the buchan field development
TRANSCRIPT
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U 3
THE
UCH N
FIELD DEVELOPMENT
by E. Darnborough, P
Petroleum
Development
Ltd.
EUROPE n
OFFSHORE
PETROLEum
conFEREnCE
EXHI ITion
© Copyright1980 EuropeanOffshore Petroleum Conference and
xhibition
This paper was presented t the European Offshore Petroleum Conference and xhibition held in London England October21 24 1980. The material is subject
to
correction by the author.
Permission to copy is restricted to an abstract
of
not morethan 300 words.
ABSTRACT
This paper
descr i bes
th e Buchan development
scheme which is
based
on
subsea
completions
t ied
into
a su bsea man ifo ld
connected
in
turn
by a flexible
r iser
to a f l o at i n g production
platform. Oil export is by
t anker
from a
single
poi nt m ooring buoy.
The system
i s
s im il ar t o
bu t more complex than t h at used
in
th e
Argyll f i e l d .
Overall
th e
project ha s exceeded budget
an d
programme largely due to
delay
in completing
th e
conversion
of a
semi-submersible
d r i l l i n g
r i g to a production
platform.
Subsea work ha s
gone remarkably
well.
The paper comments on
t hese
f act s
and
suggests
that
improved
front
end engineering and g r eat er c on tr ol o f con
t r a c t o r s
would
have
improved
performance. t
suggests that a purpose-built
platform i s
a
b e t t e r proposition than a conversion. t
concludes t h at d es pi te t he
problems
encountered
t h i s
type of development is
viable.
INTRODUCTION
The Buchan f i e l d
i s
located approximately 160.
kI
E.N.E. of Aberdeen an d
approximately 55
km
W.N.W.
of the
Forties
f i e l d . The water
d ep th o ve r
th e
f i e l d varies
between
112 m.
to
118 m.
The f i e l d was
discovered
in
1974 by
th e
Trans
world Group and
appraisal
wells were
d r i l l e d
between then an d 1976 t o d el in ea te th e f i e l d .
In early 1977 th e author s Company BP
Petroleum
Development Limited farmed in to t he l ic en ce
and t oo k o ve r
operatorship of the
development.
The
majority
of the f i e l d l ies th block 21/1
with a small
proportion
in b lo ck 2 0/ 5.
The
partners
in
th e consortium
involved in th e
177
development
were
apart
f rom BP St. Jo e Pe t r o l
eum, CanDel Petroleum Natomas I n t er n at i o n al
Gas Oil Acreage Limited Charterhall Oi l
Limited
CCP
North
Se a Associ at es Lochi el
Exploration
UK Limited
City
P et r ol e um C or
poration. The c on so rt i um h a s since
been j oi ned
b y T ex ac o l icencees
f o r block
20/5
~ r
a
Unitization
Agreement.
RESERVOIR CHARACTERISTICS
The majority
of the oil in
place
estimated
to
be
390 million b ar r el s l ies within a horst
b lo ck w it h sm al l
amounts
lying
in
flank areas.
The mos t probable figure
fo r
recoverable reserves
i s 50 million b ar r el s equi val ent
to approximately
1 ;6 recovery.
The
horst
block of Devonian sandstone is
over
pr essur ed an d highly
fractured.
The to p of the
reservoir
l ies
some 2 600 m. below
sea
be d
an d
th e o il water
contact
some 3 200 m. below sea
bed.
The
pa y
zone is up to 500 m. thick.
Pr essur e
a t th e bottom
of the r eser v o i r is
7 500 p si
an d
th e
maximum
well head
cl osed
i n pressure 4 900
p s i . D r i l l i n g
in t h i s
r eser v o i r
pr esent ed
con
s i de r ab l e t e ch n i ca l
problems
which
have
been
discussed
in
other
SPE
papers.
t is no t
yet c l e a r what th e
ext ent
o f th e re
covery will
in f act be .
t
is known t h a t oil
flows freely from th e f r act u r es bu t what
ha s
yet
to be
established
is t h a t oil in th e
matrix
of
th e rock w i l l make up t he lo ss es from th e
f r act u r es. f t h i s does no t happen
th e
amount
o f oil recovered
will
be very small an d the p ro
ject w i l l be tot lly uneconomic.
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They were -
DEVELOPMENT ~
The
crude
ha s a low s u lp h u r c o nt en t and ha s th e
following
average
c ha r a c t e r i s t i c s
-
1)
Capital investment ha d to be kept to a
minimum
Unusually significant
in
t h e c o ns id e ra ti on s
was th e
question
of abandonment costs. Again
due to th e sho rt l i fe of th e
fi e l d
these
costs
have a
greater
impact than normal. The us e of
a floating
platform s im p li fi es t he
abandonment
considerably.
I) P ip el in e t o
Forties
an d thence
to
shore
2
By
tank.ers
The
other
major
choice
which
ha d
to be made was
th e
methoa
of export.
