Note: Descriptions are shown in the official language in which they were submitted.
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~ method of towing a pipeline structure in a body of water
and a struct~re for use therein.
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This invention relates to a method of towing a pipeline
structure suspended between two tugs at a controlled depth
in a body of water, e.g. to bring this structure from a
point of assembly to a zone where it has to be laid on the
sea bottom, and to a structure for use in this methocl.
Such a method is e.g. known from USP 4.363.566~ Therein
a pipeline structure is provided with weighting means,
consisting of chain lengths. The pipeline structure itse,lf
is given a slight buoyancy and together with the weighting
means the total structure is given a submerged weight to
such an extent that, as soon as part oF the chain lengths
rest on the bottom, the pipeline structure is kept floating
at a short distance above the bottom, in which condition it
is towed by a tug to the site to be reached, there being a
trailing tug;connected to the trailing end of the structure
as usual. If obstacles on the sea bottom, such as ship
wrecks or reefs are encountered, the restraining force
exerted by the trailing tug will be increased to lift the
structure off the bo~ttom to pass the obstacle. It is also
possible to exert this restraining force continually during
towing to maintain the structure at a controlled distance
above the bottom during towing.
The present invention aims at improving such known
methods. I~t was found that a much better control of the
plpeline st;ructure~during towing~as to~deflections is
possible, also allowing considerably longer pipeline
structures to be towed in one~operation without considerable
increase of towing~and restraining forces. This ls based on
the insight that weightlng~means such`as chains together
with the pipeline structure itself, if moving at sufficient
speed with respect to the surrounding water, are subject not
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only to drag forces but also to lifting forces, which may be
used as a major contribution to decrease deflections and
allow an efficient and well-controlled towing at the
required depth without high towing and hold back forces.
In view thereof, a method as given in the preamble is
according to this invention characterized in that the
pipeline structure is given a weight and volume so as to
have nett buoyancy, that weighting means are connected to
the pipeline structure in a number of points along its
length so that the combination of pipeline and weighting
means has a submerged weight, said combination having
protruding parts with surfaces, which during towing are
subjected to lift forces caused by the water flowing along
them, the towing speed with respect to the ambient water
being at least during part of the tow path such as to give
the said combination a submerged weight being decreased by
at least 40O of and with respect to the submerged weight of
the combination in the stationary position of the pipeline
structure .
The protruding parts subjected to the lift forces may be
combined with the weighting means entirely or in part.The
weighting means may be chains as in said known method, being
flexible by having links easily pivotable with respect to
each other, and such chains cause such lift forces when the
structure is towed at sufficient speed through the water,
~causing the chains to take up inclined positions trailing
with respect to their vertical suspended positions in
stationary condition of the structure.
; Instead thereof or together therewith there may be other
protruding parts giving such lift forces such as hydrofoils
or inclined vanes connected r~igidly or pivotably to the
pipeline structure itself and/or to the weighting means.
Preferably the said towing speed is chosen so as to give
the said combination a vertical de~lection between its ends
of 30-70 m.
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If the towing speed becomes too high, the combination
may rise to the surface by the fact that the said lift
forces become high enough to reduce the submerged weightto
zero. In many cases, this might be less desirable under
adverse weather conditions in view of the deflecting forces
of waves thereon. In view thereof, it is often preferred to
keep said towing speed at a level, which is at least 2o
below the speed at which the submerged weight of the
combination is zero.
Further preferred features and details about the
realisation of this method will be given in the following
description of the annexed drawings. Therein:
Fig. 1 is a diagrammatic elevational view of a pipeline
structure with leading and trailing tugs during towing
through a body of water;
Fig. 2 is a graph showing the vertical load (submerged
weight) of the pipeline structure with weighting means
versus tow speed.
In Fig. 1 there is a pipeline structure 1, of which the
buoyancy may be controlled, e.g. by filling or emptying
parts thereof with a suitable liquid or gas such as
nltrogen. Such pipeline structures usually are complex~ i.e.
they consist of more than one pipe one within the other~and
often also one to the side of the other. The normal pipeline
structure has a surroundlng carriel pipe~ in which there are
one or mDre pipes e.g. for guiding oil or gas, if desired
entirely or in part also to be used as TFL-lines
(through-flow-line pipes for guiding tools etc.) and one or
~more pipes for other purposes, cables, control lines etc.
