Method for the casting of anchors and mooring of platforms, and an anchor casting unit

Abstract: This invention concerns a system for the casting of anchors meant for the mooring of floating oil producing and/or drilling platforms, for which an anchor laying craft is used, whose general arrangement consists of a deck with wells for anchors, a capstan for dealing with chains, cable drums of an improved kind, lockers for the stowing of chains built into the underside of the deck of the craft, which are led by a capstan from such lockers or back to them, along hawsepipes, and drums for winding cables, mounted on a bed. When a floating platform is being moored, an improved laying device is used which prevents any slipping or dropping after each platform chain has been fastened to the previously laid line whenever the latter has to be lowered to its place in the sag. The invention also provides an improved line fisher.

Description: This invention concerns a system for the casting of anchors intended to moor floating drilling and producing oil rigs, for which an anchor casting unit is used that is specially designed to store all the material usually kept on the platform for such purpose. Such unit is equipped to enable anchors and cables to be cast beforehand continuously, to enable line tautness tests to be carried out, and to enable cables to be fastened to platform chains when the platform is being positioned. The conventional method of mooring floating offshore drilling and production platforms, particularly when intended for use in deep waters, uses a combination of chains and large gauge cables, which means that platforms must be provided with storage space and heavy-lift cranes to handle them. Usually at least eight mooring lines are needed, and two main winches are needed for each mooring line. Some examples of systems most used are described in FR-A-2208373, US-A-3985093, US-A-3967572 and US-A-4722293. Though the best optimised of known systems is the one described above, they have one very great defect when they are used for production platforms. Once a production platform has been put into place it will not be moved again for the next 10 or 15 years, which means that the equipment is hardly being used at all, and if so then only very occasionally, thereby rendering it highly expensive. To this fact must be added the drop in cargo-carrying capacity, the loss of deck space, and the loss of compartment room, etc., amongst other things. In the conventional system the chains, cables and anchors are stored on board the platform and they are dropped by transferring the anchor, already fastened to its chain, into a special kind of tug with the aid of a device known as an anchor chaser tied to a tug hawser. After such transfer the tug travels away from the platform to wherever it has been agreed upon beforehand that the anchor is to be dropped, thereby towing the chain that the windlass on the platform has released. After an agreed length of chain has been paid out, if the system is not of the continuous kind the chain is tied to the steel cable (on the platform) and work goes ahead, this time with the aid of the capstan for the cable whereupon, at the spot where the anchor is to be taken hold, it is lowered and the chaser retrieved. To do this requires a very strong tug, able to make its way against the weight of the anchor and the sag of the cable and chain. It is a big disadvantage of this system that production platforms lie moored by their cables, which are more prone to wear, fatigue and corrosion than are the chains, particularly at the parts that pass through fairleads and at the splash zone where waves and the salt sea air do the most harm, thus calling for greater maintenance or more frequent replacement thereof. For drilling platforms this kind of trouble is not so serious since such kinds of platform are shifted from place to place more frequently, the vulnerable parts referred to above thus being moved around to different locations along the cable, even when the platform is moved between positions in waters of like depth. Experience has shown that the ideal minimum requirements to keep a production rig in place consist of --working from the sea bottom upwards -- an anchor, a first length of chain, an intermediate length of cable, a second length of chain, a fairlead, a windlass, and a locker for the second length of chain. The foremost advantage of this system is that the platform is moored by the chain, instead of by the cable as in the conventional system, and therefore corrosion and metal fatigue troubles are reduced. However to put this system into effect meant having to overcome the problem of casting anchors, for which the conventional methods could not be employed; all solutions so far devised turned out to be extremely costly and complicated. This invention aims to produce an improved solution to such problems. One object of this invention is to provide an anchor-casting craft able to store all the gear needed to cast the anchors which are to be taken off the platform deck; such craft are equipped to enable all of the anchors, chains and cables to be cast overboard beforehand and the lines tested for tension, and afterwards for cables to be fastened to production or drilling platform chains upon positioning of such platform on station. Accordingly one aspect of the present invention provides a platform-mooring process, characterised by the following steps:- recovering, with the aid of laying craft, standby buoy, and chain windlass, the line to be dealt with and fastening an auxiliary cable thereto; shifting the laying craft towards the platform while laying auxiliary cable, and picking up close to the platform the end