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(11) | EP 2 514 878 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Jack-up offshore platform and method for reducing lateral oscillating movements thereof |
| (57) The invention relates to a jack-up offshore platform (1). The platform comprises
a work deck (2) supported by spud poles (3) and adjustable in height above the water
surface, wherein at least one spud pole is provided with through-channels (23,24)
for the seawater from an upstream side of the spud pole to a downstream side of the
spud pole. The jack-up platform can be utilized at greater water depths than the known
platform. The invention also relates to a method for operating a jack-up offshore
platform in a water mass with strong current.
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[0001] The invention relates to a jack-up platform comprising a work deck supported by spud poles and adjustable in height above the water surface. The invention also relates to a method for operating the platform in a water mass with strong current.
[0002] A jack-up platform of the above stated type is utilized for carrying out work operations at sea. The spud poles, which are generally situated at the corner points of the work deck of the platform, can be raised or lowered relative to the work deck by jacks provided on the work deck. The platform can thus be easily transported, with the spud poles in raised position, to the location of the operations to be performed. Once the platform has been moved to the desired position, the platform is anchored on or in the seabed by lowering the spud poles with the jacks until they find support on or in the seabed. The work deck is then carried by means of the jacks to a position above the water surface in which the work deck is supported by the spud poles and is clear of the water surface. If desired, the height of the work deck above the water surface can be adjusted by further jacking up the work deck relative to the spud poles (whereby the work deck is raised relative to the water surface) or by lowering the work deck (whereby the work deck is lowered relative to the water surface).
[0003] It will be apparent that a j ack-up platform of the known type can only be employed in relatively shallow waters. This is because the maximum feasible water depth is determined by the spud pole length, which must after all exceed the water depth in order to enable placing of the work deck at different height positions relative to the water surface. The length of the spud poles is however limited by structural considerations. If they were to be given a structurally stronger form, for instance by increasing the wall thickness, they can then become too heavy, whereby transport of the platform is made more difficult or becomes uneconomic.
[0004] An object of the present invention is to provide a jack-up platform which can be utilized at greater water depths. An additional object of the present invention is to provide a platform which can be applied in water masses with a strong current.
[0005] These and other objects are achieved by providing a platform according to the appended main claim. Particularly provided according to the invention is a jack-up offshore platform comprising a work deck supported by spud poles and adjustable in height above the water surface, wherein at least one spud pole is provided with through-channels for the seawater from an upstream side of the spud pole to a downstream side of the spud pole. Providing at least one spud pole of the platform with such through-channels according to the invention achieves that the spud poles can have a greater length than is the case in the known platform. It is hereby possible to work at locations with greater water depth. The through-channels are located at least in that part of the spud pole assumed to be situated in the water.
[0006] A further advantage of the invented platform is that it can be applied for the same known spud pole length in a water mass with strong current where it is no longer possible to work with the known platform in such a current. Halting operations on a platform is very costly and must therefore be avoided.
[0007] Water flowing along the spud poles of a platform can cause vortices on the downstream side of the spud pole. This vortex formation is periodic and causes oscillating forces in the longitudinal direction of the spud pole (these forces are relatively limited) and in the lateral direction of the spud pole (substantially perpendicularly of the spud pole). The lateral forces provide for an oscillating lateral movement of the spud poles and the platform. The frequency of the vortex formation is given by:
wherein:
fK = Von Karman or vortex formation frequency [Hz]
D = external diameter of the spud pole [m]
V = average speed of the current [m/s]
Sn = Strouhal number
[0008] The value of the Strouhal number varies slightly with the Reynolds number but typically equals about 0.2.
[0009] The periodic vortex formation causes an oscillating lifting force on the spud pole in a direction perpendicularly of the current. The maximum force is given by:
wherein:
FK = Von Karman force [N]
CK = coefficient of force [-]
V = average speed of the current [m/s]
A = projection of the spud pole volume in a plane perpendicularly of the current [m2]
ρ = density of the water [kg/m3]
w = 21t*fK
[0010] These lateral movements in a direction perpendicularly of the current can occur with a high frequency in the case of strong current such that they approach the eigenfrequency of the platform and thus cause problems. By providing a platform according to the invention the operation of the platform, and in particular of the spud poles, remains substantially unaffected and lateral oscillating movements of the platform, in particular of the work deck of the platform, particularly also in the case of strong current, are better reduced than is the case in the known platform. When flowing seawater strikes against the spud poles, a part of this seawater will be admitted to the through-channels s via the entrance openings located on the upstream side of the through-channels, after which this seawater flows in the through-channels to the downstream side of the spud pole and leaves the spud pole along this side via the exit openings of the through-channels. A turbulent flow is hereby created locally at the position of the exit openings on the downstream side of the spud pole. The turbulence created prevents or impedes vortex formation, whereby the oscillating movement of the spud pole is also suppressed or reduced.
