[0001] The present invention relates in general to the transport of prefabricated offshore
structures and, more particularly, to self-propelled sea-going vessels for loading
and transport of prefabricated offshore structures such as drill jackets and to methods
of submerging vessel main decks.
[0002] A drill jacket is an elongated offshore structure made up of a plurality of tubular
members with cross bracing, which structure is installed by lowering it to rest on
the sea bottom. The jacket is secured in this position and functions to support a
deck unit and hydrocarbon production equipment above the water line. The deck unit,
which usually comprises a flat deck area with a plurality of legs extending downwardly
therefrom, provides support for the oil drilling and recovery equipment. These components
make up an offshore facility.
[0003] The manufacture of prefabricated components of such an offshore facility has in the
past required that the fabrication yards or sites be built relatively close to a final
desired location for the facility. Most of these yards are located near shallow water
areas. When components are fabricated in such yards, the components may be loaded
onto barges having shallow drafts. The barges are then towed to the final location
for installation of the facility. Such a procedure may be reliably accomplished safely
if the seas are calm and if the duration of such a tow is no more than a day or two
since weather forecasts for such short durations are usually fairly reliable. However,
even though a tow may be of short duration, long delays may be experienced while waiting
for calm seas. When these tows do arrive at the installation site, the barges may
be moored in shallow water alongside the construction equipment at the sitw-, which
equipment may include, for example, cranes mounted on flat barges.
[0004] Offshore facility components may often be built more economically at fully integrated
fabrication yards. However, due to existing favorable conditions at such fabrication
yards and the quantity thereof being limited by the cost of building them, such fabrication
yards may be located at extreme distances from the final offshore facility locations.
The technical and safety risks of long tows, however, make it difficult for these
remote yards to compete with fabrication yards located near the final offshore facility
sites. Shallow draft barges for loading offshore structures in the typically shallow
waters, because of the size and bulk of such structures, may not have enough stability
for the open sea, thus requiring periods of calm weather for towing safety and, therefore,
long delays may be experienced while waiting for these calm conditions. On long tows
such as over an ocean, there is also no assurance, as previously noted, that good
weather and calm seas will prevail throughout the transport. Rough seas may in addition
cause severe fatigue or other damage to the offshore components during a tow by barge.
[0005] The fatigue stresses during such transport are increased with higher accelerations
during the roll of the transport structure. Barges typically have a low period of
roll with resulting high accelerations during the roll as compared to a self-propelled
sea-going vessel. Furthermore, the total fatigue stresses on an offshore structure
during a transport of specified length is related to the duration of transport. The
speed of a barge being towed is typically slow as compared to the speed of a self-propelled
sea-going vessel resulting typically in more than twice as many fatigue cycles when
an offshore structure is towed by barge. Thus, in calm seas as well as rough seas,
the number of and severity of fatigue stresses on an offshore structure being towed
by barge may be much greater, resulting in increased risk of damage over what would
be the case if the offshore structure were being carried by a sea-going vessel over
the same distance.
[0006] In addition to loading of prefabricated offshore structures at such fabrication yards
for transport over the ocean, it is also desirable that the same vessel have the capability,
of loading prefabricated offshore structures that are floating in the ocean for transport
to another area of the ocean. For example, jack-up drilling rigs are commonly used
for exploratory drilling, and are therefore designed for transport from one location
to another as the exploratory needs determine.
[0007] In order to maneuver the vessel into position at a fabrication yard for loading of
a prefabricated offshore structure, it is desirable that the main deck on which the
structure is to be loaded extend all the way to either the forward or after end and
that therefore there not be a raised deck on either the forward or after end of the
vessel. However, in order to load a floating offshore structure, it is necesary that
the main deck be sunk to permit floating thereof onto the main deck. In order to accomplish
this objective, the vessel must have sufficient stability such as may otherwise be
provided by forward and after raised decks to maintain the vessel afloat.
[0008] In order to achieve the foregoing objectives, a transport vessel and method for long
distance ocean transport of such prefabricated offshore structures is desirable which
is both fast and safe, thus ensuring that the components will arrive undamaged and
on time, and which affords the flexibility of loading such components which are either
at a fabrication yard near shallow water or floating in deep water.
[0009] The present invention provides a sea-going self-propelled vessel for loading and
transporting of prefabricated offshore structures, the vessel comprising a main deck
for supporting at least one prefabricated offshore structure, means for submerging
the main deck to a selected depth for floating of prefabricated offshore structures
onto and off of the main deck for loading and off-loading thereof, a raised deck at
one end portion of the vessel, and means for mounting at least one portable stabilization
tank on the main deck at the other end portion of the vessel.
