1992 Consideration for Design of Drilling Conductors for the New Generation of Deepwater Harsh Weather Jackups_WetmoreHalbleib

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IADCISPE
lADC/SPE 23856
Consideration for Design of Drilling Conductors for the New
Generation of Deepwater, Harsh Weather Jackups
S.B. Wetmore and B.L. Halbleib, Global Marine Drilling Co.
Copyright 1992, fADC/SPE Drilling Conference,
This papar was prepared for presentation et the 1s92 lADC/SPE Drilling Conference held in New Orleans, Louisiana, February 1S-21, 1992.
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ABSTRACT
As the next generation of harsh weather
jackups for the North Sea push the
operating water depth limits from 300 feet
(91m) upwards to 500 feet (150m), the
environmental criteria also increase. At
these more severe depth and environmental
conditions the increased dynamics of both
platform and drilling conductor will
increasingly affect drilling operations.
When drilling in water depths greater than
several hundred feet the conductor must be
tensioned to resist buckling and increase
its stiffness, the amount of tension being
proportional to water depth and
environmental conditions. The tensioning
device is usually placed at the top of the
conductor and just below wellhead/BOP
=~==.k# ~~~s creating a point of lateral
fixity. As currents and waves act on the
tensioned but unstayed conductor, the
wellhead/BOP stack rotates about the point
of support to suit the structural
deflection of the conductor below. At some
limiting sea state, the top of the BOP
stack no longer lines up with the rotary
table and drilling must be suspended. If
the top of the BOP stack is also laterally
fixed (or hydraulically snubbed), a bending
momement is induced in the BOP stack, which
under severe storm conditions, can exceed
the structural limit of the BOP stack
.-----~ :---GUJU8W!SLA“’,-.
Reference and illustrations at end of paper
A frequency domain computer program was
used to parametrically analyze the dynamics
of the system and to calculate the maximum
stress levels in the various components.
The paper examines over 75 combinations of
physical configurateion, water depth,
environmental criteria, drag coefficient,
conductor diameter, vertical tension and
joint stiffness. Appropriate graphical
representation of the results are
presented. Based on the analysis, the
authors conclude that the design of the
drilling conductor for winter storm
conditions in the North Sea and for water
depths greater than 300 feet will be
significantly more complex than present
shallow water designs. An ongoing
analytical program is postulated to study
composite configurations, fatigue effects,
and drilling downtime.
INTRODUCTION
The first units of the new generation of
deepwater, harsh environment jackups has
entered the North Sea market and in so
doing has extended the water depth
limitations, for year round operation, out
to 360ft (llOm). Other mobile harsh
weather jackup type units with water depth
capabilities of 500ft (150m) have been
proposed. The deeper water depths in the
North Sea are generally coincidental with
mnwe. ---- - n=~~&r~y ~~~i~~d~~ and harshEtg
environmental criteria. This double jump
in both water depth and weather criteria
should be cause to re-examine all facets of
137
drilling from a jackup platform as the
extrapolation of practices previou~iy u~ed
in Sh~~~QWer: more benign? areas is not
necessarily linear. Figure 1 graphically
portraye the general trend of increasing
water depth and wave criteria with
increasing latitude in the North Sea.
Table 1 shows the same data in tabular
form ● The particular aspect of jackup
drilling discussed in thie paper are the
structural design considerations of the
well conductor, wellhead, BOP stack,
tensioning system, and the internal
casings. To analyze a reali8tic model of
the system, the well proven D.E.R.P.
computer program (References 1-3) was
utilized. This program accounts for both
the static and dynamic aspects of the
system and has been proved in many simiiar
analyses. In the subject analysis, the
program was used to parametrically study
the effect of a large number of variables
in the system.
The intent of the subject analytical
program was not to formulate the detail
design of a drilling conductor and casing
system, but to provide the upper bound of
tension and lateral load inputs into the
design criteria for a new harsh weather
jackup, the Glomar Gibraltar Class
(Reference 4). The Glomar Gibraltar Class
is based on the basic Marine Structures
Consultants (MSC), CJ-62 design. This unit
must have the capability to drill in 360ft
(llOm) of water depth anywhere in the North
Sea on a year round basis and in up to
almost 400ft (120m) on a seasonal basis.
The Glomar Gibraltar Class jackup would be
*nmWem-m+n+{we of*-K. -w...---.- th~ n~w generation of
North Sea rigs presently being deployed in
the North Sea and the analytical
considerateions herein are considered
generic to this class.
BASIC HARDWARB SYSTBM
A cantilever jackup usually operates in one
of three drilling modes --- drilling
multiple wells over a platform, drilling
single exploratory w~iiB, Gr d=iiiiii~
multiple wells over a subsea template. The
latter two cases are assumed to utilize a
mudiine sus~nsion uy=tem with ~==ifi~ ti~”
backs to a surface BOP eystem. A typical
well program in shallower water utilizes a
30” conductor pipe either drilled or driven
into competent soil and is unstayed and
free standing from the mudline to the rig’s
cellar deck or drive pipe support platform.
