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4 , O c t . 1 9 8 0, p p . 3 7 1 - 3 8 4

Ap proaches to the D esign

t P . F o r t i n o 1

of H opp er Dredges

T h r e e n e w h o p p e r d r e d g e s o f d i ff e r e n t s i z e s a n d m i s s io n

p e r fo r m a n c e a r e c u r r e n t l y u n d e r c o n s t r u c t i o n f o r

t h e

U . S . A r my C o r p s o f En g i n e e r s . T h i s

p a p e r d e s c r i b e s t h e

p a r t i c u l a r f e a tu r e s o f

t h e d r e d g e s t h a t c o n s t i -

t u t e

a d v a n c e s i n

t h e s t a t e o f t h e

ar t , and

re v ie w s t he ph i los oph y be h ind t he i r de s ign . In a dd i tion , t he

a u th o r

d e m o n s t r a t e s h o w t h i s n e w d r e d g e f l e e t , th o u g h

sm al le r than ear l i e r f lee ts , w i l l

i m p r o v e t h e

C o r p s ' c a p a b i l i-

ty , fo r bo th c i v i l i an and mi l i ta ry

p u r p o s e s , t o

main ta in a l l U . S .

c o a s t a l w a t e r w a y s

a s w e l l a s l o c a t i o n s o v e r -

s e a s t h a t a r e o f

in t e re s t t o t he Un i t e d S t a t e s .

F i n a ll y , d a ma g e s ta b i l it y r e q u i r e me n ts a n d n o i s e a b a te m e n t

s p e c i f ic a t i o n s a r e t r e a t e d i n s e p a r a t e a p p e n d i c e s .

I n t r o d u c t i o n

T a b l e 1 T h r e e n e w h o p p e r d r e d g e s - - g e n e r a l d a t a

I N JU N E O F 1 97 6, t h e U . S . A r m y C o r p s o f E n g i n e e r s e m b a r k e d

t o d e s i g n th r e e n e w h o p p e r d r e d g e s a s a fi r s t s t e p

n t h e e s t a b l i s h m e n t o f a r e d u c e d b u t m o d e r n s t a t e - o f - t h e - a r t

o p p e r d r e d g e f l e e t. T h e f l e et r e d u c t i o n i s i n c o n f o r m a n c e w i t h

e c e n t l y e n a c t e d l e g i s l a ti o n ( P L 9 5 -2 6 9 ) w h i c h i n d i c a t e s t h a t a n

i n c r e a s in g s h a r e o f t h e n a t i o n a l d r e d g i n g w o r k i s t o b e p e r f o r m e d

b y c o m m e r c i a l l y o w n e d p l a n t , w h i c h i s t h e r e a s o n w h y c o n t r a c -

t o r s a r e n o w b u i l d i n g h o p p e r d r e d g e s .

T h e t h r e e n e w d r e d g e s w h ic h h a v e b e e n d e s i g n e d a n d a r e n o w

u n d e r c o n s t r u c t i o n f o r t h e C o r p s a r e o u t l i n e d i n T a b l e 1 , a n d

p r e l i m i n a r y d e s i g n p r o f i l e s a r e s h o w n i n F i g s . 1, 2 a n d 3 .

T h e p h i l o s o p h y g o v e r n i n g th e r e d u c e d f l e e t is th a t t h e n e w

d e s i g n s w i l l b e c a p a b l e o f o p e r a t i n g o n a l l c o a s t s a n d w i l l b e

a v a i l a b l e o n s h o r t n o t i c e f o r n a t i o n a l d e f e n s e n e e d s o r f o r m i l i t a r y

o r c i v il e m e r g e n c i e s a n y w h e r e i n t h e U n i t e d S t a t e s o r i n a n y o f

i t s o v e r s e a s i n t e r e s t s . I n t h e a b s e n c e o f s u c h e m e r g e n c i e s , t h e

d r e d g e s w i l l b e k e p t i n r e a d i n e s s b y c o n t i n u o u s o p e r a t i o n i n

r e g u l a r d r e d g i n g a s s i g n m e n t s . N e v e r t h e l e ss , s o m e s p e c i a l c a p a -

b i l i ti e s m a y b e p r o v i d e d . T h e

Yaquina

i s d e s i g n e d t o b e e s p e c i a l l y

s u i t a b l e f o r o p e r a t i o n a t s h a l l o w d r a f t i n s u c h i n l e t s as a r e f o u n d

o n t h e W e s t C o a s t. T h e l a r g e -c l a s s h o p p e r d r e d g e ( L C H D ) i s

i n t e n d e d t o h a v e e n o u g h p o w e r t o o p e r a t e a l l f o u r o f i t s d r e d g e

p u m p s s i m u l t a n e o u s l y . I t h a s th e u s u a l t w o s i d e d r a g a r m s , e a c h

c a r r y i n g a s u b m e r g e d d r e d g e p u m p , a n d , i n a d d i t i o n , a t h i r d

d r a g a r m i n a c e n t e rw e l l c a p a b l e o f fe e d i n g t w o d r e d g e p u m p s .

W i t h a l l fo u r p u m p s o p e r a t i n g , th e d r e d g e c a n b e e s p e c i a ll y e f -

f e c t i v e w h e n w o r k i n g i n t h e a g i t a t i o n m o d e , t h a t i s , s i m p l y o v -

e r fl o w i n g fr o m t h e h o p p e r s c o n t i n u o u s l y , a l l o w in g th e m a t e r i a l

t o b e b r o a d c a s t o v e r th e w a t e r o n b o t h s i d e s o f t h e d r e d g e a n d

l e t t i n g t h e c u r r e n t c a r r y t h e s o l i d s t o b e d e p o s i t e d o u t s i d e o f t h e

c h a n n e l b o u n d a r i e s . T h i s d r e d g e i s e x p e c t e d t o b e p a r t i c u l a r l y

e f f e c t i v e i n d e a l i n g w i t h t h e s h o a l i n g w h i c h o c c u r s i n t h e M i s -

s i s s ip p i R i v e r p a s s e s , w h i c h o c c a s i o n a l ly h a s r e q u i r e d a t t a c k b y

a s i z e a b l e d r e d g i n g f l e et .

T h e m e d i u m - c l a s s h o p p e r d r e d g e (M C H D ) c a n be co n s i d e r e d

a g e n e r a l - p u r p o s e d r e d g e , d e s i g n e d t o b e e f f i c ie n t a n d e f f e c t iv e

i n a ll a s s i g n m e n t s . I t s d i m e n s i o n s a r e a v e r a g e , a n d i t s l o a d e d

d r a f t c a n b e t o l e r a t e d b y t h e m a j o r i t y o f a u t h o r i z e d d r e d g i n g

p r o j e c t s .

T h e a u t h o r h a s b e e n c o n t i n u o u s l y i n v o lv e d w i th t h e d e s i g n ,

1 Chief, M ar ine D es ign Divis ion (Ret . ) , Phi lad elph ia Dis t r i c t , U . S.

A r m y C or ps o f Eng i ne e r s .

Presented a t the December 7 , 1979 meet ing of the P hi ladelphia Sect ion

of TH E SOCIETY OF NAVALARCHITECTS AND MARINE ENGINEERS.

The op i n i ons e xp r e s s e d he r e in a r e t he v i ew s o f the a u t ho r a nd no t

necessar i ly those of the U. S. Arm y Corps of Engine ers or the U. S. Army

at l a rge .

Yaquina M C H D L C H D

Len gth OA, f t 200 350 409

Len gth BP, f t 193 333 384

Beam , mo lded, f t 58 68 78

De pth amid ships , mo lded, f t 17 35 39

H o p p e r v o lu m e , y d~ 825 6000 8400

H op pe r c a pa c i t y , s a nd ( m i n . ) , yd 3 650 4818 6000

Prop uls ion power, shp 2 1125 2 3500 2 5200

Design draf t , loaded (es t .) , f t 12 27 29.5

Displa cem ent a t loade d draf t , LT 2963 13 950 19 000

Speed , loade d (es t . ), knots 10 13.4 14.5

D r e dge Pum ps , i nboa r d :

num be r 2 2 2

discharge dia , in . 16 26 30

bhp , each 565 3000 3600

D r e dge pum ps , d r a ga r m - m oun t e d :

num b e r 0 2 2

d i s c ha r ge d i a m e t e r . . . 26" 26 /

Bh p, each . 1450 1600

M axim um dredgin g dep th , f t "4'5 80 80

No. in crew 28 38 40

t e s t in g , o p e r a t i o n a n d m a i n t e n a n c e o f C o r p s o f E n g i n e e r s h o p p e r

d r e d g e s s i n c e 1 9 3 9 . D u r i n g t h a t p e r i o d h e h a s o b s e r v e d a n d

c o n t r i b u t e d t o c h a n g e s t a k i n g p l a c e t h a t c o u l d b e c a ll e d k e e p i n g

u p w i t h t h e s t a t e o f t h e a r t , a n d i n a fe w i n s t a n c e s e v e n p u s h i n g

t h e s t a t e o f t h e a r t . T h e C o r p s c a n r i g h t f u l l y t a k e c r e d i t f o r p i -

o n e e r i n g t h e f o ll o w i n g f e a t u r e s o n h o p p e r d r e d g e s :

( a ) P i l o t h o u s e c o n t r o l o f p r o p u l s i o n , w i th t h e i n t r o d u c t i o n

o f e l e c t r ic a l l y p r o p e l l e d d r e d g e s i n 1 9 2 4 o n t h e f o u r Kingman-

class d r e d g e s .