In
th e case of
Buchan
there
we re two realis t ic
options
-
Of perhaps
greater
significance was
th e question
of timing. By following t he o pt io n 2 ) route
d r i l l i n g
and completion
of th e v l o p m ~ w l l s
proceed
i n p a ra ll el
with
t h e c o ns tr u ct io n
of
th e production
platform
and, i f one s planning
i s r i ght , as the d ri ll in g r ig
leaves
th e fi e l d
th e p r od u ct io n p l at fo r m
comes
over the horizon.
A ll that
is
t h en n e ce s sa r y is
to anchor
th e
ri g , connect
up
th e r i ser
an d
s tar t th e o il
flowing.
In
t h i s
case
cost
was
th e
dominating
factor,
the overall cost of i n s t a l l i n g , connecting,
operating eventual abandonment of the pipe
line t o F or ti es was fa r
in
excess of th e com
p ara ble c ost s of
i n s t a l l i n g
a single point
moor ing s ys tem and using
tankers.
Reviewing th e two
o p tio n s
there was
l i t t l e
d i ffi c u l t y , bearing i n
mind
th e
objectives,
i n selecting option 2).
In
th e location
under
c o n s id e r at i o n t h e i n s t a l l e d cost of a fixed
platform
which would
need
d ri l l i n g
an d pro
duction
facil i t ies was s u b s ta n t ia l ly g r e a te r
than that
of
a
floating p r od u ct io n p l at f or m .
Within
these options
number
of other choices
ha d to
be made, i n p ar ti cu la r t ha t of th e
means of exporting th e o i l from th e fi e l d .
2 a floating
p ro d uc ti on p l at fo r m w it h
sub
se a w ells, the d r i l l i n g of
which
could
be s t a r t e d long
before the i n s t a l l a t i o n
of
th e
platform.
a fixed
p la tf or m w it h th e
d r i l l i n g
of
th e
development
wells
subsequent
to i n st al
lation
In
re a l i t y there
w er e two
basic
development
options to
be
considered
-
How
well
we have met these aims will
be
des
cribed.
To r e c ap i t ul a t e t h er e fo r e th e p ri me a im s were low
cost an d swift
development
to give the
e a rl i e s t
production.
0
310 s c f per ba r r e l
API gravity
Gas/Oil ra t i o
ai content
2 The time
fo r
th e development ha d to be
as
short as
feasible.
The
second point is p er ha ps n ot quite
so obvious.
Those
fa m ilia r
with
DCF
discounted cash
flow)
methods
of
economic evaluation will be aware
t ha t
th e
sooner positive
cash
flow
i s
achieved
th e
b e t t e r
th e rate o f r et ur n on th e investment.
There is to sa y
th e
sooner oil flows af ter com
mitment is f i rs t made
the b e t t e r th e
economic
retu rn. In a
fi e l d
such as Buchan, with a
very
short l i fe an d consequently a s ho rt p er io d
of
positive cash flow, delays
i n s ta rt -u p could
in
th e
extreme
lead
to a position where financing
costs swamp income.
I t
w i l l
be
seen t ha t t h i s reservoir combining
d i ff i c ul t d r il li n g conditions with an
uncertain
recovery needing
th e
complication of ga s l i f t
to produce only 50 million barrels and a l l i n
re l a t i v e l y
deep
water is
a
t ru l y
marginal
fi e l d .
The commercial r i s ks associated
with
i t s
development ar e considerable.
The f i r s t point. is something of a
truism
and
c l e a r l y is an
objective with
any project; in
th e
case
of
a
fi e l d
as marginal as Buchan with
a re a l
ri s k
of commercial fa i l u re i t is
par t i -
cularly important.
The development scheme adopted i s
in principle
th e one selected by th e
original
operator.
Eefore accepting
i t
however
alternatives
considered. Keeping in mind
th e
high commercial
risk presented
by
the
development of
th e
Buchan
reservoir
the major objectives were very clear.
In
order
to
achieve
a
reasonable
rate
of
pro
d u ct io n d u ri ng th e
l a t te r ha l f
of
th e l i fe
of
th e
f i el d ga s l i f t
is
to be
introduced. I t is
n o t
feasible to
introduce
pressure maintenance
by
e i t h e r water
i nj e c t i on
o r
ga s injection.
The fractured
nature
of
th e reservoir leads
to
water
or ga s
being dissipated
through
the
fractures without
effectively
displacing oi l
Assumihg t ha t there i s makeup from
th e
matrix
th e fi e l d ca n
be
produced a t
a peak rate
o f
72,000
ba r r e l s a day
with
a n a ve ra ge
maximum
daily ra t e of 48,000 ba r r e l s . This rate
ca n
be maintained fo r approximately I years a f t e r
which
production
declines
with
th e anticipated
fi e l d
l i fe
b ei ng n ot
more
than
five
years.