In deeper water, axial tension must be
applied to the conductor to alleviate
buckling problems in the lower portion of
the conductor and some form of horizontal
support at the cellar deck level is used to
mitigate lateral motions of the conductor’s
top end. A 26” hole is drilled through the
COiX51GCftG= Io~“dsiig z p~~g~~re diverter
mounted on the conductor as protection
against low pressure, shallow gas biowouts.
For wells in deeper water depths, the
casing weights dictate the use of a mudline
suspension system such that the entire
weight of the uncemented casing string can
be supported at the mudline suspension
hanger and not from the wellhead at the top
of the conductor. The lower end of the
tie-back casing is then anchored at the
mudline suspension hanger, pretensioned
slightly with the blocks, and seated in the
wellhead casing hanger at the rig’s cellar
deck level. In this manner the wellhead
and conductor only supports the weight of
the casing from the wellhead to the mudline
and the siight ~E=t=FISiGis. mace ~~,e 2QW
casing is run and cemented and the 20”
wellhead attached, a 21-1/4” 5000 psi BOP
stack is attached, (flanged or clamped) to
the wellhead and a 17” hole is drilled for
the 13-3/8” casing string. After the 13-
3/8” ,casing is run and cemented, the 21-
1/4” BOP stack is removed and a 20” x 13
5/8” crossover spool is installed. A 13-
5/8” 10,000 psi (or 15,000 psi) BOP stack
is then attached to the new wellhead. The
remainder of the well is drilled and
completed with this arrangement.
Although unitized wellheads have become
more popular with jackup drilling, the
prevalent practice in some areas is to
still utilize the two separate BOP stacks.
The next logical advancement in drilling
efficiency will be to utilize the unitized
wellhead in combination with asingle, 18-
3/4” 15gO00 psi BOP stack, similar in
concept to most subsea drilling
applications. Such an arrangement
eliminates many of the time consuming BOP
handling steps and opportunities for
accidents and mechanical problems. Once
the 18-3/4” BOP is set on the 20” unitized
wellhead it never needs to be removed until
the well is completed. If properly
arranged, the choke, kill and BOP control
hoses never need to be detached from the
m a “- “1. +.. .+=.k i= m~~=~ between theDC= -&a&= as ~,,=.b=w
test stump and wellhead and vise-versa.
The Glomar Gibraltar Class jackup utilizes
stick~m arzarxgarwmt.
As jackup operations move into deeper
waters, greater tensioning of the conductor
becomes necessary to prevent excessive
stresses and deflections. Depending on
water depth and weather criteria, it might
also be necessary to provide a system of
lateral restraints to the conductor and BOP
stack. Preliminary design considerations
for these potentially large and critical
items of hardware are developed in the
following sections.
138
.
ANALYTICAL PROGRAM
The authors initiated a broad brush
parametric analysis of the physical
hardware in order to gain an underntan~ing
of the system’s dynamics and the resultant
stresses and motions. The D.E.R.P. riser
analysis program (Reference 5) was used for
the analysis. This frequency domain
program was initially developed in 1978 and
later refined in 1981 for floating drilling
ricer operations. It has also been used
extensively for parametric analyses of
tendon and production risers for tension
leg platforms. The use of a frequency
domain program provides a cost effective
method of examining the parametric effects
of a number of variables in the system.
Over 75 individual cases were examined for
a range of water depths from 250 to 500ft,
(76 to 152m), three storm conditions of
98.4, 82.0, and 49.2ft (15, 25 and 30m)
maximum waves, two conductor diameters of
30” and 36”, two drag coefficients of 0.7
and 1.0, six tension levels of O to 850
kips (O to 385T), and two top end
conditions i.e. fixed and free. In
addition, several special cases of varying
lateral stiffness, BOP weights and sea
spectrum inputs were studied. Table 2
indicates the major cases studied.
Comvuter Model
Figure 2 indicates the two physical
arrangements that were modeled. The two
conductor sizes modelled were 36” diameter
x 1.5” wall and 30” x 1.5” wall. The
tensioner support point was placed 101.7ft
(31m) above the water line in all cases.
The model also included a lateral spring at
the tensioner level whose stiffness was
assumed to be 1.0 x 10IOlb/ft (5.34 x 10°
N.In/m) i.e. very stiff. Each conductor
included the contributory stiffness of a
20”- 1331b/ft casing that was assumed to be
concentric and mechanically centralized
within the conductor. The 20” wellhead and
18-3/4” 15,000 psi modularized BOP stack
was placed atop the conductor tensioner
support point with an assumed stiffness of
100 times greater than the conductor. The
conductor tensioner was laterally and
vertically supported at the rig’s cellar
deck. The weight of the BOP stack and
wellhead was assumed to be 300 kips (136T)
evenly distributed over the 28.4ft (8.66m)
height of the stack. A mathematical ball
joint was placed at the top of the annular
preventor on the BOP stack. A 6.Oft
(1.83m) long slip joint assembly attached
the ball joint to the diverter housing
which in turn was fixed to the rig’s rigid
subbaee. The inner barrel of the slip
joint was assumed to be a 25” OD x 2.5”
wall ‘=== *(Vaa C5 c...Cw.aa; &&~= and the =~ter
barrel was a 31” OD x 1.75” wall (788 x
44.5mm) tube with its upper end rigidly
fixed to the diverter housing. The
distance between the rigid diverter housing
~n~ the .-.A,,~+nv teaSiQner support point““..--”.-.
is thus 34.4ft (10.49m).