and (part of) such pipes may have a surrounding heat
insulation layer,~ sleeve~ pipes around such layers and
suitabIy coatings.There may also be a carrier pipe with
external pipes and lines connected thereto or a combination
of both possibilities. All this is known as such in many
dlfferent embodiments so that it IS not shown in detail. At
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least part of the pipes and of the spaces between the pipes
may in known manner be closed at the end of the structure,
be connected to means to fill them with gas such as
nitrogen, whether compressed or not, ancl/or liquid such as
oil or sea water. Together with the possibility to apply
floats or weighting means, to apply closed or closable
bulkheads in (part of) the structure to fill or empty
separate compartments with (different) fluids alonq the
length of the structure, and with the possibility to vary
wall thicknesses and materials chosen, all this allows to
choose and to vary the buoyancy of the structure in the
water.
At preferably regular intervals, chain lengths 2 are
connected to this structure to be suspended therefrom. The
chains may be normal link chains with or without studs. They
may be connected to the pipeline structure by having their
top link engaging a lug welded to said structure or a strap
or bracket surrounding the (outer) pipe of the structure.
Instead thereof the top link may be connected to the
pipeline structure by a short steel wire or cable. Instead
of chain lengths only consisting of the usual linl<s one
immediately engaging the other there may be thin steel wires
in each chain length, connecting two adjacent links in one
or more points of the chain length to allow easy adjustment
of the length of each chain length. In this case the chains
with their connections to the pipeline structure form the
only weighting means, but there may be additional weighting
means such as heavy straps, weighting blocks or the like
rigidly connected to the pipeline structure. The chain
lengths 2 in this case form the only protruding parts
subjected to lift forces~but, as stated~ there may be
additional such means~such as protruding vanes, hydrofoils
or the like connected to the structure.
In usual manner the pipeline structure may have tow
heads 3 at its ends. A leading tug 4 will be connected by
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tow line 5 to the leading tow head 3 of the structure and a
trailing tug 6 will be connected by tow line 7 to the
trailing tow head 3. The tow lines 5 and 7 7 the tow heads 3
and the tugs 4 and 6 may be of known and usual design, the
tow lines 5 and 7 may in part consist of NylonR and may
consist of or be provided with the usual parts, such as
shackles, hawsers, pennants, bridles ancl floaters.
In Fig. 1 the sea bottom is shown at 8.
When this pipeline structure is moved through the water,
the chains 2 will take up inclined positions as shown by
lines 2' for the two leftmost chainlengths. This will give
lift forces relieving part of the weight of these chains by
the water flowing along them. This effect is used to
advantage by the present invention, which will now be
described in more detail with reference to Fig. 2.
This gives the tow speed 9 e.g. in m/s of the pipeline
structure with respect to the water along the abscissa and
the resultant vertical load on the pipeline structure with
chains in the water along the ordinate, e.g. in N/m length
of the structure. In calculating said vertical load the tow
force and the hold-back force exerted thereon by the tow
lines S and 7 are assumed to be horizontal at the towheads
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It will be seen that the vertical load on the entire
structure at zero speed is at a certain maximum (negative)
value (maximum submerged weight) and at increasing speed
there is at first not much decrease of this vertical load.
At higher speeds, this load decr~eases gradually until it
r~eaches zero value, at which speed the structure will just
be floating, so that it will reach the surface of the water.
According to the invention, the tow speed is chosen so
as to be at least at the value, at which the submerged
weight is decreased by at least 40O of and with respect to
the submerged weight in stationary condition of the
structure, indicated by the horizontal dashed line lQ.; At
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these ~0O remaining submerged weight the tow speed is
represented by the vertical dashed line 11, which speed is
indicated as v min. For short combinations this 40O is a
good practical limit. In particular for long combinations
with a high submerged weight it is preferred to tow at
higher speeds in view of maximum deflections to be kept
sufficiently low by the lift action of the protruding parts
such as the chains.
Preferably, the tow speed is chosen so as to decrease
the vertical load (submerged weight) so much that the
vertical deflection between the ends of the pipeline
combination is between ~û and 70 m. This will for a
particular case e.g. be so at dashed line 12, at which the
speed Vp is represented by vertical line 13.
To be on the safe side in view of water currents,
deviations in temperature and salinity and thus specific
gravity of the water and normal tolerances in the structure
as to weights and dimensions, the chosen tow speed will in
many cases have an upper limit below the speed at zero
vertical load, and this upper-limit is preferably at 98,o of
the speed at zero vertical load ( 2o below the speed at zero
~vertical load), as indicated by vertical dashed line 14.
