[0001] The current invention relates to a structure and a method for decommissioning an
offshore platform.
[0002] The invention can be implemented in the oil and gas extraction activities, in particular
for disassembling end-of-life platforms located in offshore oil and gas fields such
as in the North Sea. However, it can also be used for dismantling any offshore structure.
[0003] Decommissioning an oil or gas extraction platform is a complex and costly activity
due to the size of the platform and its offshore location. The dismantling of platform
comprises different stages. The first one consists in the well plugging and abandonment.
Then the platform topsides are removed after having been separated from the platform
jacket. Eventually, the jacket is removed. Currently, the topsides removal usually
requires the mobilization of a heavy lift vessel and transportation barges. However,
this operation requires favorable weather conditions and in oil fields such as those
in the North Sea favorable weather windows are limited. Therefore, in order to save
time, the topsides are removed in one piece which requires the involvement of heavy
lift vessel with sufficient lifting capacities. There are not so many vessels with
sufficiently large crane for removing in one piece the topsides of an oil platform
and they are very expensive.
[0004] The object of the present invention is to propose a solution to the problems of decommissioning
offshore platforms which is safer, less sensitive to weather conditions and cheaper
than the currently used methodology and equipment.
[0005] To this aim, the invention relates to an offshore decommissioning structure for disassembling
an offshore platform presenting topsides mounted on a jacket having a plurality of
jacket legs, wherein said decommissioning structure comprises on the one hand a jack-up-vessel
which comprises a hull, a main deck, an elevating jacking device and a plurality of
jack-up legs, wherein said jack-up vessel can be elevated on its legs above the sea
level by the elevating jacking device; and, on the other hand a carrying and skidding
system for shifting the topsides of the platform to be disassembled from the platform
jacket onto the main deck of the jack-up vessel.
[0006] The invention concept lies in the utilization of a jack-up which is converted from
its usual drilling activities to platform decommissioning. A jack-up vessel can be
lifted on its legs sufficiently high above the sea level for enabling most of the
preparation works to be performed in unrestricted weather conditions. While said preparation
works can be done with the jack-up main deck at the same level as the platform main
deck, namely at survival level, only the installation of the skidding system and the
shifting of the platform topsides requires favorable weather conditions even though
the resulting restriction are not so stringent as when floating vessel are used. This
means that a less expensive equipment is mobilized over a shorter period of time as
less interruption caused by weather deterioration will occur.
[0007] Advantageously, the carrying and skidding system comprises a skid beams set for bridging
the platform jacket and the jack up vessel.
[0008] Preferably, the skid beams set comprises: a plurality of jack-up skid beams which
are fastened on the jack-up main deck; and a plurality of jacket skid beams which
can be transferred from a first position where they rest on the jack-up main deck
to a second position outside of the jack-up main deck where they can be attached to
the jacket legs of the platform. This arrangement generates a skidding plan over which
the topsides can be skidded for being transferred from the jacket to the jack-up main
deck.
[0009] Additionally, the skid beams set may comprise a plurality of link beams, each of
said link beams being attached by a hinged end to an aft end of a jack-up skid beam,
wherein said link beams can be lowered for connecting said jack-up skid beam to a
corresponding jacket skid beam, when it is in the second position, and raised for
disconnecting them. The hinged link beams make it possible to connect or disconnect
as appropriate the jack-up vessel to the platform, enabling disconnection of the two
structures in case of weather deterioration.
[0010] Preferably, the skid beams set comprises at least two parallel skid beams subset
each comprising a pair of jack-up skid beams aligned with a corresponding pair of
jacket skid beams, each one being adapted to be positioned on opposite sides of at
least two jacket legs when they are in said second position. This skid beams arrangement
allows a balanced load transfer on the platform legs.
[0011] Advantageously, the offshore decommissioning structure comprises further skid beam
support assemblies which can be attached on the jacket legs for supporting the jacket
skid beams when in said second position.
[0012] Preferably, each skid beam support assembly comprises: a support ring which can be
welded on a jacket leg, and a skid beam support clamp which can be mounted on the
jacket leg above the support ring and which presents lateral skid beam brackets for
supporting the jacket skid beams on both side of the jacket leg. This arrangement
optimizes the load transfer from the jacket skid beams to the jacket legs via the
welded support rings.
[0013] Additionally, the offshore decommissioning structure comprises skid assemblies for
shifting the platform topsides from the jacket to the jack-up main deck, said skid
assemblies being able to move on the skid beams top surface when the jacket skid beams
are in said second position. The skid assemblies comprise skid shoes positioned on
the top surface of the skid beams; vertical jacks mounted on the skid shoes for lifting
the platform topsides; and horizontal jack assemblies for shifting the platform topsides
to and off the jack-up main deck.
[0014] Preferably, the horizontal jack assemblies are push-pull units which can move back
and forth the platform topsides on skid tracks formed on the top surface of the skid
beams, and the jacket skid beams on the jack-up skid beams for moving between said
first and second positions. Push-pull units can be found easily by different vendors
and are completely autonomous.
[0015] Advantageously, the vertical jacks and the horizontal jack assemblies are hydraulic
jacks and are actuated by a hydraulic pump controlled by a control unit.
[0016] Additionally, the offshore decommissioning structure comprises further: at least
a supporting crane for lifting loads between the jack-up vessel and the platform topsides,
and/or at least a mobile working platform for reaching working areas in particular
below the platform topsides. This equipment enhances the safety and facilitates the
preparation works on the platform.
[0017] Advantageously, the jack-up vessel further comprises: at least a gangway/bridge for
enabling access from the jack-up vessel to the platform; and/or walkways attached
on at least one side of each jacket skid beam for reaching all areas along the jacket
skid beams.