( b ) A u t o m a t i c c o n t r o l o f p r o p u l s i o n h o r s e p o w e r , f i r st i n t ro -

d u c e d o n t h e d r e d g e

Goethals

i n 1 93 7, a n d l a t e r i m p r o v e d o n t h e

Comber c l a s s , Essayons a n d Markham. T h i s f e a t u r e e n a b l e s t h e

v e s s e l s t o a d j u s t p r o p u l s i o n m o t o r r p m a u t o m a t i c a l l y so t h a t 1 0 0

p e r c e n t p r o p u l s i o n p o w e r is u s e d w h e n r e t u r n i n g l i g h t f r o m , a s

w e l l a s w h e n p r o c e e d i n g l o a d e d t o , t h e d i s p o s a l a re a .

( c) P r o c e s s in s t r u m e n t a t i o n , i n c l u d i n g t h e d e v e l o p m e n t o f

t e c h n i q u e s f o r t h e d i r e c t m e a s u r e m e n t , d i s p l a y a n d r e c o r d i n g

o f h o p p e r l o a d , f l o w v e l o c i t y a n d d e n s i t y , a n d s o l i d s f lo w r a t e ,

d u r i n g t h e d e c a d e o f t h e 1 9 5 0 's .

( d ) S e l f - u n lo a d i n g o f h o p p e r d r e d g e s b y d i r e c t p u m p o u t , f i rs t

t o a r e h a n d l i n g v e s s e l (1 9 54 ) a n d l a t e r d i r e c t l y t h r o u g h l o n g

p i p e l i n e s t o s h o r e d i s p o s a l a r e a s , b e g u n i n 1 9 63 .

( e ) C o n t r o l l a b l e - p i t c h p r o p e l l e r s , f i r s t i n s t a l l e d d u r i n g r e -

p o w e r i n g o f th e d r e d g e Harding i n 1958 .

( f) B o w t h r u s t e r s , f i r s t i n t r o d u c e d o n t h e d r e d g e

Markham

i n 1 95 9. T h e s e i m p r o v e d r e d g e c o n t r o l a t s lo w d r e d g i n g speeds

O C T O B E R 1 9 8 0

0 0 2 5 3 3 1 6 1 8 0 1 1 7 0 4 0 3 7 1 5 0 0 . 5 3 10

3 7 1

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, . . . . . . . . . . " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o : '

I I Y '

Fig. 1 Hoppe r dredge,

Yaquina

200 by 58 by 17 ft

and greatly facili tate docking, especially for dredges engaged in

direct pumpout operation which are required to moor, without

assistance, to a disposal facility perhaps a half-dozen times each

24-hour (h) period.

(g) Waste management systems for shipboard use, first pro-

vided on the dredge M a r k h a m in 1959.

(h) Semi-automatic operation of the dragarms, a feature de-

signed into the dredge M c F a r l a n d in 1965. At the touch of a

button each dragarm can be deployed out to the dredging position

from its stowed position on deck, and vice versa.

(i) Acoustic habitability. First steps in this direction were

taken in 1955 after conversion of the old steam hopper dredge

N e w O r le a n s to a diesel-powered sump rehandler. As a result of

this experience, specifications for the dredge M c F a r l a n d con-

tained stringent requirements for acoustic engineering and

testing.

Other innovations and improvements have been incorporated

on existing dredges as a result of research and development work

done by the Corps or observation of European practice.

A number of new features or departures from previous practice

mark the design of the three new dredges. The rest of this paper

is devoted to a description of these features.

Dam age s tabi li ty

Heretofore, the U. S. Coast Guard has had no rules enforcing

the subdivision of cargo ship hulls. The Load Line Regulations

make provision for the assignment of lesser freeboards to vessels

if they are designed to "remain afloat in a satisfactory condition

of equilibrium after flooding" of any single or any adjacent pair

of damaged compartments. However, such subdivision is not

mandatory. In the absence of any such regulation the Corps long

ago, commencing with dredges built in the 1930's, adopted a

policy that all dredge hulls would, where practicable, be designed

to survive flooding of any two adjacent compartments. An ex-

ception was made for small dredges like the H a i n s class and the

Paci f ic , which were designed to a one-compartment standard of

subdivision. The rules followed in implement ing this policy were

the Rules and Regulations for Passenger Vessels, Part 73, Wa-

tertight Subdivision, and Part 74, Stability. Since the Corps

hitherto has been the only builder of hopper dredges in the

United States it did in effect thus set a standard for design of

dredge hulls, thereby relieving the Coast Guard of having to es-

tablish a regulation.

With the e ntry of commercial firms into hopper dredging, this

picture has changed. Since non-Government-owned hopper

dredges would be operating in harbor channels, with the atten-

dant risk of collision due to traffic and bad weather, the Coast

Guard had to become concerned. In Washington, D. C.~ in De-

cember 1975 Corps personnel had a preliminary meeting with

Coast Guard marine safety representatives. In the 2 August 1976

issue of the F e d e r a l R e g i s t e r there appeared an Advance Notice

of Proposed Rule Making issued by the U. S. Coast Guard enti-

tled "Damage Stability Standards for Hopper Dredges." The

Corps responded to this proposal with a documentation of its

experience record and with the recommendation that "as a

minimum, a one-compartment subdivision standard under

maximum draft loaded condition be established as the rule

governing damage stability standard for hopper dredges."

In the meantime, both private industry and the Government

were rapidly developing plans for the design of new dredges, and,

in due course, designers and prospective owners began applying

to the Coast Guard for consideration of reduced "working free-

boards" such as permitted by Bureau Veritas for Europea n de-

signs. The Coast Guard's reply was that it had no authority to

deviate from the Load Line Regulations. Nevertheless, as the

Coast Guard further considered this problem, it developed a set

of conditions under which it could agree that a "working free-

board up to one-half the tabular B-100 freeboard may be au-

thorized .. . . "T hes e conditions appear in a letter dated 23 June

1977 from the U. S. Coast Guard to the C. F. Bean Corporation

and are reproduced here as Appendix 1.

3 7 2 M A R I N E T E C H N O L O G Y

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o o o o o o o o

r , ; j l

, , i . . . . ~ ~ . . . . . : 7 ~ : . . . . . . . . , ~ . . . . . . , " . . . . . , ;o : : '~ - : - - : ,~ : - - x '

c c ~ s a , ~ E ~ S

U S AR M Y

Fig, 2

,2 _)"

Hopper dredge, medium class, 34 4 by 68 by 35 f t

At the time that the U. S. Coast Guard made this letter public,

the Corps had virtually completed the design of the shallow-draft

dredge Yaquina to a one-compartment standard. Considerable

redesign was necessary to meet the two-compar tment subdivision

required by the conditions of Appendix 1, including an increase

in beam from 56 to 58 ft and the introduction of some longitudinal

subdivision and cross-flooding provisions.

Examina tion of the conditions set forth in Appendix 1 reveals

two interesting points:

(a) A length of damage is specified, namely, 0.495L2/3. In some

areas of a hull, this length may very well span three compart-

ments.

(b) There is no reference to a margin line as the limit of sub-

mergence. Thus the decks may very well be awash. It is simply

required that an equilibrium position be established with the

final angle of heel not exceeding 30 deg, with positive righting arm

of at least 4 in. over a range at least 20 deg beyond equilib-

rium.

As the Yaquina hull was recalculated in accordance with Ap-

pendix 1, it further developed that advantage could be taken of

the spillage which occurs from the hoppers as the vessel heel in-

creases. This represents an appreciable help because the hoppers

are considered to be loaded with a fluid having a specific gravity

of 2.0. This is a penalizing factor when figuring free-surface effect

in calculations of intact stability, but it helps in calculations of

damage stability.

S upers truc tures

A major change from previous practice is the location of the

_2

, , a . . . . . , . . . . , [ /

- : :. . . . . . . . : : : : : : i . . . . . . : . . . . . . . . . . . . . . . . .

- . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .

J,

: : , . . .

F ig .

3 Hopper dredge, large class, 409 by 78 by 39 f t

O C T O B E R 1 9 8 0 3 7 3

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Tab le 2 L igh t t r im data

Es t i m a t e d L i gh t Tr im , f t

Vessel LBP, f t @ 0% B ur nou t @ 90% B ur nou t

Yaquina 193 2.86 5.33

M CH D 333 10.12 13.93

LC HD 384 9.33 9.67

e n t i re s u p e r s t r u c t u r e a f t. W i t h t w o e x c e p t io n s , b o t h d a t i n g b a c k

t o t h e l a t e t h i r t i e s , it h a d b e e n g e n e r a l C o r p s p r a c t i c e t o h a v e a

s u p e r s t r u c t u r e a n d p i l o t h o u s e fo r w a r d a n d a n o t h e r s u p e r s t r u c -

t u r e a f t , w i t h t h e h o p p e r s p a c e b e t w e e n s u p e r s t r u c t u r e s .

T h e p r o v i s i o n o f t w o s u p e r s t r u c t u r e s m a d e i t e a s i e r t o a c -

c o m m o d a t e t h e l a r g e r c r ew s , w h ic h f o r t h e o l d e r d r e d g e s n u m -

b e r e d b e t w e e n 5 0 a n d 1 1 0 p e r s o n s . T h e y a l s o m a d e i t a l it t l e

e a s ie r t o m a i n t a i n r e a s o n a b l e t r i m f r o m t h e f u l ly lo a d e d ( e v e n

k e e l) t o t h e l i g h t (9 0 p e r c e n t b u r n o u t w i t h h o p p e r s e m p t y ) c o n -

d i t i o n .

T h e n e w d r e d g e s w i l i h a v e r e d u c e d c r ew s o f b e tw e e n 2 8 a n d

4 2 p e r so n s , s o t h a t a s in g l e s u p e r s t r u c t u r e a f t c a n a c c o m m o d a t e

a l l c r e w s a n d t h e n e c e s s a r y f a c i l it i e s . B y c a r e f u l l y s p o t t i n g t h e

h o p p e r l o a d a n d f o r w a r d t a n k s , a n a c c e p t a b l e li g h t t r i m c o n d i t i o n

c a n b e e s t a b l i s h e d . T h e d r e d g e s a r e d e s i g n e d t o b e a t e v e n k e e l

w i t h a f u l l l o a d o f w e t s a n d i n t h e h o p p e r s a n d w i t h a l l t a n k s f u ll .