[0010] The present invention also provides a sea-going self-propelled vessel for loading
and transporting of prefabricated offshore structures, the vessel comprising a main
deck for supporting at least one prefabricated offshore structure, means for submerging
the main deck for floating of prefabricated offshore structures onto and off of the
main deck for loading and off-loading thereof, a raised deck at one end portion of
the vessel, and at least one portable stabilization tank mountable on the main deck
at the other end portion of the vessel.
[0011] Further, the invention provides a method of submerging a vessel main deck to a selected
depth for loading or off-loading a structure, the method comprising providing the
vessel with a raised deck on one end portion thereof, mounting at least one stabilization
tank on the other end portion of the vessel, the raised deck and the stabilization
tank each having a height to provide freeboard when the main deck is submerged to
the selected depth, and submerging the main deck to the selected depth whereby the
structure may be floated onto or off of the main deck.
[0012] A preferred embodiment of the present invention described hereinbelow provides quick,
safe, and reliable transportation over an ocean of prefabricated offshore structures
from a fabrication yard near shallow water, minimizes the number and severity of fatigue
stresses on an offshore structure during ocean transport thereof so as to minimize
the risk of damage to the offshore structure, and provides flexibility in the transport
vessel for loading of offshore structures from a fabrication yard near shallow water
and for loading floating offshore structures in deep water.
[0013] The invention will now be further described, by way of illustrative and non-limiting
example, with reference to the accompanying drawings, in which:
Figure 1 is a perspective view of a sea-going self-propelled vessel embodying the
present invention, carrying a prefabricated offshore structure;
Figure 2 is a side view of the vessel loading a prefabricated offshore structure from
a fabrication yard near shallow water;
Figure 3 is a side view of the vessel with a main deck thereof submerged and receiving
a floating offshore structure thereon;
Figure 4 is a plan view of the vessel as depicted in Figure 3;
Figure 5 is a plan view of a portion of the main deck of the vessel, showing means
thereon for mounting a stabilization tank;
Figure 6 is a plan view illustrating the mounting of a stabilization tank to the main
deck; and
Figure 7 is a side view, partially in section, of the means for mounting the stabilization
tank.
[0014] Figure 1 shows a sea-going self propelled vessel 10 for transporting large, bulky
prefabricated offshore structures such as, for example, a drill jacket 12. The vessel
10 includes a suitable power plant for propulsion, and a main deck 14 disposed rearwardly
of the vessel and extending between the sides 16 and 18 thereof, as shown in Figure
4, and to the after end 20 of the vessel for receiving and supporting prefabricated
offshore structures. The vessel 10 also includes a raised forward deck 22 upon which
is located various crew quarters, the bridge, and the like. For the purposes of this
description and the claims, a main deck is defined as the uppermost continuous deck
of a vessel, and a raised deck is defined as a deck which is higher than the main
deck and to the height of which the vessel portion over which is extends is water
tight.
[0015] Figure 2 illustrates the loading onto the vessel 10 of a prefabricated offshore structure
12 from a fabrication yard 24 which is adjacent to shallow water 26. The vessel 10
is maneuvered to a point as illustrated where its after end 20 is adjacent the fabrication
yard 24 and its main deck 14 is at substantially the same height as the fabrication
yard 24. The offshore structure 12 can then be loaded onto the vessel 10 by skidding
using suitable skid means, illustrated schematically at 25 in Figure 4, commonly known
to one of ordinary skill in the art to which this invention pertains. In Figure 2,
the offshore structure 12 has been partially skidded onto the vessel 10.
[0016] As shown in Figure 2, the depth of the vessel, illustrated at 28, is suitably small
enough to allow the vessel 10 to maneuver in the typically shallow waters near fabrication
yards. The depth 28 (that is, the depth of the main deck at mid-length of the vessel)
is preferably less than 35 feet (10.7 metres). For example, the depth 28 at the main
deck may be 32 feet (9.8 metres), and the vessel may have a maximum operating draft
loaded of 22 feet (6.7 metres).