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PRINCIPAL
FEATURES
OF·
THE
DEVELOPMENT
The development scheme cal l ed
fo r
t he d ra in in g
of
th e
reservoir by
8 wells. Si x o f the se were
to be
in
th e
horst and
th e o th er 1wo to be
d r i l l e d in
th e
flanks
as semi
appraisal
wells
which hopefully would
provide additional
pro
duction.
The intention was to have
the
six
horst wells i e
th e main producers,
d r i l l e d
through
a
template immed iately o v er
which would
be i n s t a l l e d
th e p r od u ct io n p l a tf o rm .
This
would
ensure that
these wells were accessible
by
divers from
th e
platform an d also
t h a t wire
l ine interventions
could be
made from th e p l a t
form.
I n p ra ct ic e we chose to convert one of the
existing exploration
wells
to
production
and
i n st a l l e d
th e t em p la t e a l on g s id e
i t
Only
f ou r w el ls
instead
of
five were eventually
d r i l l e d through
th e template
f or
programming
reasons
which are described
in
more
d e t a i l
later .
Fo r a number of reasons th e dr illing of th e
eighth
well
was
delayed
and
is
scheduled
to
be spudded
in
October 1980.
The
p r od u ct io n p l at f or m
was to be a conversion
of a semi-submersible d ri ll in g r ig an d of
those
available a t th e
time
th e
Pent
agone r ig
D r i l l
Master was :::;elected. t s basic advantages
were
Great
s tabi l i ty
Good
load carrying
capacity
Spaciousness
Experience
ha s borne
ou t
ou r thinking on th e
f i rs t
two
items
bu t
th e apparent
spaciousness
was
deceptive.
Fo r
th e s in gle po in t
mooring
system
a
Catenary
Anchor Leg Mooring
CALM)
buoy was
ultimately
chosen
on th e basis of
lowest i n st a l l e d
cost.
Engineering studies
indicated
t h a t an a r t i c u
l at ed tower despite i ts
greater
cost
might
have
b e t t e r
operating c h a r a c t e r i s t i c s allowing
loading
to continue to more severe weather
than
th e LM buoy. These b en ef i t s were mar
ginal an d a t
th e
time e x i s t i n § , a r t i c u l a t e d
towers
were experiencing problems. The buoy,
of Imodco design, is the
largest
of' i t s type
in
th e world
and
is to operate
i n
d ee pe r w at er
than e ve r b ef or e.
Two
tankers h av e b ee n chartered
an d
modified
fo r
d e di ca te d u se
on th e f i e l d . The
tankers
of
100,000
DWT
have
been
modified
to
have
bow
mooring and
l oa d in g, s e gr eg a te d
b a l l a st ,
i n e r t
gas,
crude
oil
w as hi ng a nd to have
pro
vision
for
slow
ast er n
steaming.
The
l a t te r
is
to ensure
c on tr ol o f t en si on on th e
mooring
hawser when operating
in
calm conditions. I n
their
modified
form
these
tankers ca n
accommodate a load of some 75,000 tonne.
9
Se a t r ia l s have demonstrated
th e
s a t i s f a c t o r y
performance of both tanker and
loading
buoy.
In th e case of th e subsea equipment the major
philosophical design point was simplicity.
The
Xmas t r e e s ar e
very
straight-forward,
th e
only sophistication
being
t h a t of spl i t
construction.
The lower portion
includes
a
manually o p er at ed m as te r v a lv e
in
th e flow
port
an d
in
th e
annulus access
port.
By
c lo si ng t he se valves
together
with the down
hole
saf et y valvesit
would
be
possible
to r e
t r i e v e th e
upper part of
th e
tree fo r replace
ment o r maintenance without th e
need to ki l l
t he w el l.
The upper pa rt of th e
t r e e
ha s hydraulically
operated
master wing and swab valves,the
hydraulics being simple, d i r e c t actuation •
There are no
e l e c t r i c s,
no p i l o t valves,
simply
a
line
from th e
surface to an
operating cy
l inder.
Flows from
a ll
t r e e s are
brought
to a gathering
point
described,
a l i t t l e inaccurately) , as
th e
su b sea man ifo ld.
The
manifold
forms
th e
interface between th e t r e e s an d th e
production
r iser Each
tree
is
associated
with a single
production r i ser an d a single ga s l i f t
l i n e .
These
l i n e s
are
disposed around a 12
c e n t r a l
r i ser which provides the export l in e f o r
d i s
charging dead crude
to
the tankers. The mani
fold
an d
r i ser
ar e described
in d eta il by
a
p a pe r b e in g presented
a t
t h i s conference by a
colleague.
WELL COMPLErIONS
Again
t he o bj ec ti ve
w it hi n t he co n st r ai n t s
ap
p l i cab l e
was
to
keep
th e well
completions
as
simple as
possible.