The foregoing mechanical arrangement wo;ld
normally be used during drilling operations
so that the BOP stack is forced to stay
aligned with the rotary table. The
analyses were intended to determine the
reaction forces at the tensioner level and
at the ball joint level. Casee 1 through
11 and Cases 15, 16 investigated parameters
in this fixed-end condition. If lateral
forces became too high, drilling would have
to be suspended and the slip joint
disconnected, thus allowing the BOP stack
to rotate about the tensioner support point
and to assume the end slope of the now
simply supported upper end of the drilling
conductor. To study the extent of the
conductor BOP motion deflection, and other
parameters of this free end condition,
Cases 12, 13, 14, 17, 18 were examined.
Environmental InDutS
The frequency domain D.E.R.P. program makes
the following major assumptions:
1.
2.
3.
4.
5.
6.
w1.
8.
It
The displacements and resulting
ang~e~ are small enouqh so that sine
= o, t208e = 1, and & is negligible
in comparison with unity.
Cross sections perpendicular to the
axis of the riser which are planar
before bending remain planar after
bending. This is the normal
assumption made for small d=fl~CtiO~sS
of beams.
The material is linearly elastic.
Rotary inertia is ignored.
Tension is constant in time.
Current is lm/sec at the sea surface
and 0.25 m/see at the seabed level.
AtidSdmass CCeffiCiellt i!? 2.0
Drag coefficients were 1.0 or 0.7
is well Proven that the DhWical sea
condition in a typical s<or% can be
reasonably approximated by combining a
finite number of harmonic oscillations of
smaller amplitude. The Pierson-Moskowitz
wave spectrum is used to exercise the
D.E.R.P. program for each of the component
waves over a epecified frequency range.
This pr~cess defines 30 or less component
139
waves which are each run past the
conductor. The responses of the riser in
each case are stored and then statistically
manipulated to determine the most likely
response during a given storm CXXM5iti3ii.
The storm wave input is defined by
specifying a maximum wave height (crest to
trough) and kne corresponding exeitatiGfi
period. The three storm wave conditions
studied were:
1) 98.4ft (301n)13.9sec;’
2) 82ft (25m) 12.36sec;
3) 49.2ft (15m) 10.81sec.
These are referred to in the text as the
-n- *c-SUln, 4 alu, mmmr14 + 4nnm.arlcl 15rc atmm “------------ .
~ cal esults
Figure 3 depicts the maximum dynamically
deflected shape of a 36” diameter conductor
in 300ft and 400ft water depths while under
the 98.4ft (30m) storm condition (cases 9
and 12). Note that in the non-restrained
(free-ended case) the maximum lateral
offeet at the top of the BOP stack,
relative to the rotary table, is
substantial. It should also be remembered
that this offset is cyclical and almost
completely reversible. i.e. back and forth
motions in step with and in the direction
of the waves.
Figure 4 depicts the bending moments in the
conductor for the same conditions. As in
the previous figure the top end constraint
greatly influences the shape of the moment
curves. However, of more importance are
&l....n.”-;+..Aes!=mayUALuus of ~p,= g~~,~=~ mnman+ ~altqom.... .... . . “ ---- ●
If the BOP stack is held rigidly in the
vertical position the bending moment
throughout the wellhead and into the stack
may well exceed the strength and sealing
capabilities of the various BOP component
connections. Conversely, if the BOP stack
is not held rigidly, the free ended motions
of the BOP are intolerable for ciriiiing.
These results beg the question of: “Why
haven’t these symptoms shown up before
now?” Two major reasons are: 1) shallower
water depth and 2) the correspondingly
decreased environmental criteria unique to
the North Sea. The majority of jackup
drilling in the North Sea has not exceeded
250ft (76m) of water depth in winter or
over 300ft (91m) in summer. Referring
again to Table 1, the maximum winter storm
(50 year returnj in 25C)ft (76m) Gf water
depth (Latitude 56°N to 58”N) has a maximum
wave height of approximately 82ft (25m) and
a typical winter storm (5 year return) has
a significant wave height of 69ft (21m).
To provide a realistic baseline of data
under these conditions and at the 250ft
(76m) waterdepths, the physical model was
changed for cases 17 and 18 to utilize a
30” diameter conductor, a lighter 13-5/8” -
15000 psi BOP stack, and only 440 kips
Gf tt%I15iCP..(2COT; ~~e ~op SW@ Qf kheBOP
stack was allowed to be free. The
statistically significant motions (average
-- h4-hn.+ n.n ~~i=~) at ~~e ~Q~ Of the BOPUL L.Ay..-raw v..”
stack were i 10” (254mm) for the 50 year
storm and only i 6.5” (165mm) for a close
approximation to the five year storm. Many
observers are familiar with this magnitude
of motion, thus giving reasonable validity
to the model.
