Patent Publication Number: US-6216627-B1

Title: Seismic survey vessels

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to seismic survey vessels, and is more particularly concerned with seismic survey vessels suitable for use in performing 3D marine seismic surveys covering large areas. 
     In order to perform a 3D marine seismic survey, a plurality of seismic streamers, each typically several thousand meters long and containing arrays of hydrophones and associated electronic equipment distributed along its length, are towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. 
     At the present time, a typical streamer array used by the Applicant comprises a 700 meter wide array of eight evenly spaced streamers, each about 4000 meters long. The streamers are towed by their lead-ins, ie the armoured electrical cables that convey electrical power, control and data signals between the vessel and the streamers, as described in the Applicant&#39;s U.S. Pat. No. 4,798,156, and their spread is controlled and maintained by MONOWING deflectors of the kind described in the Applicant&#39;s U.S. Pat. No. 5,357,892. 
     An array of this relatively large size allows 3D seismic surveys of large areas to be performed very efficiently. However, because the cost of performing such surveys is so high, there is continuous pressure to improve efficiency still further. 
     SUMMARY OF THE INVENTION 
     One way of achieving this is to use even wider streamer arrays, containing even. 
     According to the present invention, therefore, there is provided a seismic survey vessel having a displacement monohull, and powered drum means and cooperating guide means for deploying and recovering seismic streamers over the stern of the vessel via a streamer deck disposed at a substantial height above the waterline of the vessel, wherein the drum means are mounted substantially amidships in the vessel, the streamer deck at the stern of the vessel projects laterally outwardly from the hull of the vessel on both sides of the centre line of the vessel and is wider than the remainder of the vessel, and the guide means includes guide devices distributed across substantially the whole width of the streamer deck to facilitate the lateral spreading of the streamers. 
     In a preferred embodiment of the invention, the streamer deck of the vessel is an upper deck (although not necessarily the uppermost deck), at a height such that when the vessel is normally docked, the deck is well above the dock. 
     Preferably the width of the streamer deck decreases progressively from the stern of the vessel until it becomes substantially equal to the width of the vessel amidships. 
     Advantageously, at least parts of the drum means are mounted in the vessel at a level lower than the streamer deck. 
     The vessel preferably includes a further deck below the streamer deck at the stern of the vessel, said further deck being provided with means for deploying and recovering seismic source means over the stern of the vessel. 
     In a particularly advantageous implementation of the invention, the guide means comprises a respective streamer handling apparatus associated with each of a plurality of the streamers, each such apparatus comprising: 
     a beam; 
     means for mounting the beam above normal head height above the streamer deck of the vessel, such that the beam extends generally longitudinally of the vessel, the mounting means including a pivotal connection to the sternmost end of the beam permitting said end to pivot about a horizontal axis extending generally transversely of the beam, and support means for lowering and raising the other end of the beam towards and away from the streamer deck; and 
     a respective one of said guide devices, each such guide device comprising pulley means secured to and movable along the beam. 
     Advantageously, the mounting means is secured to the underside of a further deck of the vessel, above the streamer deck. 
     Conveniently, the pulley means comprises carriage means movable along the beam, a pulley device, and means for suspending the pulley device beneath the carriage means. 
     Preferably, the suspension means comprises a collar which is rotatably supported on the carriage means and which is rotatable about the longitudinal axis of the beam. 
     Additionally, the pulley device is preferably pivotable about a horizontal axis which extends generally transversely of the beam. 
     The carriage means preferably includes at least one hydraulic motor arranged to move it in both directions along the beam, and is advantageously provided with hoisting means at its sternmost end. 
     Conveniently, the support means comprises a telescopic member which is pivotally connected at one end to the mounting means and at the other end to said other end of the beam, and is hydraulically operated. 
     The apparatus may further comprise downwardly projecting guidance means which is positioned at said other end of the beam and which engages and pushes down the streamer when said other end of the beam is lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
     FIG. 1 is a plan view of a deep marine seismic survey vessel in accordance with the present invention, substantially at the level of its waterline; 
     FIG. 2 is a plan view of the streamer deck of the vessel of FIG. 1; 
     FIG. 3 is a stern view of the vessel of FIGS. 1 and 2; 
     FIG. 4 is a side view of streamer handling apparatus incorporated in the vessel of FIGS. 1 to  3 ; and 
     FIGS. 5 and 6 are top and end views respectively of the streamer handling apparatus of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The deep marine seismic survey vessel shown in the drawings is indicated generally at  10 , and has a displacement monohull  12  which is typically just over 80 meters long at the waterline (and just under 90 meters long overall), with a beam of about 24 meters. The maximum displacement of the vessel  10  is typically about 7500 tonnes. 
     As can be seen in FIG. 1, the shape of the hull  12  at the waterline is fairly conventional, in that the width of the hull increases smoothly and progressively from the bow  14  to a centre section  16  of substantially uniform width, and then progressively decreases, but to a much lesser extent, from the centre section to a wide cut-off stern  18 . Below the waterline, the taper towards the stern  18  increases progressively with depth. 