W i t h h o p p e r s e m p t y a n d c o n s u m a b l e s 0 a n d 9 0 p e r c e n t b u r n e d

o u t , t h e p r e s e n t l y e s t i m a t e d t r i m s b y t h e s t e r n a r e a s g i v en i n

T a b l e 2 . R e t e n t i o n o f t h e i n b o a r d d r e d g e p u m p s i n a c o m p a r t -

m e n t j u s t f o r w a r d o f t h e h o p p e r s p l a c e s so m e h e a v y m a c h i n e r y

a n d p i p i n g f o r w a r d , a n d t h i s h e l p s to m a k e t o l e r a b l e th e c h a n g e

i n t r im f r o m t h e l o a d e d t o t h e l i g h t c o n d i t i o n .

T h e s u p e r s t r u c t u r e , i n e a c h i n s ta n c e , i s l o c a t e d o v e r t h e m a -

c h i n e r y s p a c e , w h i c h h o u s e s a l l o f t h e d i e s e l e n g i n e s f o r p r o p u l -

s i o n , p u m p i n g a n d s h i p s e r v i c e p o w e r , a s w e l l a s o t h e r n o i s e

g e n e r a t o r s l ik e h y d r a u l i c p u m p s a n d c o n t r o l li n e s. T h i s h a s g i v e n

a d d e d i m p o r t a n c e t o c o n s i d e r a t i o n s r e l a ti v e to a c o u s t ic h a b i t a -

b i l i t y . S p e c i f i c a t i o n s f ir s t d e v e l o p e d i n 1 9 6 5 f o r t h e d r e d g e

M c F a r l a n d h a v e b e e n f u r t h e r r e f i n e d a n d i n c l u d e d w i t h t h e r e -

q u i r e m e n t s f o r th e t h r e e n e w h o p p e r d r e d g e s . A s a n e x a m p l e ,

A p p e n d i x 2 p r e s e n t s t h e n o i se a t t e n u a t i o n r e q u i r e m e n t s e x -

c e r p t e d f r o m t h e M C H D s p e c i fi c a ti o n s . N o t e p a r t i c u l a r l y t h e

r e q u i r e m e n t f o r p r e c o n s t r u c t i o n e n g i n e e r i n g a n d p o s t c o n -

s t r u c t i o n t e s t i n g .

A u t o m a t i o n a n d r e d u c e d c r e w s

A n o b j e c t iv e o f t h e n e w d r e d g e d e s i g n s h a s b e e n t o r e d u c e

o p e r a t i n g c o s t s , w h i c h , o f c o u r s e , b e c a m e t r a n s l a t e d i n t o d e -

s i g n in g d r e d g e s r e q u i r in g f e w e r o p e r a t i n g p e r s o n n e l . T h i s o b -

j e c t i v e c o u l d b e r e a c h e d b y t r a v e l i n g e a c h o f s e v e r a l ro u t e s :

( a ) A d o p t n e w o p e r a t i n g p h i l o s o p h i e s , s u c h a s c o m b i n i n g

d u t i e s a n d e l i m i n a t i n g c l a s s if i c a t i o n s , a n d r e d u c i n g c o s m e t i c

m a i n t e n a n c e l ik e c le a n i n g , c h i p p i n g a n d p a i n t i n g .

T a b l e 3 S u c t i o n /d i s c h a r g e v a l ve s e t u p s w i t c h L C H D )

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1 d r e dg i ng ove r s ide hopp e r l oa d i ng

2 d r e dg i ng i nboa r d hopp e r l oa d i ng

3 un l oa d i ng i nboa r d po r t d i s c ha r ge

to shore

4 un l oa d i ng i nboa r d s t a r boa r d d i s c ha r ge

to shore

5 d r e dg i ng i nboa r d ove r boa r d

( a g i ta t i on ) ove r s i de t o hoppe r s

and overflows

6 d r e dg i ng i nboa r d t o hoppe r s

ove r s i de t o hoppe r s

7 d r e dg i ng i nboa r d ove r boa r d

(agi ta t ion) overs ide overboard

( b ) C h a n g e f r o m t h e t r a d i t i o n a l 4 -h w a t c h e s t o o th e r d u t y

c y c l e s , w i t h s o m e s c h e d u l e d o v e r t i m e .

( c) I n t r o d u c e e x t e n s i v e a u t o m a t i o n i n t o o p e r a t i o n o f b o t h t h e

s h i p a n d t h e d r e d g i n g m a c h i n e r y .

T h e d e s i g n s h a v e a c c o m p l i s h e d t h is b y a c o m b i n a t i o n o f a ll

t h r e e m e t h o d s . T h e r e s u l ts a r e i n d i c a t e d b y t h e f o l lo w i n g c o m -

CLASS VESSE L CREW SIZE

La r ge Essayons 110

La r ge LC H D 42

M e d i u m Comber 90

M e d i u m M C H D 3 8

Sm a l l Pacific 42

Sm a l l Yaquina 28

p a r i s o n :

T h e c o m p a r i s o n s i n th e l a r g e a n d m e d i u m c l a s s e s a r e n o t t o t a l l y

f a i r , a s t h e E s s a y o n s a n d C o m b e r a r e s t e a m t u r b i n e - e l e c t r i c

v e s s el s w i th f i r e m e n , w a t e r t e n d e r s a n d e l e c t ri c i a n s i n t h e i r w a t c h

c o m p l e m e n t s , w h e r e a s t h e n e w d r e d g e s a r e d i e s e l - p r o p e l le d . '

A n a p p r e c i a b l e p a r t o f t h e c r e w r e d u c t i o n i s a t t r i b u t a b l e t o t h e

d e c i s i o n t o i n t r o d u c e e x t e n s i v e a u t o m a t i o n . A d d i t i o n a l l y , i t w a s

d e c i d e d t o b u i l d i n t o t h e s e v e s s e ls t h e c a p a b i l i t y fo r u n a t t e n d e d

e n g i n e r o o m o p e r a t i o n , f o ll o w i n g t h e p r e c e p t s o f U . S . C o a s t

G u a r d N a v i g a t i o n a n d I n s p e c t i o n C i r c u l a r N o . 1 -6 9 d a t e d 8

J a n u a r y 1 96 9. T o t h i s e n d , t h e v e s s e l s a r e to b e i n s p e c t e d a n d

c l a s s e d b y t h e A m e r i c a n B u r e a u o f S h i p p i n g ( A B S ) w i t h M a l t e s e

C r o s s A C C U 2 c e r ti f ic a t io n . H o w e v e r , i t i s n o t i n t e n d e d t o o p e r a t e

i m m e d i a t e l y w i t h o u t e n g i n e r o o m w a t c h p e r s o n n e l , a n d t h e f ig -

u r e s t a b u l a t e d i n t h e p r e c e d i n g d o n o t r e f le c t su c h p o s s i b l e f u r-

t h e r p e r s o n n e l r e d u c t i o n s .

E x t e n s iv e a u t o m a t i o n o f d r e d g in g m a c h i n e r y a n d e q u i p m e n t

o p e r a t i o n i s sp e c i f ie d . T o b e g i n w i t h , a p r o c e s s w i ll b e d i a l e d , b e

i t h o p p e r l o a d in g , a g i ta t i o n , d ir e c t p u m p o u t , e t c. F o r e x a m p l e ,

T a b l e 3 o u t l i n e s t h e p r o c e s s e s w h i c h c a n b e s e l e c t e d o n t h e

L C H D . U p o n d i a l in g a n d p r e ss i n g t h e " e x e c u t e " b u t t o n , t h e

p r o p e r v a l v e s w i ll b e a u t o m a t i c a l l y o p e n e d a n d c l o se d , a n d l i g h ts

o n a m i m i c d i a g r a m w i ll s h o w t h e s t a t u s o f ea c h v a l v e a n d t h e

f l ow p a t h .

I n a d d i t i o n , p o w e r f o r th e d r e d g e p u m p s i s d e r i v a b l e fr o m m o r e

t h a n o n e s o u rc e . A g a i n u s i n g th e L C H D a s a n e x a m p l e , F i g . 4 ( a )

s h o w s t h e m a i n p u m p p o w e r s y s t e m d i a g r a m a n d F i g . 4 ( b ) th e

p o w e r r o u t e s t h a t c a n b e s e l e c te d . T h e p r o p e r c i r c u i t b r e a k e r s

w i ll b e o p e n e d o r c l o se d u p o n s e l e ct i o n a n d e x e c u t i o n o f p o w e r

p a t h s , a n d l i g h t s w i l l s h o w t h e s t a t u s o f e a c h c i r c u i t b r e a k e r .

A d r e d g e c o n t r o l o f f i c e r ( D C O ) o n t h e b r i d g e w i l l m a k e a l l

p r o c e s s d e c is i o n s a n d o p e r a t e t h e d r e d g i n g s y s t em . D u r i n g a c t u a l

d r e d g i n g h e w i l l b e s e a t e d a t a c o n s o l e h o u s e d i n a g la s s b l is t e r ,

" f r o n t " a n d c e n t e r o f t h e p i l o t h o u s e , a t a l o w e r l e v e l s o a s n o t t o

i n t e r f e r e w i t h t h e h e l m s m a n ' s v i s io n . B e f o r e i n i t i a t i n g d r e d g i n g

t h e D C O w i ll f i rs t n o t e a t h i s c o n so l e w h e t h e r t h e p u m p i n g p o w e r

e n g i n e s a r e w a r m . I f s o, a n d i f t h e y a r e n o t r u n n i n g , h e c a n r e s t a r t

t h e m a t t h e c o n s o le . If t h e y a r e n o t w a r m , t h e y m u s t b e s t a r t e d

b y t h e e n g i n e e r .