of the second part of the platform chain which is fastened to the laying craft chain, where one joins the other, with the aid of a triple link; after fastening is finished, laying a suitable length of chain from the platform while the laying craft starts to pick up the laid cable, up to where fastening of such cable to the line rises to the level of the working platform, and locking the cable capstan drum; fastening triple links of the chain and cable, respectively, lying more or less at the same level, such fastening being effected by introducing a small length of chain together with a laying device which helps in the lowering of the fastened line down to its place in the sag; after completion of the fastening, transferring the weights of chain and cable to the laying device, unfastening chain from the laying craft and from the links of the auxiliary cable and tying auxiliary cable to said laying device and transferring the weight of the laying device to the auxiliary cable, releasing said laying device from the means used to support it, lowering the line down to its level in the sag, and recovering said laying device. US-A-4348975 discloses a vessel having a winch 22 for cable to assist in laying an anchor attached to that cable, and also includes disclosure of on-deck storage for buoys to be used during anchor laying. The pre-characterising portion of claim 1 is based on the disclosure of this US document. GB-A-1471131 discloses a ship having storage for a plurality of chains. Accordingly a second aspect of the invention provides a laying device for lowering anchoring lines in a platform mooring device characterised by consisting of a support base; and a sheave crown which fits on said support base by means of a spindle, said crown having notches in its lateral flanges to act as guides for breakable ropes which are fixed, at one end, into a common lug, and then in the middle of the ropes are moved away by a guide mounted on said spindle, and which at the other end are held by individual turnbuckles, said breakable ropes enabling links of chain to be tied to said sheave crown at different angles. Further objects, features and advantages of this invention will be easier to follow from the detailed description thereof given below, with reference to the accompanying drawings which form part of this specification. In the drawings:- FIGURE 1 is a general layout of the deck of the anchor-dropping craft of this invention; FIGURE 2 is a side view of a platform standing aft of the craft; FIGURE 3 is a view showing a mooring line, the middle of which consists of a cable, with two loops at either end for fastening to the two lengths of chain or to a standby buoy; FIGURE 4 is a front and partly sectional view of an improved capstan drum for the mooring cable; FIGURE 5 is an aft view of the anchor-dropping craft in action; FIGURES 6A-6H, 7 and 12, are views of the anchor-dropping stages; FIGURES 13A-13H are views of the stages of the platform mooring system of this invention; FIGURES 14A-14B are front and side views of the improved dropping device used for the platform mooring system; FIGURE 15 is a view of the improved line-fishing tool used for the platform mooring system; FIGURE 16 is a partly sectional view of the line-fishing tool shown in Figure 15; FIGURE 17 is a view showing how the improved line-fishing tool on the platform is installed; and FIGURES 18A-18D are views showing how the improved line-fishing tool of this invention works. This invention consists of a process for use in the mooring of floating production or drilling rigs. The process uses a craft equipped so as to enable anchors, chains and cables to be cast overboard beforehand, and the lines to be tested for tension, and then afterwards such cable can be fastened to the producing or drilling platform chains when such platforms are being positioned on station. Figure 1 represents the general layout of the deck of such a dropping craft 1, on which anchors 2 in their wells 3 are shown, as is a capstan 4 for chains 5, and two improved kinds of drums 6 for steel cable 7 operated by drive 8 from motors 9. Chains 5 are stowed in lockers 10 built into the underside of the deck of the craft, and are led to the capstan 4, or back to the lockers 10, along hawsepipes 11 optionally under power to make return into the lockers easier. Cable 7 is wound on stowage drums 12 mounted on a bed 13, which drums are provided with devices, also power-driven, and brakes and fairleads (not shown in Figure 1), to enable cable 7 wound on the improved drums 6 of capstans 4 to be paid out when casting, and the opposite sense when mooring cables are being brought in or exchanged. The capacity of the improved drums 6 and of the stowage drums 12 is high, though practical and economical enough to enable long stretches of rope to be laid; even greater lengths may be laid if formed of several lengths joined together. The main buoys 14 lie on their rails 15 and the buoys of smaller size and lesser weight, anchors and anchor weights can be left on deck, within cribs or special beds. In order to make it easier to shift such gear on deck, a mobile track-laying crane may be provided. As can be seen from Figure 2, a frame 16 bears a full-length girder along which any required sheaves and running tackle are provided. Working platform 18, mounted aft of the craft 19, runs on rails 20 and is powered to enable it to take up any position across the stern of the craft whenever cables and chains are being fastened or unfastened, and the platform may be provided with wedges for the chains and cable to help such operations. The foregoing description with reference to Figures 1 and 2, is of a version of the invention which is