[0018] According to a second aspect of the invention, it is proposed a method for disassembling
an offshore platform using the offshore decommissioning structure as defined above
and comprising the following steps: position the jack-up vessel close to the platform
with the stern oriented toward the platform, lift the jack-up vessel at working height;
position skid beams on jack-up main deck relative to the platform legs; push the jacket
skid beams toward the platform jacket legs in said second position; cut the platform
jacket legs below the bottom part of the platform topsides; and shift platform topsides
on the skid beams to the jack-up main deck.
[0019] Additionally, a preparation stage may be performed on the platform before the step
of cutting the platform jacket legs, the preparation stage comprising the following
steps: perform the plug and abandonment campaign if not already done before; remove
a communication tower of the platform if not already done during the plug and abandonment
campaign; remove a vent boom of the platform; remove at least a part of a helideck;
remove all elements of the platform bottom part which clash with the skid beams; and
cut all secondary parts between platform jacket and topsides for providing a passage
for the jacket skid beams and the topsides shifting.
[0020] Preferably, skid beams support assemblies are installed before the step of pushing
jacket skid beams toward the jacket platform legs, the installation of the skid beam
support assemblies comprising the following steps: weld the support rings on the platform
jacket legs; bolt together half tubular parts of the skid beam support clamps around
the platform jacket legs above the support rings; and put grout between the skid beam
support clamps and the platform jacket legs for compensating the diameters difference
if necessary.
[0021] Advantageously, the following steps are performed before the step of pushing the
jacket skid beams toward the platform: lower the jack-up vessel to a level below a
platform shifting elevation; verify the skid beams position relative to the platform
legs and reposition as appropriate; and install support and stiffening structures
on the lower of the platform topsides as required for lifting the topsides.
[0022] Preferably, the following steps are performed before the step of cutting the platform
jacket legs: position the link beams to connect the jack-up skid beams to their respective
jacket skid beams; move the skid assemblies under the platform topsides supporting
points; connect to a pump and test hydraulic jacks for lifting and shifting the topsides;
and extend the vertical jacks for preloading the topsides fastening to the jacket
legs.
[0023] Additionally, the following steps are performed before the step of pushing the platform
topsides to the jack-up vessel: lift the platform topsides with the vertical jacks
for checking the weight of the topside with an integrated weighting system; if topsides
weight is within an acceptable threshold, then continue to lift the topsides until
the defined clearance with the jacket is reached; if topsides weight is above the
acceptable threshold check, then the platform topsides are lowered back and secured.
[0024] Preferably, the following steps are performed when the platform topsides have reached
their final position on the jack-up main deck: lower topsides on transport support
points on the jack-up main deck; open skid beam connections at link beams; fasten
topsides on jack-up vessel main deck; pull back the jacket skid beams on the jack-up
main deck; lower the jack-up vessel on the sea; and tow the jack-up vessel with the
topsides on jack-up main deck to the offloading location.
[0025] Additionally, the following steps may be performed before critical stages of the
method: check weather forecast; if weather is deteriorating disconnect the jack-up
vessel from the platform and lift up the jack-up vessel to safe elevation, or if weather
forecast is favorable, then perform the planned critical stage.
[0026] Further features and advantages of the invention will be revealed in the below description
of non-limiting examples of achieving the different aspects of the invention. The
description will be supported by figures also provided as non-limiting examples.
- Figure 1a and 1b represent a top and side view of a jack-up vessel of an offshore
decommissionary structure according to one embodiment of the invention;
- Figure 2a and 2b represent a top and a side view of a platform PKA;
- Figures 3a represents a top view of the jack-up vessel positioned with respect to
the platform;
- Figures 3b represents a top view of the jack-up vessel with the platform topsides
shifted on the jack-up main deck;
- Figure 4 illustrates a skid beam support assembly mounted on a platform jacket leg;
- Figure 5a illustrates a pair of link beams with the connection in open situation;
- Figure 5b illustrates said pair of link beams of figure 5a in connected position;
- Figure 6 represents a side view of jacket skid beams with skid assemblies;
- Figure 7 represents a side view of a skid assembly;
- Figures 8a to 8c represent different stages of the jacket skid beams positioning;
and
- Figure 9 represents in perspective the skid beams arrangement in skidding position.
[0027] In the following, the description and figures are based on the removal of the platforms
Pickerill A (PKA) or Pickerill B (PKB) topsides. All the indicated values are based
on these two examples and are not limitative for the definition of the invention and
it must be understood that they may vary according to the kind of offshore structure
to be dismantled by the structure of the invention. The weight of the PKA topsides
is estimated at 1.536 tones and the weight of PKB topsides is estimated at 1.398 tones.
However, in order to provide sufficient safety margin and to allow the reutilization
of the invention equipment the decommissioning structure should be able to remove
topsides weighting 1.800 tones. This allows the removal of the topsides for this kind
of platform in one piece and one operation.
[0028] As shown in figures 1a and 1b, the invention concept is based on the utilization
of a converted drilling jack-up (JU). The example chosen for describing the invention
is the JU Energy Endeavour which is a drilling platform comprising a JU vessel 1.
The jack-up vessel 1 has a hull 2 which can float on the sea so that the JU vessel
1 can be towed to the drilling area on an oil or gas filed located. It further comprises
three legs 3 which can be jacked down to the sea bed for lifting the hull 2 above
sea level 8. Similarly, the legs 3 can be jacked up for lowering the hull 2 as appropriate.
In general, JU Energy Endeavour operates on the North Sea. Once the jack-up vessel
1 has reached the drilling area, the three legs 3 are lowered to the sea bed 4 (see
figure 2b) by an elevating jacking device for lifting the JU vessel 1 above the sea
level 8 at the desired working level or at survival level in case of storm when the
work to be performed is below the height of the waves plus a safety margin. The JU
vessel 1 comprises further spuds at the lower end of the legs 3 and ballast tanks
which are filled with a determined amount of water in order to preload the JU vessel
1 on the sea bed 4 and stabilize the jack-up vessel on the sea bed or in floating
condition.