The
elements
of th e
com-
pletion,with one
exception,
are standardised
throughout th e f i e l d a nd c om pr is e 5 tubing,
hydraulically
set packer,
sliding
s le ev e, s id e
pocket
mandrels
fo r
ga s
i n j e c t i o n
v al ve s, t ub in g
r e t r i e v a l
downhole saf et y valve, mechanically
ru n an d
set
tubing hanger, single j o i n t 2i OD
fo r
annulus access, fac i l i t ies fo r wire
l i n e
retrievable
downhole saf et y
val ve
in th e annulus
access. The tubing retrievable
v al ve d es ig n
incorporated fac i l i t ies enabling a wire
line
retrievable
valve
to be i n st a l l e d inside
the
tubing
r et r i ev ab l e valve
i n
th e
event t h a t
th e
l a t te r
malfunctioned. Additional fac i l i t ies
are
provided for plugs
in
th e
upper j o i n t of
t he t ub in g
an d in
th e annulus access j o i n t also
in
t he t ub in g
hanger s u p po r ts t h em s el v es .
The exception is th a sa te l l i te well d r i l l e d into
th e flank which ha s tubing
bu t
i s
otherwise
completed
exactly as
th e others.
A p o t e n t i a l problem
with
th e co mpletio n ad op ted ,
except
of
course i n
th e
flank w e l l , i s th e fact
t h a t
a t
those
points where wire
l i n e s
tools may
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4/8
be used th e
tubing
str ing is
reduced
to 4 ~ O D
There i s
therefore an
inherent
risk
that
in
th e
event of
a
wirel ine breakage
a birds
nest could be formed
below
one of the 4i OD
restrictions
making
fishing very diff icul t .
Tubing re trievable
downhole
safety
valves
were
selected for
a number of reasons but th e
two most
significant
were
a) ou r
experience
to date
has indicated
that
this
type i s
th e most
rel iable.
b)
they pe rmi tt ed th e
running of perfora
t ion
tools.
This meant
that
th e overall
safety
of th e
perforation
operation was
increased
as, in addition to tree valves
and
th e wire l ine BOP we had th e safe
guard of
a downhole
safety valve.
I t
is of course to o e ar ly t o
make
any
comment
on th e performance of the
completion
during
production. Problems were
experienced
during
installation
and
th es e a re described later .
PROGR J VlME
Work on
th e
development s ta rt ed i n March 1977
and
called
for th e field to be on-stream by
th e end of September
1979,
i e
a
3
month
programme. At
th e time this
was
recognised
to
be ambitious
and
with hindsight we realise
that
some
key
elements
were
misjudged and
that
th e
proposed
timing was barely,
i f
at
al l
possible.
The
in i t ia l cr i t ical path
la y
through dri l l ing
and completion.
With this
in
mind
efforts
were concentrated on design manufacture
and
installation of th e
dril l ing
template in
order to be
able
to s tar t dri l l ing
as soon
as
possible.
Once again simplicity·;was th e key
note to success
which
in
th is
instance
was
certainly achieved.
The
template
was
installed
on th e
seabed in
August 1977 and dril l ing
commenced in
early
September 1977, i e
within
six
months of start ing
th e
project.
Concurrently
with the
design and
manufacture
of
th e
template
specifications for th e sub
se a equipment, the production equipment and
of
th e
export system
were being
drafted and
tenders
being invited. Enquiries were also
ini t iated
fo r
semi-submersible
dril l ing
rigs
ava il ab le for c ha rt er and
o n v e r ~ i o n
By
November 1977 contracts had been le t for de
sign
and procurement
of the
subsea
equipment
and for the
management
of a ll
work
concerned
with
the
product ion plat form, including i ts
installation. By
th e beginning of February
of
1978 agreement had been
reached
to bare
boat
charter th e r ig, Dri l l
Master. Just
prior to this th e o rde r for the s in gle p oint
mooring system had been placed. Also in March
1978
enquiries for suitable tankers to be
dedicated to th e
export of
o i l from Buchan
were issued.
8
At
th is date
work, with th e exception of
th e
dril l ing programme was
proceeding
on
target
and
unfortunately,as
will be recalled,
dril l ing
and completion la y on th e
cri t ical path.
A
decision
was therefore taken to
reduce th e
number of wells dril led
through
th e
template
from
five to
four
and to dr i l l
a wel l dev ia ted
into
th e
horst block
from th e
satel l i te lo
cation.
In th is way i t
would
be
possible
to
clear th e template si te in time for th e
scheduled
arrival of Drill
Master in the
third
qua rt er o f 1979 and
s t i l l have the
desired
total
number of
wells. This decision incurred
disadvantages
namely,
a) th e need to la y
additional flowlines
and
hydraulic control l ines.
b)
removing
th e well
from th e
operating
area of Drill
Master,
which means
that
any
wireline
interventions
reqUired
in t he f ut ur e
will
necessitate
the
hire
of a special
semi-submersible.
