The Buchan Field Development

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    1/8

     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.

      8

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    3/8

    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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    5/8

    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.

  • 8/9/2019 The Buchan Field Development

    6/8

    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

  • 8/9/2019 The Buchan Field Development

    7/8

    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.

  • 8/9/2019 The Buchan Field Development

    8/8

    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.