OTC 6488
Upheaval Buckling Failures of Insulated Buried Pipelines:
A Case Story
N-J.R. Nielsen and B. Lyngberg, Maersk Olie & Gas AS, and P.T. Pedersen,
Technical U. of Denmark
Copyright 1990. Offshore Technology Conference
This paper was presented at the 22nd Annual OTC in Houston. Texas, May 7-10,1990.
This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(3). Contents of the paper, as ç nted, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, doesnot necessarily retiedi any ;ion of the Offshore Technology Conference or its officers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrationsmay not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented.
Laboratorium voor Scheepshydromecbanica
Archief
Makelweg 2,2628
CD Delft
Tel.: 015-7868'73 -Fax:015.781836
terfield pipelines in the Danish Sector of
the North Sea.
It is the objective of this paper to pre-sent the case story of this pipeline up-heaval giving a chronological description
of all actions taken, in order to
re-estab-lish the pipeline integrity.
Based on the upheaval buckling experience within Mmrsk Olie og Gas AS, a recommenda-tion for the design and installarecommenda-tion of buried, hot pipelines is presented.
INTRODUCTION
L_zing the annual pipeline inspection survey in July 1986 along the buried Rolf A/Gorm E two phase pipeline in the Danish
Sector of the North Sea, a pipeline section
was discovered to have protruded the sea-bottom, and was standing in an arch. The 17 km long pipeline is an O.D. 8.625" x WT 14.3 mm carbon steel line (API 5L grade X 52). The pipe is insulated with a 2" thick polyurethane foam (PUF) of min. density 96
kg/m3 encased in a high density polyethylene
(PE) jacket. The exposed PUF at the ends
of the PE jacket is sealed with water tight end cap sleeves. A 2" thick concrete weight
coating is applied on top of the PE jacket resulting in an overall pipe diameter of 0.45 m. A 3" gas lift line is "piggy bac-ked" to the 8" pipeline. The pipeline
References and illustrations at end of
paoer.
The pipeline was laid in the summer of 1985 in 40 m water depth utilizing conventional lay barge techniques. Trenching of the line was carried out using water jetting equip-ment.
The pipeline was brought into service January 1986 for transport of unstabilized
hydrocarbons with a temperature of up to
180°F (82°C) from the Rolf satellite field
to the central process facility at Gorm.
Upheaval buckling analysis of the 8" line was carried out as part of the design documentation resulting in a lowering requirement of 1.15 m to top of the con-crete coated pipe. The certified upheaval
buckling calculations were performed accor-ding to the state-of-art at that time, i.e.
using the "classical" upheaval buckling
analysis, where the design criteria against
upheaval snap buckling is based upon post
buckling equilibrium curves assuming a pipe
of uniform weight on a rigid foundation.
Following the detection of the exposure an upheaval buckling research programme was immediately initiated at the Technical
University of Denmark. The results from the
study showed that the "classical" design approach did not always yield conservative
results. Thus, a new and improved
theoreti-cal model was developed and made available late 1986, and formed the basis for the repair work. The new design approach has been published in ref. [2], [3] and [4].
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ABSTRACT
The first recognized upheaval buckling of build-up is shown in Figure 1, and a
detai-a subsedetai-a, buried pipeline took pldetai-ace in led description of the pipeline is given 1986 in one of Mmrsk Olie og Gas AS' in- by Pallesen et.al. (1985) in ref. [1].
DETECTION OF EXPOSED PIPE AND ACTIONS TAKEN AA-found Survey (let Upheaval).
f0F.n During the 1986 annual inspection survey along the buried 8" Rolf A /Gorm E
pipe-line, an exposed pipe section was detected from side scan sonar records at a distance of 0.3 km (KP 0.3) from the Rolf platform.
A subsequent diver inspection of the ex-posure revealed a very localized pipeline
upheaval,where the apex of the buckle
mea-sured to the bottom of the pipe protruded
1.1 m above seabed level leaving the
pipe-line free spanning over a 10 in section,
see Fig. 2. The apex of the buckle coin-cided with a field joint, and it became evident that the steel pipe at this loca-tion was subjected to a relatively severe bending curvature. An estimation of the
induced strain showed that a strain in the
order of 3.5-8.0% was likely to have oc-cured, which meant that the pipe cross
section had undergone plastic deformation.