Poin t a ) re pr ese nt ed
a
substantial
increase
in c ap it al
cost
whi ls t point b) will
lead to
increases in operating
costs.
The
decision
to
modify th e dril l ing programme was taken
in th e ful l l ight
of th is knowledge
and reflects
th e fact
that
start ing production on
time
was
th e most
s ign if i can t fac tor in th e
profitabil i ty
of the
development.
The
budgeted
cost
for
th e development
at
th is
time eariy 1978 was £135 million. Of this
some £40 million was
for
dril l ing, test ing
and completing th e
production
wells.
The
actual cost soared as
a
consequence of the
planned
programme not
being
met.
The
three
most significant
problems
in
th is
respect started to emerge in early 1979 and
were
a)
dril l ing and
completion
b)
design
and
fabrication
of
subsea
equipment
c) conversion of Dri l l Master.
Of
these
by
far th e
most
significant, without
question, was th e conversion of the dri l l ing
r ig,
item
c ).
Some
detail
of
how
th e
programme
in fact deve
loped is
given in th e following sections.
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DRILLING
ND
COMPLETION
The
d r i l l i n g
programme ha d
originally
been
scheduled
to finish
in
August/September
1978. In fa ct dr illing of template
wells
finished in early November some two months
l a t e w hil st t he sa te l l i te
wells
were
finished,
on programme in September.
The rig over th e template went
immediately
onto
completion
of the template
wells
and
of
t h e e x pl o ra ti on well
21/1-2ST.
I t ha d to
~ u s p e n
work
a t
th e end
of May
1979
to permit
the
laying
of subsea
flowlines
an d other
subsea construction
work.
Completion work on th e
t em pl at e w e ll s
was
resumed
at
th e end of
th e
construction work
in October and t he w el ls were le f t
perforated,
cleaned
up and ready to flow a t
th e
end of
December 1979 i e three months
af ter
th e
originally scheduled start-up of the f i e l d .
On
the
satell i te location completion
work
commenced l a t e March 1979 and finished
i n
mid July 1979.
Most of th e
time lo s t in
th e overall dr i l l ing/
completion
programme was during th e
completion
phase. Delays during dr illing were l ar g el y
due to
mechanical
problems with dr illing
equipment.
The
major problem
during
completion
was with
o r ie n ta t io n o f t he t ub in g hanger. The cause
of the dif f iculties
la y in
a f au l t y design
o f tu bin g h an ge r r u nn in g
tool
which
fortu
n at el y was f air ly easily r e c t i f i e d
once
i t
ha d been i d e n t i f i e d .
In two
of
th e
wells
problems were experienced
with
downhole safety
val ves
when satisfactory
pressure
t e s t s
could no t
be
achieved.
In
b ot h c as es t h i s was a sc ri be d t o heavy mud
used during completion, s ettling ou t on
working p ar t s
and
preventing
correct
operation.
In th e case of one well i t was eventually
necessary to lock open th e tubing
retrievable
valve
and i n se r t
t he w ir el in e
retrievable
valve.
In the
other
case
i t
was
possible
to
retrieve
t he t ub in g
and re-run
with a
replacement valve.
Summing , up ,t he re fo re , t he wells were ready
to flow at th e end of 1979 i ns te ad o f May
1979 an d i f D r i l l Master had be en complete,
as
scheduled, i n September
1979
i n st a l l a t i o n
of th e
f i e l d
would
have
ha d to
have
been
delayed
to
permit completion
of the
well
programme.
D r i l l i n g
and completion in f act took months
longer
than
originally planned
representing
an overrun
of
approximately 15 Whilst
disappointing
a t
th e
time
th e
performance,
on r e f l e c t i o n ,
is
no t wholly unsatisfactory
bearing in mind th e following factors:
The d i f f i c u l t i e s
of dr illing in
th e
Buchan r eser v o i r
I t was th e f i rs t
time
deviated wells
ha d
been
d r i l l e d from a
f l o a t e r
in
th e North
Sea.
Deviations were up
to
45°
and d r i l l e d depths
as
much
as
3,500 m
11,500
f t
I t was t he O p er at or s f i rs t experience
of
su bsea co mpletio n s in
th e
North
Sea.
I t should be
noted
t h a t
th e delay ca n
be
ascribed
e n t i r e l y to mech anical p rob lems: des
p i t e th e d i f f i c u l t c on di ti on s t he planned
dr illing procedures proved t o t a l l y s a t i s f a c t o r y .
FLOWLINES
The dynamically positioned r e e l
ship
Apache
was used fo r laying twin 4
flowlines
to each
sa te l l i te well. I t also l a i d t he h y dr au l ic
control
umbilicals. Apart from tangling
th e
ends
of the
control
umbilicals,which
fortu
nately presented no
r e a l
problem,the flowline
laying programme was
extremely
successful.