[0029] This JU vessel 1 has been converted for offshore structure removal, in particular,
but not exclusively, for topsides removal operations in the North Sea. However, it
must be understood that other type of JU vessel can be used and that the decommissioning
operations according to the invention can be performed in offshore oil or gas fields
in all seas and oceans. The intended main tasks of the converted JU vessel 1 are the
plugging and abandonment, the preparation works on platforms and in particular the
removal and transportation of the topsides 12. For this purpose, the drilling equipment
such as drilling tower and support cantilevers are removed and a grid of load distribution
beams 35 are installed on the JU main deck 5 for receiving the platform topsides 12
once removed from the platform jacket 10. Furthermore, the JU vessel 1 is equipped
with a new crane 6 located at the JU stern 7 for assistance works. The JU vessel 1
is for example 68 m long and 70 m wide. It weights 11.603 tones.
[0030] The supporting crane 6 at the stern 7 of the JU 1 is mainly used for preparing the
topsides before their removal. In the following the crane 6 used as example for describing
the invention is a 300mt PMC-6200-300 from Huisman. The crane 6 can lift a maximum
load of 300 tones with a 12,6 m outreach. The maximum working radius for the hoist
is 48 m with a maximum load of 50 tones. The wind speed that the jack-up vessel can
stand is an important design parameter, for example a 20 m/s wind speed during operation
at JU main deck level, 36,7 m/s in survival conditions and 30 m/s during transport.
[0031] Besides the stern 7 of the JU vessel 1, a platform 9 to be decommissioned, for example
PKA, is shown in figure 3a. The PKA platform operates in the Southern North Sea. As
shown in figures 2a and 2b, the platform 9 comprises a jacket 10 with four legs 11
anchored in the sea bed 4. The jacket 10 supports the topsides 12 which are fastened
at the top end of the jacket legs 11 at sufficient elevation above the sea level 8.
The upper part of the topsides 12 consists in a main deck 13 located at 30,5 m above
the sea bed and the lower part consists mainly in a cellar deck 14. Above the main
deck 13 of the topsides 12, there is a helideck 15 where a helicopter can land, a
communication tower 16 and a vent boom 17. The water depth at PKA location is between
20,3 m (lowest astronomic tide) and 25,54 m (highest astronomic tide) with storm surge
it can go from 22,13 m (LAT) to 27,37 m (HAT).
[0032] As shown in figure 3a, the jack-up vessel 1 is positioned beside the platform 9 with
the stern 7 pointing to the platform. The jack-up vessel 1 is used for preparing the
platform topsides 12 for its removal from the jacket 10 and the topsides towing to
its offloading place on the shore or on a barge where it will be dismantled. The distance
between the jack-up vessel hull 2 and the platform topsides 12 should be comprised
for example between 0,75 m and 2,25 m, e.g. 2,15 m. The removal sequence is mainly
the same for removing any kind of platforms besides PKA such as Pickerill B (PKB)
for example. The main difference between the removal of different platforms lays mostly
in the preparation works which need to be adapted to the features of the platform
to be decommissioned.
[0033] As shown in figures 8a a set of skid beams 18 and 19 are installed on to the JU main
deck 5. The skid beams set consists of two parallelly positioned subsets of skid beams
18 and 19. Each subset comprises two parallel adjusted skid beams 18 and 19. The skid
beams are split into JU skid beams 18, which are located on the JU main deck 5 via
the load distribution beams, and jacket skid beams 19 which are moved from a first
position on the JU skid beams 18 on the JU main deck 5, as shown in figures 3a and
8a, to a second position outside of the JU main deck 5 onto the existing jacket legs
11, as shown in figures 8c and 9. When the jacket skid beams 19 are in the second
position, the topsides 12 supported by the skid beams 18 and 19 can be skid from the
jacket 10, as shown in figure 3a, to the JU main deck 5, as shown in figure 3b. As
shown in figures 5a and 5b, each JU skid beam 18 is connected to its respective jacket
skid beam 19 by a link beam 21 which is connected by a hinged end 22 with pin connection
to the JU skid beam 18 at the JU aft edge 23.
[0034] At the beginning of the removing sequence, the jacket skid beams 19 are in the first
position on the top of the JU skid beams 18 on the JU main deck 5, as shown in figure
8a. Then the jacket skid beams 19 are pushed toward their second position on the jacket
10 by skidding on the JU skid beams 18 as shown in figure 8b.
[0035] As shown in figure 8c, when they reach their second position, the jacket skid beams
19 are supported and connected to the jacket legs 11 by support assemblies 20. The
support assemblies 20 are connected to each one of the four jacket legs 11 below the
topsides cellar deck 14. As shown in figure 4, each of the support assemblies 20 comprise
a support ring 24 welded on the jacket leg 11 and a support clamp 25 consisting of
two half tubular parts 26 bolted together around the leg 11. Grout can be put between
the support clamp 25 and the jacket leg 11 if the jacket legs outer diameter is smaller
than the inner diameter of the support clamp 25. The half tubular parts 26 present
sidewise support brackets 27 on which the jacket skid beams 19 of a sub set of skid
beams will rest on each side of the jacket leg 11.
[0036] The JU skid beams 18, the link beams 21 and the jacket skid beams are welded box
profile. For example, the dimensions of the jacket skid beams in mm can be: height
1500, width 500 x vertical walls thickness 40 and horizontal walls thickness 70, and
the dimensions of the jack-up skid beams and link beams profile can be: height 900,
width 500, vertical walls thickness 40 and horizontal walls thickness 70. The skid
beams 18 and 19 are planned to be fabricated as welded boxed girder made from material
with yield strength of 355 MPa. Local parts of the skid beams are designed with yield
strength of 460 MPa.
