Abstract:
A swing arm type chain support with a hook arrangement integral with a swing arm which allows a mooring chain to ratchet over the hook while tensioning, then automatically engage the hook when tension is reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon provisional application 60/354,600 filed on Feb. 5, 2002, the priority of which is claimed. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to mooring systems for offshore structures such as platforms and vessels and in particular, to a device which supports the mooring chain in such systems. 
     2. Description of the Prior Art 
     Offshore structures such as platforms (e.g. tension leg and SPAR platforms) and vessels (e.g., floating storage and offloading vessels; floating production storage and offloading vessels; and floating storage drilling production and drilling units) at times, require a moored (or anchored) connection to the sea floor. Such moored connections enable the offshore structure to remain in a relatively stable position resisting forces caused by environmental conditions such as wind, waves, and the like. To establish the moored connection, an anchor is embedded into the seafloor, and a mooring chain or the like is attached thereto. The moored chain is then attached to a guideline from the offshore structure and “reeled” in until a desired tension is created on the chain between the buoyant uplift of the offshore structure and downward pulling force of the anchor. To facilitate this reeling, a plurality of different chain guides and devices are known in the art. To maintain the tensile connection, a chain stopper or device is needed. 
     U.S. Pat. No. 5,845,893, issued to Groves discloses two devices for maintaining tension. The first, “a latch mechanism,” includes a latch housing and bending shoe, which are gimbaled to the offshore structure. The chain is passed through the latch housing; and then, when a desired tension is reached, a latch engages the chain via a manual intervention or a remotely operated hydraulic device. The second, “a pelican hook,” works in a similar manner, but engages the chain via a book using a hydraulically actuated arm. 
     IDENTIFICATION OF OBJECTS OF THE INVENTION 
     A primary object of the invention is to provide a simple and inexpensive chain support that requires no powered actuator for latching the chain. 
     Another object of the invention is to provide a chain support with minimal moving parts. 
     Another object of the invention is to provide a chain support that provides a latching mechanism with no moving parts. 
     Another object of the invention is to provide a chain support assembly that has only one moving part, no manually manipulated chain stopper, using readily available materials at a cost lower than designs currently used. 
     SUMMARY OF THE INVENTION 
     The objects identified above along with other advantages and features are incorporated in a chain support that latches a mooring chain without an actuator required The swing arm with a hook fixed thereto, is mounted so that it can pivot about a horizontal axis with respect to the offshore structure. The swing arm has a mouth in which two plates are mounted parallel to each other to form a passage between each other that is separated by a distance large enough to allow a guideline to pass, but small enough to prevent a guide device or a perpendicularly oriented chain link from passing through. Chain links which are oriented parallel to the passage pass through the passage while the chain is being pulled up through the swing arm. Chain links oriented perpendicularly to the passage are too wide to pass through the passage. The plates include sloping guide ramps which are arranged so that when a perpendicular oriented chain link passes through the mouth and meets the ramps through upward pulling tension, the force of the perpendicular chain link causes the swing arm to pivot upwardly. The swing arm pivots upwardly and downwardly for ratcheting of the chain links when they are pulled altematingly through the passage between the plates and over the ramps of the plates. When tension is relaxed on the chain, the swing arm rotates downwardly and a link is captured by the hooks of the plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side section view of a chain support/latch mechanism which shows a chain support and a swing arm/hook arrangement according to the invention at a time prior to the chain entering the swing arm; 
     FIG. 1A is a section view taken along lines A—A of FIG.  1  and which shows the mouth of the swing arm with plates and hooks mounted thereto, and with a guide device of a guideline entering the mouth; 
     FIG. 1B is a section view taken along lines B—B of FIG. 1; 
     FIG. 2 shows a side view of the chain at a position in the chain support where the guide device of the chain has contacted the guide ramps while being pulled upwardly by a guideline and as a result the swing arm has been rotated such that the guide device is at the tip of the hook; 
     FIG. 3 shows a side section view of the chain being pulled through the chain support while ratcheting over the hooks; 
     FIG. 4 shows a side section view of the hooks engaging and supporting the chain after tension on the guideline has been released; 
     FIG. 5 shows a side view of an alternative arrangement according to the invention where the chain support includes an integral chain guide with the swing arm; 
     FIG. 6 shows a section side view of the alternative arrangement showing a perpendicular link of a chain being pulled over ramps of parallel ramp/hook plates; and 
     FIG. 7 shows a section side view of the alternative arrangement showing a parallel link being pulled through a passage between the two plates. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4 show section side views of a chain support  10  of a first embodiment of the invention where swing arm  12  and chain guide  16  are separate devices. The chain guide  16  is mounted to offshore structure  100  above chain support  16  and includes grooves  18  designed and arranged to orient links L such that alternating links are parallel (in line) or perpendicular to a surface  19  in chain guide  16 . The chain support  10  swing are  12  includes hooks  14  integral with plates  50  which are fixed in the mouth  20  as shown in FIGS. 1,  1 A, and  2 - 4 . The plates  50  define a hook passage  36  which is characterized by a distance d which is large enough to pass the guideline  26  and a link L of chain  22  that is in line or parallel with passage  36 . The hook passage  36  communicates with interior passage  37 . The distance d is too small for a guide device  28  or a perpendicular link L of chain  22  to pass through it, so as the guideline  26  is pulled upwardly via interior passage  37 , a guide device  28  (or a perpendicular link L) meets ramps  30  of plates  50  and causes swing arm  12  to pivot upwardly about pin  24  as shown in FIG.  2 . 