W i t h e n g i n e s r u n n in g , a n d w i t h d r a g h o i s t m o t o r - g e n e r a t o r s e t s

r u n n i n g , t h e D C O n o w p r e ss e s a b u t t o n t o i n i t ia t e d r e d g i n g - -

a c t u a l ly , a b u t t o n f o r e a ch d r a g a r m . T h i s d e p l o y s t h e d r a g a r m ,

l i f t in g i t f r o m i t s s t o w e d p o s i t i o n o n t h e d e c k , b r e a s t i n g i t o u t -

b o a r d , t h e n l o w e r i n g i t d o w n t h e s h i p ' s s i d e t o a h o r i z o n t a l r e f -

e r e n c e p o s i ti o n , w i th t h e t r u n n i o n p r o p e r l y s e a t e d i n t h e o p e r -

a t i n g p o s i t io n . T h e m e c h a n i s m s w i ll s t o p a t t h i s p o i n t a n d a l i g h t

w i l l a d v i s e t h e D C O t h a t a l l is r e a d y f o r d r e d g i n g .

N e x t t h e D C O w i ll p r e s s a " l o w e r " b u t t o n . T h e d r a g a r m w i ll

n o w p i v o t a r o u n d t h e t r u n n i o n , t h e r e b y l o w e r i n g t h e d r a g h e a d

t o d r e d g i n g d e p t h . W h e n t h e l a t t e r to u c h e s b o t t o m , t h e s w e ll

c o m p e n s a t o r w i l l a s s u m e c o n t ro l , s t o p p i n g t h e l o w e ri n g a c t io n .

T h e s w e l l c o m p e n s a t o r w i l l a u t o m a t i c a l l y k e e p t h e d r a g h e a d i n

p r o p e r c o n t a c t w i t h t h e b o t t o m , a n a d j u s t m e n t h a v i n g b e e n

2 Pa r a g r a p h 41 .7 o f A B S r u l e s de f ine s t h i s s ym bo l a s " A u t om a t i c

C on t r o l Sys t e m fo r U na t t e nde d E ng i ne R oom C e r t i f i e d . "

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p r e s e t t o d e t e r m i n e h o w b i g a " b i t e " t h e h e a d w i ll t a k e , t h a t i s,

t h e d e g r e e o f e m b e d m e n t i n t o t h e b o t t o m m a t e r i a l . T h i s a d -

j u s t m e n t e s t a b l i sh e s w h a t p o r t i o n o f t h e d r a g h e a d w e i g h t is

s u p p o r t e d b y t h e h o i s t i n g c a b l e s a n d w h a t p o r t i o n b y t h e c h a n n e l

b o t t o m . T h e s w e l l c o m p e n s a t o r i s a h y d r a u l i c c y l i n d e r w i t h a

s t r o k e o f 1 2 f t, o r 4 - 6 f t a b o v e a n d b e l o w a m e d i a n . T h i s p e r m i t s

t h e d r a g h e a d t o f o l lo w u n e v e n n e s s o f t h e c h a n n e l b e d w i t h i n

t h e s e l i m i t s w i t h o u t o p e r a t i o n o f t h e d r a g h o i s t w i n c h e s . I f a l a r g e

h o l e o r h i ll is e n c o u n t e r e d , t h e h y d r a u l i c r a m w i l l t r a v e l t o o n e

o r t h e o t h e r e n d o f t h e s t r o k e a n d a u t o m a t i c a l l y s t a r t th e w i n c h e s

t o h o i s t o r l o w e r t h e d r a g h e a d u n t i l t h e r a m r e t u r n s t o t h e m e d i a n

p o i n t .

O t h e r a u t o m a t i c d r a g a r m d e v i c e s k e e p t h e d r a g h e a d f r o m

d r i f t i n g u n d e r t h e s h i p ' s h u l l .

A t c o m p l e t i o n o f d r e d g i n g , t h e s a m e a u t o m a t i c f e a t u r e s p r e -

v a i l, a s n e e d e d , i n t h e r e v e r s e d i r e c t i o n f o r s to w i n g t h e d r a g a r m s .

O r , i f t h e d r e d g e i s t o b e t u r n e d 1 8 0 d e g f o r a r e v e r s e c u t, r a i s i n g

t h e d r a g a r m s o u t o f t h e w a t e r i s a l so a p u s h b u t t o n o p e r a t i o n .

S t i l l o t h e r a u t o m a t i c d e v i c e s w il l g o v e r n t h e l o a d i n g i n t h e

h o p p e r s a n d r a i s i n g a n d l o w e r i n g o v e r f lo w le v e l s f o r w a r d o r a f t ,

p o r t o r s t a r b o a r d , i n r e s p o n s e t o p r e s e t i n s t r u c t i o n s a n d t o v e s s e l

t r i m o r l i s t ,

W h e n t h e v e s s e l i s d u m p i n g , a u t o m a t i c c o n t r o l s w il l o p e n t h e

d o o r s i n t i m e d s e q u e n c e i n a c c o r d a n c e w i t h p r e s e t i n s t r u c t io n s ,

a u t o m a t i c a l l y i n t e r r u p t i n g t h e d u m p i n g o p e r a t i o n a n d a d v i s i n g

t h e D C O i f t h e s h i p s t a r ts t o t a k e a n i m p r o p e r a t t i t u d e o r i f s o m e

o t h e r f a u l t o c c u r s .

I f t h e v e s s e l i s u n l o a d i n g i t s h o p p e r s b y d i r e c t p u m p o u t , t h e

a u t o m a t i c s a r e d e s i g n e d t o p r e v e n t l i n e p lu g g i n g , a s w e l l a s t o

e f f e c t u n l o a d i n g c o n t r o l .

T h e D C O i s p r o v i d e d w i t h r e m o t e c o n t r o l s a t h i s s t a t i o n o r in

t h e b r i d g e f o r a ll d e t a il s o f t h e d r e d g i n g , lo a d i n g a n d u n l o a d i n g

o p e r a t i o n s . T h e r e i s a l s o a w e a l t h o f i n s t r u m e n t a t i o n t o t e l l h i m

w h a t i s g o i ng o n a n d h o w w e l l h i s e q u i p m e n t a n d s y s t e m s a r e

f u n c t io n i n g . T h e h o p p e r l o a d i s c o n t i n u o u s l y d i s p l a y e d , a n d s o

i s t h e l o a d i n g ra t e , s o th a t h e c a n q u i c k l y d e d u c e w h e n h e h a s

a c h i e v e d a n " e c o n o m i c l o a d " i n th e h o p p e r s . P o s i t i o n i n d i c a t o r s

a n d r e c o r d e rs v e ri f y t h a t t h e s h i p a n d t h e d r a g h e a d s a r e a c t u a l ly

w o r k i n g e x a c t l y w h e r e d e s i r e d o n t h e s h o a l e d a r e a s. A t t h e m a t e ' s

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o p t i o n , c o u rs e c o r r e c t io n i n f o r m a t i o n c a n b e f e d t o t h e a u t o m a t i c

p i l o t to h o l d t h e d r e d g e o n t h e c u t .

B e c a u s e o f t h e p l a n e v e n t u a l l y t o o p e r a t e o n t h e b a s i s o f a n

u n a t t e n d e d e n g in e r o o m , m u c h a u t o m a t i o n h a s b e e n i n t r o d u c e d

i n t h e a r e a o f p ro p u l s i o n a n d a u x i l i a r y m a c h i a e r y o p e r a t i o n . D u e

t o l i m i t a t i o n s o f s p ac e , h o w e v e r, t h i s p a r t i c u l a r a u t o m a t i o n

c a n n o t b e d i s c u s s e d h e r e . A s a f i n a l n o t e o n t h i s m a t t e r , h o w e v e r ,

r e c o g n i t i o n s h o u l d b e g i v e n to t h e p r o v i s i o n o f s o m e d e g r e e o f

a u t o m a t i o n i n k e e p i n g t h e s h i p ' s lo g . I n t h e p i l o t h o u s e t h e o p -

e r a t o r w i ll h a v e a v a i l a b l e a n a u t o m a t i c e v e n t l o g g e r , h a v i n g a

k e y b o a r d , a p r i n t e r a n d a c lo c k m e c h a n i s m . B y s t r i k i n g t h e a p -

p r o p r i a t e l y n u m b e r e d k e y , h e c a n c a u s e t o b e p r i n t e d t h e i d e n t i t y

a n d t h e t i m e o f o c c u r re n c e o f s u c h e v e n t s a s s t a r t i n g a n d s t o p p i n g

o f d r e d g e p u m p s , g o i n g t o a n d r e t u r n i n g f r o m t h e d u m p , i n t e r -

r u p t i o n a n d r e s u m p t i o n o f o p e r a t i o n , a n d a n y o t h e r e v e n t s t h a t

t h e d r e d g e m a s t e r m i g h t c a r e t o h a v e a u t o m a t i c a l l y r e c o r d e d .

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T h e l a r g e -c l a s s h o p p e r d r e d g e w i l l b e p a r t i c u l a r l y d i s t i n -

g u i s h e d b y a t h i r d d r a g a r m l o c a t e d i n a c e n t e r w e l l d e s i g n e d i n t o

t h e h u l l i n t h e h o p p e r a r e a a l o n g t h e c e n t e r li n e . T h e d r a g a r m i s

r a i s e d a n d l o w e r e d a n d p e r m a n e n t l y l o c a t e d i n t h is w e ll .

D e s p i t e t h e p r e s e n c e o f t h e c e n t e r w e l l , t h e h u l l m o d e l t e s t s

i n d i c a t e t h a t a f u l l -p o w e r l o a d e d d r a f t s p e e d o f a b o u t 1 4 k n o t s

m a y b e e x p e c t e d . T h e m o d e l t e s t in g r e v e a l e d t h a t t h e c e n t er w e l l

a d d s a b o u t 2 1 p e r c e n t r e s i s t a n c e a t s p e e d s o f 12 k n o t s a n d h i g h e r.

