Abstract:
A seismic ocean bottom cable array is provided for use in subsurface exploration. The array includes receiver stations for measuring seismic signals, and a cable including conductors for data transmission and an externally attached stress member. The array is assembled during deployment by attaching the data transmission cables and receiver stations to the stress member as it is lowered into the water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application Serial No. 60/334,284, filed Nov. 30, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to ocean bottom cable arrays (OBC arrays) used in marine seismic surveys and in particular to OBC arrays adapted for permanent placement on the ocean floor.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    Marine seismic exploration surveys are conducted in two ways: seismic sensors (e.g. pressure sensors, motion sensors, or a combination of both) are deployed behind a vessel and towed just below the ocean surface, or they are deployed and placed on the ocean floor. A survey where sensors are placed on the ocean floor is often referred to as an ocean bottom seismic (OBS) survey. The OBS survey involves the steps of: placing an OBC array, collecting data, and the optional step of retrieving the OBC array.  
           [0004]    The OBC array  2 , as shown schematically in FIG. 1( a ) is generally comprised of one or more ocean bottom cables (OBCs)  4  that are mechanically and electrically connected together. OBCs are generally constructed with a plurality of electrical or optical conductors  6  for transmitting signals, one or more stress members  8  for transmitting axial forces along the OBC, connectors  10  for terminating the conductors and stress members at each end of the OBC and transmitting electrical signals and mechanical forces between OBCs, a waterproof jacket  12  that surrounds the conductors and stress members, and one or more receiver stations  14  each having one or more seismic sensors. The receiver stations usually contain the sensors in a hermetically sealed housing. The receiver station is often rigidly attached to the OBC such that it is designed to survive deployment and retrieval operations without encountering mechanical or electrical failure.  
           [0005]    The stress member is designed to provide the primary axial load carrying capability of the OBC. The stress member is generally manufactured into the OBC construction such that it is integrated into the OBC and the OBC is handled as a single unit when it is loaded onto a vessel and later deployed. A problem with this approach is the size and weight of the integrated OBC and the size and complexity of the handling equipment required to deal with the cable.  
           [0006]    Each end of the OBC typically has a connector that terminates the ends of the conductors and stress members at substantially the same point so that multiple OBCs may be connected end-to-end to create an OBC array. An OBC of this type is seldom longer than 200 meters. An OBC array comprised of this type of OBC does not have a continuous conductor or stress member along its length because the conductors and stress members terminate simultaneously at spaced apart connectors along the length of the OBC array.OBCs have long been constructed to survive many deployment and retrieval operations. Deployment and retrieval specifications normally require that an OBC be mechanically robust, because it must support it&#39;s own weight in tension while being waterproof and carrying the required number of conductors. Terminations and connectors for OBCs tend to be bulky and therefore complex to design, expensive and prone to failure.  
           [0007]    A consequence of the traditional OBC construction is the cable&#39;s high mechanical rigidity. The high rigidity allows noise transmitted into one part of the OBC to migrate throughout the cable to receiver stations along the cable, reducing the system signal-to-noise ratio. In particular, stress members provide an ideal path for noise transmission. Traditional OBCs with receiver stations that are rigidly coupled to the cable provide little or no damping mechanism between the cable and the receiver station.  
           [0008]    A desirable OBC includes receiver stations that are rigidly coupled to the cable during deployment, but become significantly decoupled prior to a survey such that signal-to-noise ratio is improved.  
           [0009]    OBCs are also used for reservoir monitoring, where multiple surveys are conducted in the same area over a period of years. OBCs may be deployed and retrieved for each survey or they may be permanently left at the survey location. Permanently placed OBCs have the advantage of not requiring a retrieval step. Retrieval processes usually place more forces on an OBC than deployment processes. Therefore, the traditional OBC is commonly overdesigned for permanent placement. A simple and inexpensive OBC is desirable for permanent placement at the ocean bottom to perform seismic surveys and reservoir monitoring.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention provides an OBC array embodying features of the invention including one or more conductor cables, a plurality of receiver stations coupled to the conductor cables, and a stress member coupled externally to the conductor cables.  
           [0011]    In a preferred embodiment, the stress member may be substantially continuous along the length of the OBC array. In another embodiment, the OBC array may include a layer of material that surrounds the stress member and the seismic cable. In another embodiment, the layer may be a yarn braid or an extruded thermoplastic.  