[0037] For supporting the topsides 12, the jacket skid beams 19 are planned to be arranged
sidewise of the legs 11, as shown in figure 9, with an eccentricity of 1000 mm to
the leg axis. Therefore, as shown in figure 4, it is planned to weld the support ring
24 onto each jacket leg 11 and the support clamp 25 with sidewise support brackets
27 will be mounted onto the legs. The bracket clamp 27 transfers the vertical loads
via the support ring 24 underneath into the legs 11.
[0038] As shown in figure 5b, each link beam 21 connects together a JU skid beam 18 with
its respective jacket skid beam 19 in order to form a continuous skidding track extending
from the extremity of the platform jacket 10 to the end of the topsides storage area
on the JU main deck 5, as shown in figure 9. As it can be seen in figure 5b, the link
beam 21 is lowered to a connected position for connecting the JU skid beam 18 to its
respective jacket skid beam 19, and it can be raised to a disconnected position shown
in figure 5a where the skid beams 18 and 19 are disconnected. The jacket skid beams
19 and the JU skid beams 18 are only connected temporarily by the link beams 21, mainly
when the topsides 12 are skidded from the platform jacket 10 to the topsides storage
area on the JU main deck 5.
[0039] After topsides preparation is completed, the four legs 11 of the platform 9 are cut
below the cellar deck 14, and the topsides 12 is lifted by means of sixteen skid beams
assemblies 28 positioned on the jacket skid beams 19 (see figure 7). Further on, the
topsides 12 are moved on the skidding track formed by the top surface of the jacket
skid beams 19, over the link beam 21 and then on the JU skid beams 18 toward the JU
main deck 5. When the topsides 12 have reached their final position, they are lowered
on dedicated transport supports on the storage area on the JU main deck 5 and are
fastened for being towed. Eventually, the JU jacking system lower the JU vessel 1
at sea level with the topsides 12 fastened on the JU main deck 5 and it can be, for
example, towed to the shore at the offloading location where the topsides will be
dismantled.
[0040] This skidding sequence is performed by skid assemblies 28 shown in figures 6 and
7. The skid assemblies 28 are interposed between the bottom parts of the topsides
12 and the top surface of the jacket skid beams 19. The skid beam assemblies 28 ensure
the lifting and lowering of the topsides 12 as well as the pushing and pulling movement
of the topsides 12 on the skid beams 18 and 19. The skid assemblies 28 are suitable
to move the topsides 12 from the jacket 10 onto the jack-up main deck 5, and then
to offload the topsides 12 to a quay side or a barge. The total topsides weight to
be skidded is 1.600 tones including the weight contingency. The skid assemblies 28
are able to deal with at least 1.800 tones topsides weight in order to provide a 200
tones safety margin. The skid assemblies 28 comprise skid shoes 29 which glide on
the skidding tracks constituted by the skid beams top surface. A sliding interface
may be provided on the skid beams top surface for lowering the friction during the
skidding stages. During the topsides skidding, the loads in the skid shoes should
be actively controlled by vertical jacks 30 with a lifting stroke of at least 150
mm. These jacks 30 can be hydraulic or of other type, e.g. electrical jack. The vertical
jacks lift the topsides above the platform jacket 12 in order to provide sufficient
clearance for skidding the topsides 12 away from the jacket legs 11 and lower the
topsides on the JU main deck 5 once the topsides skidding is completed. Push-pull
units 31 comprising horizontal hydraulic jacks control the topsides movements on the
skid tracks provided on the top of the skid beams 18 and 19. The horizontal jack of
the push-pull units 31 are hydraulic in this example but other type of jacks can be
used such as electrical jacks.
[0041] A control system is required to operate and control the vertical hydraulic jacks
30 and the horizontal hydraulic jacks of the push-pull units 31 remotely from a central
control console.
[0042] The topsides skidding distance from the platform jacket 12 to the final position
on the storage area of the JU main deck 5 is about 30 to 40 m. The topsides traveling
speed of the skid assemblies is at least 20m/h. This results in a required time window
of less than 2 hours for the topsides skidding operation itself.
[0043] The skid beams arrangement creates two skid tracks on each side of the jacket leg
11 to avoid eccentric loading of the legs 11 during the topsides skidding. A total
of sixteen skid assemblies 28 are arranged underneath the topsides 12. The vertical
jacks 30 are grouped in order to apply the same hydraulic pressure to the jacks within
the same group. The vertical jacks grouping is defined for optimizing the required
jacking forces considering the center of gravity location.
[0044] The skid assemblies 28 should comply with different technical requirements for obvious
safety reasons. For example, the skidding should be possible in the two directions
on the one hand for pushing the jacket skid beams 19 to the platform jacket legs 11
and to pull them back on the jack-up main deck once the topsides skidding is completed,
and for skidding the topsides 12 from the jacket 10 to the JU main deck 5 and for
offloading the topsides 12 from the jack-up main desk 5. In particular, the push-pull
units 31 must generate a sufficient pulling and pushing force. The vertical hydraulic
jacks 30 must provide sufficient lifting force for lifting the topsides 12 and weighting
of the topsides should be possible during lifting. Given that weather can deteriorate
very quickly, the skid assemblies 28 should make it possible to stop and resume the
topsides skidding at every stage of the skidding sequence and at any position. A robust
and adaptive design of the skid assemblies 28 is of high importance as the structure
of the invention is supposed to be reusable for decommissioning other platforms than
PKA and PKB. Robustness is particularly important because the structure will operate
offshore. For this reason, the skid assemblies 28 must be easy to handle in an offshore
context and provide sufficient system redundancy in case of failure of components
(such as pumps, valves, hoses, hydraulic cylinders, control unit). Moreover, the equipment
must be exchangeable in offshore conditions.