By increasing the water depth from 25C$ft
(76m) to 300ft (91m) the corresponding
significant motions increased to * 15” and
i 9.7” (381/mm and 246/mm respectively).
The statistically maximum excursion, for 1
in a 1000 waves (about every 4 hours), is
1.86 times larger than the significant
motions cited. During actual drilling
operations in 250ft (76m) water depths, the
range of motions is usually accommodated
without restraining or snubbing the top of
the BOP stack. Since very little winter
season exploratory drilling has been done
by jackups in 300ft (91m) water depths, the
more severe motions have not been
experienced. The motions for the foregoing
conditions cited are summarized in Table 3.
Moving into deeper water of the North Sea
and the correspondingly harsher environment
causes the free ended motion of the BOP
stack to dramatically increase. For these
conditions the model conductor was
intuitively increased to 36” diameter and
the CQndUCtor ten~ion increased to 550 kips
(250/tonnes). At the 360ft (llOm) design
water depth of the new generation of
jackups, the maximum wave height for the 50
year storm is 98.4ft (30m) and 82ft (25m)
for the common 5 year storm (reference 6).
If left unrestrained, the significant
motions of the top of the BOP stack would
be + 26= (6.’i2mm) afid S 23.5” (5981MT,j fcr
the 50 year and 5 year storm respectively.
The corresponding maximum motions would be
i 49.2” (1250mm) and i 43.8” (l13mm).
Figure 5 indicates the range of maximum
motions of the top of the BOP stack when
the conductor is free-ended.
From this comparative analysis, it becomes
obvious that some method of restraining the
top of the BOP stack is necessary to keep
the motions within acceptable limits for
A..:ll;nm
u&&&A&Lby. Ek%+’evex, as ee9p. in ~igu~~ ~f
the consequences of restraining the BOP is
to change the shape of the bending moment
curve and to induce large bending moments
into the BOP stack and upper end of the
conductor. Restraint also induces iarge
lateral forces into the rig’s cellar deck
140
.
and substructure due to the generated
couple. The magnitude of these lateral
forces is on the order of several hundred
kips but iS considered structurally
manageable. The magnitude of the bending
moments induced in the upper end of the
conductor, including the resultant streaaee
in the 20” casing, the integrated wellhead
and in the BOP stack elements dictate a
more rigorous stress analysis of these
elements as part of the development effort.
Since the majority of this stress is cyclic
bending, fatigue considerations in both the
20” casing and the conductor also warrant
further study.
The following paragraphs discuss the
specific results of the analytical effort
in the establishment of criteria for:
1) Selection of conductor diameter
2) Conductor tensioner capacity
3) Lateral restraint capacity
4) Bending stresses in 20” casing
5) Bending moments in BOP stack
Smecific Umner Bound Desiun Results
By understanding the general tendencies of
the typical results, an upper bound case
was developed to determine a reasonable
range of design inputs for the new
generation jackup. The specific case
selected for design specifies a water depth
of 400ft, with 50 year return winter storm
conditions above latitude 61°N in the North
Sea. The specified storm condition thus
had maximum waves of 98.4 feet(30m) and a
predominant excitation period of 13.9
seconds. Superimposed on this wave
condition was a surface current of 1.94
knots (lm/see) decreasing linearly to about
0.5 knots (0.25m/see) at the seabed. A
drag coefficient of 1.0 was used as an
upper bound where it impacted the upper
limit for design although 0.7 is probably
more realistic in practice. The weight of
a 20” unitized wellhead and a modularized
18 3/4” - 5000 psi BOP stack was assumed to
be 308 kips (140T).
Conductor Diameter Selection
As stated previously, the traditional
drilling conductor is a 30” diameter tube
with a 1.5” wall thickness. From past
experience and the typical analytical
results cited earlier, it is recognized
that the top of the BOP stack must be
restrained in order to keep it vertically
aligned with the rotary table. Table 4
summarizes the data for the 30” conductor
in 400ft (122m) water depths under the
specified conditions (cases 1-4). It iS
apparent that a 30” conductor cannot
survive without a minimum level of tension.
Even with 550 kips (250T) tension in the
conductor, the maximum cyclic stresees in
the riser and 20” casing are substantial.
It should be noted that these maximum
weather conditions will exist only during a
small fraction of the drilling time and
that the statistically significant stresses
will only be about one half the maximums
cited.
Use of a 36” diameter, 1.5” wall conductor
was examined under the same conditions
(Cases 5-8) and the results showxi in Table
5. Comparing the results shown in Tables 3
and 4 for the 30” and 36” conductors, it is
seen that the induced bending moment from
the 36” conductor is somewhat higher (17%
to 32%) because of its greater stiffness.
The stresses in the 36” conductor are
slightly lower (5% to 15%) depending on the
riser tension, and the stresses in the 20”
casing are significantly lower (20% to 30%)
when using the 36” conductor. The maximum
deflections at mid-span and the stresses at
the mudline are also reduced when using the
larger conductor.
