Rigid-stem lead-in method and system

Disclosed are methods and systems for using a rigid-stem lead-in assembly comprising a plurality of interconnected rigid stems in a marine geophysical survey. An embodiment discloses a method of towing a survey device from a survey vessel, comprising: coupling the survey device to the survey vessel with a lead-in comprising a rigid-stem lead-in assembly, the rigid-stem lead-in assembly comprising a plurality of rigid stems that are interconnected and each comprise a stem both defining one or more interior chambers; and towing the survey device through a body of water. Also disclosed are marine geophysical survey methods and marine survey systems.

BACKGROUND

The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to using a rigid-stem lead-in comprising a plurality of interconnected rigid stems in a marine geophysical survey.

Techniques for marine surveying include marine geophysical surveying, such as seismic surveying and EM surveying, in which geophysical data may be collected from below the Earth's surface. Geophysical surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, may include towing an energy source at a selected depth in a body of water, typically above the seafloor. One or more geophysical sensor streamers also may be towed in the water at selected depths by the same or a different vessel. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced-apart locations. A lead-in typically couples the sensor streamer to the survey vessel. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom with changes in the energy field due to this interaction detected by the geophysical sensors positioned on the streamers. The detected energy is used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.

Since the introduction of three-dimensional seismic surveying, there has been interest in towing wider and longer spreads of sensor streamers, which require more cables, larger lateral separation, deeper tows, and longer or bigger cables with more and more external equipment. To achieve the desired lateral spread between the sensor streamers, spreading devices have been used, which may include lateral depressors, such as inclined plates or wings. Some lateral depressors typically the larger one may be connected to the survey vessel using a separate tension member while others may be attached to the lead-in connecting the sensor streamer to the survey vessel. For towing sensor streamers, the lead-in can take the shape of an umbilical with or without fairing connected to one or more wings (also reformed to as depressors) for achievement of both lateral, vertical, or combined offset from the vessel trajectory and may be used in combination with weights, flotation devices, and sometimes active propulsion to achieve deep towing/large offsets.

When towing sensor streamers up to several kilometers in length and from 1 to 3 inches in diameter, as may be done in three-dimensional surveying, a tension of a little over 1 ton may be normally required at the industry standard of 5 knots transversal speed. The tension increases with increased speed. In order to keep the cables at as lateral spread of more than 1,000 meters, the tension often exceeds 10 tons on the outermost lateral depressor because it typically sees its own drag and the drag from the lead-in or tow wire in addition to the required lateral lift for the sensor streamer. The tension will typically be the highest in the outermost members and the surveys with the widest or largest spreads. For the purpose of storing these lead-ins and other tension members and to be able to deploy as much cable as desired while stopping at any position under tension, a high-torque and often brake-able winch may be used. In instances where the lateral depressor is not connected to a sensor streamer, as pure wire with high strength and smaller bend radius than for the lead-ins may be used for the tension member. In other instances, a steel, or Aramid-armored umbilical cable with copper and fiber fibers inside has been used.

However, these cables often traverse through the water with up to 45° of cross flow. Uneven water flow around the cables may produce alternating shedding forces which may cause transverse vibrations known as “strumming” or “vortex index vibrations” in the cables. Strumming may be problematic with lead-ins as the adding drag due to strumming results in higher load with corresponding lower lifting performance on the lead-ins. In addition, the turbulent flow within the water caused by the strumming generates acoustic noise that may interfere with data collection. Strumming may further generate stresses at equipment connection points and can accelerate equipment failure. A number of techniques have been developed to reduce problems associated with strumming as a cable is towed through the water. One technique involves attachment of fairings to the cables to reduce strumming in the water. There are number of different types of fairings in use, including hard fairings and hairy or fabric fairings. Hard fairings may include a streamlined shell or other structure attached to sections of the cable. Drawbacks to hard fairings may include increased complexity of the cable-handling system as a faired cable cannot be stored directly on a drum when large or in long lengths. Hairy or fabric fairings may include fairing hairs attached to the cable. While hair or fabric fairings may remove vibration, very little reduction in drag may be achieved as the reduced drag is typically compensated by the increased diameter/area of the faired cable.

Accordingly, there is a need for improved techniques for reducing drag forces which may reduce strumming noise interfering with data collection and increase towing efficiency.