T w o o t h e r i n t e r e s t i n g p o i n t s w e r e e v i d e n c e d b y t h e m o d e l t e s t s :

( 1) I t i s b e t t e r t o k e e p t h e d r a g a r m l o w i n t h e c e n t e r w e l l , f o r m i n g

a s o r t o f r o u g h c l o s u r e o f t h e w e l l w h e n t h e s h i p i s u n d e r w a y . T h i s

p r e v e n t s o s c i l l a t i o n o f t h e w a t e r l e v e l i n t h e w e l l , w h i c h i f a l l o w e d

w o u l d r e p r e s e n t a d d e d r e s i s t a n c e a n d a d i s s i p a t i o n o f e n e r g y . ( 2)

B y e l i m i n a t i n g tr a n s v e r s e k e e l s o n s b e t w e e n t h e h o p p e r b o t t o m

d o o r s , h i th e r t o u s e d t o f o r m i n d i v i d u a l h o p p e r d o o r w e l l s, o r

r e c e s s e s , t h e h u l l r e s i s t a n c e w a s d e c r e a s e d a b o u t 1 3 p e r c e n t a t

s p e e d s o f 12 k n o t s a n d h i g h e r . T h u s , a n e w f e a t u r e fo r t h e L C H D

a n d M C H D w i ll b e a s i ng l e h o p p e r d o o r t r o u g h o n e a c h s i d e , i n -

s t e a d o f i n d i v i d u a l d o o r r e c e s s e s .

T h e c e n t e r w e l l d r a g a r m w i ll f e e d t h e t w o i n b o a r d d r e d g e

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o

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o

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EXISTING ELONGATED

DRAGARM DRAGARM

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4 0 5 0 6 0 7 0 8 0 9 0 ] 0 0

DREDGING DEPTH-FT

F i g . 5 C a l c u l a te d p u mp p e r fo r ma n c e o f d r e d g e

Essayons

wi th ex i s t i ng

36- in . - i ns ide-d iamete r d ragarms and w i th added p ipe leng ths hypo thes i zed

fo r dep ths g re a te r than 60 f t )

p u m p s . E a c h o f t h e s e p u m p s h a s a d i s c h a rg e d i a m e t e r o f 30 in .

T h e d i a m e t e r o f t h e d r a g a r m s u c t i o n p i p e i s 4 2 i n. , w h i c h i s j u s t

1/2 i n . s h y o f t h e d i a m e t e r r e q u i r e d t o p r o v i d e a c r o s s s e c t i o n e q u a l

t o t h a t o f t w o 3 0 - i n . s e c t i o n s . T h i s s u c t i o n s o u r c e t o g e t h e r w i t h

t h e t w o s i d e d r a g a r m s w i l l p e r m i t s i m u l t a n e o u s o p e r a t i o n o f al l

f o u r d r e d g e p u m p s . T h e p o w e r s e t u p i s s u c h t h a t t h i s c a n b e d o n e

w i t h o n e d r e d g e p u m p g e n e r a t o r p o w e r i n g t h e t w o o v e r s id e p u m p

m o t o r s a n d t h e o t h e r g e n e r a t o r p o w e ri n g th e t w o i n b o a r d p u m p

m o t o r s .

A s p r e v i o u s l y s t a t e d , n o r m a l d r e d g i n g c a n b e c a r r i e d o n w i t h

t h e i n b o a r d p u m p s i f t h e o v e r s i d e p u m p s a r e d i sa b l e d . H o w e v e r,

a p r i n c i p a l r e a s o n f o r p r o v i d i n g c e n t e r w e l l s u c t i o n i s t o a l l o w t h i s

d r e d g e t o b e e s p e c i a l l y e f f e c t i v e i n a g i t a t i o n d r e d g i n g . I n t h i s t y p e

o f d re d g i n g , p u m p i n g t a k e s p l a c e c o n ti n u o u s l y , a n d t h e e x c a v a t e d

m a t e r i a l s a r e b r o a d c a s t o v e r a w i d e a r e a o f t h e o u t s i d e w a t e r

s u r f ac e . T h e s o l i d s a r e o f s u c h f i n e g r a i n a s to r e m a i n s u s p e n d e d

l o n g e n o u g h f o r t h e c u r r e n t s t o c a r r y t h e m e i t h e r to d e e p w a t e r

o r o u t s i d e o f t h e c h a n n e l . I n t h i s m o d e o f d r e d g i n g , t h e l a r g e- c l a ss

d r e d g e w i ll h a v e a b o u t t w i c e t h e c a p a b i l i t y o f th e d r e d g e Goe-

thals, w h i c h i s p r e s e n t l y t h e s e c o n d l a r g e s t d r e d g e i n t h e C o r p s

f l e e t .

T h e a d d e d c e n t e r w e l l s u c t io n c a n b e u s e d a d v a n t a g e o u s l y f o r

c u t t i n g d o w n t h e t i m e t o l o a d t h e h o p p e r s a l s o . O r d i n a r i l y , it

t a k e s a b o u t 1 5 m i n o n m o s t d r e d g e s f o r th e i n i t i a l f i ll i n g o f t h e

h o p p e r s b e f o r e o v e r fl o w co m m e n c e s . W i t h t w i c e th e u s u a l

n u m b e r o f p u m p s i n o p e r a t i o n th e t i m e c a n b e c u t i n h a lf . T h i s

r e d u c t i o n c a n b e s i g n i f i c a n t . I f , f o r e x a m p l e , t h e d r e d g e i s w o r k i n g

o n a 3 - h ( 1 8 0 m i n ) c y c l e, s u c h a s 1 h p u m p i n g a n d 2 h g o i n g to a n d

f r o m t h e d u m p , a r e d u c t i o n o f 71/,2 m i n i n a 1 8 0 - m i n c y c l e g i v e s

a t h e o r e t i c a l p r o d u c t i o n g a i n o f 4. 3 p e r c e n t . ( O f c o u r s e , a s s o o n

a s t h e h o p p e r s a r e i n i t i a l l y f i l le d , t w o o f th e f o u r p u m p s w i l l b e

s h u t d o w n ; o t h e rw i s e , t h e t o t a l f l o w w i ll o v e r w h e l m t h e h o p p e r

a r e a a n d p r e v e n t s e t t l i n g o f s o li d s , a n a l o g o u s t o t r y i n g t o f i ll a

s m a l l b u c k e t w i t h a f i r e h o s e . )

J u s t a s t h e t w o s i d e d r a g a r m s a r e d e s i g n e d t o b e r a is e d e n t i r e l y

a n d s t o w e d o n d ec k , w h e r e m a i n t e n a n c e c a n b e p e r f o r m e d , t h e

c e n t e r w e l l d r a g a r m c a n b e s i m i l a r l y r a i s e d a n d l o w e r e d . A l a d d e r

s t r u c t u r e a r o u n d t h e s u c t i o n p i p e a n d g u i d e s i n th e c e n t e r w e l l

s t r u c t u r e w i ll r e s t r a i n l a t e ra l m o v e m e n t o f t h e d r a g a r m w i t h i n

t h e c o n f i n e s o f t h e w e l l .

O v e r s i d e p u m p s

T w o o f t h e t h r e e n e w h o p p e r d r e d g e s w il l h a v e d r e d g e p u m p s

m o u n t e d o n t h e d r a g a r m s , so th a t d u r i n g d r e d g in g t h e se p u m p s

w i l l b e w e l l s u b m e r g e d . T h i s , o f c o u rs e , g r e a t l y i n c r e a s e s t h e

w e i g h t o f e a c h d r a g a r m a n d m a k e s f o r i n c r e a s e d d r a g a r m h a n -

d l i n g p r o b l e m s , s u c h a s w i n c h p o w e r , w i r e r o p e s i z e s a n d s u p -

p o r t i n g d a v i t s t r u c t u r e . H o w e v e r , t h e y a r e e s s e n t i a l, fo r d e e p

d r e d g i n g p r o j e c t s w h i c h a r e p l a n n e d f o r t h e f u t u r e , a n d t h e i r

i n c r e a s e d e f f e c t i v e n e s s is u s a b l e o n p r o j e c t s a t c u r r e n t d e p t h s .

T h e p e r f o r m a n c e o f c e n tr i f u g a l p u m p s i s l i m i t e d b y s u c ti o n

l i f t c o n s i d e r a t i o n s . H e r e t o f o r e , d r e d g e p u m p s o n h o p p e r d r e d g e s

h a v e b e e n i n s t a l l e d in b o a r d . T h e p u m p s h a v e b e e n s o i n s t a l l e d

a s t o p l a c e t h e s u c t i o n s a s lo w i n th e h u l l a s h u l l s t r u c t u r a l c o n -

s i d e r a t i o n s w o u l d p e r m i t . E v e n s o , w h e n d r e d g i n g a t , s ay , 4 0 - f t

d e p t h a n d p u l l i n g in a s o l i d s / w a t e r m i x t u r e o f p e r h a p s 1 .2 a v e r a g e

s p e c i f i c g r a v i t y , t h e p u m p w i l l p u l l a v a c u u m i n t h e r a n g e o f 22

t o 2 8 in H g , r e g a r d l e s s o f t h e f a c t t h a t t h e p u m p c e n t e r l in e m a y

a c t u a l l y b e s e v e r a l fe e t b e l o w t h e o u t s i d e w a t e r s u r f a c e .

T h e o r e t i c a l l y , if t h e p u m p w e r e t o b e p l a c e d a t t h e c h a n n e l

b o t t o m , t h e p u m p s u c t i o n w o u l d a lw a y s b e u n d e r a h i g h st a t i c

h e a d a n d t h e p u m p w o u l d n e v e r p u ll a v a cu u m . T h e l i m i t a t i o n

o n p u m p p e r f o r m a n c e t h e n w o u l d b e t h e a v a i l a b l e p o w e r f o r

p u s h i n g t h e s o l i d s / w a t e r m i x t u r e u p t h e d r a g a r m p i p i n g a n d i n t o

t h e h o p p e r s .