As a prudent design measure it was decided
that the conductor tensioner system would
be configured such that it would have a
redundancy factor of 2 and also be able to
maintain a minimum capacity of 330 kips
(lSOT). At this minimum condition, the
maximum stresses in the 36” conductor and
casing are still within acceptable limits
for a short term drilling situation.
Selection of Conductor Tensioner Camacitv
The previous section established that the
minimum tensioner capacity should be 330
kips (150T) when using a 36” conductor
under the assumed conditions. However, the
maximum tensioner capacity has a direct and
inverse impact on the combined cantilever
ioaci raking of the jackup ~r,~ -..e+s,,--*
therefore be chosen judiciously. It can be
seen that higher tensions result in reduced
moments and stresses in the conductor, but
there are practical and economic
limitations. A summarization of various
critical parameters as a function of
conductor tension, as gleaned from cases 5,
6, 7, 8, 10 and 11, are shown in Table 6
and depicted graphically in Figure 6. It
is apparent from Figure 6 that a point of
diminishing return is to be found in the
tension range of 550 to 700 kips and that
greater increases in tension do not result
in proportionally decreased stresses and
AaGlamt4m”8.U=&A=W.*”.. Wnv.-- cmnduc~~r deSiQn
purposes, the lower figure of 550 kips
(250T) iS suggested for the normal
operating condition and should be the load
which is used in the jackup rating
equation. For the design of the riser
tensioner unit itself and its interface
141
..,,
with the supporting structure, a larger
maximum capacity is recommended.
Sizinu the Lateral Restraints
As discussed previously, drilling in
shallower waters and in less severe sea
condition, results in small lateral motions
at the top of the BOP stack which can
usually be managed with little or no
restraint devices. Restraint devices that
have been used in the past were simpie and
rudimentary at best. No hard data is
available that documents unacceptable
lateral motions or forces that can be
correlated to weather conditions. Varioustugger wires, chains, screw jacks and the
like have been used to provide effective
restraint and/or snubbing of the motions.
However, in one instance a 1-1/4” (38mm)
diameter wire rope was reported to have
parted, but there was no other information
on the point of failure, condition of the
wire, etc. Other factors that might cause
differences between the analysis and the
physical hardware are: 1) the conductor
support platform (or cellar deck) for
exploratory wells is usually very near the
vessel transom and therefore can absorb
more load, and 2) the vertical distance
available to restrain the forces of the
couple is usually somewhat larger than the
28.4ft (8.66) used in this model.
An analysis of the unrestrained 36”
conductor motions in 250 and 300ft (76 and
91m) water depths, with the 82.0 and 49.2ft
(25 and 15m) storms , yields almost
ide~a++~=1 I-nalll+a.*”-* .-” . . ..- ~~ fQr &he 30”
conductors. Although the added stiffness
of the 36” conductor would intuitively
yield smaller motions, the added diameter
increases the deflecting forces. Figure 5
graphically illustrates the maximum cycii~
offset of the top of an unrestrained BOP
stack for various water depths and storm
conditions. From this data it is
reasonable to conclude that the top of the
BOP stack does not need to be restrained
when using a 30” or 36” conductor in 250ft
of water or less in the winter, or in 300ft
of depth or less in the summer.
However, because the excessive lateral
cyclic motions at the top of a free-ended
EOP stack -----Would a..t7l:w-prizcliide u&&A&&aay
operations in deeper waters, restraint
devices should be provided in the design.
The total horizontal force on the conductor
due to current and waves alone is resisted
proportionately at the mudline and at the
rig’s tensioner platform or cellar deck.
The lateral environmental force at the
tensioner for various water depths and
storm conditions are shown in Table 7. The
lateral environmental force at khe
tensioner “is small compared with the
concurrent force induced by the restraint
couple.
The magnitude of the forces necessary to
induce the necessary couple is a function
of the wave induced bending moment in the
conductor and the vertical separation
distance of the restraining forca ‘@ifitf5.
For analytical purposes, it was assumed
that the lateral restraint points were at
,the tensmner fWppOrG LALLY WLA -SS= -O..--.w.-- -~ -: -- -- bk- - *A.,m+fi*
and at the ball joint just above the
annular element of the BOP stack, a
distance 28.4ft (8.66). (See Figure 2)
The maximum lateral forces at the ball
joint and at the tensioner for various
water depths and storm conditions are shown
in Table 8. The force at the tensioner
includes the concurrent environmental
forces from Table 7. The magnitude of
these restraining forces (50 to 250 kips)
dictates that thorough engineering and
design considerations be targeted for these
devices and their interfaces with both the
BOP/Diverter units and the rig’s structure.
Sizinu the BOP/Wellhead Connectors
Due to the necessity to physically restrain
the BOP stack near the vertical position,
the bending moment induced in the conductor
must be removed between the two lateral
restraint points. Figure 7 indicates the
placement of these forces and the
distribution of the moments in relation to
the BOP stack. The maximum moment for the
most severe storm condition is plotted for
operations in 300ft (91m), 350ft (107m),
and 400ft (122m) water depths. Assuming
that the BOP/Wellhead is a rigid body, the
moment is linearly reduced to zero at the
upper restraint point, i.e. the ball joint.