DETAILED DESCRIPTION

The present invention relates generally to the field of marine geophysical surveying. More particularly in one or more embodiments, this invention relates to using a rigid-stem lead-in comprising a plurality of interconnected rigid stems in a marine geophysical survey. A rigid-stem lead-in assembly in accordance with embodiments of the present invention may be used to couple a sensor streamer to a survey vessel. A rigid-stem lead-in assembly in accordance with embodiments of the present invention may also be used to couple other towed devices, such as energy sources, sensor assemblies, samples, or transducers. The rigid stems may be assembled on a survey vessel to form one or more rigid-stem lead-in assemblies that can be deployed in a body of water. The rigid stems may include wings that create lateral lift as moved through the water to place the front ends of the sensor streamers at their lateral position. The rigid stems may have a round cross-section or have other shapes, such as a wing-shaped cross-section for a streamlined, low-drag profile.

FIG. 1illustrates a marine geophysical survey system5in accordance with embodiments of the present invention. In the illustrated embodiment, the marine geophysical survey system5may include a survey vessel10that moves along the surface of a body of water15, such as a lake or ocean. The survey vessel10or a different vessel (not shown) can tow a source cable20that includes one or more energy sources25. As illustrated, the energy sources25may be towed above the water bottom35, wherein the energy sources25are disconnected from the water bottom35. In some embodiments (not shown), one or more of the energy sources25may be mounted to the hull of the survey vessel10. The energy sources25may be any selectively actuable sources suitable for subsurface geophysical surveying, including without limitation seismic air guns, water guns, vibrators or arrays of such devices, or one or more electromagnetic field transmitters. As energy is emitted by the energy sources25, it travels downwardly through the body of water15and rock formations30below the water bottom35.

In the present example, a plurality of geophysical sensors40may be disposed at spaced-apart locations along the sensor streamer45. A lead-in50may couple the sensor streamer45to the survey vessel10. The type of geophysical sensors40is not a limit on the scope of the present invention and may be, without limitation, seismic sensors such as geophones, hydrophones, or accelerometers, or electromagnetic field sensors, such as electrodes or magnetometers. In one embodiment, the sensor streamer45may include a lateral force and depth (“LFD”) control device (not shown) configured to, for example, regulate streamer depth so that the sensor streamer45may be kept as level as possible while towed through the body of water15. The LFD control device may be any of a variety of different devices suitable for regulating streamer depth, including “birds” having variable-incidence wings. The geophysical sensors40may generate response signals, such as electrical or optical signals, in response to detecting energy emitted from the energy sources25after the energy has interacted with the rock formations30. Signals generated by the geophysical sensors40may be communicated to equipment on the survey vessel10, shown generally at55and referred to for convenience as a “recording system.” The recording system55typically includes devices (none shown separately) for navigating the survey vessel10, for actuating the energy sources25, for example, electrical controller with swept frequency alternating current or other signal, and for recording signals generated by the geophysical sensors40.

The lead-in line50may be used, for example, to deploy the sensor streamer45from the survey vessel10and to maintain the sensor streamer45at a selected distance behind the survey vessel10. As illustrated, the lead-in50may be coupled to the sensor streamer45at an axial end closed to the survey vessel10(“forward end”). The lead-in50may include, for example, a rigid-stem lead-in assembly60. In some embodiments (not illustrated), more than one rigid-stem lead-in assembly60may be coupled end-to-end to form the lead-in50. The rigid-stem lead-in assembly60may comprise a plurality of interconnected rigid stems65coupled end-to-end. In some embodiments, stem joints70may be disposed at the intersection of adjacent rigid stems65for mechanically joining the adjacent rigid stems65together. While not illustrated, a stem joint70may not be used, in some embodiments, to couple at least one pair of adjacent rigid stems65leaving an open joint. In some embodiments, components, such as sensors, electronics, actuators, transducers and other devices, may be disposed in the rigid stems65. In some embodiments, the rigid-stem lead-in assembly60may include one or more inline retrievers that can be used for retrieval, for example, in the event of a damaged or detached streamer or lead-in assembly60. The inline retriever may include a ballasting system, for example, to lift the rigid-stem lead-in assembly60to a different depth or even the surface. Inline retrieval may be needed in a number different circumstances such as if wings or other device on the rigid-stem lead-in assembly60or the sensor streamer45have stopped working, there is a risk of entanglement, or if service/repair is needed and retrieval cannot be performed by another technique. In some embodiments, the lead-in50may communicate power and/or signals between the recording system55and the various electronic components (e.g., geophysical sensors40) on the sensor streamer45. For example, lead-in termination75at an axial end furthest away from the survey vessel10(“distal end”) of the lead-in50. Electrical and/or optical connection between the recording system55and electrical components on the sensor streamer45may be made through the lead-in termination75. In some embodiments, the lead-in termination75may flexibly couple the lead-in50to the sensor streamer45so that the sensor streamer45can extend in a different direction in the body of water15than the lead-in50. WhileFIG. 1illustrates, the sensor streamer45as a cable, it should be understood that the sensor streamer45may be have other configurations, including, for example, being formed from one or more rigid-stem assemblies coupled end-to-end.