[0017] A typical cargo vessel may have a width equal to 11 times its depth. However, for
the shallow waters in which a transport vessel embodying the present invention is
expected to operate, it is expected that the width of the vessel (i.e., the greatest
width overall of the hull in a direction parallel to the pitch axis as illustrated
at 30) should be at least 21 times the depth 28 in order to provide suitable stability
for the depth of the vessel and to furthermore provide greater stability to the vessel
than would be the case if it were necessary to have a typical ocean going ship carrying
prefabricated offshore structures. For example, for a vessel having a depth of 32
feet (9.8 metres), the width may be 104 feet (31.7 metres). Such a vessel may also
have a length overall of 380 feet (115.9 metres), 745 shaft kW (1000 shaft horsepower),
a cargo deck space of 215 feet X 104 feet (65.6 metres X 31.7 metres), a maximum deck
loading of 3000 lbf/in2 (20.7 MPa), and an operating speed of 14 knots (7.2 m/s).
The major frames and bulkheads are on 10, 20 and 40 foot (3, 6 and 9 metre) spacing
to provide an advantageous hull space loadout condition. Alternately, the length overall
may be 500 feet (152.5 metres). Such a vessel with a speed of 14 knots (7.2 m/s) can
move over twice as fast as a typical tug/barge combination, thus reducing the number
and severity of fatigue stresses on the offshore structure.
[0018] The vessel 10 is suitably outfitted with a pair of pivot supports 32 which extend
rearwardly from the skids 25 for launching the jacket 12 when it has been transported
to the desired location. This is accomplished in a manner commonly known to those
of ordinary skill in the art to which this invention pertains by sliding the jacket
12 rearwardly so that its center of gravity moves onto the support 32. With suitable
ballasting while maintaining stability in accordance with principles of common knowledge
to those of ordinary skill in the art to which this invention pertains, jacket 12
is rotated into the water through the pivoting of support 32 thereby launching it
into the sea at the desired location. Smaller structures may typically be lifted for
launching with a derrick from a derrick barge.
[0019] The jacket 12 illustrated in Figure 1 covers the entire area of the main deck 14.
Thus, the main deck 14 should be clear of any raised structures during loading and
transport of such a jacket. In addition, the main deck 14 should be clear of any raised
structures during launching of barge jackets. However, if it is desired to submerge
the main deck for floating an offshore structure such as a generator plant carried
by a barge or an exploratory jack-up drilling rig onto it for loading and for transport
to another offshore location, the vessel 10 will typically require water plane area
at both the forward and after ends to afford sufficient stability during the submergence
of the main deck 14 and the loading of the offshore structure thereon. In the preferred
embodiment shown, such water plane area is suitably provided forward of the vessel
10 by raised deck 22.
[0020] Referring to Figures 3 and 4, such water plane area is afforded, in accordance with
the present embodiment, jaft of the vessel 10 by one or more stabilization tanks such
as the pair illustrated at 34. Such tanks 34 may each weigh typically 20 tons and
are portable such that they can be mounted and dismounted with cranes and removably
affixed to the main deck in a manner which will be described hereinafter. For the
purposes of the description and claims, a "stabilization tank" is defined as a member
which is watertight up to the water level when the vessel on which it is located is
submerged to a selected depth and which is sized to provide waterplane area for stabilizing
the vessel during such submergence of the main deck thereof. Although such a tank
may typically be hollow so that it weighs less and is easier to handle, it may also
be of solid construction. The height, illustrated at 36, of a tank 34 is sufficient
if it provides some freeboard when the main deck 14 has been submerged to the desired
depth. For the vessel the characteristics of which were previously described, there
may be provided two such tanks 34, as shown in Figures 3 and 4, the height 36 of each
of which is 18 feet (5.5 metres), and the length and width of which are each 19 feet
(5.8 metres), for example. The specific dimensions and shapes of such stabilization
tanks may vary and can be calculated for a specific vessel using principles of common
knowledge to those of ordinary skill in the art to which this invention pertains.
For example, the tanks 34 may be rectangular or circular in plan view.
[0021] In a preferred embodiment of the present invention, as shown in Figure 4, the vessel
10 is provided with two such portable stabilization tanks 34, one on each side to
thus allow room or space illustrated at 38 between the tanks, which in the typical
example previously described may be 60 feet (18:3 metres) for loading and unloading
of smaller offshore structures such as those having a width less than 60 feet ( 18.3
metres) while the tanks 34 remain attached to the vessel should that be necessary
or desirable.