ML & ha rlmww4aAby~-ne Msfidhg fiIctMrk tlmt mzueb -= -S..-S=
each connector in the BOP stack is a
function of the exact vertical stack-up
dimensions of the tensioner, wellhead, and
BOP elements. The stack-up shown in Figure
7, is reasonably to scale and considered
representative of a 20” unitized wellhead,
hydraulic wellhead connector, an 18 3/4” -
15,000 psi four ram modularized BOP,
annular BOP, and a fixed diverter. Table 9
summarizes the maximum moment induced by
the 98.4ft (30m) storm waves through the
wnnnnm+nl.avar~cus “-------“--- and fQr va~i~u~ watez
depths.
The seal data for the 18 3/4” flange, shown
as an inset bar graph on Figure 7, was
excerpted from reference 7. Although the
18-3/4” API flanged connections are
structurally adequate, it can be seen from
Figure 7 that the 6BX steei seaiing ~iii~
will allow leakage or be damaged if the
142
.
SPE 25856
bending moment is combined with internal
bore pressure. The use of a recently
developed flexible seal element in the
flange in place of the 6BX ring would be a
suitable solution based on data also
presented in reference 7. However, it can
be inferred that use of a smaller BOP stack
with the same conductor/wellhead
arrangement would have even less moment
carrying and sealing capability.
20” Casina Stresses
The structural integrity of the internal
casing strings is of primary concern in any
well - especially in the presence of
cyclic, stress reversing bending
conditions. Utilizing a 36” conductor in
place of a 30” conductor attracts more
environmental load and imparts more bending
moment into the BOP stack, but the larger
conductor also attracts a larger portion of
the bending stress in the concentric nest
of tubes. The maximum bending stress in
the internal strings is proportional to the
ratio of the diameters. The conductor is
not intended to take internal pressure so
its major loads are the axial tension
applied by the tensioner and the bending
moments induced by the environmental
forces. Table 10 tabulates the maximum
combined stresses in the 36” riser for
various storm conditions and in various
water depths.
It is assumed that a mudline suspension
system will be used during drilling and
that only the portion of the 20” casing
between ~h~ mudline and the tenSiOller ‘ill
be supported from the wellhead in addition
to a slight pretension (20,000 kips). The
maximum internal pressure anticipated in
the 20” casing is 2,000 psi. The stresses
in the 20” casing caused by bending,
pressure, and the pretension are tabulated
in Table 11. If one assumes that a P-11O
grade casing steel is used, the maximum
stress ranges from 30% to 38% of the yield
stress of the material. As noted
previously, the number of storm stress
cycles associated with a typical drilling
program is very small and the superposition
of maximum BOP closure pressure coincident
with such storms is even more remote.
However, the levels of stress do indicate
that a more rigorous fatigue analysis,
based on known weather histograms for the
area would be warranted. The relatively
high levels of stress also dictate that
strict quality control and inspection
procedures should be exercised in selecting
the caeing string, especially in the
critical joints subjected to bending
moments.
Conclusions
*
*
*
*
*
*
*
*
*
The new generation of deep water,
harsh weather jackups should be
capable of drilling out a subsea
template and be able to adequately
support the drilling conductor
anywhere in the rig’s drilling
pattern.
Generally, as jackups move to deeper
waters in the North Sea, the weather
criteria also increases.
The lateral motions at the top of an
unrestrained BCP -A.-i.mcuGK baeoime
excessively large to maintain
drilling operations in water depths
greater than 250ft in winter and
300ft in summer.
The conductor tensioner and BOP stack
restraint devices will become
significant pieces of equipment and
the substantial anchoring forces on
the rig’s structure must be
accommodated.
The conductor bending moments,
deflections, and stresses are all
reduced with increased conductor
tension, but reach a point of
diminishing return at effective
tension levels of 250 to 400 kips
(115 to 150T) greater than the weight
of the BOP stack and wellhead.
A minimum tensioner design criteria
should be to provide a minimum of
support equal to or greater than the
weight of the BOP stack and wellhead.
A 36” conductor is more suitable for
operationin deeper waters as it
lowers the induced bending stress in
the 20” casing.
In view of the high cyclic bending
moments induced in the system by the
restraint devices. all elements of
the wellhead, wellhead connector, BOP
stack, and diverter should be
reassessed for structural adequacy,
sealing capacity, and fatigue
resistance.
A program of gathering quantitative
stress/motion data from existing
deepwater jackup conductors shouldbe
undertaken to confirm the analytical
model used and to corroborate the
analtical predictions.
143
References
1.
2.
3.
4.
5.
6.
7.
Young , R.D., Fowler, J.R.;
“Mathematics of the Marine Riser”,
Presented at the Energy Technology
Conference and Exhibition, Houston,
Texas, November 5-6, 1978.