     As shown in FIGS. 1-4 and  1 B, the swing arm  12  is mounted for rotation with respect to a bracket  27  by means of pivot pin  24  through swing arm  12  and bracket  27 . The bracket  27  is mountable on offshore structure  100 . A high strength homogenous composite bushing is provided between the pin  24  and the bracket  27  and the swing arm  12 . The high strength of the bushing material reduces the required bearing area and diameter. The low elastic modulus of a composite bearing improves load distribution, particularly when the chain is pulled off center. Since the bushing is homogenous, the low friction properties exist throughout the entire thickness of the bushing, thereby allowing for greater wear. 
     The hooks  14  are formed into the top of guide ramps  30  of plates  50  and each includes a groove  32  which faces away from mouth opening  20 . The hooks are placed on the swing arm  12  well below the pivot axis (that is, pin  24 ) of the swing arm  12 , requiring very little side load on the chain  22  to cause the swing arm  12  of chain support  10  to rotate about the pin  24 , thereby reducing chain wear. 
     FIGS. 1-4 also illustrate the operation of the chain support  10  while pulling in and secuing an anchor chain to an offshore structure. The chain guide  16  and swing arm  12  are mounted as depicted in FIG. 1, and a guideline  26  is connected to a reeling mechanism (not shown) on the offshore structure  100 . The guideline  26  is fed through interior passage  37  of the swing arm  12  and between plates  50  via hook passage  36  and connected to the end of the chain  22  by a guide device  28 . The guideline  18  is pulled upwardly through chain guide  16  until the guide device  28  contacts ramps  30  of plates  50  causing swing arm  12  to rotate upward as illustrated in FIG.  2 . The tension on chain  22  by guideline  26  holds the swing arm  12  up and prevents hooks  14  of plates  50  from fully engaging link L. In other words, the chain ratchets over the hooks  14  as illustrated in FIG.  2 . During such ratcheting, chain links L can potentially rest on the hook  14  (if the reel on the guideline or top of the chain stops pulling) such that the hook  14  is inserted into a link L, thereby preventing the chain from moving downwardly. FIG. 3 shows hook  14  inserted in the interior of a link L with the link L resting on groove  32  of hook  14 . As long as the chain  22  is pulled upwardly, parallel links L pass through hook passage  36 , perpendicular links ride up and over ramp  30  and the links L continue to ratchet over hook  14 . When the desired level of tension is achieved, tension is relaxed on the guideline  26 , the hook  14  holds chain  22  as shown in FIG. 4, and the swing arm  12  rotates downwardly. The chain  22  is now latched to hooks  14  and secured to the offshore structure  100  by connection of swing arm  12  to the offshore structure  100 . The longitudinal axis of the chain  22  is in line with pin  24 . 
     Release of the chain support  10  is accomplished by tensioning the chain  22  until the swing arm  12  pivots to the position of FIG.  3 . Manual release is accomplished by pivoting the swing arm upwardly to prevent chain engagement of hooks  14  as the chain is lowered. An eyelet (not shown) on the frame can be supplied for connecting a winch line or the like. 
     An alternative embodiment of a swing arm chain support  10 ′ is illustrated in FIGS. 5-7. The chain guide  16  of FIGS. 1-4 is incorporated within swing arm  12 ′ of the alternative embodiment with guide grooves  18  providing alternating perpendicular and parallel orientation guidance for links L of chain  22  while chain  22  is pulled upwardly. The swing arm  12 ′ is pivotably mounted via a pin  24  and mounting support  40  to an offshore structure. The guide ramps  30 ′ and hooks  14 ′ within the interior of swing arm  12 ′ are oriented differently from the embodiment of FIGS. 1-4. As illustrated in FIG. 5, the guide line  26  pulls the chain  22  via the guide device  28  to the mouth  20 ′ of the swing arm  12 ′. FIG. 6 illustrates a perpendicular link being pulled over ramp  30 ′. FIG. 7 illustrates a parallel link being pulled through the passage between the plates on which hooks  14 ′ are mounted. While upward tension is maintained on the chain  26 , the hooks  14 ′ do not capture one of the links L of chain  22 . When tension is released, the swing arm  12 ′ rotates downwardly, and hooks  14 ′ latch one of the links L of chain  26 , and the chain  22  is secured to the floating structure via the support  10 ′.