     The vessel  10  has most if not all of the major features common to a vessel of its type and size, eg multiple diesel engines, bow thrusters, electric generators, accommodation for 60 to 70 persons, a helideck, winches/derricks, emergency equipment, etc. However, since these features can take any of several well known and conventional forms, and are not germane to the present invention, they will not be described in detail for the sake of simplicity. 
     Additionally, the vessel  10  is arranged in accordance with the invention for performing deep marine seismic surveys, as will now be described. 
     Thus as can be seen in FIG. 2, the vessel  10  has a streamer deck  20  which extends aft from the centre section  16 , increasing linearly in width as it does so from about 24 meters at the centre section to about 37.5 meters at the stern  18 , the increase in width taking place symmetrically on each side of the fore-and-aft centre line of the vessel  10 . The overall length of the streamer deck is about 36 meters, while the height of the streamer deck  20  above the waterline of the vessel  10  is typically about 9 meters. 
     Within the centre section  16  of the vessel  10 , sixteen large powered streamer drums  22  are distributed across the width of the vessel, each drum having its axis extending substantially horizontally and transversely of the vessel  10  and being capable of storing a respective streamer up to 6000 meters long, together with its lead-in. The outer four drums  22  on each side of the centre line of the vessel  10  are mounted at least partly below the streamer deck  20 , while the inner eight drums are mounted on the streamer deck itself, the drums being partly interleaved to fit them in the available space. The lower drums  22  overspool, while the upper ones underspool. 
     At the centre of the stern  18  of the vessel  10 , beneath the streamer deck  20 , is a deck  24  known as the gun deck. From the gun deck  24 , seismic sources  26 , typically multiple airgun seismic sources of the kind described in the Applicant&#39;s U.S. Pat. No. 4,686,660, are deployed, typically using a handling system of the kind described in Applicant&#39;s U.S. Pat. No. 5,488,920. 
     In use, the streamers, indicated rather diagrammatically at  28 , each pass over a respective guide  30  and then along the length of the streamer deck towards the stern  18  of the vessel  10 . As they pass over the streamer deck  20 , the streamers  28  pass via respective tension relieving devices  32 , and at the stern  28  they each pass over a respective further guide, this time in the form of a spooling block  34  with its axis extending substantially vertically. To save space, the spooling blocks  34  can be of the type described and claimed in the Applicant&#39;s PCT Patent Application No PCT/IB97/00156 (WO 97/29302). Each spooling block  34  forms part of a respective streamer handling apparatus, which will be described in more detail hereinafter with reference to FIGS. 4 to  6 . 
     Once deployed over the stern  18  of vessel  10 , the twelve streamers  28 , towed at about 5 knots by the vessel, are formed into an array typically up to 1100 meters wide, using Applicant&#39;s MONOWING deflectors as described hereinbefore. This wide streamer array is then used, in conjunction with the seismic sources  26 , to perform a 3D marine seismic survey as described earlier. 
     The vessel  10  has a number of advantages over prior art seismic survey vessels of comparable size. For example, the fact that the vessel  10  has a substantially conventional displacement monohull means that it is not much more expensive to build than the prior art vessels. Further, the powered drums  22  with the streamers  28  wound on them are extremely heavy, so their location amidships, as low as conveniently possible, contributes to the stability and seaworthiness of the vessel  10 . 
     Another major advantage of the vessel  10  lies in the substantially increased width of the streamer deck  20  at the stern  18  of the vessel. This facilitates the formation of the extra wide streamer array, inter alia by reducing the angles through which the lead-ins of the outer streamers  32  in particular are required to bend at the stern of the vessel to form the array. This latter feature reduces the stresses to which the lead-ins and the streamers  32  are subjected, so reducing the possibility of breakage. The increased width also provides more space at the stern  18  for working with the increased number of streamers  28  used to form the extra wide array. But because the streamer deck  20  is 9 meters above the waterline of the vessel  10 , it does not significantly interfere with the docking of the vessel, since the deck will normally be well above the dock and will therefore merely project over it. 
     The extra space across the width of the streamer deck  20  at the stern  18  of the vessel  10  also creates sufficient room for the installation of new and improved streamer handling apparatus, one for each streamer: this new and improved streamer handling apparatus is shown in FIGS. 4 to  6 . 
     The streamer handling apparatus of FIGS. 4 to  6  is indicated generally at  40 , and comprises an elongate beam  42  which extends generally longitudinally of the vessel  10 . The beam  42  is pivotally mounted at its end  44  nearer the stern  18  of the vessel to mounting structure  46 , which is firmly secured to the underside  48  of the deck  50  of the vessel immediately above the streamer deck  20 : the deck  50  is, in fact, the upper stern deck of the vessel. 