installed on the deck of a float since the width thereof enables adoption of the layout outlined, thus rendering operations easier; however, any other kind of craft could have been employed. The main advantage of this solution is that existing rafts can be used subject to a few modifications while the cost of any such modifications, inclusive of the cost of the windlass and other fixtures, should be less than the amount saved in the building of an individual semi-submersible production platform, taking into account the fact that the intended system means that there will be no need for eight stowage capstans and eight cable winding capstans on board. In addition to dealing with the mooring of other like kinds of platforms, other advantages provided by the craft 1 consist of: a) being able to operate continuously, having to return to base only to refuel; b) being able to carry out tension tests on lines; c) being able to undertake inspection and/or replacement of mooring lines; d) being able to use piggybacks and intermediary buoys for the lines; e) being able to use ordinary tugs; f) being able to use the craft itself for hauling and stowage purposes, loading it at regular wharfs or those belonging to factories, usually provided with cargo handling facilities, thereby doing away with the need for handling gear and equipment which would otherwise be needed at the supply base since, because made-to-order items are involved, the group in charge of operations cannot do without a suitable stock of such items. In Figure 3 the cable 7 in the middle of the mooring line is provided with loops 21 and 22 at either end, for fastening to the two lengths of chain or to a standby buoy, as will be explained later with reference to the anchor-dropping system. After cable 7, which had until then been wound on the capstan drum, has been laid a loop 22 should lie on the working platform 18 aft of the craft, waiting to be fastened subsequently. Placing of the loop 22 cannot however be performed with the aid of an extension to the cable (for instance, a smaller gauge one), since the dimensions of not only the loops but also the fastening accessories lying on the first few turns of cable on the drum when being wound, are of a given size, and this would lead not only to the subsequent turns being unevenly wound but also, worse still, to damage brought about by squeezing and twisting which is fatal to the life of cables. As can be seen from Figure 4, capstans for mooring cable are provided with an improved kind of drum which is divided into two parts (an auxiliary part A and a main part B), by means of a central flange in the middle 23 including a groove 24 to receive a cable 7 passing from one part A to the other B. The auxiliary part A which holds less cable, has a hub which is narrower than that of the main part B, such difference in the diameter of the hub being meant to stow the extension cable 25 (of smaller gauge) which is wound on this part until the build-up of cable 25 grows to give the hub of part A the same effective diameter as that of the hub of the main part B, thereby diminishing any risk of twisting. The diameter of the root of the groove 24 is the same as that of the hub of the main part B. Thus, after having been laid, the main cable 7 shifts from the main part B to the auxiliary part A where it is fastened to the extension cable 25 (by means of a triple link 26 and other conventional accessories lying outside the main part B) which will lead loop 22 to the working platform 18. Because of all this, the shape of all stowage drums 12 must be the same. For an anchor-laying system where the anchors are laid beforehand, standby buoys must first of all be dropped and moored, their job being to hold, at the surface, the end of the laid mooring line, or the end of a cable leading to the end of such lines, until the platform arrives at the site. Laying of the mooring means for such buoys is done in the same way as is described below for anchors, while the choice as to the quantity and types of buoys is dealt with later on. Anchors are laid in pairs so as to lie (approximately) opposite to one another, as regards the mid-point of the array (the rig site), in order to enable line pulling tests to be carried out as will also be dealt with later herein. The manner of laying the anchors in accordance with this invention is shown in Figures 5 and 12. As can be seen from Figure 5, the anchor 2 is first of all shifted out of its well, already fastened to the end of the chain 5, and is brought in front of the chain capstan 4 with the aid of the travelling block and tackle 27. At this point the weight of the anchor 2 is transferred to the chain 5, and the block and tackle 27 is released for paying out (Figure 6A) and a start made on casting the chain 5 overboard (Figure 6B). The length of chain 5 to be dropped is measured not only by instruments but also visually when the special triple link 28 (Figure 6C) arrives at the edge of the working platform 18 (Figure 5), whereupon the loop 21 at the end of cable 7 is fastened to it. After the cable 7 has been fastened to the improved drum 6, an extra length of chain (extension 29 in Figure 6D) is cast in order to transfer the weight of the chain 5 to the cable 7 (Figure 6D), and the fastening is then hoisted (this time by means of the cable drum) up to the level of the working platform 18, in order to release the extension chain 29 from the chain 5. Operation goes ahead, in that the cable 7 is cast, and the total length of line already dropped (chain 5 and cable 7) is controlled until the anchor 2 reaches the sea bed (Figure 6E), whereupon the laying craft 1 should be lying over the position agreed upon for the anchoring