[0011] A further advantage of the invention is that the height adjustment of the work deck relative to the spud poles can take place in essentially the same manner as is the case with the known platform. If means for preventing vortex formation are arranged externally on the spud poles, they have to be removed, possibly temporarily, during raising and lowering of the work deck relative to the spud poles, which is time-consuming and can even be dangerous in strong current. The through-channels for the seawater would be expected to weaken the strength and the stiffness of the associated spud pole. The through-channels are however found to reduce the oscillating movements of the platform so effectively that the average load on the spud poles can decrease to at least a level corresponding to the load-bearing capacity of the spud poles provided with the through-channels.
[0012] In a preferred embodiment of the platform according to the invention the at least one spud pole comprises an internal cavity and the through-channels are connected to the cavity. The seawater is thus admitted to the through-channels via the entrance openings of the through-channels located on the upstream side, after which this seawater enters the cavity and is held up there, wherein additional turbulence (and so energy dissipation) can occur, after which the seawater leaves the cavity and flows via the through-channels to the downstream side of the spud pole and leaves the spud pole along this side via the exit openings of the through-channels.
[0013] In a further preferred embodiment the cavity comprises an internal obstruction for the seawater passing through the cavity, for instance in the form of transverse baffles and/or partly perforated transverse baffles, and preferably in the form of a cylinder extending with its longitudinal axis in the lengthwise direction of the spud pole. The seawater flowing into the cavity is diverted in the cavity because it has to flow round the internal cylinder. This provides for additional dissipation of energy.
[0014] In a further preferred embodiment of the invention a platform is provided wherein the entrance and exit openings of the through-channels are ordered in a regular pattern in the peripheral direction of the spud pole. Not only are lateral oscillating movements of the platform hereby better damped, such a spud pole is also found to be mechanically stronger. Still better results are achieved when the entrance and exit openings of the through-channels are ordered in a regular pattern in the lengthwise direction of the spud pole and/or when the entrance and exit openings of the through-channels are arranged in a spiral pattern in the spud pole. The angle which the spiral pattern forms with the vertical (the lengthwise direction of the spud pole) can be chosen within broad limits, but preferably lies between 10° and 80°, more preferably between 10° and 60°, and most preferably between 15° and 45°. In the context of the present invention 'lies between' is understood to mean a range including the indicated end values.
[0015] The overall surface area of the casing surface of the spud pole taken up by the entrance and exit openings of the through-channels must preferably not be made too large since the through-channels then take up too large a part of the spud pole volume and the spud pole is weakened too much. Conversely, too small an overall area will perhaps damp the lateral oscillating movements of the spud pole insufficiently. A platform is preferably provided in which the entrance and exit openings of the through-channels have an overall area which is less than 10% of the casing surface of the spud pole, more preferably less than 6% and most preferably less than 4%. A platform is preferably provided in which the entrance and exit openings of the through-channels have an overall area which is more than 1% of the casing surface of the spud pole, more preferably more than 2% and most preferably more than 3%.
[0016] In a preferred embodiment the platform according to the invention is characterized in that the entrance and/or exit openings of the through-channels are closable. By making at least a part of the entrance and/or exit openings closable the above described action of preventing vortex formation by the spud pole can be adjusted in height, which can be important in the case of a variable spud pole length or when the weather conditions - and so the strength of the current - change. The spud pole length will indeed depend on the specific conditions at the location of operations. Making the entrance and/or exit openings of the through-channels closable can be implemented in any known manner, for instance by arranging slides over the openings. If desired, it is possible to provide all spud poles with through-channels, or only some of them.
[0017] In addition to providing the through-channels, it is also possible to provide the platform, and more particularly the spud poles thereof, with a mass which is connected to the spud pole by means of damping means. Suitable damping means comprise for instance a spring-dashpot system arranged in or around the spud poles. In the context of the present invention a spring is understood to mean a mechanical device which resists movements by means of viscous friction. Dashpots generally comprise a cylinder in which the hydraulic fluid is situated and in which a piston is movably received.
[0018] The invention also relates to a method for operating a jack-up offshore platform in a water mass with strong current, the platform comprising a work deck supported by spud poles and adjustable in height above the water surface. In the method according to the invention at least one spud pole is provided with through-channels for the seawater from an upstream side of the spud pole to a downstream side of the spud pole. The advantages of such a method have already been discussed above in the context of the device and will not be repeated here.