           [0012]    Also in accordance with the invention a method is provided of deploying a marine seismic array from a vessel into a body of water. The method includes the steps of deploying a continuous stress member from the vessel and attaching a seismic cable to the stress member before the seismic cable and the stress member are deployed into the body of water. The method of attaching the cable to the stress member may be done optionally on the vessel. The method of attaching may further include applying a braid around the stress member and the seismic cable. The method may also be done by feeding the stress member and the seismic cable into a braiding system.  
           [0013]    For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appending claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    FIGS.  1 A- 1 C are side views of an ocean bottom cable.  
         [0015]    [0015]FIG. 2 is a side view of a receiver station attached to the ocean bottom cable.  
         [0016]    [0016]FIG. 3 is a top view of the receiver station of FIG. 1C.  
         [0017]    [0017]FIG. 4 is a cross-section view of the ocean bottom cable shown in FIG. 2.  
         [0018]    [0018]FIG. 5 is a side view of a vessel with OBC deploying equipment.  
         [0019]    [0019]FIG. 6 is a side view of an alternative embodiment of OBC deploying equipment. 
     
    
     DESCRIPTION  
       [0020]    An improved OBC array  16  is represented schematically in FIG. 1( b ). The OBC array includes a stress member  18  with first and second ends  20  &amp;  22 . Conductor cables  24  include receiver stations  26  at spaced apart locations and are coupled end-to-end with electrical connectors  28  that transmit electrical or optical signals between the conductor cables. As opposed to prior art designs, electrical and mechanical terminations are not necessarily co-located as in FIG. 1( a ) in order to assemble the OBC array.  
         [0021]    An OBC array  100  is also shown in FIG. 1( c ) and includes multiple, spaced apart receiver stations  102  positioned along a cable  104 . FIG. 2 shows an exemplary version of the receiver station  102  attached to the cable  104 . Referring to FIG. 2 and FIG. 4, the cable  104  includes a stress member  202 , a first conductor cable  204 , and a second conductor cable  206 . The stress member  202  provides substantially all of the axial load bearing capacity for the cable  104 . The first conductor cable  204  and the second conductor cable  206  are both constructed of multiple, insulated, electrical and/or optical conductors surrounded by an extruded waterproof jacket. The stress member  202  is preferably external to the first conductor cable  204  and the second conductor cable  206  such that the first conductor cable  204  and the second conductor cable  206  are constructed independently of the stress member.  
         [0022]    Ocean bottom cables are traditionally attached end-to-end by connectors that transmit electrical signals from electrical conductors in one ocean bottom cable to electrical conductors in another ocean bottom cable. Connectors generally also transmit mechanical forces held by internal stress members between two connected ocean bottom cables. Ocean bottom cables with internal stress members are traditionally expensive and complex in part because the connectors must terminate stress members and conductors. A benefit of an external stress member is that it may terminate at a point independent from an electrical conductor termination. In other words, a stress member  202  may terminate at a connector that does not terminate a conductor. The external stress member may be of any length independent of the electrical conductor length. In a preferred embodiment, the stress member  202  terminates only at each end of the OBC array  100  and is therefore substantially continuous along the OBC array  100 . A continuous length of the stress member  202  may be up to 10-15 kilometers and would significantly reduce the number of connectors and simplify their design.  
         [0023]    The stress member  202  may be constructed of synthetic fiber or steel and is preferably continuous along the length of the cable  104 . The first conductor cable  204  and the second conductor cable  206  are secured to each other by an inner braid  400 . The inner braid  400  is preferably a yarn material that is wound around the cables. Alternatively, the first conductor cable  204  and second conductor cable  206  may be secured by other means such as an extruded layer of thermoplastic or thermoset material. The cables may also be secured by discrete clamps spaced along the length of those cables. The stress member  202  is secured to the first conductor cable  204  and second conductor cable  206 . The outer braid  200  surrounds the cables and the stress member  202  and is also preferably a yarn material.  
         [0024]    Referring to FIG. 2 and FIG. 3, the receiver station  102  includes a mechanical coupling member  208  that mechanically connects the stress member  202  to the receiver housing  210 . A retainer  214  couples the mechanical coupling member  208  to the stress member  202 . The retainer  214  also couples the first conductor cable  204  and the second conductor cable  206  to the mechanical coupling member  208 .  