[0045] The maximum jacking force per skid shoe is 138 tones considering a topsides weight
of 1,800 tones. Vertical hydraulic jacks with at least 150 tones capacity and 150
mm stroke are considered for jacking the topsides 12 after that the jacket legs 11
are cut. During skidding, the topsides 12 is supported by activated vertical jacks
30 which are pressure/force controlled. Thereby, the load in the vertical jacks 30
is monitored and controlled during skidding. This avoids overloading of skid assemblies
28, jack-up vessel 1 and topsides 12. During the topsides skidding stage, deflections
and installation tolerances are compensated by the stroke of the vertical jacks 30.
Furthermore, the hydraulic vertical jacks 30 are capable to withstand 5% of their
vertical capacity as horizontal load.
[0046] The vertical jacking system is able to determine the topsides weight and center of
gravity with an accuracy of 5% for enabling the structure to be usable for other platforms
than PKA and PKB.
[0047] Push-pull units 31 are used for skidding the topsides 12 because the push-pull-units
are able to move the topsides 12 back and forth without additional preparation. This
is beneficial for offloading operation and can also be considered as a safe return
option during topsides removal offshore. The push-pull units 31 are combined with
horizontal guiding system. However, alternatives to push-pull units such as horizontal
hoist could be considered for moving the jacket skid beams and/or the topsides 12.
[0048] The push-pull unit capacity is about 300 kN, which is sufficient to overcome the
friction loads of maximum vertical capacity, a slope of 1/100 and wind loads. The
topsides traveling speed during the skidding operation is about 24 m/h and the topsides
skidding distance is about 40 m which results in the topsides skidding being performed
within 2 hours. Another advantage of using push-pull-units 31 for skidding the topsides
12 is that they can also be used for installing the jacket skid beam 19 on the jacket
legs 11 and for bringing the skid assemblies in position underneath the topsides.
In addition to skidding longitudinally the topsides from the jacket 10 to the jack-up
main desk 5, it may also be useful to move the topsides 12 transversally after that
it has been skidded on the JU main deck 5, e.g. for balancing the load on the jack-up
vessel.
[0049] Different equipment may be needed for completing the offshore decommissioning structure,
such as lifting devices, mobile working platforms and walkways for provide a safe
working area and supporting the preparation work on the topsides, the removal and
the tow preparation. For example, a bridge or a gangway is required to provide access
form the jack-up vessel 1 to the platform 12 and also to guide service lines (e.g.
electrical power). As the main preparation work is performed below the topsides which
is not easily reachable by the JU crane 6, a mobile working platform is foreseen which
is connected to the stern of the jack-up vessel and which can reach the different
working areas. With this mobile platform, rope access works can be avoided, and scaffolding
reduced which also enhances safety. Similarly, a walkway will be attached to the side
of each jacket skid beam 19 opposite to the platform leg 11. From these walkways all
areas along the jacket skid beams 19 and below the topsides 12 can be reached safely
and conveniently.
[0050] According to the invention, a method for disassembling an offshore platform using
the offshore decommissioning structure described above is explained in the following,
in particular the removal of the platform topsides 12 from the platform jacket 10
to the main deck 5 of the jack-up vessel 1. This topsides removal sequence can be
split in several stages: the jack-up vessel positioning, the platform preparation,
the installation of support assemblies for the jacket skid beams, the installation
of the jacket skid beams, the installation of topsides supports, the installation
of the skid assemblies, topsides lifting and skidding on the jack-up main deck 5,
and jack-up vessel lowering and towing.
[0051] In the following the jack-up positioning stage, as all the other stages, is described
based on the example of decommissioning the PKA platform. As shown in figures 3a,
the jack-up vessel 1 is positioned north of the platform 9 using, for example, the
same position as planned during the plugging and abandonment activities. Distance
range between the stern of the jack-up vessel and platform has been given above. The
jacket skid beams 19, the skid assemblies 28 and all other elements of the decommissioning
structure, such as the support assemblies 20, are located on the jack-up main deck
5.
[0052] After positioning of the jack-up vessel 1, the jack-up legs 3 are lowered to the
sea bed 4 and the jack-up hull 2 is lifted above the sea level 8. Preloading is performed
considering maximum loads resulting from survival conditions including the removed
topsides12 on jack-up main deck 5. After preloading, the jack-up vessel 1 is lifted
to a higher elevation used for the preparation activities. If the jack-up vessel has
been previously located beside the platform, spud cans 32 shall be positioned into
the existing depressions formed during the first jack-up vessel positioning. The ballast
tanks are filled for preloading the jack-up legs 3 and then the jack-up hull 2 is
lifted at the required elevation for performing the preparation works of the platform
and the plugging and abandonment activities if still required.
[0053] Once the jack-up vessel 1 is in position and before removing the topsides 12, several
activities need to be performed. This includes removing a part of the helideck 15
and the vent boom 17, which both would clash with parts of the jack-up vessel during
the skidding. These elements can be cut and lifted off with the jack-up crane 6 to
the JU main deck 5 and afterwards cut in smaller pieces. The communication mast 16
is removed if not already done during plugging and abandonment campaign. Furthermore,
secondary platform elements located between the jacket 10 and the topsides 12 like
risers, ladders are also removed, if not already done, because they can clash with
the skid beams 18 and 19.
[0054] Before that the skid beams can be installed, four skid beam support assemblies 20
are fastened to the jacket legs 11. For each support assembly 20, the support ring
24 is welded to the jacket legs 11 to provide a vertical support. Then, a support
clamp is mounted around the legs 11 by bolts. The support assembly parts are lowered
from the cellar deck 14 and temporary secured by lifting devices before being finally
welded and bolted to the legs 11. Access to the area for performing these activities
is provided by the mobile work platform.
[0055] The support clamp inner diameter is 1.25 m in order to fit jacket leg of other platforms
as well. The gap between the clamp and the PKA jacket legs (outer diameter 0.9 m)
will be filled with grout or similar material.