0
[0022] With the stabilization tanks 34 securely attached to the main deck 14 at the after
end portion 40 thereof as shown in Figures 3 and 4, the vessel 10 may then be ballasted
by means of suitable ballast tanks some of which are schematically illustrated at
42 to submerge the main deck 14 to a suitable depth, as shown schematically by the
water surface 41 in Figure 3, for loading a floating offshore structure or for off-loading
a structure such as, for example, a generator plant schematically illustrated at 43
carried by barge 45. Such ballasting may be accomplished utilizing principles of common
knowledge to those of ordinary skill in the art to which this invention pertains.
[0023] In the example which has been provided, the vessel 10 may be ballasted to a depth
which will allow 16 feet ( 4.9 metres) of water above the main deck 14, the forward
structure 44 up to the raised deck 22 of the vessel as well as the stabilization tanks
34 are sealed to a super structure height of 5.5 m above the main deck thus allowing
about 0.6 m of freeboard during submergence of the main deck, and the forward structure
44 and stabilization tanks 34 thereby act as stabilizers for the vessel 10. For a
vessel having an overall length of 152m and as otherwise previously described, the
distance between the tanks 34 and forward superstructure may be sufficient to load
a floating structure having a length up to about 285 feet ( 86.9 meters).
[0024] A preferred means for mounting a portable stabilization tank 34 on the main deck
14 is shown in Figures 5, 6, and 7. A housing 46, welded or otherwise fixedly attached
to the main deck 14 and extending below the surface 48 of the main deck a suitable
distance and reinforced by member 50, is constructed to provide apertures 52 and 54
opening upwardly for receiving a stab point member 56 and a pin 58 respectively of
the respective tank 34 for each corner thereof as will be hereinafter described.
[0025] The stab point member 56 is welded or otherwise suitably fixedly attached to the
tank 34 by members 60 and 62 and has a lower frustro-conical tip 64 converging to
a smaller cross-sectional area at its lower end for insertion in similar shaped apertures
52 for lining aligning tank 34 at the desired position on the main deck 14 for securing
it to the main deck as will now be described.
[0026] Welded or otherwise suitably fixedly attached to the tank 34 is a member 66 which
has a slot 68 which slot is positioned directly over the aperture 54 of the main deck
when the tank 34 has been properly aligned therewith and which receives the shank
portion of a hold-down pin 58. The hold-down pin 58 is threaded to receive a lock-nut
71 and a nut 70 along with washer 73 on the end portion 72 which is to protrude upwardly
from the slot 68. Pin 58 is provided with a T-shaped other end portion 74 for engaging
the aperture 54 in the main deck 14. Extensions 76 and 78 of housing 46 extend partially
over the upward opening of aperture 54 to provide a slot 80 in the main deck 14 for
engaging the hold-down pin shank 82 with the T-shaped end portion 74 below the extensions
76 .and 78 whereby the hold-down pin 58 may secure the tank 34 to the main deck 14
by engagement of slots 68 and 80 by the hold-down pin shank 82 and by securely tightening
hold-down nut 70. Such a tank mounting means may be provided at each corner of a tank
34 for firmly securing the tank 34 to the main deck 14 whereby the tank 34 may easily
be dismounted therefrom for removal from the main deck 14 and for storage.
[0027] In order to load a floating offshore structure on to the above- described vessel,
the stabilization tanks 34 are hoisted to the general location for mounting on the
main deck 14 by a derrick or other suitable means after which the tank stab point
members 56 are aligned with and inserted in the respective apertures 52. The hold-down
pins 58 may then be inserted in the respective slots 68 and 80 of the tank 34 and
main deck 14 respectively, and the washers 73, lock-nuts 71, and nuts 70 mounted and
tightened to firmly mount the tanks on the main deck. The vessel 10 is then ballasted
so that the main deck 14 is submerged to a suitable depth for loading the offshore
structure but wherein the forward structure 44 and the stabilization tanks 34 have
some freeboard and thus provide suitable stability to the vessel 10 during the submerging
of the main deck 14 and the loading of the offshore structure. Then, the offshore
structure is floated onto the main deck 14 afterwhich the vessel 10 is deballasted
and the main deck 14 with its load is raised above the water for transport. As long
as they are not in the way, the stabilization tanks 34 may be allowed to remain mounted
on the main deck 14. However, if it should become necessary to load an offshore structure
at a fabrication yard near shallow water or should it become necessary for any other
reason such as for launching an offshore structure, the tanks 34 may then be dismounted
and stored for use at a later time.