Pr i o r
to laying the 4 flowlines, an
orthodox
la y
barge, Choctaw ha d b ee n used to th e
12 export l i n e from th e
template
to the buoy.
Apache should have
been
used bu t was
not
available
in
time.
Between th e
completion
of th e laying o f th e
12 l ine an d th e s tar t of th e flowline laying
th e LM
buoy was
i n st a l l e d ,
w it h o nly minor
problems.
Concurrently
with
th e buoy i n st a l l a t i o n Bred
ford Dolphin was stationed over th e template
carrying ou t
the
i n st a l l a t i o n of the manifold
base
an d
o f i n te r co n ne c ti n g
pipewor k between
th e Xmas
t r e e s
an d manifold
base.
The diving
programme was h ig h ly s u cc e ss fu l with
divers
a ve ra gi ng o ve r 22 hours on th e
seabed
ou t of
every 24 over a period
of
some 45 days. The
originally planned
work programme ha d
been
65 days.
The
f i e l d
construction work where
we ha d
ex
pected
p ro bl em s a nd
delays
went
very
smoothly
an d was highly
successful.
SUBSEA
EQUIPMENT
A
single
management contract was le t fo r th e
d e s i g n , f a b r i c a t i o n , i n s t a l l a t i o n
an d
commissioning
of subsea
equipment
which comprised-
Tubing hangers
Xmas
trees
Manifold
Production
r i ser including
tensioning
equipment
Wire
l i n e r i ser an d
BOP including
h n l i ~
an d
tensioning equipment.
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ful l saturation diving fac i l i t ies
three stage
separation
plant
The
fac i l i t ies
on
th e
platform
include
oily water t rea tment
plant
th e
fa ct th at
th e
work was
of
much
greater complexity and ext en t than en
visaged by any
of
th e part ies involved,
wireline
r i ser handling and tensioning
system
What
then
been
th e
problems which had le d
to a
conversion
programme originally
planned
to
o ccu py 8 mo nt hs
taking
months? The two
major
reasons ar e
-
At th e time
of
writing
th e
rig
is
scheduled to
leave this year,in th e f i rs t
week
of
September
w hich would
lead to oil
flow
a t th e beginning of
October
1980.
production
r i ser handling and tensioning
system
On
the question of space th e r ig
i s
now in a l l
areas somewhat cramped
and particular problems
have
been experienced
in
laying
ou t
th e
moon
pool. Space
h er e h as
been
taken
up
by th e
diving equipment which is separated from th e
rest of the
moonpool
by
a
f i re wall. In th e
space lef t we have had to accommodate th e
tensioning
arrangements for
th e production
r i ser ,
which
in i t s complete
form
will
have
19
separate
l ines, tensioning equipment for
guidelines, provide fac i l i t ies
fo r
running
and t ension ing the wir el ine r iser , for
storing
the
wirelineBOP
and for
th e
flexible connections
between r isers and
platform.
gas
l i f t
compressor with treatment plant
and
distr ibution manifold
this equipment
to be
installed af ter
two
years of pro
duction
duplicated flare systems for high, medium
an d low
pressure venting
metering skid
for measurement
of crude
o il won and saved
Very care fu l contro l
of
weight
has been exer
cised throughout design and
construction
to
ensure that
wei ght a nd
stabi l i ty
ar e wi
thin
th e limits
of
th e
rig.
Master,
namely
stabi l i ty,
load carrying
capacity, s p c i o u s n ~ s s are sat isf ied with
our judgement
regarding
th e f i r s t two
factors
bu t the
space
required
fo r a ll
th e
faci l i t ies
proposed on
th e platform
has proved
extremely
t ight
despite th e removal of almost a l l
dri l -
l ing equipment other
than
th e derrick and
dra w w orks .
t
is perhaps
now
worth recalling
th e
tech
nical reasons for t he s el ec ti on , o f Dril l
The wireline
r i ser
and BOP were constructed
and
made
available in time for th e perforation
of
t he wel ls . Despite the
sat isfactory per
formance
of
this equipment
in
these operations
adequate time
was
not g iv en to the design in
the
context
of
serv ice operat ions
from
th e
production platform. As currently designed
the
BOP
in particular, will
present opera tiona l
problems in
terms
of i t s bulk and weight.
Turning
now
to Dril l Master,
th e
rig entered
th e conversion
yard
in Stornow ay on
14th
October
1978
and th e
original programme
agreed by
a ll
parties ,
i e BP
th e
managing
contractor and
the
conversion contractor,
was for completion by th e end of May 1979.
At that time i t was planned that
construction
work on
th e template should
be carried from
the
r ig. t very qui ck ly
became
apparent
that
delays
in del ivery o f ma te ri al s, no t
helped
by
th e
transport
s tr ik e a t
t he beg in
ning
of
1979
precluded
any
possibil i ty of
the r ig being available for this work. A
separate
construqtionprogramme
on
the tem
plate as
r ef er re d t o
a bove was established
and
completion of Drill Master was rescheduled
for September 1979.