[0056] The JU skid beams 18 are positioned on the JU main deck 5 for being aligned with
the jacket legs and then fixed with brackets. Afterwards, the jacket skid beams 19
are positioned on the top of the JU skid beams 18 and fixed by lateral guiding. After
the JU vessel 1 has been lowered, the jacket skid beams 19 are pushed towards the
jacket 10 by hydraulic jacking units, such as push-pull units 31. The jacket skid
beams 19 are secured against uplifting before the second support assembly 20 on the
jacket 10 is reached. The support assemblies 20 on the jacket legs 11 will also have
some lateral guides to prevent the jacket skid beams 19 from capsizing/falling of
the supports.
[0057] When the jacket skid beams 19 are in their second position, they are linked together
and secured to the legs 11.
[0058] After the jacket skid beams 19 are installed, the link beams 21 are installed at
the aft edge 23 of the JU skid beams 18. The link beams 21 can be closed and opened
depending on the planned operation and required airgap between the sea level 8 and
JU hull 2.
[0059] For lifting the topsides, the vertical hydraulic jacks 30 which mounted on the skid
assemblies 28, will be used after their positioning on the jacket skid beams 19 below
the topsides 12. To this end, primary beams 33 on the topsides 12 are used as supports
wherever possible (see figure 6). They can be reinforced with local stiffeners if
necessary. In areas were only secondary beams are available on the topsides 12, there
is a need to add a more robust section. Topsides supports 34 are also fastened to
the topsides structure where the vertical jacks cylinders will exert their lifting
force. All the reinforcements and added supports will be welded to the existing structure
and can be installed from the walkway below, which is connected to the jacket skid
beam as indicated above.
[0060] The skidding of the topsides 12 is done by the skid shoes 29 of the skid assemblies
28 which are moving on the skidding track formed by the skid beams top surface (see
figures 6 and 7). The skid assemblies 28 include the vertically oriented hydraulic
jacks 30 which can lift the topsides 12 and horizontal hydraulic jacks included in
push-pull units 31 for moving horizontally the topsides 12. The skid shoes 29 which
are part of the decommissioning structure are preinstalled on the skid beams pre-positioned
on the JU vessel 1. During the operation, they can move from the JU main deck 5 to
the platform for being positioned underneath the topsides 12 by their own horizontal
jacks. The skid assemblies 28 are accessible from the walkway (not illustrated) connected
to the jacket skid beams 19. Afterwards, the skid assemblies are installed and tested
and the vertical jacks cylinders are extracted until they come in contact with the
topsides supports 34. Eventually, the weather is checked and if it is favorable, then
the JU vessel 1 is lowered from the jacked skid beams installation level (see figure
8c) to the skidding elevation where the top surfaces of the jacket skid beams 19 is
at the same level as the JU skid beams 18 (see figure 6 and 9). Then, the link beams
21 are installed on the JU skid beams 18, as shown in figure 5a, and lowered to connect
the JU skid beams 18 with the jacket skid beams 19 as shown in figure 5b. In an event
of bad weather, the connection can be opened by lifting the link beam 21 and the Jack-up
vessel may be lifted to safe elevation. Once the skidding track on the top the skid
beams and link beams, the skid assemblies 28 are moved under the topsides supporting
points 34. Again, the weather forecast is checked and if weather is deteriorating,
the link beams are lifted for disconnecting the skid beams 18 and 19 and the JU vessel
1 is lifted to safe elevation. If weather forecast is favorable, then the operation
can proceed further with the topsides removal.
[0061] The removal of the topsides 12 will start with the cutting of the platform legs 11
below the cellar deck 14. Cutting may be done by flame cutting, water jet cutting
or diamond wire. The vertical jacks 30 will be regulated to keep the vertical level
of the topsides 12 constant during the cutting step and to exert a preload on the
jacket legs 11. After that the four legs 11 have been cut, the weight of the topsides
12 can be estimated by reading the pressure in the hydraulic system of the vertical
jacks 30.
[0062] If the topsides weight is considered too high for the removal, then the topsides
12 will be lowered back on the jacket legs 11. In this event, a temporary fixation
is added to secure the topsides 12 until weight shedding has been performed. A survey
with regards to topsides weights will be performed during the preparation and the
plugging and abandonment phase to verify the weight report. However, as all skidding
equipment and strength checks are based on a topsides weight estimation of 1,800 tones
while the expected weights of the topsides is, for example,1536 tones for PKA and
1398 tones for PKB including weight factors, an overweight occurrence for the topsides
is unlikely. Therefore, it is not expected that weight shedding is required.
[0063] If, as very likely, the topsides weight is considered acceptable for the removal,
the topsides 12 will be further lifted by 75 mm to provide sufficient clearance underneath
the topsides 12. Subsequently, the topsides skidding is performed moving the skid
assemblies along the skid beams 19, 21 and 18 until it reaches the final position
on the JU main deck 5. Finally, the topsides 12 is lowered on transport supports on
the JU main deck 5.
[0064] When, the topsides 12 is completely moved onto the JU main deck 5, as shown in figure
3b, the link beams 21 are lifted for disconnecting the skid beams 18 and 19 and the
JU vessel 1 is lifted to a higher elevation.
[0065] When the topsides have been put down on the JU main deck 5, it will be finally secured
for the tow by sea-fastening. In parallel, the jacket skid beams 19 which are still
located on the jacket 10 are removed, for example by lifting them with the JU crane
6 to the JU main deck 5. After confirmation of adequate weather forecast, the JU vessel
1 will then be ballasted to equalize heel/trim and lowered to sea. Then a tow line
is connected to tugs and jack-up legs 3 are lifted. Eventually, the Jack-up vessel
1 with topsides 12 on main deck 5 is towed to the offloading location.