point. Then the craft 1 travels (Figure 6F) towards the future position intended for the platform, while dropping and laying the chain and the cable 7 to the bottom of the sea, until the triple link 26 (Figures 6G-6H) is at the level of the working platform 18 ready to be fastened to a standby buoy. This fastening can be done in two ways:- in the first, shown in Figure 7, the cable 7 itself is fastened directly to a cable 30 hanging from the buoy 31 (a high capacity independent buoy and installed before hand in the laying route); in the second, shown in Figure 8, the cable 7 is laid in its entirety on the sea bottom and is fastened to a buoy 32 which is a common buoy, by means of a hanging cable 33 which is of smaller diameter and therefore lighter than the cable 7. In this second way all the hanging cables 33 for the anchors on one side of the platform can be fastened to a common buoy 32 (in such instances placed beforehand beyond the route of the mooring lines). The two buoys 32 can then also be used as markers when bringing the floating platform into position. A fastening 34 between the two buoys 32 is carried out after dropping work has been done, so as not to affect pulling tests, and is withdrawn after the floating platform has been moored on site. After the cable has been laid, the craft 1 will be a certain distance away from the standby buoy as shown in Figure 9. Buoy 31 is brought close to the laying craft 1 and the rope 35 is pulled by means of the auxiliary capstan, then fastened to the buoy 31 with the help of the tug. Once the buoy 31 has been reached it is moored to the craft 1 and the auxiliary windlass is released for rotation. Then, with the aid of an auxiliary capstan, or a block and tackle 27 (Figure 10), the triple link 36 of extension chain 37 to the hanging cable 30 of the buoy 31 is brought to the level of the working platform 18. The extension 29 of the chain is fastened so as, with the aid of windlass 4, to bring plate 38 of the buoy hanging cable 30 up to the level of the working platform 18 to enable it to be fastened to the triple link 26 of cable 7 (Figure 11). The procedure is the same if a hanging rope is used (with the main rope laid on the sea bottom) for mooring to the buoy. After fastening to the buoy has been completed, the procedure of transferring weight and releasing the extensions goes on until the cable 7 is hanging from the buoy 31 and the extension 37 of the hanging cable is fastened to the buoy. The configuration at completion of the laying operation is shown in Figure 12. After every pair of opposite lines has been laid the mooring test should be carried out by pulling, with the laying craft winches, one line against the other at a preset force. The test is carried out by mooring the first line of the pair directly on to the standby buoy, while the second line is being laid. When the second line has been laid its end is fastened to a cable of the same gauge and length (second drum) and the operation goes on as described, this other cable now being laid being intended for the first line standby buoy. Upon being reached, the first line is fastened to the chain extension of the winch, and the buoy is released. The laying craft 1 now returns, picking up the cable and laying the chain, until it comes to a predetermined point. The chain is locked and the first pull made with the cable capstan drawing on the second line; the brake is then applied to the winch drum and the second line is pulled, using the first line capstan, until a set force is achieved; this state is kept up for the time defined under the test. To complete the test the pull in the lines is eased, first of all by slackening the chain (to diminish the pull) and then by slackening the cable. The next step starts off with picking up the chain while at the same time laying the cable (again), up to the end of the first line; then the chain is unfastened and the cable is tied on to the buoy directly (or by means of a hanging cable). Afterwards the craft 1 starts picking up the auxiliary cable until it gets to the end of the second line and ties it to its standby buoy. The platform mooring procedure is shown in Figures 13A-13H. First of all with the aid of the chain capstan the laying craft 1 picks up the line that is to be dealt with at the standby buoy (Figure 13A), and fastens the auxiliary cable to it (Figure 13B). Craft 1 then moves off bound for the platform while laying an auxiliary cable (Figure 13C) and taking on, close to the platform, the end of the second part of the platform chain, which is then fastened to the laying craft chain. A triple link 39 (Figure 13D) is introduced where the two chains meet. After such fastening, the length of chain available at the platform is dropped; the craft 1 at the same time begins to pick up the auxiliary cable it has just laid, up to the stage where this cable meets the line at the level of the working platform 18, whereupon the drum is locked (Figures 13E and 13F). If the chain is very short, more pull will have to be exerted upon the auxiliary rope to get to the meeting point. With regard to the step shown in Figure 13G, where the triple links 39 and 26 of the chain and cable respectively lie more or less at same level, the two are fastened together with the aid of a small length of chain 40, together with a laying device 41 for helping in the lowering of the fastened line until it gets down to its point in the line sag (Figure 13H). To do this, once the fastenings have been made the weights of chain and cable are transferred to a laying device 41, and triple links 39 and 26 are withdrawn from the craft chain and the auxiliary cable, the latter being fastened to the laying device 41 