In the known method as described above, the hold back
force when lifting the structure from the bottom so as to
pass obstacles is relatively high. When applying the
invention the hold back force can be much lower and is
preferably between 100 and 300 kN even for very long pipe
structures.
The following example, for which values of speeds and
vertical loads have been given in Fig. 2, will explain a
possibl~e embodiment of the method of the invention in more
detail.
A p~ipeline consisting of an outer carrier pipe and a
number of oil lines and control lines therein has an outer
diameter of the carrier~ pipe of ~76 mm (including coating)
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and a length of 500U m.
The total weight in air of the pipeline structure without
chains i5 5800 N/m and its displacement expressed in weight
of sea water of a given normal temperature and salinity is
6100 N/m, so that there is a nett buoyancy of 300 N/m.
The chains 2 are connected to the pipeline structure in
lengths of about 4,15 m, at mutual distances of 12 m. The
chains 2 consist of usual anchor chain with links having a
thickness of 78,5 mm. The submerged weight o-f each chain
length of 1~ links is 4200 N. This gives per unit length of
the structure an additional submerged weight of 350 N/m. The
total submerged weight of the combined structure with the
chains will thus be 350-300 = 50 N/m in stationary
condition.
When towing this structure with chains the chain lengths
will, in their inclined positions (as 2' in Fig. 1) be
subjected both to drag forces and to lift forces and these
can and have been determined rather easily by experiments.
It will be clear that at zero speed of the struct~re in the
water both drag and lift will be zero and that at increasing
speed both the drag force and the lift force will increase.
The lift force will be low at lower speeds and rise
ccnsiderably at higher speeds, which is reflected by Fig. 2.
In this example, the structure will float at a speed of 2,48
m/sec, the towing speed should be at least 1,73 m/sec for
40O decrease in vertical load (line 11 in fig. 2) and is
preferably 2,25 m/sec for a 85Z decrease in vertical load,
corresponding to a maximum deflection of the~bundle between
its ends~of approximately~60 m. For keeping the structure
~sufficiently submerged, the speed should be below 2,35 m/sec
(98~ of 2,48m/sec). ~ ~
As compared with the known method as indicated in the
preamble of this specification and as stated before the tow
and hold back forces may be considerably lower when appIying
the invention for the same length of structure, or the
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structure may be much longer for the same tow force.In said
known me-thod, the hold back force will be relatively high
with respect to the towing force. In applying the invention
it is preferred that the hold back force is a set value
which results normally in a value between 10o and 40O of the
towing force. In the given example, the towing force at the
speed chosen according to the invention (of 2,25 m/sec) is
about 1200 kN and the hold back force is 150 kN. One of the
grounds for a high hold back force in the known method is
the necessity to limit deflections in the structure by means
of tension forces at its ends. When applying the invention,
the deflections appear to be so low that the hold back force
need not be high to counteract such deflections. A lower
hold back force also means a lower towing force and this is
thus an important advantage of applying this invention.
The tow heads 3 may also be of known and usual design,
having means for adjusting their buoyancy, skids to rest on
and slide over the sea bottom when starting towing in
shallow water and when sinklng the structure at the required
site, means to connect the pipes of the structure to
underwater connections, e.g. for oil or gas at oil wells,
submarine storage means etc. The values given above of
course relate to the structure together with such tow heads
in their normal operation.
The structure may be assembled on shore entirely or in
part on shore and in part in shallow water, where
adjustments oÇ the chain weights may be performed. When
towing begins, the towing and hold back forces may either
immediately be set at the values whic;h in equilibrium
conditions will give the structure the chosen speed or the
towing and hold back forces may be increased gradually or in
steps with, as soon as the hold back fo~ce reaches the
require~d value, a further increase of the towing force only.
If towing begins in shallow water, where (part of) the
chains rest on the bottom to keep the structure floating in
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submerged condition by part of the weight of the chains
being taken up by the sea bottom, the forces may be adapted
thereto e.g. by being at the beginning of towing equal and
at a level higher than the final hold back force at speed
towing according to the invention, to l:ift the structure
with the chains entirely from the bottom as soon as possible
before any considerable towing speed is reached, after which
the hold back force is decreased to the required value.
In general, it is preferred that the submerged weight of
the combination in stationary condition of the entire
structure with chains as both weighting means and lift means
is between 8 and 5~O of the nett buoyancy of the structure
without chains and between 7 and 40O of the submerged weight
provided by the chains. These values are preferred in view
of practical design considerations, tolerances and lift
requir~ments for the chains.
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