Patent Application
20190106307
This invention relates to the method of providing motion compensation to equipment being landed on the seafloor utilizing depth compensated components proximate the subsea location.
When equipment is landed on the seafloor, it characteristically is supported from a vessel at the surface which tends to heave with the waves and swells of the ocean. This means that the motion of the vessel will tend to be duplicated in the subsea equipment at the time of landing as lifting cables or pipes are relatively stiff. For example, if the vessel is moving up and down about five feet, the subsea landing package will be moving up and down five feet also.
The packages to be landed can vary from a few hundred pounds to a few million pounds. As there are frequently connecting interfaces between the package to be landed and the package to be landed on, the package to be landed can impact the package to be landed on several times as it is slowly landed in place. This can be very damaging to connecting interfaces.
Several methods have been tried to solve this problem including actively powering a supporting winch in and out in opposite timing with the vessel motion. This can be done when the supporting means is a steel cable, but becomes more complex as the package gets heavier and must be done on every winch to be used. The steel cable is relatively good for this service as being bent repeatedly across a sheave during a process like this has minimal effect on its service life.
When the supporting means is a steel pipe, especially a pipe called coiled tubing, repeated yielding of the pipe around a sheave under tension is very damaging to the service life of the steel pipe. Characteristically the wire rope and steel pipe wrap around a spool and go over a single sheave to change its direction to vertical as it goes into the ocean. As the ocean depths increase, the weight of the steel cable or pipe adds up and limits the amount of load which can be handled. In one case in 6,000 feet of water, 75% of the capacity of the wire rope was consumed in just holding the wire rope up. Only 25% of the rating was available to do useful work.
Newer synthetic ropes are advantageous to be used in the service as they are near neutrally buoyant. What this means is the approximately 100% of the capacity of the synthetic rope is available for lifting capacity. However, a synthetic rope winch is very different from a steel cable winch when requiring multiple layers and high capacity. Repeated high-tension wraps of synthetic rope tend to “knife” the upper layers into the lower layers and damage the synthetic rope. What is the normal “winch” becomes a light duty storage reel and the tension is taken by a different kind of winch. The synthetic rope winch will comprise two parallel drums with at least six grooves in each one. The synthetic rope is wrapped around one and then the other in grooves successively and will end up having six oval loops around the two drums. The repeated wrapping around the drums will accumulate enough friction so that the synthetic rope will be able to take a load. U.S. Pat. No. 8,322,691 gives a description of one of these winches.
As the synthetic rope comes off the storage spool onto the first drum it will have a very low load, e.g. 500 lbs. on a large system. As it exits the last groove on the second drum if will be under full tension, e.g. 2,000,000 lbs. One foot of synthetic rope under 500 lbs. tension can be twelve and one-half inches long under 2,000,000 lbs. tension. That means the cable is moving faster when it exits the synthetic rope winch than when it enters by about 4%. Said another way, the synthetic from in the last groove is moving 4% faster than the synthetic rope in the first groove. All the grooves are in the same drums, so they are all moving at the same speed. This means that the synthetic rope is constantly sliding a little bit in the grooves. This means the operation is somewhat inefficient and the sliding friction generates a little heat. This heat is not of a consequence in normal operations.
However, when a synthetic rope winch is used for motion compensation service, the area of this repeated sliding friction is relatively short. This means heat buildup which can be serious and can potentially destroy the synthetic rope and cause the load to be dropped. For this reason, a better solution needs to be available for synthetic rope winches.
A new approach has been needed for the motion compensation of packages landed in a subsea environment as long as packages have been landed. This need has been amplified over the past 60 years when progressively larger and heavier packages have been required to be stacked on the ocean floor with critical interface connections. The use of synthetic rope has made the need more pressing.
The object of this invention is to provide a method for motion compensation of subsea landing packages.
A second object of this invention is to provide a method for motion compensation of subsea landing packages which will work with steel cable, steel pipe, or synthetic rope.