which is released from the means that holds it up, the weight being transferred to the auxiliary cable. The line is lowered until it reaches its spot in the sag and the laying device 41 is recovered. During the lowering, this device prevents any slipping and/or falling of the line which, owing to its weight, might mean a jerk that could seriously damage the platform and its capstan. However, this work cannot be done with the aid of any of the chasers currently available on the market since they have no kind of locking arrangement to prevent any slipping of the line supported thereby. Figures 14A and 14B provide details of the improved laying device 41 according to this invention, as illustrated in Figure 13G, and specially meant for this operation. The device consists of a bearing base 42, and a sheave crown 43 which fits in the bearing base 42 by means of a spindle 44. In the flanges in the sides of crown 43 are notches 45 which act as guides for breakable ropes 46 (two of them) which are fixed in the following way:- at one end they are fixed to a common central lug 47, their central sections are guided radially outwardly from the sheave spindle by a guide 48 mounted on spindle 44, and at their other ends they are fixed to individual turnbuckles; the breakable ropes thus enable links of chain to be tied to the sheave crown 43 at different angles. Thus, as the line is lowered to its point in the sag, the chain wrapped around the sheave crown 43 tends to spread out, links at the topmost side of the sag shifting away from it first of all. The strength (size) of the breakable ropes 46 is such as to ensure that the chain is held, but they are unable to withstand the weight of the line when the held link begins to shift away from the sheave crown. Thus, when the last breakable rope bursts, the line will have practically reached its point in the sag and any sliding or slipping of the improved laying device 41 will not harm the platform or capstan. Whenever any part of a mooring line undergoes regular checking or replacement the conventional procedure requires the anchor to be displaced, this being done with the aid of an anchor chaser; after this the required length of line is pulled in from the platform. Such work calls for a high-powered tug, particularly to re-lay the line afterwards, and there are several disadvantages, the more so if cables have to be changed. Where the lines are laid beforehand, as in the system according to this invention, the same work can be done with the laying craft 1 in a simpler way, by hoisting upon a line at the point where the second part of the platform chain joins the cable (middle part of the line), for which an improved line fisher 49 shown in Figures 15 and 16 is used. The line is untied at such point, and the chain is given up and hauled up on to the platform, while the previously laid part is picked up by the laying craft 1. Any change of line parts, including the platform chain (second part) can be done successively with the chain stowed on board the laying craft 1. The line fisher 49 consists of a sheave and crown 50 held in place by a spindle 51 which passes through the body 52 of the fisher, there being in the upper half of the fisher, in the reinforced part thereof, a hoisting eye 53 in the shape of an inverted groove which slides over the chain, a flared front part 54 which acts as a guide, and a nozzle-shaped back part 55 through which only one link at a time of the chain can pass. As shown in Figure 17, such improved line fisher 49 is fitted into the platform chain (second part) below fenders 56 lying at the water line of the floating platform and is fastened to the platform by means of a hanging cable 57. To use it, the hanging cable 57 is delivered (with the aid of a crane) to laying craft 1 lying close to the platform on which the hanging rope is fastened to the cable of the main drum of the platform capstan, and by means of which it is lowered until it gets to the meeting point. As the line fisher 49 is lowered the laying craft 1 is shifted along the route of the line so as to keep the cable more or less vertically over the line fisher 49. When the nozzle 55 gets to the first triple link 39 (Figure 18A) of the fastening, the line fisher will be in place for hoisting. The craft 1 should continue to travel along its route and reach a position where it is able to pull the line fisher 49 towards the lowest side of the sag (Figure 18B), whereupon the groove will lift and the hoisting eye 55 will wrap the chain around, and fit it into, the sheave crown 50 as the line is being hoisted (Figure 18C). Upon reaching the surface, the triple link 39 is fastened to the chain (or extension) of the laying craft 1, the weight of line being transferred to the craft. The line fisher 49 is fastened directly on to a block and tackle or other holding means; the cable of the line fisher is unfastened and tied to the triple link 26 where the chain part 40 joins the previously laid cable (Figure 18C). Then the part 40 of the chain is unfastened and put on to the craft 1 and, where a procedure opposite to that described above is concerned, previously laid line is fastened to the platform, it is returned to the end of the chain together with the fisher on to the platform. The craft 1 then picks up the laid cable while getting closer to the platform, fastens and transfers the end of the previously laid cable that is to be recovered, to the extension cable of the second main drum (then empty) so as to start recovery work. All work after this is a repetition of what has already been described above, including lowering of the line already fastened to the platform, down to the level of the sag, at the end of the operation.