To determine the overall buckling
confi-guration, diver probing of the burial depth
on both sides of the exposed pipe was carried out. The results of the probing showed that the buckle wave length was
confined to 24 m corresponding to two pipe joints.
As the burial depth to bottom of the
"un-disturbed" pipeline sections to either side
of the upheaval was approx. 1.5 m, the
overall buckling amplitude became of the order of 2.6 m. Fig. 3 shows the buckled pipe section after having been retrieved
from the seabottom and transported to shore. It is noticed that the resulting up-heaval configuration has similarities with
a plastic "hinge" failure.
Immediate Safety Measures
At the time, in July 1986, when the
pipeli-ne upheaval was discovered, the lipipeli-ne was
operating at a pressure of 1,000 psig
tran-sporting approx. 10,000 barrels daily of unstabilized oil with a flowing tubing head temperature (FTHT) of 176°F (80°C).
During the service period (7 months) prior to detection of the upheaval buckle the pi-peline had been subjected to 15 major shut-downs varying between 1 and 17 hours
(aver-age duration was 5 hours). Assuming that
the upheaval had occured at an early stage
in the service period, the susceptibility
to "low cycle high strain" fatigue was eva-luated to be critical, in particular in the welding area, where relatively large stress
concentrations exist.
Consequently, in order to minimize the risk
of a rupture in the plastically deformed
pipe, the following safety precautions
were taken:
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Maintain steady production (constant
temperature and pressure) .
Monitor the upheaved pipeline configura-tion.
Dedicate a guard vessel to patrol at the upheaval location for protection against third part damage (e.g. fishing activi-ty).
Establish procedures defining remedial actions in case of observed
abnorma-lities.
The 3" piggy back line had not yet been taken into service as a gas lift line and therefore did not present a potential
safety hazard.
The relevance of precaution No. 1
(elimina-tion of low cycle fatigue) was verified upon shut-in of the line prior to the
repair work as contraction of the line upon cooling down lowered the apex of the
uphea-val section from 1.1 m to 0.55 in above seabed level, see Fig. 2,
ASSESSMENT OF DAMAGE
In order to find the technically and
econo-mically most feasible repair strategy, a detailed assessment of the damage caused
by the upheaval was undertaken. In
particu-lar the investigations were concentrated
on
Strain/ovalization Low cycle fatigue Amount of foam damage
Determination of time ot upheaval
The ratio between the outer diameter and
the wall thickness of the 8" steel pipe is
15.3. Such "compact" pipes normally have sufficient capacity to form a plastic
bending mechanism without significant
flattening.
Thus, it was concluded that thecross section was not subjected to severe
ova lization.
The "low cycle high strain" fatigue life of the pipe at the apex of the buckle was
estimated to 20-25 cycles of 3-3.5% strain. Based on the production history (number of shutdowns), it was calculated that: the
remaining fatigue life at the time of detection was 10-15 strain cycles of 3-3.5%. The strain of 3-3.5% was estimated to correspond to one complete shut-down assuming an initial induced strain of 8%. However, as no "low cycle high strain"
fatigue data was available for the
materi-al in question, these cmateri-alculations were subjected to some uncertainty.
2 UPHEAVAL BUCKLING FAILURES OF INSULATED BURIED PIPELINES - A CASE STORY OTC 6488
ue to the longitudinal expansion of the 8" steel pipe at the upheaval location, relative movements occurred between the PUF/PE-sleeve interface causing the foam to slide approx. 50 mm out of the sleeve and damaging the end cap sealings. This bond breakage could have taken place for a considerably distance on both sides of
the upheaval. Also, there were indications
that a shrinkage of the foam had taken place. After having retrieved the damaged
section, the severity of the foam shrinkage
became evident. The foam had collapsed to
half the original thickness, see Figure 4.
By evaluating the biological parameters of the marine growth on the exposed pipe
section, it was concluded that the upheaval
was most likely to have taken place in January 1986, i.e. within the same month
the pipeline had been brought into service.