[0028] Certain features of this invention may sometimes be used to advantage without a corresponding
use of other features. While specific embodiments of the invention have been shown
and described in detail to illustrate the application of -the principles of the invention,
it will be understood that the invention may be embodied otherwise without departing
from such principles. For example, the raised deck may be on the after end of the
vessel and the stabilization tanks may be mounted or mountable on the forward end
of the vessel resulting in the main deck extending to the forward end of the vessel
for loading of offshore structures at the forward end thereof. For another example,
a different means for mounting the tanks 34 other than the specific means described
may be employed.
1. A sea-going self-propelled vessel for loading and transporting of prefabricated
offshore structures, the vessel (10) comprising a main deck (14) for supporting at
least one prefabricated offshore structure (12; 43), means for submerging the main
deck (14) to a selected depth for floating of prefabricated offshore structures onto
and off of the main deck (14) for loading and off-loading thereof, a raised deck (22)
at one end portion of the vessel, and means for mounting at least one portable stabilization
tank on the main deck (14) at the other end portion of the vessel.
2. A vessel according to claim 1, which has a width which is at least 21 times its
depth (28).
3. A vessel according to claim 1 or claim 2, which has a depth which is less than
10.7 m.
4. A vessel according to claim 1, claim 2 or claim 3, comprising means (25) for skidding
a prefabricated offshore structure onto the vessel.
5. A vessel according to any one of the preceding claims, wherein the tank mounting
means comprises an aperture (52) in the main deck (14) for receiving a stab point
member (56) attached to the tank for aligning the tank for attachment to the main
deck.
6. A vessel according to claim 5, wherein the tank mounting means comprises a slotted
second aperture (54) in the main deck (14) for engagingly receiving one end portion
of a hold-down pin (58), the other end of which pin is engageable with the tank for
securing the tank to the main deck.
7. A vessel according to claim 6, comprising at least one portable stabilization tank
(34) mounted on the main deck (14) at said other end portion of the vessel, the tank
(34) including at least one stab point member (56) for engaging the first-mentioned
aperture (52) and a hold-down pin (58) attached between the slotted second aperture
(54) and the tank to attach the tank to the main deck.
8. A vessel according to any one of claims 1 to 4, comprising at least one portable
stabilization tank (34) mounted on the main deck (14) at said other end portion of
the vessel.
9. A vessel according to claim 7 or claim 8, wherein the raised deck (22) and the
stabilization tank (34) each have an height to provide freeboard when the main deck
(14) is submerged to the selected depth.
10. A vessel according to any one of the preceding claims, wherein the raised deck
(22) is located on the forward portion of the vessel.
11. A vessel according to any one of the preceding claims, comprising means including
a pivot support (32) for launching an offshore structure.
12. A vessel according to any one of the preceding claims, comprising a pair of stabilization
tanks (34) spaced from the raised deck (22) for floating of at least one offshore
structure (43) onto the main deck (14) between the raised deck and the pair of stabilization
tanks, the pair of stabilization tanks being spaced apart for loading of offshore
structures from a fabrication yard (24) between the pair of stabilization tanks.
13. A sea-going self-propelled vessel for loading and transporting of prefabricated
offshore structures, the vessel (10) comprising a main deck (14) for supporting at
least one prefabricated offshore structure (12; 43), means for submerging the main
deck (14) for floating of prefabricated offshore structures onto and off of the main
deck (14) for loading and off-loading thereof, a raised deck (22) at one end portion
of the vessel, and at least one portable stabilization tank (34) mountable on the
main deck (14) at the other end portion of the vessel.
14. A method of submerging a vessel main deck to a selected depth for loading or off-loading
a structure, the method comprising providing the vessel (10) with a raised deck (22)
on one end portion thereof, mounting at least one stabilization tank (34) on the other
end portion of the vessel, the raised deck (22) and the stabilization tank (34) each
having a height to provide freeboard when the main deck (14) is submerged to the selected
depth, and submerging the main deck (14) to the selected depth whereby the structure
may be floated onto or off of the main deck (14).
15. A method according to claim 14, wherein the step of mounting the stabilization
tank (34) comprises inserting a stab point member (56) from the tank (34) into a first
aperture (52) in the main deck (14) to thereby align the tank for attachment to the
main deck, inserting one end of a hold-down pin (58) in a second aperture slot (54)
to engage the second aperture slot, and attaching the other end of the pin (58) to
the tank (34).
16. A method according to claim 14 or claim 15, comprising dismounting the stabilization
tank (34) from the main deck (14) so that a prefabricated offshore structure (12)
may be skidded onto the main deck from a fabrication yard (24).
17. A method according to any one of claims 14 to 16, comprising providing the vessel
(10) with a width which is at least 2) times its depth.