A third objective of this invention is to provide a method for motion compensation of subsea landing packages which will work with conventional winches or synthetic rope winches, especially with pre-existing winches.
Another objective of this invention is to provide a method for motion compensation of subsea landing packages which does not require modification of the winches.
Another objective of this invention is to provide a method for motion compensation of subsea landing packages which is compensated for the depth to be landed.
Referring now to
Depth compensated cylinder 40 is connected to supporting cable 28 at 42 and has attachment hook 44 illustrated as connecting to padeye 46 on subsea landing package 48. Subsea landing package 48 comprises one or more connectors 50 which will engage one or more mandrels 52 on seafloor package 54 which is landed on seafloor 56. One or more connectors 50 comprise a seal ring 58 which will engage seal surface 60 on mandrel 52. Locking dogs 62 will engage locking grooves 64 on mandrel 52 to secure subsea landing package 48 to seafloor package 54.
Connectors such as 50 can be expected to contain high loads often over one million pounds and high pressures up to fifteen thousand p.s.i, and so can be finely machined and polished interface surfaces. If a two million-pound subsea landing package comes down and repeatedly hammers the interface surfaces due to surface vessel motions, there is a high probability that significant damage can occur. Therefore, the subsea landing package needs to be motion compensated against the vessel motion. Prior art would have attempted to do this motion compensation at the surface by dynamically and reversibly driving the winches or with surface cylinder arrangements to move the sheave 30.
Referring now to
Chamber 130 is connected to a vacuum pump at port 132, a vacuum is drawn and then the port 132 is plugged, or a connected valve is closed. As a practical matter, port 134 can be pressured to move piston assembly 136 up until shoulder 138 contacts shoulder 140 and the vast majority of air will be expelled from chamber 130, making it for all practical purposes a vacuum when the piston assembly 136 moves back down to an intermediate position.
Chamber 150 is filled with enough inert gas, such as nitrogen, that when the piston 110 is moved approximately to the position as shown which chamber 130 being twice as long as chamber 150 and the resulting force on the piston is equal to the weight of the subsea landing package when it is underwater. This can involve some extensive calculations and predictions as to what the subsea landing package 48 will weigh when it is underwater.
Upper rod 108 and lower rod 114 are approximately of the same diameter and are both exposed to the environmental or seawater pressure at all times. This means that the environmental or seawater pressure is balanced at all times and its effects are cancelled, or it is “depth compensated”. This would be contrasted to a cylinder with only a single rod (i.e. having no upper rod), and the environmental or seawater pressure on the single rod end would be acting to collapse the cylinder at all times.
This means that chamber 150 of the depth compensated cylinder 40 can be charged with approximately the right pressure or even no pressure at all when the subsea landing package 48 is picked up off the deck of the vessel 22. When the subsea landing package 48 is lowered off the side of the vessel 22 and is below the surface of the ocean 24, the pressure in chamber 150 can be adjusted to position the piston 110 in the nominal or central motion compensation position. As the effect of the seawater is cancelled out, the subsea landing package 48 will be motion compensated at whatever the depth it is lowered. The subsea landing package 48 cannot effectively be motion compensated until it is underwater as its effective weigh changes due to buoyance when lowered into the water.
Referring to
Referring now to
Referring now to
These increases and reduction of the pressure in chamber 150 impact the quality of the motion compensation movement directly in proportion to the changes in pressure. A longer chamber 150 will tend to minimize the changes in pressure for the same strokes, but causes expensive polished bore cylinders to become longer. A second solution to this is to add a simple gas tank to the outlet 134. This gives reduced changes in pressure with the same cylinder length.
As can be readily appreciated, a variety of connection methods can be implemented between the depth compensation cylinder 40 and the support cable 28. Similarly, alternate connections can be provided between the depth compensated cylinder, including making the depth compensated cylinder 40 an integral part of the subsea landing package 48.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
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