DETAILED PIPELINE SURVEY
ollowing the detection of the pipeline
upheaval at KP 0.3, it was decided to carry out a comprehensive sub-bottom profile
(SBP) survey along the entire length of the
pipeline in order to determine the in-situ condition of the pipeline burial and
pipe-line profile.
The SBP survey started in September 1986 with pipeline crossings at approx. 10 in
intervals utilizing a remotely operated vehicle (ROV). The 10 in crossing interval
was selected as a compromise between survey
costs and required number of SBP measure-ments in order to resolve pipeline undula-tions with 20-30 m wavelength.
During the execution of the SBP survey a second pipeline exposure was found at a distance of 2.2 km (KP 2.2) from the Rolf
platform.
This exposure did not exist at the time of -he annual pipeline survey in July 1986,
_.e. the pipe had upheaved between late July and early September 1986. The sub-sequent diver inspection revealed a 5 in
pipeline exposure with a maximum height of 0.2 m above seabed level measured to the bottom of the pipe. As for the first up-heaval at KP 0.3 the imperfection wave length was confined to 24 m. The overall height of the buckle was of the order of
2 m.
No further exposures were found along the pipeline. However, at 26 locations the
pipeline was subjected to severe vertical undulations, i.e. imperfection amplitudes
of 0.5-1.0 in with a corresponding wave
length of 50-70 m. The minimum depth of
cover at these locations varied between 0.4
m and 1.4 in as compared to the generally obtained lowering depth of 1.6-1.9 in to top of pipe.
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Thus, upon completion of the detailed SBP survey the state of the pipeline could be summarized as follows:
Upheaval buckle at KP 0.3
Upheaval buckle at KP 2.2
Excessive vertical undulations of unex-posed pipeline sections at 26 locations.
CAUSES OF OBSERVED UNSTABLE PIPELINE BE-HAVIOUR
Installation Induced Imperfections
In order to explain the causes of the
observed unstable pipeline behaviour the
history of the pipeline was closely exami-ned.
It was known and documented that the
pipe-line had been subjected to trawl gear
im-pact at about 10 locations along the pipe-line route prior to trenching of the pipe-line. The upheaved section at KP 0.3 had
experi-enced such trawl gear impact causing da-mage to the 8" line and laterally displa-cing 6 joints (72 m) of the 3" piggy back line up to 2 in from the 8" pipe, leaving
a permanent bend in the 3" line. Trawl gear "pull-over" calculations indicated that also the 8" steel pipe had been stressed beyond the elastic limit. At a further 5 locations the 3" line had been damaged
with displacements up to a maximum of 10 m from the main line.
During lowering of the pipeline all sec-tions, which had been hit by trawl boards
were temporarily left untrenched to provide
access for detailed inspection and sub-sequent repair work, if required. Upon
trenching of the repaired sections, the
resulting pipeline configuration in the transition zones becomes as shown on Fig.
5.
An analysis of the bending stresses induced
in this condition is shown in Fig. 6, as a function of the untrenched length for
different depths of burial. In the present
case, it was found that the linear cal-culation model predicted stresses which
exceeded the yield stress by typically 23%. Therefore, some plastic deformation of the pipeline was expected to have been introdu-ced during the subsequent trenching
opera-tion. Furthermore, the starting and stop-ping of the trenching at these locations
would almost certainly have introduced some
level of foundation imperfection over and above that to be found in sections where
trenching had been performed in one conti-nuous pass.
A change in the operating parameters of the
trenching equipment and variable soil
conditions along the pipeline route could further result in an irregular pipeline
profile.
for this idealized model, where the foun-dation is assumed to be rigid. The bifur-cation load is infinitely high. As the limiting permissible temperature rise is taken the minimum temperature on the U-shaped postbuckling equilibrium curves in
the temperature-buckling wave length plane,
such as Fig. 8, or temperature-buckling amplitude plane. For perfect pipelines without initial imperfections such a pro-cedure will be conservative.