Design
and fabr ica tion o f
the
production r i ser
ra n many
months
behind schedule.
f
th e pr o
duction
platform
had been
installed
on time
lack of
vi ta l components in th e r i ser would
have
inhibited
the field
start-up. The
inter
facing of this
equipment
with
th e
r ig presented
problems w hich w ere
no t addressed
s oon e nough.
The
tensioning
arrangements,
t he f leXibl e
connections
are a l l
complex,occupying
c o n ~
siderable
space
which will make access to
the
moonpool,during
operations,difficult.
G iven more
engineer ing design
time i t is
conceivable
that a r adi cal ly d i ff e ren t so
l ut io n to
th e
production
r iser problem might
have
been adopted.
PRODUCTION
PLATFORM
The leSs orthodox items
of
equipment caused
problems. The subsea manifold despite i t s de
s ign being based
on
that of t he Argyl l in
stal lat ion presented design and fabrication
problems:
in
part.1.cular
diff icul t ies
were
ex
perienced in obtaining
a
sat isfactory pressure
test
on
th e
complex pipework. Consequently
this item
was
not
available for instal lat ion
during
th e
construction
programme. t was
in
fact
installed
by
th e
dri l l ing
rig
which sub
sequently continued completion work on th e
template wells.
The manufacture and supply
of
hangers and
trees
whilst
on a very
t ight
schedule kept
pace
with
th e
dri l l ing and completion
p r o ~
gramme.
8
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i e BP, i t s managing contractors and
th e
conversion
yard
2) low
productivity
in
th e conversion yard.
Deal ing with th e lat ter point f i rs t our ex
perience was
t h t d ~ s p i t e th e
r ig
being
inshore ,overa l l productiv ity of
th e
construc
tion
was
no
better than
that achieved
in off
shore construction
work.
Factors
contributing
to
th is were:
The
yard had to be remobilised
af te r
having been mothballed
for
some time
The rig
had to be anchored in th e
harbour
with access by ferry,not
directly
from
th e quayside.
Working
space
was restricted
limiting
th e amount of labour which
could be
used at
any
work face.
The
design
work was
inadequate
leading
to
many
clashes of different trades,
e.g.
piping with
H
V
electrical
cable pulling with pipework.
Turning
to point
1 i t
is
worth
l is t ing
some
of
th e areas where work scope
has been
greater
than anticipated.
1
The condition
of
ancillary
services
was
no t satisfactory. Considerable unscheduled
work
has
gone
into th e re sto ra tion to
satisfactory
workirtg condition
of
f ire
mains,
compressed
a ir
systems, cooling
water systems
and steam generation systems.
2)
Some basic
equipment
needed extensive modi
fication
to
bring i t into
line
with regu
la tions
promulgated
s in ce t he
building
of
th e
r ig.
A prime example
is th e
main
electrical
distribution
system
which whilst
originally
acceptable was
considered
unsaf e both by th e
certifying
authorities
and
by BP.
3
The extraction
of existing
equipment and
insertion of new
has
proved much more dif-
ficult and
more
time
consuming
than ex
pected.
This
stems from th e
integrated
design
of the Pentagone;
i t ha s been
neces
s ar y to
cu t
and
reweld
decks and walls
in many
areas in
order
to
remove and
ins ta l l equipment.
4
Safety
zoning
for
production
differs from
safety
zoning for
dril l ing.
Many areas
on th e r ig originally safe
ar e now
hazardous and in order
to
make
use
of
existing
electrical equipment a complex ,
highly instrumented
venti lat ing system
ha s been necessary.
5
To
meet
regulations relat ing to
accommo-
8
dation on
production
plat forms extensive
modifications
have been
necessary
including
th e installation o f s ev er al new
prefabri
cated modules. On
a
product ion pla tform
there
may
be no t more than two
men
per
cabin
compared
to
the
four
men
per cabin
permitted on a dri l l ing
rig.
Again this
had entai led substan tia l ancil
lary work in
th e
way
o f s er vi ce s
such
as
heating and v.entilating.
6)
The
complications in themoonpool area,
which
have already been touched upon.
7) A totally
unexpected problem
was
that an
inclination
test
carried
ou t
on
acceptance
of the
r ig indicated
th e weight
of
th e
s tructure to be approximately
100
tonne
in
excess
of the figure
provided by
th e
owners
and
which
had
been
used in th e design
of
th e
conversion. To
counter this
i t
was
necessary
to
remove
or move
some equipment
e.g. th e
potable
water
storage
tank had to
be
moved from
th e deck to th e base of
a
leg.
8
The
weight
of
P1Plng
instal led is over
100
tonne
greater
than
t hat o r ig ina l ly
estimated.