     The beams  42  of adjacent streamer handling apparatuses  40  diverge slightly, or fan out, towards the stern  18  of the vessel, to follow the increasing width of the stern. 
     As will become apparent hereinafter, the beam  42  normally extends parallel to the streamer deck  20  and the upper deck  50 , ie substantially horizontally, at a height of nearly 4 meters above the former. To this end, the other end  52  of the beam  42  is pivotally connected to one end  54  of a hydraulically operated extendable telescopic arm  56 , whose other end  58  is pivotally connected to the underside  48  of the upper deck  50 . In its unextended state, the telescopic arm  56  securely holds the beam in its normal, horizontal, position, while extension of the telescopic arm  56  lowers the end  52  of the beam  42  to a height of about 1 meter above the streamer deck  20 , ie to about waist height. Both the raised and lowered positions of the beam  42  are shown in FIG. 4 
     A curved guide  59 , effectively a 90° segment of a pulley wheel, is pivotally secured to the joined ends  52 ,  54  of the beam  42  and the telescopic arm  56  respectively. 
     Mounted on the beam  42  so as to be movable in both directions along it is a carriage  60 . Motive power for the carriage  60  is provided by hydraulic motors  62 , which typically move the carriage, on suitably positioned wheels (not shown), along the beam via a rack and pinion drive arrangement (not shown). Pressurised hydraulic fluid for the motors  62  is supplied via a long flexible pressure hose  63 , which unfolds as the carriage  60  moves along the beam  42 . 
     The carriage  60  rotatably supports a collar  64 , which coaxially surrounds both the carriage and the beam  42  and is rotatable about the longitudinal axis of the beam. The collar  64  supports in turn the spooling block  34 , which is effectively a particularly compact equivalent of a large radius pulley or fairlead. The spooling block  34  is pivotally suspended beneath the collar  64 , so as to pivot about an axis extending generally transversely of the beam  42 . 
     The end  66  of the carriage  60  nearer the stern end  44  of the beam  42  is provided with a winch  68 . 
     In normal use, ie when the streamer  28  handled by the apparatus  40  is deployed and being towed, the beam  42  is held in its raised horizontal position, and the carriage  60  is locked at the stern end  44  of the beam. The streamer  28  passes over the spooling block  34 , which is free by virtue of the ability of the collar  64  to rotate about the carriage  60  and the pivotal connection between the spooling block and the rotatable collar  64  to pivot both sideways and fore and aft, to accommodate lateral and vertical changes of direction of the streamer as it leaves the stern  18  of the vessel  10 . 
     When it is desired to work on the streamer  28 , eg to connect or remove a depth controlling “bird” or other device to it during deployment or recovery, the carriage  60  is moved towards the other end  52  of the beam  42  by the motors  62 , and the telescopic arm  56  is extended, thus lowering the end  52  of the beam towards the streamer deck  22 . As the end  52  of the beam  42  is lowered, the curved guide  59  engages the streamer  28 , and pushes it down towards the streamer deck  22  until it reaches about the same level as the top of stern rail  70  of the streamer deck. The stern rail  70  extends substantially the whole width of the streamer deck  22 , and is formed with a rearwardly-facing curved surface  72  which supports the lowered streamer  28 . The surface  72 , as well as each of the other curved surfaces which engage or are engaged by the streamers  28 , is of sufficiently large radius to ensure that the minimum bending radius of the streamer is not exceeded. 
     Before or after the streamer  28  is fully lowered, the winch  68  can be coupled to a suitable attachment point on it, and then reeled in a little to take the strain off the portion of the streamer on the vessel, ie to the right of the attachment point to the hoist, as viewed in FIG.  4 . Then, once the streamer  28  is fully lowered and no longer under tension, it is very easy to work on, being typically at waist height above the streamer deck  22 . If desired, a bench or the like can be provided in the working area on the streamer deck  22 , to make such work even easier. 
     The winch  68  is also used to raise and bring onto the streamer deck  22  auxiliary equipment associated with the streamer  28 , eg tailbuoys, acoustic positioning equipment and the like. 
     The streamer handling apparatus  40  is thus particularly ergonomically advantageous. In its normal (or towing) condition, it is mostly disposed above head height above the streamer deck  22 , leaving the deck relatively clear for personnel to move about. But in its lowered condition, it provides particularly good access to the streamer  28  passing through it. 
     Many modifications can be made to the streamer handling apparatus  40 . For example, the hydraulic motors  62  can be replaced by electric motors, while the telescopic arm  56  can be replaced by any other support device capable of moving the end  52  of the beam  42  up and down between its raised and lowered positions. And the spooling blocks  34  can be replaced by any other suitable fairlead or pulley device. 
     Additionally, if there is no upper deck above the streamer deck  22 , the mounting means for mounting the beam  42  above the streamer deck can comprise a suitably shaped frame supported on the streamer deck itself. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.