During the design phase also the effect of
imperfections was discussed. This discus-sion was related to Fig. 8, which
schema-tically shows how the response will differ
from the one predicted for the perfect
geometry case. It was concluded that since the adopted "trough" criterion is not very
sensitive to imperfections the proposed design should be conservative also for
pipelines, which invariably posses initial imperfections.
The result of these numerical calculations was that for a design temperature of 200°F
(93°C) and an internal pressure of 3,000
psig (20.7 MN/m2) the required minimum soil
cover was 0.90 in to top of the 8" pipe. The design trench depth was then taken as
1.60 in giving approx. 1.15 in soil cover to
top of the 8" line.
The trenching analysis performed for the pipeline was based on a method suggested by Mousselli [5]. The theoretical back-ground for this analysis is a beam model,
where the pipe is assumed elastically sup-ported by a concentrated soil spring at the
edge of the trench, see Fig. 9. Assuming
a relatively rigid
foundation
and anallo-wable maximum bending stress equal to 305 MN/m2 (0.85% SMYS), it was found that the
maximum trenching depth was 1.6m, and that an increase in trench depth of 20 cm would
increase the stress level by 4%.
A New Design Procedure
In order to get a consistent theoretical basis for the repair of the pipeline, it was obvious that a new mathematical model needed to be developed.
During this analysis work it was shown that a buried and heated pipeline section with an initial imperfection can lift
itself upwards upon experiencing the
opera-ting temperature, by slightly lifopera-ting the overburden without necessarily being able to break out of the soil, i.e. without upheaval snap buckling. During a Later
shut-down the line will cool down and try to return to its original position. Now, during the period with uplift, sand
par-ticles will have tended to fill the "cavi-ty" below the pipeline created by the
uplift. Thereby, a complete recovery to the
original position is prevented. Hence, at
sections of the pipeline with initial
imperfections above a certain limit the Time Dependant Upheaval behaviour
Whereas the first upheaval at KP 0.3 was
considered a classic upheaval buckling case (upheaval snap buckling) the second
uphea-val at KP 2.2 could not be so easily ex-plained owing to its apparent time
depen-dency, i.e. the upheaval took place between
the annual inspection survey in July 1986
and the SBP survey in September 1986. Also,
from the time of detection of the first
upheaval until the decision of maintaining
steady production was taken the line was
subjected to 4 additional shut-down
situa-tions of which two were major.
By comparing the detailed SBP (1986) survey with the as-built (1985) survey documenta-tion, it became evident that certain pipe-line sections with initial vertical "imper-fections" had moved their way upwards through the soil. An example of such
obser-ved imperfection growth is presented in
Fig. 7, showing that an initial
"imper-fection" height, which in 1985 was 0.5 m, had grown to 1.0 in in 1986, reducing the
local soil cover from 1.2 m to 0.7 m. Rel-ating the 0.5 in increase in "imperfection" height to the totally 17 major shut-downs,
the average growth rate becomes 30 mm per
temperature cycle.
Following the above observations, it became clear that the classical upheaval buckling
analysis applied during the design phase was insufficient. It did not model the
detrimental effects of plastic deformation
of the pipeline in combination with lack of straightness and it could not explain
that the geometric imperfection amplitudes
were growing with time.
DEVELOPMENT OF A THEORETICAL MODEL
Analyses performed prior to installation
Prior to installation, upheaval buckling analyses had been carried out in order to
determine the necessary depth of burial and an independent analysis appraisal had been
performed by a classification society. In
both cases the analysis procedure was
iden-tical to the "classical" calculation per-formed for analysis of vertical stability of railroad tracks.
Following this approach the problem was formulated as a heavy, elastic beam on a rigid flat foundation where the weight of
the beam includes the weight of the 8" and 3" pipes and the weight of the cover soil. The solution procedure places the pipe into
a buckled configuration and seeks to find
the temperature and internal pressure which
can maintain equilibrium with a prescrited length L of the pipe in the buckled
posi-tion. This analysis produces postbuckling equilibrium curves which can be reached, if the pipe is sufficiently disturbed. A
normal bifurcation behaviour does not exist
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4 UPHEAVAL BUCKLING FAILURES OF INSULATED BURIED PIPELINES - A CASE STORY OTC 6488
'-perfection amplitudes will grow with the .mberof temperature changes such that the
local soil cover will decrease and at a certain stage upheaval snap buckling can
take place.