9
The
f lare
booms,
of
which
t he re a re
two
in
order
to
accommodate a l l wind
directions,
have turned
ou t to be
much larger and more
massive
than
planned in order to keep radi
a ti on l ev el s
a t
an
acceptable
level .
10
The
ballast ing system ha s had to
be
refur
bished,
including
re-commissioning
of the
control system.
11
Our
problems have
of course
been
further
compounded
by th e disaster to th e sister
vessel,
Alexander
Kielland,
which cap
sized
with
great
loss
of
l i fe
on
27th
March
1980.
As
a
resul t
of
that
incident
very ex tens ive i ns pe ct io n o f t he
r ig
struc
ture has been carried
out,
des ign rev iews
have been
made
to confirm
the
soundness
of the design, and a large number of modi
fications , a l l relat ively minor, e.g.
reshaping of
st i ffeners,
removal of
re
dundant penetrations, have been carried
out.
I t is roughly estimated that
this
workload
has del ayed tow-out
by some two
months and
increased
costs by
between
£7 - £8 million.
PRESENT
POSITION
At
th e time of w riting th e
Buchan
field
i s
expected to
be on-stream just
over
twelve
months
behind
th e
original
schedule, i e at
th e
end of 42 months
instead of 30
months.
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The
tota l capital cost of
the development
ha s increased by
almost ~ f t
Despite this
we
ar e
p le as ed t o
report
that , helped by
increasing
oi l
prices,
i t
is
s t i l l
an economic
development.
The
bulk of the delay and o f th e in crea se d
cost has
resulted from
th e problems
experi
enced
in
converting a d r il li ng r ig to a
production
platform.
CONCLUSIONS
Despi te our
somewhat bi t te r experience with
Buchan we
are firmly of
tl;J.e
opinion that
t he b as ic approach
to th e
development
is
sound
and
would
no t hes it at e t o adopt i t
again in developing a marginal
field.
There
are
lessons
of
c9urse)to
be learnt which
we t hi nk a re as
follows:
With increas ing ly s t ringent
regulations and
realis ing
more
fully now
the
comprehensive
differences
between dri l l ing rigs and
pro
duct
ion pla tforms
we
would
not contemplate
another
conve rs ion. Bea ri ng
in mind
that.
ou r
original programme was
extremely opti
mistic and that i t is unlikely that a develop
ment of this
type
can
be
completed
in less
than 36
months
i t
is
better in
a ll respects
to
build
a
purpose-designed rig.
Based on
Buchan
experience
a
purpose-built r ig
would
cost substantially
less than
a
conversion
and
could have
been completed
in
th e time
scale actual ly at ta ined.
t is th e
author s view
that to o great an
emphasis was
put
on th e
achievement of
a
short
programme which meant committing to
the manufacture
of hardware bef or e t he
com-
plet ion
of
adequate
studies.
A
period
of
say six months, a t th e beginning of th e
pro
ject devotedto
a
des ign s tudy of
a l l
th e
pro
blems associated
with
th e floating
production
platform,
would almost
certainly have high
lighted
some of
the problems which we hit
later
8
A
particular aspect of design of
cr i t ica l
importance which would
receive
much greater
a tt en ti on i n
a
future project is
the
co
ordination of design
work on
interfaces. E.g.
additional
time
spent
on
th e study of the
interface between th e subsea equipment and
th e
rig in th e
moonpoo1 would have been
well
justified.
The
author
would strongly
recom
mend on future
projects of this nature the
bui ld ing o f
a
scale
model
to
assist
in th e
design.
A
study of this nature
carried
ou t
ear l ier in th e
project might well have le d
to a radical rethink of th e
production
r i ser
design.
Apart from
th e specific point
raised
above
modelling
would have been
of
immense
help
in th e design of the conversion
and of
i t s
planning.
Time spent on this
in
th e early
stages
would have been most
valuable.
To
recapitulate
as
succinctly as
possible
on
t he l es sons we believe
Buchan to
have
taught
us
stop
and
think
before
finally
committing
to
a development programme
consider
and
evaluate a l l real ist ic options.
2.
Pay particular
a tt en ti on t o i nt er fa ce s -
ensure that as an Operator you have
fu l l
control of their
design.
3.
In
complex
design si tuat ions
make
us e
of
scale models.
4. Be
very
wary of major conversion projects
purpose-built
equipment i s
l ik el y t o
be
much more economic.
ACKNOWLEDGEMENTS
The author wishes to acknowledge BP Petroleum
Development
Limited
and
their
partners
in
the
Buchan
development,
namely
s t Joe Petroleum
Corporation,
CanDel Petroleum
Limited,
Natomas
International, Gas and
Oil
Acreage Limited,
Charterhall Oi l Limited,
Lochiel
Exploration UK
Limited, CCP
North Sea
Associates and
City
Petroleum
Corporation
fo r
permission to
produce
this paper.