[0066] After removal of the topsides 12 from the jacket 10 and transport on the jack-up
main deck 5, eventually, the topsides 12 must be offloaded from the jack-up vessel
1. Different options are possible and four examples are listed below.
[0067] The first option consists in offloading the topsides 12 onto a quay with the jack-up
vessel elevated at quay level skidding the topsides directly on the quay. The second
option is to offload the topsides on a barge with the jack-up floating. The third
option consists in lifting the topsides 12 from the jack-up vessel 1 with a floating
crane and then to lower it on a quay. The fourth option consists in cutting the topsides
in small piece while being on the jack-up main deck 5 and then removing the small
pieces with the jack-up crane 6 or any other mobile crane directly on a quay or on
a barge.
[0068] The first offloading option is the preferred one, as it does not require an additional
barge or vessel and the dismantling of the topsides on land is the easiest way. In
this scenario, the jack-up is elevated beside a quay at quay level. The jacket skid
beams are installed on the quay and are connected to the jack-up skid beams by the
link beams. Then, the topsides is skidded on the quay as a reverse operation to the
offshore removal from the platform jacket. Afterwards topsides can be dismounted on
the quay. Jacket skid beams can be moved back on the jack-up main deck as soon as
the topsides is lowered to support points on the quay. The advantages of this option
are the usage of the same skidding equipment as for the offshore skidding, and a relatively
short duration until the topsides is on land and the jack-up vessel can be used for
the next job. However, it requires that an adequate quay side can be found with respect
to the loading capacity and height, and additional structure on the offloading quay.
Moreover, this may require the jack-up vessel ballasting when the topsides is skidded
to land to avoid floating up the JU.
[0069] Offloading the topsides on a barge requires that the jack-up vessel is floating in
a sheltered area and a barge is moored to the stern of the jack-up vessel. Then, the
topsides is skidded on the barge exactly as on the quay for the first option. Eventually,
the topsides can be dismounted on the barge and the parts lifted to quay by mobile
cranes. This option requires a barge with sufficient strength and adequate height
with ballasting capacities. It also requires sufficient ballasting capacity of the
jack-up vessel in order to overcome change of the center of gravity. The advantages
of the second option are that the same skidding equipment as for the offshore topsides
removal can be used, and the skidding to the barge is independent from the water depth
and tide level. The drawbacks are the hiring costs for the barge until final removal
of topsides, and more complex operation compared to skidding to quay due to the hydrostatic
stability of the barge and jack-up vessel during the topsides skidding.
[0070] For the third option, the jack-up vessel is floating in a sheltered area (e.g. a
harbor). A floating heavy lift crane approaches the jack-up vessel from the stern
and lifts the topsides. Afterwards, the floating crane is moved to the quay and lowers
the topsides on the quay for final dismounting. The constraints of this option lie
in finding a large floating crane with sufficient lifting capacity, outreach and hook
height. The advantages of the third option are a relatively short duration until topsides
is on land which means that the JU can be used quickly for next job. However, the
disadvantages are in the hiring costs for the crane vessel and finding appropriate
quay side.
[0071] The fourth option consists in removing the topsides in small pieces with the jack-up
crane. This means that the jack-up vessel must be elevated beside a quay, the topsides
is dismantled on the JU deck in small parts and the parts are then lifted by the jack-up
crane to the quay. The advantages of this solution are that no additional equipment
is required, and it does not involve any critical operation. The main drawback is
that this option is very much time consuming and the jack-up vessel cannot be used
for the next job until dismantling of the topsides.
[0072] Despite that, in the above description, specific aspects of the invention have been
presented in the context of the decommissioning of PKA and PKB, it can be implemented
in different contexts requiring technical adaptation which would not escape from the
scope of the invention. All the values indicated in the above description are given
as example based on the removal of PKA platform. It must be understood that they may
vary for decommissioning other platforms or offshore structures.
1. Offshore decommissioning structure for disassembling an offshore platform (9) presenting
topsides (12) mounted on a jacket (10) having a plurality of jacket legs (11), said
decommissioning structure is
characterized in that it comprises:
• a jack-up-vessel (1) comprising a hull (2), a main deck (5), an elevating jacking
device and a plurality of jack-up legs (3), wherein said jack-up vessel (1) can be
elevated on its legs (11) above the sea level (8) by the elevating jacking device;
and
• a carrying and skidding system for shifting the topsides (12) of the platform to
be disassembled from the platform jacket (10) onto the main deck (5) of the jack-up
vessel (1).
2. Offshore decommissioning structure according to claim 1 wherein said carrying and
skidding system comprises a skid beams set for bridging the platform jacket (10) and
the jack up vessel (1).
3. Offshore decommissioning structure according to claim 2 wherein said skid beams set
comprises:
• a plurality of jack-up skid beams (18) which are fastened on the jack-up main deck
(5); and
• a plurality of jacket skid beams (19) which can be transferred from a first position
where they rest on the jack-up main deck (5) to a second position outside of the jack-up
main deck where they can be attached to the jacket legs (11) of the platform (9).
4. Offshore decommissioning structure according to claim 3 wherein said skid beams set
further comprises a plurality of link beams (21), each of said link beams (21) being
attached by a hinged end (22) to an aft end (23) of a jack-up skid beam (18), wherein
said link beams (21) can be lowered for connecting said jack-up skid beam (18) to
a corresponding jacket skid beam (19), when it is in the second position, and raised
for disconnecting them.
5. Offshore decommissioning structure according to claim 3 or 4 wherein said skid beams
set comprises at least two parallel skid beams subset each comprising a pair of jack-up
skid beams (18) aligned with a corresponding pair of jacket skid beams (19), each
one being adapted to be positioned on opposite sides of at least two jacket legs (11)
when they are in said second position.
6. Offshore decommissioning structure according to anyone of claims 3 to 5 comprising
further skid beam support assemblies (20) which can be attached on the jacket legs
(11) for supporting the jacket skid beams (19) when in said second position.
7. Offshore decommissioning structure according to claim 6 wherein each skid beam support
assembly (20) comprises:
• a support ring (24) which can be welded on a jacket leg (11), and
• a skid beam support clamp (25) which can be mounted on the jacket leg (11) above
the support ring (24) and which presents lateral skid beam supports brackets (27)
for supporting the jacket skid beams (19) on both side of the jacket leg (11).