This upheaval creep mechanism resembles the
well known phenomenon seen in farm fields in the spring time where the frost drives buried stones up to the surface.
The theory behind this new design procedure
against upheaval creep is semi-analytical and easy to apply. It was implemented on a PC, and the theoretical background has
since been published in [3). It was decided
that in order to avoid that heating and cooling of the pipeline shall cause a growing foundation imperfection, a design criterion should be that, at the maximum pipeline pressure and temperature, the
maximum uplift displacement should be restricted to typically 20 mm. At KP 0.3 the foundation imperfection amplitude was timated to be 0.2-0.4 in and the wave
-_,ngth to be 24.5 m (two pipe joints). Due to the large bending moments induced in the pipeline during trenching, a plastic
defor-mation of the same form was assumed. Se-veral shapes of the imperfection were
analyzed. As an example Fig. 10 shows
relations between the minimum trench depth H and the allowable temperature differenti-al AT for three different imperfection
amplitudes, all prop-shaped. From Fig. 10, it is clear that for a maximum temperature rise equal to 80°C and the estimated
imper-fection amplitudes upheaval creep had to be expected to take place for a trench depth of 1.5 m.
This semi-analytical linearized analysis procedure [3] is rapid and easy to apply.
This is also illustrated in ref. [2], where
the procedure was subsequently used to study the design and installation of
va-rious pipelines and where requirements were 9-itab1ished for acceptable
out-of-straight-3s of these pipelines. Of course, the linearized analysis procedure can only be expected to be sufficiently accurate for
analyses connected with design against gradual upheaval caused by temperature
fluctuations, where the upheaval
displace-ments are small.
To overcome this shortcoming, a non-linear theoretical model was developed, which could be used to study the final upheaval event where the pipeline comes out of the sea bottom, see Fig. 11. This non-linear analysis procedure was also implemented on a PC. A detailed description of the mathematical background for the method has been presented in [4].
The iterative procedure uses, as start configuration, the linearized solution
produced by the upheaval creep analysis and
the mathematical model includes non-line-arities caused by:
585
Geometric non-linearities due to large deflections of the pipeline.
Non-linear stress-strain behaviour of the pipeline material.
A soil uplift resistance, which varies with the uplift displacement.
Variable or deformation-dependent axial
friction forces.
Numerical results are presented in Fig. 11
showing temperature difference AT versus
upheaval clearance A between the
founda-tion and the pipeline at the apex, for the
case of a trench depth equal to 1.5 in in "cemented" sand. The foundation imperfec-tion amplitude is 0.20 in and, as before, the pipeline is assumed to be stress free in this shape. Included in this figure is the result of the classical heavy elastic beam calculation. It is noted that the
equilibrium curve produced by the classical
procedure differs considerably from the result predicted by the more correct cal-culation procedure.
Besides the above-mentioned limitation on the allowable pre-buckling displacements, in order to avoid upheaval creep, it is also necessary to impose restrictions on how close the post-buckling equilibrium
curve should be to the points of operation
on the pre-buckling path. The latter
cri-terion is necessary in order to avoid snap-through buckling due to small disturbances.
Verification of Mathematical Model
To illustrate the correlation between predictions based upon the theoretical model and the observed upheaval creep behaviour, the "imperfect' pipe section shown in Fig. 7 will be considered. The basis is the assumption that the observed as-built out-of-straightness corresponds to the shape of a "propped" pipe section.
The free span equilibrium wave length
asso-ciated with a prop height of 0.5 in can be calculated to 46 in (airfilled pipeline).
Such an imperfection configuration has been
drawn in Fig. 7. It is seen that the shape of a "prop imperfection" shows good
agree-ment with the as-built survey measureagree-ments.