O n e d r e d g e w a s b u i l t i n E u r o p e w i t h t h e d r e d g e p u m p a c t u a l l y

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

t h a n t h i s on e p l a c e d th e d r e d g e p u m p a b o u t h a l f w a y u p t h e

d r a g a r m , a p p a r e n t l y t o r e d u c e h a n d l i n g p r o b l e m s a n d p r o b le m s -

a r i s in g f r o m s e n d i n g h y d r a u l i c o r e l e c tr i c p o w e r d o w n t h e d r a -

g a r m t o s u c h d e p t h s . O n t h e C o r p s ' m e d i u m - a n d l a r g e - c l a s s

d r e d g e s t h e p u m p s a r e p l a c e d a b o u t h a l f w a y u p e a c h d r a g a r m .

F i g u r e 5 d e m o n s t r a t e s t h e n e e d f o r t h e s u b m e r g e d d r e d g e

p u m p s f o r d e e p d r e d g i n g . I t sh o w s t h e c a l c u l a t e d p u m p p e r f o r -

m a n c e o n t h e d r e d g e

E ssay ons

( p r e s e n t l y t h e C o r p s ' l a r g e s t

d r e d g e ) a t v a r i o u s d r e d g i n g d e p t h s , w i t h e x t e n d e d s u c t io n p i p e

h y p o t h e s i z e d to r e a c h t h e g r e a t e r d e p t h s . T h e p u m p s a r e p la c e d

i n b o a r d o n t h i s d r e d g e a n d t h u s a r e l im i t e d i n p e r f o r m a n c e b y

t h e i n c r e a s e d v a c u u m e x e r t e d a t g r e a t e r d e p t h s . P e r f o r m a n c e

i s s e e n t o b e a c c e p t a b l e u p t o a b o u t 6 0 - t o 6 5 -f t d e p t h , b u t a s h a r p

d r o p - o f f i n p e r f o r m a n c e i s s e e n t o s t a r t a t a b o u t 7 0 f t.

T h e m e d i u m - a n d l a r g e- c l as s h o p p e r d r e d g e s w i ll h a v e i n b o a r d

d r e d g e p u m p s i n a d d i t i o n t o th e d r a g a r m - m o u n t e d p u m p s . T h e s e

a r e n e e d e d p r i m a r i l y f o r d i r e c t p u m p o u t o p e r a t i o n s o t h a t t h e

d r e d g e s c a n u n l o a d t h e m s e l v e s i n t o a sh o r e d i s p o s a l ar e a . H o w -

e v e r, p i p i n g c o n n e c t i o n s a r e s u c h t h a t e i t h e r o r b o t h o f t h e i n -

b o a r d p u m p s c a n a l s o b e u s e d f o r d r e d g i n g i n t h e e v e n t t h e o v -

e r s i d e p u m p s a r e d o w n f o r r e p a i r s. I n s u c h a c a se , th e p u m p /

m o t o r m o d u l e o f t h e d r a g a r m i s r e p l a c e d b y a l e n g t h o f p ip e , o n

t h e m e d i u m - c l a s s d r e d g e . O n t h e l a r g e - c l a s s d r e d g e , t h e i n b o a r d

p u m p s w i ll t a k e s u c t i o n f r o m t h e c e n t e r w e l l d r ag , a n d t h e s i d e

d r a g a r m s w i l l b e s t o w e d o u t o f s e r v ic e :

T h e p u m p s a n d m o t o r s o f t h e d r a g a r m - m o u n t e d u n i ts w i ll b e

c l o s e -c o u p l e d . T h e p u m p s w i ll h a v e 2 8 - in . i n s i d e d i a m e t e r s u c t i o n

a n d 2 6 - in . i n si d e d i a m e t e r d i s c h a r g e . T h e m o t o r s w i l l b e 3 - p h a s e

s q u i r r e l - c a g e i n d u c t i o n t y p e , o p e r a t i n g a t 2 4 0 0 V a n d o i l -c o o l e d .

E n c l o s u r e s a n d s e a ls w i ll b e s p e c i a ll y d e s i g n e d f o r s u b m e r g e d

o p e r a t i o n .

M o t o r s p e e d i s c o n t r o l l e d b y c o n t r o l o f e n g i n e s p e e d , t h a t i s ,

b y c o n t r o l o f a c f re q u e n c y p r o d u c e d b y t h e g e n e r a t o r . S p e e d

c o n t r o l is a u t o m a t i c , b u t t h e o p e r a t o r c a n c h a n g e o v e r to m a n u a l

c o n t r o l i f t h e s i t u a t i o n s o r e q u i r e s .

P u m p p o w e r

C a r e f u l c o n s i d e r a t i o n h a s b e e n g i v e n t o t h e d r e d g e p u m p

3 7 6 M A R I N E T E C H N O L O G Y

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T a b l e 4 C o m p a r i s o n o f h o p p e r d o o r s i z e s

D r e dge A r e a o f ba y , f t2 V o l um e o f B a y , yd 3 Area of Door , f t2 A r e a o f D oor % o f a r e a A r e a o f D oor % o f V o lum e C

Comber

class 300 255 19.5 6.5 7.6

Essayons

450 345 23.86 5.3 6.9

Goethals

369 401 23.43 6.3 5.8

Lyman

clas s 196 115 13.77 7.0 12.0

Harding

368 340 21.0 5.7 6.2

McFarland

286 262 18.0 a 6-3a 6.8

Markham 433 340 20.2a 4.7 a 5.9

Pacific

304 125 16.0 5.3 12.8

W es t Co ast 315 150 24.0 7.6 16.0

La rge class b 404 to 644 516 to 743 66.0 16.3 to 10.2 12.8 to 8.9

M ed ium class 425 502 64.0 15.0 12.8

a The se are p rojec ted areas , ha l f of ac tua l . Doors are on a s lope of 60 deg.

b N O TE: H op pe r ba ys no t un i f o r m due t o ho ppe r a r r a nge m e n t . D a t a r e p r e s e n t s pa n o f c ha r a c t e ri s t ic s .

c Square footage of hopp er do or area p er 100 yd 3 .

p o w e r r e q u i r e m e n t s , e s p e c i a ll y f o r d r e d g in g . T h e r e w a s a t i m e

w h e n i t w a s t h o u g h t t h a t t h e m o r e p o w e r o n e c o u l d a p p l y t o a

p u m p , t h e b e t t e r t h e o p e r a t i o n . T h i s t h i n k i n g w a s ju s t i f i e d w h e n

p u m p s d r i v e n b y l a rg e l o w - p r e s s u re r e c i p r o c a ti n g s te a m e n g i n e s

w e r e a c t u a ll y u n d e r p o w e r e d . T h e a d v e n t o f t h e c o m p a c t e l e c tr i c

m o t o r d r i v e e n c o u r a g e d t h e u s e o f h i g h e r p u m p p o w e r , a n d

e v e n t u a l l y so m e d r e d g e s w e r e b u i l t w i th t o o m u c h p u m p p o w e r.

T h i s r e s u l t e d i n f l u i d b e i n g d i s c h a r g e d i n t o t h e h o p p e r s o f a s t a s

t o c r e a t e a d e g r e e o f t u r b u l e n c e t h a t i n h i b i t e d t h e s e t t l i n g o f

s o l id s i n t h e h o p p e r s . U n d e r t h e s e c o n d i t i o n s , s o li d s w e r e b e i n g

w a s h e d p a s t t h e o v e r f lo w s a t a r a p i d r a t e . T h e p u m p s h a d t o b e

s l o w e d d o w n i n o r d e r to i m p r o v e t h e h o p p e r r e t e n t io n r a t e , t h a t

i s, t h e s e t t l i n g r a t e o f s o li d s in t h e h o p p e r s .

A n a n a l y s i s o f t h i s s i t u a t i o n r e v e a l e d t h a t t h e r e i s a n o p t i m u m

a v e r a g e d i s c h a r g e r a t e f r o m t h e p u m p s i n t o t h e h o p p e r s a n d t h a t

t h i s r a t e is p r o p o r t i o n a l n o t t o t h e h o p p e r v o l u m e b u t t o t h e

h o p p e r s u r f ac e a r e a . M a t h e m a t i c a l l y , t h i s is e x p r e s s e d a s

Q = KA

I f A i s t h e h o p p e r a r e a i n s q u a r e f e e t, Q w i l l b e t h e t o t a l d i s c h a rg e

r a t e i n t o t h e h o p p e r s , i n c u b i c fe e t p e r s e c o n d . K i s a f a c t o r h a v i n g

t h e d i m e n s i o n c u b i c f e e t p e r s e c o n d p e r s q u a r e f o o t o f h o p p e r

a r e a . T h e a n a l y s i s , b a s e d o n a s t u d y o f e x p e r i e n c e w i t h a l l

C o r p s - o w n e d h o p p e r d r e d g e s, s o m e b e t t e r t h a n o t h e rs , i n d i c a te s

t h a t t h i s f a c to r s h o u l d n o t e x c e e d 2 . 5 t o a s s u r e a n a c c e p t a b l e

s o l id s s e t tl i n g r a te . T h i s i s k n o w n a s t h e " p o n d i n g r a t i o . " W i t h

t h e a i d o f t h i s f ig u r e , a n d a c a l c u l a t i o n o f t h e t o t a l p u m p h e a d

w h e n f lo w i s t a k i n g p l a c e a t d e s i g n v e l o c i t y a n d d e n s i t y ( u s u a l l y

2 4 fp s a t t h e p u m p d i s c h a r g e a n d 1 .2 s p e c if i c g r a v i t y o f m i x t u r e ) ,

t h e p u m p p o w e r is o b t a i n e d f r o m t h e w e l l - k n o w n f o r m u l a

b h p = QWH/550 E

w h e r e W i s th e m i x t u r e d e n s i t y i n p o u n d s p e r c u b i c f o o t, H i s t h e

t o t a l p u m p h e a d , i n f ee t o f f l u id , a n d E i s t h e a s s u m e d p u m p

e f f i c i e n c y .