Young, R.D., Fowler, J.R., Fisherq
E.A. , Luke, R.R.; “Dynamic Analysis
as an Aid to the Design of Marine
Risers”; Journal of Pressure Vessel
Technology, Transactions of ASME,
Vol. 100, May 1978, pp 200-205.
Young, R.D.; “Methods of Analysis for
Marine Ricer Design and Operations”;
Presented at the 37h Petroleum
Mechanical Engineering Workshop and
Conference, Dallas, Texas, September
13-15, 1981.
“Glomar Gibraltar Class Jackup”;
A---- T-A....&”..Wuw=lb~41uu=b&x,October, ~~~~t nn 11-== .-
32.
Young, R.D., Miller, C.A., Fox, S.A.;
“D.E.R.P. Users Manual”, Stress
Engineering Services, 13800 Westfair
East Drive, Houston, Texas.
“Offshore Installations; Guidance on
Design Construction and
Certification”, Fourth Edition, 1990,
U.K. Dept. of Energy, Petroleum
Engineering Division, London
“Capabilities of API Integral Flanges
Under Combination of Loading”, .
Project No. 82001; Report No. 90-
61019 submitted to API Production
Dept. - End Connector Task Group by
Venitas Marine Services (USA) Inc,
Houston, Texas, December 1990.
144
$PE 23856
Table 1 General Trends in North Sea
Betwaen 0° and 4° E
IATITUDE WATER DEPTH 50 YEAR WAVE CRITERIA
Degrees N Matars Significmt - m Wlnwm - m
54- !55 20-40 10-11 18.5-20.5
55-56 40-80 11-13 20.5-24
!56-57 60-;00 12-74 9*3-9R66. ” - -“
57-58 60-100 14 26
58-59 100-120 14- 14.5 26-27
59-60 100-120 14.5 -15.5 27 -28.8
60-61 100-150 15,5-16 28.8-30
LATITUOE - “ N
FIGURE 1 - GENERALIZED TRENDS IN NORTH SEA
BETWEEN O ● AND 4’ E
TABLE 2 - DRILLING CONDUCTOR CASES STUDIED
Table 3 BOP Motions - Non Restrained BOP
E
CASE
17
17
18
18
WAVES
25m
25m
15m
15m
DEPTH MAX MOTION SIGNIFICANT MOTION REMARKS
* inches * inches
250 I 19“ I 1o“ I (50yearl Winter
Storm
300 29” 15“ Beyond Experience
250 12“ 6.5’ (5 year)Whter Storm
300 I 18“ I 1o“ Max Summer Storm
,
145
..-,,-- -.,-.–-––. —.” —.. ,A—
LOCATION
Bending Moment at Tensioner
Bending Stress at Tensioner
Bending Stress in 20” Casing
Maximum Deflection in Middle
Bending Stress in Middle
Bending Moment at Mudline
Bending Stress at Mudline
Table 5 36” Diameter Results
TENSlONER LOAD
ml—INo I --A v
I
aau n I A An V:-. I CEA I/:R.Uuw nlpa I
Tensioner (150 T) I ~OOm~)= I (250 T) I Units
54.8
637,000
425,000
239
701,000
64.9
754,000
7.4
86,100
57,400
21.5
55,200
6.77
78,700
6.12
71,100
47,400
~6.0
39,400
5.23
60,800
5.22
60,300
40,200
12.!5
30,100
4.24
49,400
x loeft#
psi
psi
ft (m)
x lo6ft#
LOCATION I TENSlONER LOAD I
I NoTensioner
Bending Moment at Tensioner
Bending Stress at Tensioner
Bending Stress in 20” Casing
Maximum Deflection in Middle
D#.-A:m” C*.,... - :- hA:AAla
DGIIUII Is QLl caa Ill IVIIUUIG
Bending Moment at Mudline
Bending Stress at Mudline
14.50
120,500
66,900
29.1
~ f 4J-)QQ
14.30
118,800
Table 6 Variation of Parameters vs.
Applied Tension using
36” Conductor
h 1 , m , m
I 5 I o 117,s00I 291I ‘*6WI 118*200
I 6 J 330 I r3%400 I 14.0 I 41,900 I 61,200
I 7 I 440 I 61,300 I 11.7 I 34,000I 51,800
I 8 I 550 I 55,700 I ‘“’I 27”’WI ‘“7W
I
10” 700
I
49,200
I
8.1
I
23,300
I
37,400
11* e50
I
45,300 8.8
I
20,500 I 33,700
330 K
I
440 Kips
(150 T) (200 T)
8.7
72,100
40,000
14.0
~~,~~~
7.41
61,500
7.67
63,700
35,300
11.6
A 1 annT ,, ”””
6.32
52,400
550 Kips
(250 T) I Units
6.88
57,100
31,700
9.9
~~,~o~
5.51
45,700
x lo6ft#
psi
psi
ft
@
x lo6ft#
txi
,
● Adjusted from Cd == 0.7 to Cd - 1.0 for consistency.
146
.. ..