Claims: A platform-mooring process, characterised by the following steps:- recovering, with the aid of laying craft, standby buoy, and chain windlass, the line to be dealt with and fastening an auxiliary cable thereto; shifting the laying craft towards the platform while laying auxiliary cable, and picking up close to the platform the end of the second part of the platform chain which is fastened to the laying craft chain, where one joins the other, with the aid of a triple link; after fastening is finished, laying a suitable length of chain from the platform while the laying craft starts to pick up the laid cable, up to where fastening of such cable to the line rises to the level of the working platform, and locking the cable capstan drum; fastening triple links of the chain and cable, respectively, lying more or less at the same level, such fastening being effected by introducing a small length of chain together with a laying device which helps in the lowering of the fastened line down to its place in the sag; after completion of the fastening, transferring the weights of chain and cable to the laying device, unfastening chain from the laying craft and from the links of the auxiliary cable and tying auxiliary cable to said laying device and transferring the weight of the laying device to the auxiliary cable, releasing said laying device from the means used to support it, lowering the line down to its level in the sag, and recovering said laying device. A platform mooring process according to claim 1, characterised in that said laying device consists of a support base; and a sheave crown which fits on said support base by means of a spindle, said crown having notches in its lateral flanges to act as guides for breakable ropes which are fixed, at one end, into a common lug, and then in the middle of the ropes are moved away by a guide mounted on said spindle, and which at the other end are held by individual turnbuckles, said breakable ropes enabling links of chain to be tied to said sheave crown at different angles. A platform mooring process according to claim 1 or 2, characterised in that displacement of the anchor in order to regularly check or replace any item in the mooring line is effected with the aid of said laying craft, said line being hoisted at the fastening of the second part of the chain from the platform to the cable (middle part) by means of a line fisher. A platform mooring process according to claim 3, characterised in that said line fisher consists of a sheave and crown fixed by means of a spindle which passes through a body a part of which is reinforced in the shape of an inverted groove; and in that said body has a front part which is more open in order to guide the chain, and a back part having a guiding nozzle and stop which prevents a triple link of the chain from passing through said guiding nozzle. A platform mooring process according to claim 3 or 4, characterised in that said line fisher lies in a second part of the platform chain, below fenders lying at the water line of said platform and fastened to the platform by a hanging cable. Laying device for lowering anchoring lines in a platform mooring device characterised by consisting of a support base (42); and a sheave crown (43) which fits on said support base by means of a spindle, said crown having notches (45) in its lateral flanges to act as guides for breakable ropes (46) which are fixed, at one end, into a common lug (47), and then in the middle of the ropes are moved away by a guide (48) mounted on said spindle, and which at the other end are held by individual turnbuckles, said breakable ropes enabling links of chain to be tied to said sheave crown at different angles.

Inventors: FUJITA TOKUME, FUJITA, TOKUME
Applicants: PETROLEO BRASILEIRO SA, PETROLEO BRASILEIRO S.A. - PETROBRAS
Application year: 1991
Main classification: B63B21
IPC codes: B63B21