         [0025]    The mechanical coupling member optionally disengages the cable  104  from the receiver housing  210  after the receiver housing  210  is deployed. The action of disengaging may be enabled through a number of different methods. The member may be made of a material that degrades in the presence of seawater, for example, certain polyurethanes. The member may be made from a material such that application of a chemical to the member would cause the member material to degrade. The member may be made of a material that has a low melting point and the member is electrically heated in situ to physically melt the material. Such a material may be a thermoplastic or a low melting-point metal such as powder metal manufactured by Serra™. Such a metal is heated to melting points of 175° F. or higher using an electrical source of heat. The member may also be a material that acts as an anode in a galvanic reaction and would thus dissolve in seawater. The member may also be made of a material that is designed to oxidize in the presence of sea water such as aluminum. The member may also be mechanically actuated to detach the receiver station from the seismic cable.  
         [0026]    Referring back to FIG. 1( b ), the OBC array may generally be assembled using coupling member  30  to attach the stress member  18  to the receiver station  26 , the electrical connector  28 , or the conductor cable  24 . In this fashion, the OBC array may be optimally assembled depending on the operating conditions.  
         [0027]    The receiver housing  210  includes one or more seismic sensors such as a hydrophone, geophone, or accelerometer and may include electronics for filtering and digitizing signals from the one or more seismic sensors. An output signal from the receiver housing  210  is coupled to the second conductor cable  206  through connectors  212 . The receiver housing  210  is preferably cylindrical in shape and its longitudinal axis is preferably aligned with the cable  104  longitudinal axis.  
         [0028]    The embodiment as described above is an inexpensive array to manufacture and deploy compared to prior art systems in which the stress member is manufactured into the seismic cable. Because the stress member is coupled externally to the seismic cable, the telemetry and second conductor cables may be assembled separately from the stress member. The embodiment eliminates a need for expensive cable manufacturing equipment and allows the designer to select an inexpensive stress member. The embodiment also reduces the typical number of electrical and mechanical terminations found in the array. Traditional systems use custom connectors that are designed to terminate electrical or optical conductors at the receiver housing while transferring axial mechanical loads to the receiver housing. The continuous stress member eliminates the need to transfer loads through the housing and results in a simple connector design.  
         [0029]    Referring to FIG. 5 and FIG. 6, a seismic cable deployment system  500  is shown. A vessel  502  deploys a seismic cable  506  from a storage bin  504  into a body of water  516 . The vessel  502  may be of the type that is typically used for deployment and retrieval of ocean bottom seismic cables. The seismic cable  506  includes one or more receiver stations  518  and one or more conductor cables. The storage bin  504  is used to secure the seismic cable  506  on the vessel deck, but the same function may be accomplished using a reel.  
         [0030]    The stress member  202  is unwound from a reel  508  over a sheave  512  and is attached to the seismic cable  506 . The sheave  512  is preferably at least 3 meters in diameter. A wire tensioner  602  deploys the seismic cable  506  from the storage bin  504 . The wire tensioner  602  is a two-wheel wire winch that preferably controls the cable deployment speed from 0-20 meters/minute. As the wire tensioner  602  deploys the seismic cable  506 , the reel  508  deploys the stress member and maintains a tension force on the stress member  202  such that the reel bears most of the weight of the seismic cable  506  as it is deployed Optionally, the stress member  202  may be deployed from a storage bin that is not shown. In that case, a back-tensioner must then be used to provide the tension force.  
         [0031]    Again referring to FIG. 5 and FIG. 6, a braiding system  514  attaches the seismic cable  506  to the stress member  202  while simultaneously deploying both. The braiding system  514  is well known in the art of cable manufacturing. The braiding system  514  may preferably be placed in the deployment system such that the stress member and seismic cable are joined just before entering the water. In this fashion, the seismic cable experiences minimal tensile or bending forces. Reduced forces allow the cable and connector design to be relatively simple and inexpensive.  
         [0032]    The resulting OBC array and deployment system are designed for cost-effective manufacturing and deployment. As opposed to simultaneous deployment, the seismic cable  506  and stress member  202  may optionally be joined at a location not on the vessel and subsequently loaded onto the vessel for deployment. While the OBC array is ideally intended for permanent placement on the ocean bottom, these concepts may be applied to a retrievable cable design.  
         [0033]    Although the invention has been described in detail in the reference to a preferred version, other versions are possible. Therefore, the spirit and scope of the claims should not be limited to the preferred version described in detail.