I t i s e m p h a s i z e d t h a t t h e p u m p p o w e r s o d e r i v e d s er v e s w e l l

t h e a v e r a g e s i t u a t i o n . T h e r e a r e s i t u a t i o n s w h e r e t h i s p o w e r

c a l c u l a t io n i s n o t b i n d i n g . F o r e x a m p l e , i f a d r e d g e i s p u m p i n g

f i n e s i lt , w h i c h d o e s n o t s e t t l e a t a l l i n t h e h o p p e r s , i t h a s b e e n

f o u n d t h a t t h e m o s t e c o n o m i c a l m o d e o f o p e r a t i o n i s s i m p l y t o

f il l t h e h o p p e r s a s r a p i d l y a s p o s s i b l e t o t h e p o i n t o f o ve r fl o w , a n d

t h e n s t o p p u m p i n g a n d t a k e t h e l o a d to t h e d i s p o s a l a r ea . A t t h e

o t h e r e x t r e m e , s e t t l i n g ta k e s p l a c e v e r y r a p i d l y w h e n p u m p i n g

v e r y c o a r s e g r a i n s a n d o r g r a v e k I n e i t h e r o f th e s e s i t u a t i o n s , a l l

a v a i l a b l e p o w e r w i ll b e u s e d . L i k e w i s e , w i t h a s u b m e r g e d p u m p ,

w h i c h c a n b r i n g u p h e a v i e r d e n s i ti e s u n l i m i t e d b y v a c u u m c o n -

s i d e r a t i o n s , p o w e r i s l i m i t e d o n l y b y m e c h a n i c a l a n d g e o m e t r i c

c o n s i d e r a ti o n s a n d b y h o w d e n s e a m i x t u r e c a n e c o n o m i c a l ly b e

" p u s h e d " u p t h e d r a g a r m b y t h e p u m p .

S o m e c o n s i d e r a t i o n m u s t a l so b e g i v en t o t h e p o w e r r e q u i r e -

m e n t s f o r d i r e c t p u m p o u t o p e r a t i o n ( D P O ) . H o w e v e r , t h e r e -

q u i r e d p o w e r m a y v a r y w i t h l in e l e n g t h , t h e d i s c h a r g e h e a d t o b e

d e v e l o p e d a n d t h e d e n s i t y o f t h e m i x t u r e t r a n s p o r t e d t o sh o r e .

T h e p o w e r l e v e l c a n b e e s t a b l i s h e d s o m e w h a t a r b i t r a r i l y , f r o m

p r a c t i c a l m a c h i n e r y c o n s i d e r a t i o n s , b u t i t is i m p o r t a n t t o c o n -

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

l e n g t h a n d d i s c h a r g e e l e v a t i o n , a s t h i s w i l l i n f l u e n c e t h e i m p e l l e r

d i a m e t e r a n d r p m . S i n c e i t w i ll b e d e s i r e d t o p u m p o u t w i t h f u l l

p o w e r o n s h o r t l i n e s a s w e ll a s in t e r m e d i a t e a n d l o n g l in e s , a r a n g e

o f r p m w i t h 1 00 p e r c e n t p o w e r c a p a b i l i t y m u s t b e p r o v i d e d . I t

i s f o r t h i s r e a s o n t h a t d c m o t o r s w i ll d r i v e th e i n b o a r d d r e d g e

p u m p s o f b o t h th e M C H D a n d t h e L C H D .

H o p p e r d o o r a r e a s

A s i g n i f ic a n t c h a n g e i n h o p p e r d o o r s i z in g h a s b e e n a c c o m -

p l i s h e d i n t h e t h r e e n e w d r e d g e s , to m a k e t h e m f a s t e r d u m p e r s ,

a g a i n t o c h o p a m i n u t e o r tw o o f f t h e c y c l e t i m e . A s t e p t a k e n o n

t w o p r e c e d i n g d r e d g e s t o s p e e d u p d u m p i n g w a s t h e e s t a b l i s h -

m e n t o f u n s y m m e t r i c a l h o p p e r s l o p e s, t o p r e v e n t a r c h i n g o f

m a t e r i a l , p a r t i c u l a r l y s a n d , in t h e h o p p e r s . T h e n e x t l o g i ca l

c o u r s e w as t o in c r e a s e th e d o o r si ze to t h e m a x i m u m p e r m i t t e d

b y s t r u c t u r a l d e s i g n .

T a b l e 4 s h o w s s t a t i s t i c s f o r th e h o p p e r d o o r s o f e x is t in g d r e d g e s

a n d o f t h e t h r e e n e w d r e d g e s . T h e i m p r o v e m e n t i n th e l a t t e r i s

e v i d e n t .

T h e s iz e a n d n u m b e r o f h o p p e r d o o r o p e n i n g s i n th e t h r e e n e w

d r e d g e s a r e a s f o ll o w s :

NO. OF DOOR SIZE OF OPEN INGS

DR E DGE OPE NING S L W

Yaquina

6 6 f t 9 in. 3 f t 8 in.

MC HD 12 8 f t 0 in . x 8 f t 3 in .

LC HD 14 8 f t 0 in . X 8 f t 3 in .

A p o s s i b i l it y c o n s i d e r e d , in t h e i n t e r e s t o f s p e e d i n g u p t h e

d u m p i n g p r o c e s s, w a s t o d e s i g n t h e d r e d g e s a s s p l i t - h u l l t y p e s ,

d e s i g n s f a v o r e d b y c o n t r a c t o r s n o w b u i l d i n g h o p p e r d r e d g e s i n

t h e U n i t e d S t a t e s p a t t e r n e d a f t e r E u r o p e a n d e s ig n s . I n t h is

m a t t e r , a s w e l l a s i n o t h e r a s p e c t s o f d e s i g n , t o p - l e v e l o p e r a t i n g

p e r s o n n e l o f t h e C o r p s w e r e c o n s u l t e d p r i o r t o c o m m e n c i n g t h e

d r e d g e d e s i g n s , a n d a c o n s e n s u s w a s o b t a i n e d t h a t f o r t h e s e

p a r t i c u l a r t h r e e d r e d g e s a d o p t i o n o f a s p li t - h u l l d e s i g n w a s n o t

a p r a c t i c a l a p p r o a c h . T h i s d e c i s io n w a s n o t r e a c h e d l ig h t ly , a n d

n o t w i t h o u t e x p e r ie n c e . T h e C o r p s o w n s a n d o p e r a t e s o n e s m a l l

s p l i t - h u l l h o p p e r d r e d g e ( th e

Currituck

W i l m i n g t o n D i s t r i c t ,

i n N o r t h C a r o l i n a ) a s w e l l a s a n u m b e r o f s p l i t - h u l l h o p p e r

b a r g e s , a n d t h e C o r p s m a y v e r y w e ll i n t h e f u t u r e d e s i g n a n d

c o n s t r u c t a n o t h e r s p l i t- h u l l t y p e o f h o p p e r d r e d g e . F o r t h e t h r e e

n e w d r e d g e s , h o w e v e r , t h e r e a s o n s f o r d e f e r r a l o f t h e s p l i t h u l l

a p p r o a c h w e r e c o m p e l l in g . T h e s e i n c l u d e d t h e r e q u i r e d v e r s a -

t i l it y o f t h e d r e d g es . I n p a r t ic u l a r , d i r e c t p u m p o u t t h r o u g h l o n g

l i n e s , t o d i s t a n c e s o f a t l e a s t 2 m i l e s , w i t h o u t a s s i s t a n c e b y e x -

t e r n a l b o o s t e r s t a t i o n s , is a c u r r e n t n e e d a n d o n e w h i c h i s c e r t a i n

O C T O B E R 1 9 8 0 3 7 7

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of wider application. This requires provision of large high-pres-

sure high-power pumping plants forward, not easily accommo-

dated in the half hulls of the split-hull design. The split hull might

have been adopted for the small dredge Yaquina which is not

equipped for direct pumpout capability, but this dredge is re-

quired to operate at shallow draft with a 300-yd load as well as

at 12- and t3-ft drafts with heavier loads. Special problems would

have been encountered designing split hulls with provision for

the self-opening and self-closing capability at several drafts,

which is normally expected of this type vessel at a single draft.

C o n t r o l l a b l e - p i t c h

p r o p e l l e r s

The application of controllable-pitch (CP) propellers for a

hopper dredge is no longer new, since the Corps began this on the

dredge Harding in 1958 and again on the dredge McFarland

completed in 1967. The Harding wheels were each 2000 hp and

those on the McFarland3000 shp. The dredge Pacific repowered

in 1977, was fitted with two CP wheels, each absorbing 1050 shp.

For the large-class dredge each wheel will absorb 5000 shp, and

on the medium-class dredge 3350 shp. On the small dredge Ya-

quina each propeller will absorb about 1050 hp.