[0019] Preferred embodiments of the method according to the invention comprise methods wherein one or more spud poles comprise an internal cavity and the through-channels are connected to the cavity; wherein the cavity comprises an internal obstruction for the seawater passing through the cavity; wherein the internal obstruction comprises a cylinder extending with its longitudinal axis in the lengthwise direction of the spud pole; wherein the entrance and exit openings of the through-channels are ordered in a regular pattern in the peripheral direction of the spud pole; wherein the entrance and exit openings of the through-channels are ordered in a regular pattern in the lengthwise direction of the spud pole; wherein the entrance and exit openings of the through-channels are ordered in a spiral pattern in the spud pole; wherein the entrance and exit openings of the through-channels have an overall area which is less than 10% of the casing surface of the spud pole; and wherein a part of the entrance and exit openings of the through-channels is closed subject to the spud pole length and/or the strength of the current.
[0020] The present invention is further elucidated hereinbelow with reference to the accompanying figures but without being limited thereto. In the figures:
figure 1 is a schematic side view of a jack-up platform according to the invention;
figure 2 is a schematic top view of an embodiment of a spud pole according to the invention;
figure 3 is a schematic side view of an embodiment of a spud pole according to the invention;
figure 4 is a schematic top view of another embodiment of a spud pole according to the invention; and
figure 5 is a schematic side view of another embodiment of a spud pole according to the invention.
[0021] Referring to figure 1, a jack-up platform 1 is shown. In the shown embodiment the jack-up platform 1 substantially comprises a work deck 2 and four spud pole jacks 3 (of which only two are visible) at the corner points of work deck 2. Each jack 3 operates a spud pole 4 which can be lowered in the vertical direction 5 until the associated spud pole finds support on or in the seabed 6. Work deck 2 is provided, by way of example, with a lifting crane 7 with an arm pivotable in the direction R. For reasons of clarity a number of structures normally present on a jack-up platform are omitted from the figure. Platform 1 is anchored at the desired position on or in the seabed 6 by lowering the spud poles 4 with jacks 3 until they find support on or in seabed 6. Work deck 2 is then carried by means of jacks 3 into a position above water surface 8 in which work deck 2 is supported by spud poles 4 and is clear of the water surface 8 as shown in figure 1. Spud poles 4 here run partially underwater and another part protrudes above the water surface. Depending on the operations to be performed and the local conditions, the height of work deck 2 is set above water surface 8 by jacking up work deck 2 further relative to spud poles 4, whereby work deck 2 is raised relative to water surface 8 or by lowering the work deck, whereby work deck 2 is lowered relative to water surface 8. If desired, the spud poles are provided with insertion openings 9 for the purpose of anchoring spud poles 4 relative to jacks 3. According to the invention spud poles 4 are likewise provided with through-channels (23, 24) for the seawater from an upstream side of spud pole 4 to a downstream side of spud pole 4.
[0022] Water flowing along spud poles 4 of platform 1 in a flow direction 11 can cause vortices 10 on the downstream side of spud pole 4 (see figures 2 and 4). The periodic vortex formation 10 causes, among other things, oscillating forces in the lateral direction 12 of the spud pole (the direction substantially perpendicularly of flow direction 11). The lateral forces provide for an oscillating lateral movement 13 of spud poles 4, and therefore also of work deck 2 of platform 1, which is after all supported by spud poles 4.
[0023] According to the invention at least one spud pole 4 comprises through-channels (23, 24) for the seawater from the upstream side of spud pole 4 to the downstream side of spud pole 4. Through-channels (23, 24) help to reduce the above-mentioned lateral oscillating movements 13 of platform 1. In the embodiment shown in figure 2 the through-channels 23 run wholly in the wall of spud pole 4, wherein they avoid an internal cavity 4a of spud pile 4. Internal cavity 4a generally runs over substantially the whole length of spud pole 4, although this is not essential. When seawater flowing in the direction 11 strikes against spud pole 4 a part of this seawater is admitted to through-channels 23 via the entrance openings 23a located on the upstream side. The seawater then flows in through-channels 23 in the direction 20 to the downstream side of spud pole 4 and leaves spud pole 4 via exit openings 23b of through-channels 23. The seawater is slowed to some extent in through-channels 23 and creates at the position of exit openings 23b a turbulent flow which prevents or impedes the above-mentioned vortex formation. The oscillating movement 13 of spud pole 4 is hereby suppressed or reduced.
[0024] Another embodiment of spud pole 4 is shown in figure 4. Spud pole 4 herein comprises an internal cavity 4a, wherein through-channels 24 are connected to cavity 4a. In this embodiment the seawater is admitted to through-channels 24 via entrance openings 24a of through-channels 24 located on the upstream side, after which the seawater enters cavity 4a and is held up there, wherein additional turbulence (and so energy dissipation) can occur. The seawater leaves cavity 4a on the downstream side of spud pole 4 via exit openings 24b of through-channels 24. Received in cavity 4a is an internal obstruction in the form of a cylinder 19 which extends with its longitudinal axis in the lengthwise direction 5 of spud pole 4. The seawater flowing into cavity 4a is here diverted in that it must flow around the internal cylinder 19 in the indicated flow direction 21.