8. Offshore decommissioning structure according to anyone of claims 3 to 7 further comprising
skid assemblies (28) for shifting the platform topsides (12) from the jacket (10)
to the jack-up main deck (5), said skid assemblies (28) being able to move on the
skid beams top surface when the jacket skid beams (19) are in said second position;
said skid assemblies (28) comprising:
• skid shoes (29) positioned on the top surface of the skid beams;
• vertical jacks (30) mounted on the skid shoes (29) for lifting the platform topsides
(12); and
• horizontal jack assemblies (31) for shifting the platform topsides (12) to and off
the jack-up main deck (5).
9. Offshore decommissioning structure according to claim 8 wherein the horizontal jack
assemblies are push-pull units (31) which can move back and forth the platform topsides
(12) on skid tracks formed on the top surface of the skid beams, and the jacket skid
beams (19) on the jack-up skid beams (18) for moving between said first and second
positions.
10. Offshore decommissioning structure according to claim 8 or 9 wherein the vertical
jacks (30) and/or the horizontal jack assemblies (31) are hydraulic jacks and are
actuated by a hydraulic pump controlled by a control unit.
11. Offshore decommissioning structure according to anyone of the preceding claims, wherein
the jack-up vessel (1) further comprises:
• at least a supporting crane (6) for lifting loads between the jack-up vessel (1)
and the platform topsides (12), and/or
• at least a mobile working platform for reaching working areas in particular below
the platform topsides (12).
12. Offshore decommissioning structure according to anyone of the preceding claims, wherein
the jack-up vessel (1) further comprises:
• at least a gangway/bridge for enabling access from the jack-up vessel (1) to the
platform (12); and/or
• walkways attached on at least one side of each jacket skid beam (19) for reaching
all areas along the jacket skid beams (19).
13. Method for disassembling an offshore platform using the offshore decommissioning structure
for disassembling an offshore platform (9) presenting topsides (12) mounted on a jacket
(10) having a plurality of jacket leg (11), according to anyone of claims 3 to 10,
said method comprising the following steps:
• position the jack-up vessel (1) close to the platform (12) with the stern (7) oriented
toward the platform (9);
• lift the jack-up vessel (1) at working height;
• position skid beams (18, 19) on jack-up main deck (5) relative to the platform legs
(11);
• push jacket skid beams (19) toward the platform jacket legs (11) in said second
position;
• cut the platform jacket legs (11) below a bottom part of the platform topsides (12);
and
• shift platform topsides (12) on the skid beams (18,19) to the jack-up main deck
(5).
14. Method for disassembling an offshore platform according to claim 13 wherein a preparation
stage is performed on the platform before the step of cutting the platform jacket
legs (11), the preparation stage comprising the following steps:
• perform the plug and abandonment campaign if not already done before;
• remove a communication tower (16) of the platform (9) if not already done during
the plug and abandonment campaign;
• remove a vent boom (17) of the platform (9);
• remove at least a part of a helideck (15);
• remove all elements of the platform bottom part which clash with the skid beams
(18, 19); and
• cut all secondary parts between platform jacket (10) and topsides (12) for providing
a passage for the jacket skid beams (19) and the topsides shifting.
15. Method for disassembling an offshore platform according to claim 13 or 14 wherein
skid beams support assemblies (20) are installed before the step of pushing jacket
skid beams (19) toward the platform jacket legs (11), the installation of the skid
beam support assemblies (20) comprising the following steps:
• weld support rings (24) on the platform jacket legs (11);
• bolt together half tubular parts (26) of skid beam support clamps (25) around the
platform jacket legs (11) above the support rings (24); and
• put grout between the skid beam support clamps (25) and the platform jacket legs
(11) for compensating the diameters difference if necessary.
16. Method for disassembling an offshore platform (9) according to anyone of claims 13
to 15 wherein the following steps are performed before the step of pushing the jacket
skid beams (19) toward the platform (9):
• lower the jack-up vessel (1) to a level below a platform shifting elevation;
• verify the skid beams position relative to the platform legs (11) and reposition
as appropriate; and
• install support and stiffening structures (34) on the lower of the platform topsides
(12) as required for lifting the topsides (12).
17. Method for disassembling an offshore platform (9) according to anyone of claims 13
to 16 wherein the following steps are performed before the step of cutting the platform
jacket legs (11):
• position link beams (21) to connect the jack-up skid beams (18) to their respective
jacket skid beams (19);
• move skid assemblies (28) under the platform topsides (12) supporting points;
• connect to a pump and test hydraulic jacks for lifting and shifting the topsides
(12); and
• extend the vertical jacks (30) for preloading the topsides fastening to the jacket
legs (11).
18. Method for disassembling an offshore platform according to anyone of claims 13 to
17 wherein the following steps are performed before the step of pushing the platform
topsides (12) to the jack-up vessel (1):
• lift the platform topsides (12) with the vertical jacks (30) for checking the weight
of the topside (12) with an integrated weighting system;
• if topsides weight is within an acceptable threshold, then continue to lift the
topsides (12) until the defined clearance with the jacket (19) is reached;
• if topsides weight is above the acceptable threshold, then the platform topsides
(12) are lowered back and secured.
19. Method for disassembling an offshore platform according to anyone of claims 13 to
18 wherein the following steps are performed when the platform topsides have reached
their final position on the jack-up main deck (5):
• lower topsides (12) on transport support points on the jack-up main deck (5);
• open skid beam connections at link beams (21);
• fasten topsides (12) on jack-up vessel main deck (5);
• pull back the jacket skid beams (19) on the jack-up main deck (5);
• lower the jack-up vessel (1) on the sea; and
• tow the jack-up vessel (1) with the topsides (12) on jack-up main deck (5) to the
offloading location.