Two non-linear upheaval buckling analyses of this "prop imperfection" are presented in Fig. 12 showing pre- and post-buckling equilibrium curves in a temperature versus uplift displacement coordinate system. The difference in the response of the two curves is caused by the effect of plastic deformation of the pipe, i.e. the upper curve corresponds to a pipe without an
initial geometric pipe imperfection whereas
the lower curve includes an initial geo-metric pipe imperfection equal to the
foundation imperfection. That is, in the latter case the pipeline is assumed to be stress free in the propped configuration.
6 UPHEAVAL BUCKLING FAILURES OF INSULATED BURIED PIPELINES - A CASE STORY OTC 6488
During the 7 months service period, the temperature difference at the "imperfect" location did not exceed 55°C. Compared to
the calculated peak (instability)
tempera-ture of 65-70°C no upheaval snap buckling should be expected, which is in agreement with observations. However, it is noticed that the crown uplift at a temperature
difference of 55°C for the pipe in the
pre-buckling stage varies between 10-40 mm dependent upon the degree of plasticity
assumed for the pipe section. With a depth
of burial at the apex of the imperfection of 1.2 m, the uplift displacement corre-sponding to peak uplift resistance in the
soil would typically be in the order of 20
mm. Consequently, for uplift movements in excess of 20 mm local shear failure could
be expected in the soil resulting in a
non-reversible flow of sand around the pipe.
Upon cooling of the line the redistributed sand particles and the non-linear pipe/soil
interaction prevents the line from
retur-ning to the original position resulting in
an upward ratcheting effect.
The conclusion is that even if it is not
possible, yet, to predict the speed of the
upheaval creep, then correlation studies such as the one presented here show that the new mathematical model is a useful tool to predict when upheaval creep is a potential problem. It also gives a good
indication of the order of magnitude of the
displacement growth for each load cycle.
REPAIR
On the basis of the damage assessment per-formed on the upheaved sections KP 0.3 and
KP 2.2, the detailed out-of-straightness
survey and the developed theoretical model the required remedial repair work was defined. The repair work included the
replacement of line pipe at the two exposed
sections and rock dumping of areas which
were susceptible to upheaval buckling
failure.
Repair at KP 0.3 and KP 2.2
It was decided to replace 6 joints of 8"
and 3" line pipe at each of the two exposed
sections by deburying the affected line
pipes, cutting out the damaged pipe joints
and replacing them by a spool piece, 6
joints (i.e. 72 m) in length using hyper-baric welding. Following the hyperhyper-baric welding of the new spool pieces rock
dum-ping was carried out to backfill the ex-cavated trenches. The rock dump was re-quired to ensure the necessary overburden on the pipeline and thus prevent upheaval
of the sections upon start-up of the
pipe-line. The repair work was carried out in October/November 1986 and each hyperbaric repair took about 3 weeks.
588
The decision to replace line pipes at the exposed areas was taken because:
The upheaved sections had been subjected to significant, but partly unknown accumulated plastic strains.
The apparent breakage of the bond
bet-ween the polyurethane foam and the
poly-ethylene sleeve at KP 0.3 reduced the longitudinal frictional resistance on either side of the upheaval increasing the pipeline's susceptibility to large upheaval movements. The upheaval at KP
2.2 would most likely have been
subjec-ted to "similar" bond breakage.
An additional important aspect of the repair at KP 0.3 was the pipeline expansion
at the Rolf riser. Because the observed bond breakage between the insulation foam and the sleeve could extend for a consi-derably distance around the region at KP 0.3 the max. allowable expansion at the riser could be exceeded. As the riser was
designed to take up the pipeline expansion
by bending of the riser pipe an excessive pipeline expansion could overstress the riser. Thus, in order to ensure a "high"
longitudinal frictional resistance for the
repair spool regardless of the condition
of the bond between the foam and sleeve the
expansion forces in the steel pipe were transferred directly to the concrete
coa-ting via "anchors" welded to the steel pipe at the spool ends. Further, all field joints in the spool were filled with con-crete instead of the original foam, in order to assure that the compressive loads were transmitted through the length of the
spool piece.
Remedial Rock Dumping
At a total of 31 locations (incl. the two repairs at KP 0.3 and KP 2.2) along the pipeline route the safety margin against upheaval buckling was found inadequate.
