 f luke a ngle to o larg e in hard soil !


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In soil such as sa nd a nd med ium to ha rd cla y, a n a nchor w ith a fluke/sha nk

ang le o f 32o w ill g ive the high est ho lding pow er. An a nchor w ith a 50o

f luke/sha nk an g le in this soil w ill not penet rat e b ut w ill drag a long the

sea bed . If used in mud a 50o fluke/sha nk a ng le is a ppropriat e. An a nchor w ith

a 32 o f luke/sha nk an g le w ill penetrat e less and g enera te low er holding

ca pa city in mud (fi g. 3-01).

The Stevpris Mk5 an chor ha s an a dd ition a l fluke/sha nk a ng le set ting of 41o ,

w hich can b e a do pted in certa in layered soil cond it ions (tab le M ).

Changing the fluke/shank angle on the Stevpris Mk3

This ca n be ca rried ou t w ithin ha lf a n ho ur w ith th e Ste vpris a nchor upside

dow n on d eck.

Secure t he a nchor on de ck. Con nect a tu g g er w ire (C) to th e ho les (D) on th e

bot tom side o f the f luke. Chang e from mud t o sa nd a ngle by removing t he

locking plat es an d th e t w o rea r pins in (B), de crea se t he f luke/sha nk a ng le

by hauling the cable (C). Reinstall the pins and locking plates in (A). Seal

w eld the lock-ing pla tes, do not w eld them t o t he pins (fig . 3-02).

f ig . 3-01

no penetrat ion !

chang e from mud to sand a ngle

f ig . 3-02

Soil type Optimal

fluke/shank

angle setting

Very soft clay (m ud) 500

C ertain layered soils 410 *

M edium to hard clay

or sand 320

* Stevpris M k5 only

table M

Setting the f luke/shank angle 60

Cha ng e from sa nd to the mud po sition, increase a ng le by veering (C), cha ng e

over pin a nd locking plat es from (A) to (B) No specia l w elding req uireme nt s

change from sand t o mud ang le


60/168

over pin a nd locking plat es from (A) to (B). No specia l w elding req uireme nt s

(fig . 3-03).

Changing the fluke/shank angle on the Stevpris Mk5 Ch a n g in g t h e

fluke/sha nk a ng le on th e Stevpris Mk5 a ncho r is even q uicker. No w elding

req uired . Veering a nd ha uling (C) to cha ng e t he f luke/sha nk an g le as

a bo ve, the pin ho w ever rema ins in (A), the locking plat e is secured by me a ns

of a cot ter pin (fig . 3-04).

f ig . 3-03

chan ge fluke/shan k a ng le Stevpris Mk5

f ig . 3-04

Connecting a swivel to the Stevpris anchor

To con nect a sw ivel to th e Stevpris a nchor several d ifferent conf igura tion s

Connecting a swivel 61

J C B A


61/168

To con nect a sw ivel to th e Stevpris a nchor, several d ifferent conf igura tion s

a re p ossible. The se a re:

Type I - The sw ivel is connected direct ly to the sha nk of the a nchor thus

om itt ing the a nchor shackle (fig . 3-05).

Jsw ivel shackle, Cend l ink, Benlarged link, A comm on l ink

Type II - The sw ivel is conne cted to the a nchor sha ckle (fig . 3-06).

Jsw ivel shackle, Cend l ink, Benlarged link, A comm on l ink