This will bring to six the total number of Corps dredges fitted

with CP propellers, constituting evidence of the eminent suit-

ability of such propellers for dredge service. This is so because

dredges operate basically in three propulsion modes: (1) While

dredging, the vessel is in a towing mode, "towing" its dragarms

gainst the resistance of the channel bot tom; (2) when loaded,

t is in a free-running mode at deep draft, going to the dump; and

(3) when light, it is in the free-running mode at light draft with

by the stern, returning to the dredging area. For maximum

ropulsion power in each of these conditions. With a fixed-pitch

em this is not possible. With a control lable-pitch

at 100 percent speed and the pi tch can be adjusted to allow

Prior to the advent of CP propellers, the Corps used dc electric

propulsion with fixed-pitch wheels. Automatic controls

he propeller

for the free-running conditions, so that full horsepower is

The availability of full power under any operating mode can

here and a minute there off the dredging

off this cycle time represents better

is lighter and less costly than the equiva-

by the CP propulsion control mechanisms are some-

offset by the elimination of propulsion generator and motor

H o p p e r c a p a c i t i e s

A hoppe r dredge is nominally designed to carry, say, 6000 yd 3

long tons (LT) per cubic yard

without exceeding the designloaded draft o f the hull when all fuel

and water tanks are full. As the consumables are expended, the

vessel can obviously carry more sand load without exceeding the

permissible loaded draft . If, for example, the vessel carries 1170

LT of fuel and water, then, at the 90 percent bur nout point, the

hoppers could contain an additional 702 yd 3 of sand without

exceeding the loaded draft, provided, of course, that the hopper

volume is sufficient to hold the extra sand.

But the dredges do not operate exclusively in sand. Frequent ly,

they dredge much lighter bottoms, in some cases encountering

bottom material densities as light as 75 lb/ft s, or about 0.9 LT/

yd 3. It would take 11 170 yd3 of this light material to have a

weight equal to 6702 yd 3 of wet sand A commonly encountered

mud has a density of about 1.2 LT/yd3; 8378 yd 3 of this material

is equal in weight to 6702 yd 3 of wet sand.

It is not practical to design a hopper dredge with hoppers of

such depth as to accommodate the entire spectrum of bottom

densities in sufficient volume always to load the hull to its de-

signed draft. A limit is reached when the center of gravity of the

load in the hoppers gets so high as to affect adversely the vessel

stability. Contributing to this effect is the fact that the weights

of the hopper distribution piping, valves, hopper structure and

all other appurtenances over the hopper area are also moving

upward as the hopper coamings are raised in the effort to enclose

more hopper volume.

Nevertheless, within this limitation the effort should be made

to a ccommodate a reasonably wide range of densities. With the

three new dredges it has been found that this could be done.

Figure 6 is a chart showing the capabilit ies in this respect of the

LCHD. This dredge is intended to carry 6000 yd3 of wet sand at

29.5-ft draf t with all tanks full. At 90 percent burnout it will carry

about 6700 yd 3 of sand. The maximum hopper volume was set

at 8400 yd3. Depending on the specific gravity of the hopper load,

the volume of material that can be carried at any condition of

burnout at 29.5-ft draft is shown in Fig. 6.

A similar chart for the MCHD, designed to car ry 4800 yd 3 of

wet sand (1.5 LT/ydU), would show the volumes of lighter den-

sities that can be carried at its loaded draft of 27 ft, up to its

maximum hopper volume of 6000 yd3.

H o p p e r o v e r f l o w l e v e l

A dredge can fill its hoppers only to the level of its overflow

weirs. At this point the hopper fluid spills over the weirs and is

conducted via overflow troughs and ducts to the sea, or sur-

rounding water. Most dredges operate with one or more fixed

overflow levels, selected by opening or closing louver-type

structures in the hopper coaming or sides. The location of the

overflow level is now determined with some regard for promoting

the maximum production capability of the dredge during the

burnout cycle. This is best illustrated by example.

One requirement of the small shallow-draft dredge Yaquina

is that it must be capable of carrying 300 yd3 of wet sand at a draft

not in excess of 10 ft when all fuel and water tanks are full. Since

there had to be provided enough consumables for 12 days of op-

eration, the tank contents, 180 LT, constitute a respectable

percentage relative to the hopper load. In this p~lrticular instance,

the sand used in design computa tions had a wet density of 1.35

LT/yd 3. Thus the weight of a 300-yd load is 405 LT. At the 90

percent burnout point, therefore, the dredge theoretically could

carry 567 LT or 420 yd3 of sand.

Consider, now, the two extremes: (1) If the overflow level is set

at 300 yd3, then the dredge can initially load up to 300 yd, but as

fuel and water are consumed it will be unable to hold any more

than 300 yd. Thus, as burnout occurs, the ship will not be loaded

to design draft. (2) At the other extreme, suppose the overflow

level is set at 420 yd. Initially, then, the hoppers mus t be limited

to that weight of sand plus water above the sand such that to-

gether the total weight of hopper contents does not exceed 405

M A R I N E T E C H N O L O G Y

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d )

123

rr

130

c h

g

85OO

M A X H O P P E R V O L U M E : 8 4 0 0 C U Y D S

. . . . -~Z . . . .

_ _ ~ ~'3_2~ ~ ~ ~

j . - / ~ / /

6 5 0 C ~ /

60C0 IO 20 30 40 50 60 70 80 90

PERCENT BURNOUT

F i g . 6 L C H D h o p p e r c a p a c i t y . C a r r y in g c a p a c i t y o f v e s s e l a t 2 9 .5 - ft d r a f t, b a s e d o n

p r e l i m i n a r y d e s i g n t r i m a n d s ta b i l i ty c o n d i t i o n s a s a f f e c te d b y ma te r i a l d e n s i t y . Bu r n o u t

i n c l u d e s p o ta b l e a n d f re s h w a te r s , f u e l o i l a n d d o me s t i c s to r e s

A s i m p l e a l g e b r a i c c a l c u l a t io n w i l l s h o w t h a t t h e i n i t i a l s a n d

b i l i t y is a b o u t 1 4 0 y d S . A t 9 0 p e r c e n t b u r n o u t , w h e n 1 62 L T

c o n s u m a b l e s h a v e b e e n e x p e n d e d , t h e p e r m i s s i b l e w e i g h t o f

p e r c o n t e n t s i s 5 67 L T , s o t h a t t h e d r e d g e c a n , a t t h i s u l t i m a t e

t i n t h e p r o v i s i o n i n g c y c l e , c a r r y 4 2 0 y d ~ o f s a n d . A s t r a i g h t

c a n b e d r a w n b e t w e e n t h e s e t w o s a n d c a p a c i t i e s t o s h o w h o w

s a n d w i ll b e c a r ri e d a t i n t e r m e d i a t e b u r n o u t p e r c e n t -

I t i s s e e n f r o m F i g . 7 t h a t A l t e r n a t i v e 1 n e v e r p e r m i t s t h e s a n d

r i s e a b o v e i t s i n i t i a l 3 0 0 - y d l e v e l . A l t e r n a t i v e 2 s t a r t s w i t h

n d a t t h e 1 4 0 - y d l e v el , b u t t h e s a n d l e v e l r i s e s p r o g r e s s i v e l y

u t u n t i l i t r e a c h e s th e m a x i m u m p e r m i s s i b l e 4 2 0 - y d

. O v e r t h e e n t i r e p r o v i s i o n i n g c y c l e , w h i c h l e v e l r e s u l t s i n

a r r y i n g t h e m o s t s a n d ? I s t h e r e a n i n t e r m e d i a t e o v e r f l o w l e v e l,

es s t h a n 4 2 0 y d b u t m o r e t h a n 3 00 y d , w h i c h o v e r th e p r o v i -

i o n in g c y c l e w i ll p e r m i t c a r r y i n g a m a x i m u m a m o u n t o f

a n d ?

T h e a n s w e r is y e s , a n d r e a d e r s w i t h a n y f o n d n e s s f o r m a t h e -

m a t i c s c a n d o w h a t t h e a u t h o r d i d a n d s e t t h i s u p a s a c a l c u lu s

p r o b l e m t o f i n d t h a t v a l u e o f o v e r fl o w le v e l w h i c h , w h e n i n t e -

g r a t i n g t h e t o t a l p r o d u c t i o n u n d e r t h e c u r v e b e t w e e n t h e 0 a n d

9 0 p e r c e n t b u r n o u t l i m i t s , r e s u l t s in m a x i m u m p r o d u c t i o n .

T h e r e s u l t is

Ss

- -

Sw Tb

C = Y , 4

Ss Ss

w h e r e C i s t h e c u b i c y a r d s a t t h e o p t i m u m o v e r f l o w l e v e l, Yn t h e

n o m i n a l d e s i g n s a n d l o a d w i t h a l l t a n k s f u l l ( 3 0 0 y d 3 i n t h e

f o r e g o in g e x a m p l e ) , Ss a n d S w t h e d e n s i t y o f s a n d a n d w a t e r

r e s p e c t i v e l y , i n lo n g t o n s p e r c u b i c y a r d , a n d Tb t h e l o n g t o n s o f

c o n s u m a b l e s e x p e n d e d a t t h e e n d o f t h e b u r n o u t p e r i o d .

A p p l y i n g th i s r e s u l t to t h e c h o s e n e x a m p l e

1 . 3 5 - 0 .7 7 1 1 62

C = 300 4 = 351

1 .35 1 .35

A c c o r d i n g l y , a f i x e d o v e r f l o w l e v e l f o r t h e

Yaquina

w o r k i n g a t

1 0 - f t d r a f t w o u l d b e p l a c e d a t 3 5 0 c u b i c y a r d s . T h i s v e s s e l is a ls o

p r o v i d e d w i t h h i g h e r o v e r fl o w l e v el s fo r w o r k i n g a t d r a f t s u p t o

1 2 f t .

I f a c o m p a r i s o n i s m a d e o f t h e t o t a l a r e a s u n d e r t h e c u r v e s o f

F i g . 7, i t w i l l b e f o u n d t h a t t h e y a r e i n t h e r a t i o o f a b o u t 1 01 t o

1 0 8 t o 1 1 7 . I t i s s e e n t h a t t h e o p t i m u m o v e r f l o w le v e l p e r m i t s a

s i z e a b le p e r c e n t a g e i n c r e a s e i n p r o d u c t i o n o v e r t h e e x t r e m e a l -

t e r n a t i v e s .

4 0 0

u ~

3oo

u

a