Table 7 Horizontal Force at Tensioner Due
to Wave/Current Loads Only
DEPTH MAXIMUM STORM WAVE HEIGHT
Feet 30m 25m 15m
Case 9 Case 15 Case 16
250 37,900 nla 18,300
300 44,900 37,800 23,400
350 52,500 44,900 27,700
400 60,000 51,400 31,800
500 74,100 nla 36,800
Table 8 Horizontal Force at Tensioner and
Balljoint when BOP is Restrained
AT TENSlONER I
DEPTH MAXIMUM STORM WAVE HEIGHT
feet 30m 25m 15m
250 136,600 nla 65,000
300 171,500 145,300 88,300
:: I :;; I:;gl ::;’:
*VU 94Q, [U
500 353,600 nla 176,500
AT BALWOINT
250 98,700 nla 46,700
300 126,600 105,500 64,900
350 159,100 134,900 83,200
400 195,100 166,400 103,300
500 279,500 nla 139,700
Table 9 Maximum Moments in BOP
Connections due to 30m
Storm Waves
(Moment = FT LBS X 1,000,000)
LOCATION I 300 * I 350 * I 400 ft*
At Tensioner
In Wellhead
Wellhead Conrwotor
I:l:;l:;
Upper Ram to Annular I 0.85 I 1.05 I 1.30
Annular to Balljoint I 0.15 I 0.17 I 0.20
Table 10 Maximum 36” Conductor
Stresses - PSI
I DEPTH I MAXIMUM STORM WAVE HEIGHTI I I
I
FEET I 30m I 2Sm I 15m I Cdm
I
1 250 I 24,300 ! nla ! I
I 300 ! 30,900 I 26,s00 I I
1 350 ! 38,500 I 32,S00 I 20,700 ! 1,200 I
1
400 ‘
----- 1
40,000 ‘
I. . . . . . - I
47,0W Lo,- 1,4W
500 66,900 nla
Table 11 Maximum 20” Casing
Stresses - PSI
1DEPTH MAXIMUM STORM WAVE HEIGHTI I n I
FEET I 30m 1 25m I 15m I Cdm
I 250 ! 28,900 I da I 22,100 I 1e,ooo 1
1 500 I 53,500 I da I 36,200 I 16.s00I
147
-4E- ,mNovclm Im9cnER
t
101.7
#hJ
W, WAVE HEwtT
1
/1+&E—row,FORcssWmER OEPnl
FREE END CONDITION FIXED END CONDITION
FIGURE 2 – CONDUCTOR MODELS
20’ 15’ 10’ 5’ 0
TENSK!NER TOP OF BOP
400’ W.D.= ,~’
500’
400’ W.D. CASE \ ,/’
I
I s
=>. Tk-;ow? TOP OF BOP
. .
/ 400’
/ /
/
RJ%TRAINE ) >J
/
/’ /’
300’ WL
I
/
—
\ TOP
\
\
, RESTRAINED
\
\
\
\
\
\
\
\
\ \
\ \
\
\\’,
\,\,
\
MUO LINE,
200’
100’
FIGURE 3 – DEFLECTED SHAPE
98.4’ MAX. WAVES
BENDING MOMENT ~-KIPS
8000 6000 4000 2000 0 2000 4000 6000
I I I I +TOP OF BOP I
‘ 500’
400’ F
RESTRANEO
400’
-~ 300’
TtNSIQNER
\
\
\ FREE
300’ \
\
WATER OEPTH 300 FEET
WATER OEPTH 300 FEET
\ /-—— —-
200’
100’
Y
‘Wfl -
‘~/y&&#tvN.iY,&
~“’-””-~”p” ““-
FIGURE 4 – CONDUCTOR BENDING MOMENTS
98.4’ MAX. WAVES
148
, ?
‘PE 23856
500 I 1 //
450
15 M. STORM
1
/
q 400
l..
I
~ 350
& /,J . . . -,---
.XJ KI>LK
/ at+ O,eco
0
& 300
5
250
<>
0
0 2.0’ 3.(Y
4.0’ 5.W 6.0’ i’.Q
MAXIMUM OFFEH AT TOP OF B.O.P. - FEET
FIGURE 5 – UNRESTRAINED MOTIONS AT TOP OF BOP STACK
MAX. BENDING STRESS – KSI
FIGURE 6 – EFFECT OF
f’nNnl ICTOR TENS!ONv- r----- . . .
;-r ,.7
. . -
r
DIVERTER
HOUSING ,
[4
SLIP JOINT
BALL JOINT 1
ANN~LAR
1-
WELLHEAD CONNECTOR
WELLHEAD
I
TENSlONER
*
CONDUCTOR
L
I I I I I I I I
-.
\
Y
—
I I 1 I I I I I I
I I I I I I I I I I I I I I I I I I I I
MOMENT TO LEAKAGE
18-3/4” 15,000 PSI 6BX FIANGE
I
I
I I I I
io,ooo 5,000 c
E PRESSURE- PSl [//////////
2-
1 I I I
,
—
-1.0 0 1.0 2.0 3.0 4.0 5.0 6.0
BENOING MOMENT - 17. LBS. X 1,000,000
FIGURE 7 – MOMENTS THROUGH BOP STACK
150