[0025] The entrance and exit openings (23a, 23b, 24a, 24b) are preferably closable by closing means (not shown).
[0026] Referring to figure 3, the entrance openings (23a, 24a) and exit openings (23b, 24b) of the through-channels (23, 24) are preferably ordered in a regular pattern in the peripheral direction of spud pole 4, and also in the lengthwise direction 5 of spud pole 4. In the embodiment shown in figure 5 the entrance openings (23a, 24a) and exit openings (23b, 24b) of the through-channels (23, 24) are ordered in a spiral pattern in spud pole 4. In the shown embodiment the spiral pattern forms an angle 25 with the vertical 5 of about 30°, although other angles are possible.
[0027] The overall area of the casing surface of spud pole 4 taken up by the openings (23a, 23b, 24a, 24b) should be relatively limited and lies preferably below 4% as shown in figures 3 and 5.
[0028] Because the direction of the flow 11 around a spud pole 4 is variable, it may be desirable to carry the through-channels (23, 24) in a number of different directions through spud pole 4 and/or to make spud pole 4 rotatable around an axis running parallel to the longitudinal direction 5 so that through-channels (23, 24) run substantially parallel to the flow direction 11.
[0029] Using the above described device it becomes possible to operate a jack-up offshore platform in a water mass with strong current. By applying the spud pole 4 with through-channels (23, 24) the lateral oscillating movements of platform 1 excited by the current are damped such that it is less dangerous to work under such conditions. This is s particularly the case for operations at great water depths.
1. Jack-up offshore platform, comprising a work deck supported by spud poles and adjustable
in height above the water surface, wherein at least one spud pole is provided with
through-channels for the seawater from an upstream side of the spud pole to a downstream
side of the spud pole.
2. Platform as claimed in claim 1, wherein the at least one spud pole comprises an internal
cavity and the through-channels are connected to the cavity.
3. Platform as claimed in claim 2, wherein the cavity comprises an internal obstruction
for the seawater passing through the cavity.
4. Platform as claimed in claim 3, wherein the internal obstruction comprises a cylinder
extending with its longitudinal axis in the lengthwise direction of the spud pole.
5. Platform as claimed in any of the foregoing claims, wherein the entrance and exit
openings of the through-channels are ordered in a regular pattern in the peripheral
direction of the spud pole.
6. Platform as claimed in any of the foregoing claims, wherein the entrance and exit
openings of the through-channels are ordered in a regular pattern in the lengthwise
direction of the spud pole.
7. Platform as claimed in any of the foregoing claims, wherein the entrance and exit
openings of the through-channels are ordered in a spiral pattern in the spud pole.
8. Platform as claimed in any of the foregoing claims, wherein the entrance and exit
openings of the through-channels have an overall area which is less than 10% of the
casing surface of the spud pole.
9. Platform as claimed in any of the foregoing claims, wherein the entrance and exit
openings of the through-channels are closable.
10. Method for operating a jack-up offshore platform in a water mass with strong current,
the platform comprising a work deck supported by spud poles and adjustable in height
above the water surface, wherein at least one spud pole is provided with through-channels
for the seawater from an upstream side of the spud pole to a downstream side of the
spud pole.
11. Method as claimed in claim 10, wherein the at least one spud pole comprises an internal
cavity and the through-channels are connected to the cavity.
12. Method as claimed in claim 11, wherein the cavity comprises an internal obstruction
for the seawater passing through the cavity.
13. Method as claimed in claim 12, wherein the internal obstruction comprises a cylinder
extending with its longitudinal axis in the lengthwise direction of the spud pole.
14. Method as claimed in any of the claims 10-13, wherein the entrance and exit openings
of the through-channels are ordered in a regular pattern in the peripheral direction
of the spud pole.
15. Method as claimed in any of the claims 10-14, wherein the entrance and exit openings
of the through-channels are ordered in a regular pattern in the lengthwise direction
of the spud pole.
16. Method as claimed in any of the claims 10-15, wherein the entrance and exit openings
of the through-channels are ordered in a spiral pattern in the spud pole.
17. Method as claimed in any of the claims 10-16, wherein the entrance and exit openings
of the through-channels have an overall area which is less than 10% of the casing
surface of the spud pole.
18. Method as claimed in any of the claims 10-17, wherein a part of the entrance and exit
openings of the through-channels is closed subject to the spud pole length and/or
the strength of the current.