Therefore to re-establish the integrity of
the line in accordance with the original design condition an increase of the over-burden at these locations was necessary.
The most feasible solution was found to be
rock dumping of the affected areas. The
rock dump requirements could be related to
the following criteria:
To prevent upheaval of the newly
re-paired sections.
To prevent upheaval buckling of sections
where the foam/sleeve bond was broken.
To avoid excessive riser expansion at
the Rolf platform.
To prevent a growing "imperfection" amplitude of buried pipeline sections with significant vertical undulations.
pon the completion of the remedial rock dump a total of 4.5 km pipeline had been
covered with a rock berm of height varying from 0.8-1.6 m. The berm plateau width was
taken as 4.0 m to account for lateral survey inaccuracy.
INSPECTION OF RETRIEVED PIPELINE SECTION
The section retrieved from KP 0.3 was taken
into the testing laboratory to test the
remaining fatigue life of the most strained
section, which had caused great concern just after the detection of the first
upheaval. The pipe was first subjected to 50 cycles of 1% strain after which the strain was increased to 3.5% and the fai-lure occurred after 6 full cycles. These tests confirmed the estimated remaining
fatigue life made at the time of the
detec-tion of the first upheaval.
Following the observed shrinkage of the nsulation foam at the first upheaval _ocation, KP 0.3, laboratory tests were carried out using foam samples from the retrieved pipe sections as well as from
spare pipe joints taken from the "emergen-cy" storage. The results of the investiga-tions showed that the foam tended to shrink
to approximately half its original thick-ness when exposed to a combination of water, temperature and hydrostatic pres-sure. None of these effects in isolation were found to cause the shrinkage.
Thus, water tight end cap sleeves are of paramount importance in order to prevent a detonation of the polyurethane foam. RECOMMENDATION FOR DESIGN AND INSTALLATION OF BURIED HOT PIPELINES
Following the Rolf upheaval buckling cident and the Mmrsk Olie og Gas AS
in-house experience from installation of "hot"
nipelines, a summary of essential aspects
1.thin upheaval buckling design of buried
pipelines is presented:
It shall be documented that buried pipelines will not be subjected to a progressive upheaval creep failure (limitation of crown uplift movement).
It shall be documented that the pipeline
has sufficient safety against a snap through buckling failure (width of
temperature-upliftdisplacementcurve). Upheaval buckling analyses shall be
based upon a consistent mathematical theory, which is capable of modelling the uplift behaviour of "imperfect"
pipelines taking into account the linear pipe/soil interaction and non-linearities of the pipe material.
587
The imperfection configurations used at the design stage should reflect typical imperfection configurations, which could
be generated during the installation (e.g. a prop type imperfection).
A scenario of critical pipe out-of-stra-ightness configurations for given burial depth shall be established (requirement
to trenching contractor).
Upon installation of the pipeline the
achieved out-of-straightness and depth-of-burial shall be measured with
suffi-cient accuracy to verify that the in-stalled pipeline fulfills the design
requirements. This will enable remedial
actions, e.g. rock dumping, to be em-ployed in case of a violation of the design requirements.
The as-built documentation shall form the basis for the subsequent annual
inspection surveys such that "critical"
pipeline sections can be monitored for possible upheaval creep (preventive
measure).
ACKNOWLEDGEMENT
The authors wish to thank the management of Marsk Olie og Gas AS for their
permis-sion to publish this paper.
REFERENCES
(1] Pallesen, T.R., Braestrup M.W., Jor-gensen 0. and Andersen J.B.:
"Insulated Pipeline Design for the Danish North Sea", 6th International Conference on the Internal and Ex-ternal Protection of Pipes, Nice,
France 5-7 November 1985, pp. 189-202.
Nielsen, N.J.R.; Pedersen, P.
Tern-drup; Grundy, A.R. & Lyngberg B.: "New Design Criteria for Upheaval Creep of Buried Subsea Pipelines", Proc. of the Seventh Inter. Conf. on Offshore
Mechanics and Artic Eng., OMAE, Hous-ton, vol.
v,
pp. 243-250, 1988. Pedersen, P. Terndrup & Jensen, J.Juncher: "Upheaval Creep of Buried Heated Pipeline with Initial Imper-fections", J. of Marine Structures, Vol. 1 pp. 11-22, 1988.
