Not applicable.
This disclosure relates to a system for hoisting a load on an offshore rig. configured
Hoisting of heavy loads on drilling rigs has traditionally been done by means of a winch accommodating multiple layers of wire rope. The wire rope is connected to a load through a draw-works including many small sheaves over which the wire runs and is repeatedly bent. During lifting and lowering, and particularly in heave compensation, the wire rope undergoes numerous bending cycles under load, and is therefore subject to considerable wear. Depending on the number of sheaves in the draw-works, i.e. the mechanical advantage, the wire rope on the winch side, the so-called fast line, travels a longer distance than the load, thus requiring multiple layers of wire rope on the winch. Overlying layers of wire rope act with great forces on underlying layers on the winch drum, thus further increasing the wear of the wire rope. Inertia loss in the great number of sheaves in the draw-works also leads to a rather slow acceleration of the load, thus slowing down the operation time. The typical lifetime of a wire rope used together with a multi-layer winch in heave compensation mode on a drilling rig is in the order of two weeks, leading to frequent stops of operation to perform a traditional cut-and-slip to replenish the wire rope.
Prior art hoisting systems on offshore rigs typically use only one wire connected to a winch in one end, running to the top of a derrick through a crown block and down to a travelling block, to which the load is connected, and further to a deadline anchor, typically anchored to the rig floor or to the derrick. When using only one wire, it is of the utmost importance that the wire does not break, as this could cause severe damage to rig and harm to personnel. The fear of wire fatigue also contributes to the frequent replenishment of wire rope. Wires used for heavy lifting operations are very expensive.
A Disclosed are systems and apparatus aimed at increasing the lifetime of wire ropes used in offshore hoisting operations, and in particular wire ropes used for lifting drill pipes and stands in active heave compensation and for lifting complete drill strings. The systems and apparatus are also aimed at improving safety in offshore lifting operations as well as reducing operation times.
Disclosed is a system for hoisting a load on an offshore rig, the system comprising
In the following the elongated hoisting member will be exemplified by a wire rope. The use of a single-layer winch offers several advantages over conventional multi-layer winches that have traditionally been used on offshore rigs. In a multi-layer winch, underlying layers of wire rope will typically be exposed to great wear and tear from overlying layers, hence reducing the lifetime of the wire. Multi-layer winches have been required when using traditional draw-works on drilling rigs, with relatively small winches and a great number of sheaves to achieve the necessary mechanical advantage. The wire rope undergoes numerous bending cycles around the sheaves. The fast line from the winch travels many times the distance of the load, connected to the travelling block. Hence, multiple layers of wire are required to accommodate a wire of sufficient length. A system hoisting a load on a drilling rig including a single layer winch preferably should include a limited number of sheaves between the load and the winch. In preferred embodiments, the sheaves in the crown block and travelling block may be arranged so as to give a transmission in the range of 2:1 to 4:1. In one embodiment, the winch may even be a so-called direct line winch with no transmission/gearing in the sheaves.
In one embodiment the system may comprise two or more parallel wires connecting said winch to said load. The use of multiple parallel wires may significantly improve safety, as the system may operate in redundancy with respect to the number of required wires. In case of wire failure, and even if a wire breaks, the system may still be operating within its capacity. The parallel wires may be connected to the same winch drum. The number of parallel wires is not limited, but in exemplary embodiments, the system may comprise two to six parallel wires.
In one embodiment said winch drum may be provided with a helical groove for accommodating said single layer of wire rope. The helical groove will prevent the wire on the winch from wear, as it prevents cross-contact between neighbouring wire layers, thus further increasing the lifetime of the wire rope. The winch drum may be provided with one groove for each wire rope, where several parallel wire ropes are used.
In one embodiment, the system may further comprise motion compensation means, such as heave compensation means. This may be preferable if the system is provided on an offshore drilling rig where there is a need to compensate for undesired movement of the load due to wind and waves. In one embodiment, the winch itself may be provided with heave compensation means. Traditionally, repeated lifting and lowering of a load, such as a drill string section during tripping, has entailed numerous wire rope bending cycles around the multiple sheaves in draw-works, leading to an extensive wear and reduced lifetime of the wire rope. Heave compensation by means of a system disclosed herein will not wear the wire rope to the same extent due to the fact that the wire rope only undergoes a few, if any, bending cycles during lifting and lowering. It is also preferable to use relatively large sheaves, implying that a large part of the wire stays on the sheave upon lifting and lowering, hence not leaving the sheave, and thus not undergoing a bending cycle.
In one embodiment, a ratio between the diameter of said winch drum in the first position of use and the diameter of said elongated hoisting member may be larger than 30, preferably larger than 40 and even more preferably in the range of 60 or larger. Said ratio is oftentimes called the D/d ratio, where D is the diameter of the winch drum and d the diameter of the wire rope. A high D/d ratio has been shown to be particularly important for offshore winch applications. Traditionally winches and wire ropes used for offshore drilling applications have had a D/d ratio of around 30. In one embodiment, an increased D/d ratio from 30 to 60 increases the lifetime of the wire rope approximately fivefold, thus contributing to increased wire rope lifetime. The use of a single-layer winch with a large winch drum significantly contributes to the increased D/d ratio. Preferably also, the sheaves in the system should have a large D/d ratio, with D now being the diameter of a sheave instead of the diameter of the winch drum. The sheave D/d ratio could also be in the range of 60 or larger. A person skilled in the art will understand that the diameter d of the wire rope will depend on the capacity of the system in which it is to be used, the number of parallel wire ropes, and the required safety factor. The safety factor of the wire rope should preferably be 3 or even larger. As an example, in a system with a safe working load of 1250 short tons, six parallel wire ropes with a diameter of 66 millimetres may run over two-parts blocks in the derrick. Sheaves and the winch drum may have a D/d ratio of 60 or even larger, thus requiring diameters in the range of four meters. Various embodiments of the disclosed hoisting system may be configured to lift from 200 to 750 short tons in well intervention applications, and even up to 2000 short tons in drilling operations.