Pedersen, P. Terndrup & Michelsen, J.: "Large Deflection Upheaval
Buck-ling of Marine Pipelines", Proc.
Behaviour of Offshore Structures
(BOSS), Trondheim, Norway, Vol. 3, pp.
965-980 June 1988.
Mousselli, A.M.: "Pipe Stresses at the Seabed during Installation and Trenching Operations", Proc. Offshore Technology Conference, Paper no. OTC
2965, 1977. f3]
(a)
(b)
2" PUP Epoxy Coating
Fig. 1 Pipeline Build-up for the 8./3. Rolf to Gorm
Piggy Back Line.
2 meter 3
Epoxy Coating 3" GAS Lift Line
2" Concrete Coating PE Sleeve
8" OIL! GAS Line
588
Geometry of the Exposed Pipe Section at KP 0.3. As-found Configuration.
Configuration after Complete Cool Down.
FloId Joint 025
rorn InfIll
455 530
309 400
Fig. 3 Retrieved Pipe Section Fig. 4 r Severe Foam Shrinkage in
from KP 0.3. Line Pipe from KP 0.3.
1
0
Fig.
(a)
.5
.4
LI
Line of symmetry
Moment distribution
Fig. 5
:Equilibrium Form and Moment Distribution
for a Partly Untrenched Pipeline.
Max. Moment normalized by ELI Lc
""\\
H/Lc ..I0
NU= .08
.06
Non-dimensional Bending Moment
as a Function of Half the
Untrenched Length.
589 :6 :8 .9 LO Li! LcFig. 6:
.3- .2 -.2 .3 .4 .7350 A T I 300 250 200 15 +100 .Gottm 50
Trench bottom
0.5m imperfection Amplitude 0 Orv---
10 20 30 40 50 meter Symbols:
As-built Survey Measurements
lune 1985
FOOLF
.3
0: SBP Survey Measurements September *88
0.5
A
P0
,k40D = 0.219'm
t 0.014 m
IniViol Impert Length 5010
Pipe Total Length .200
Rigid Flat Foundation
46 m
25 50 75, 100 125 150 175
Uplifted Length. tit
Fig, a : Example of "Classical Upheaval Response Calculations.
590
Sea bed
Curve fitted
As-built "Prop imperfection
through 1986 SBP Survey
1m
Fig_ 1 Observed Upheaval Creep, of an Initially 'Imperfect" Pipe Section-1
Fig. 9 Pipe Configuration at the
of the Trench., 11
2:rn
LU q.
'Edge !I100-0 A.0.02 m Sea Floor _ No A.2cm 6f -0-'
.../
....../
.../.t<C(3'''k".""'
./..-. .. ...°- ...-. ..------
--- 40,
---
.---._----"bk''
---
--
.-------_---
--
--
--- --...---0--
--
....--_
----
---
.,--
---
--I 1
2L0 = 24.5m
Noe-°
50-Heavy, linear beam on flat foundation.-'
Non-I ineor method. elasto-plastic pipe (X52)
Upheaved Pipeline Sea Floor
Uplift A (m)
Fig. 11 : Post-buckling Equilibrium Curves
for the 8" Rolf Pipeline.
591
115 2.0
04 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2D
Minimum trench depth H cm)
Fig. 10 : Critical Temperature to prevent
Upheaval Creep of an "Imperfect", Buried Pipe Section.
0 10 Lift-off oint X
T.
(C)
100 50 200AT°
(C°)
150-6I-0.2m I60 50- 40- 30- 20- 10-0 o
Temperature at "IMPERFECT LOCATION"
II
Sea bed 11.7m i05m 46m 0:1 0.2 0;3 0:4 05 0.6 0.7 08 UPLIFT DISPLACEMENT (m)Fig. 12 : Influence of Geometric Pipe
Imperfection on Upheaval Behaviour.
592
80
AT Pipe with NO GEOMETRIC IMPERFECTION
70- "STRESS FREE" PIPE