In one embodiment, the winch drive means may be a plurality of electric drive means. By using a plurality of smaller drive means, instead of one large drive means, system safety and flexibility may be further improved. In case of failure in one of several electric drive means, the system may still be run within its safety limits, thus also reducing downtime.
Said electric drive means may be permanent magnet motors, such as permanent magnet AC synchronous motors. Such permanent magnet motors are known to be compact, reliable and cost-efficient, while at the same time requiring little maintenance.
Each of said plurality of permanent magnet motors may be connected to said winch via a separate gear. Each of said gears may further be connected to a separate gear shifting means configured to shift gears and to disconnect said permanent magnet motor from the winch. A mal-functioning non-disconnected permanent magnet motor will rotate with the winch drum, produce energy, and therefore constitute a potential safety hazard. It is therefore advantageous to be able to disconnect each of the permanent magnet motors, should it be required. In contrast to traditional draw-works, at least some gearing according to this embodiment is done directly at the winch. Load acceleration will also be significantly better compared to traditional draw-works, thus leading to a quicker response and less energy-consumption. The permanent magnet motors may therefore be run at a fixed, optimized speed, while wire speed is regulated through the winch gears. Said separate gear may be a two-step gear wherein in a first gear, the winch is configured to lift and/or lower a first load, and wherein in a second gear, the winch is configured to perform multiple lifting and lowering operations of a second load. The first gear and the first load may correspond to a mode where the system is used for lifting and/or lowering a drill string, where much power but not so much speed is required. The second gear and the second load may correspond to tripping with a drill stand, where less power but more speed is required. The gear shifting means may be configured to switch between the two gears and to disconnect the permanent magnet motor, to which the gear is connected, from said winch.
In one exemplary embodiment, the gear shifting means may be configured to shift gear under load. This may be done by running each motor, one or a few at the time, consecutively in a slight overspeed, and change gear while in overspeed, while the rest of the motors are under load. This may speed up processing time and the transmission between slow and fast speed. A person skilled in the art will understand that the motors and motor speed may be controlled by a winch control unit, which may be a PLC or the like.
In one embodiment, the elongated hoisting member may be a galvanized steel wire. Galvanized steel wires have not conventionally been used in heave compensation on offshore rigs. The fact that wires typically have been worn out in about two weeks did not justify, from a cost perspective, the use of galvanized wires. However, together with an offshore hoisting system accordingly to exemplary embodiments disclosed herein, where the lifetime of wires is significantly increased, the use of galvanized wires may further increase the lifetime of the wire, and thus further reduce costs over time.
In one embodiment, said elongated hoisting member may be a wire rope including more than six strands. This will result in a smoother surface of the wire rope and thus reduced risk of the wire rope getting tangled compared to wire ropes with fewer strands which have traditionally been used in these kinds of operations. Together with the fact that the wire is provided in a single-layer, and preferably in a helical groove, on the winch drum, this will further lead to reduced wear and thus increased lifetime of the wire rope.
In one embodiment, said winch may comprise a removable shell defining an outer layer of said winch drum, and wherein the winch with said shell defines said first position of use, and wherein said winch without said shell defines a second position of use wherein the winch is configured to accommodate multiple layers of said elongated hoisting member. In this second position of use, the winch may also, for instance, serve as a subsea hoisting winch, i.e. for lowering or lifting loads to or from a seabed. Such operations will require a much longer wire rope than for instance heave-compensated derrick operations. Still, subsea hoisting operations are not frequently performed, hence wear of the wire rope is not a big issue. This embodiment may save cost and space as one winch may be used for several purposes for which, traditionally, two or more winches have been required.
In one embodiment, said removable shell may include a plurality of shell segments configured to be assembled to define said removable shell. This may make it easier to assemble and dis-assemble the shell. In one embodiment, the shell may include three segments of substantially equal size, i.e. each covering a sector of substantially 120° around the winch drum core.
It has been found that by implying one or more of the various embodiments described above, the average lifetime of a wire used on an offshore drilling rig may be increased from two weeks, which is the current situation, up to as much as five years and even more.
There is also described an offshore drilling rig comprising a system according to any of the embodiments described above.
In the following description, the various embodiments are illustrated in the accompanying drawings, wherein:
In the following description, identical reference numerals indicate identical or similar features on the figures, which are shown simplified and/or in schematic form.
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This Preliminary Amendment is filed concurrently with the National Phase Entry patent application that claims priority to PCT/NO2014/050113 having the international filing date of Jun. 25, 2014 and U.S. Provisional Application No. 61/839,194 filed Jun. 25, 2013. Prior to calculating the fees and initial examination of the above-styled case, the Examiner is requested to enter the following amendments.
Filing Document | Filing Date | Country | Kind |
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PCT/NO2014/050113 | 6/25/2014 | WO | 00 |
Number | Date | Country | |
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61839194 | Jun 2013 | US |