The configuration of the energy sources25and sensor streamer45shown inFIG. 1is only meant to illustrate an example embodiment of the marine geophysical survey system5. In alternative embodiments (not shown), the marine geophysical survey system5may include additional vessels which may tow energy sources in addition to the energy sources25shown onFIG. 1. The energy sources25may also be towed by a vessel different than the survey vessel10that tows the sensor streamer45. In some embodiments, the survey vessel10may tow a plurality of sensor streamers45arranged in a laterally spaced-apart array. For example, in some embodiments, 8 or more laterally spaced-apart sensor streamers45may be towed by the survey vessel10, while in other embodiments, as many as26or more laterally spaced-apart sensor streamers45may be towed by survey vessel10.

WhileFIG. 1illustrates use of the rigid-stem lead-in assembly60for coupling the sensor streamer45to the survey vessel10, it should understood that embodiments of the present invention may be used to couple other survey devices that may be used for measuring properties of the water itself or actively/passively measuring properties of the Earth. Non-limiting examples of such survey devices include energy sources, sensor assemblies, samplers, and tranducers, among others.FIG. 2illustrates use of lead-in50comprising rigid-stem lead-in assembly60for coupling one or more survey devices52to the survey vessel10.

FIG. 3illustrates a marine geophysical survey system5that utilizes lead-ins50that each comprises a rigid-stem lead-in assembly60to couple a plurality of sensor streamers, such as outer sensor streamers45aand inner sensor streamers45b, to the survey vessel10. As illustrated, the marine geophysical survey system5may include a plurality of laterally spaced-apart sensor streamers45a,45bon which the geophysical sensors (not shown) may be disposed at spaced-apart locations. “Lateral” or “laterally,” in the present context, means transverse to the direction of the motion of the survey vessel10. In the illustrated embodiment, the marine geophysical survey system5includes two outer sensor streamers45aand four inner sensor streamers45b. Lines80, such as a roper or other cable, may be used to secure the forward end of the sensor streamers45a,45bto the lead-ins50. As illustrate, spreader lines or some other type of lateral connector that extends between the outer sensor streamers45amay be omitted in accordance with embodiments of the present invention as each rigid-stem lead-in assembly60can be selectively placed in a desired lateral position. Accordingly, each of the rigid-stem lead-in assembly can be independently driven up, down, or laterally. In alternative embodiments, spreader lines or some other type of lateral connector (not shown) may extend between the outer sensor streamers45a. In some embodiments, only the lead-ins50to the two outer sensor streamers45a(as opposed to the lead-ins50to the inner sensor streamers45b) comprise rigid-stem lead-in assemblies60.

In one embodiment, the methods and systems may be used to tow sensor streamers45a,45bat a depth of up to about 25 meters or more. In some embodiments, the sensor streamers45a,45bmay be towed at as depth of at least about 25 meters and at a depth of at least about 100 meters, in another embodiment. In one particular embodiment, the sensor streamers45a,45bmay be towed at a depth up to about 500 meters or more. Advantageously, example embodiments of the rigid-stem lead-in assemblies60may be used to achieve larger depths for the sensor streamers45a,45bwithout the drawbacks to having an increase in vertical cable lengths and drag for conventional lead-ins formed from cables, as well as problems associated with horizontal line restrictions. In some embodiments, the sensors streamers45a,45bmay be towed at two or more different depths. In one embodiment, the methods and systems may be used to achieve a spread between the sensor steamers45a,45bat the outermost lateral positions (e.g., the outer sensor streamers45a) of at least about 150 meters, at least about 500 meters in another embodiment, and at least about 1,000 meters in yet another embodiment. In one particular embodiment, the methods and systems may be used to achieve a spread between the sensor streamers45a,45bat the outermost lateral positions of up to about 1,500 meters or more.

The lead-ins50comprising the rigid-stem lead-in assemblies60may be deployed from the survey vessel10using any suitable technique. For example, a rigid-stem lead-in assembly60may be assembled and deployed from the survey vessel10. Prior to deployment, the distal end of the rigid-steam lead-in assembly60may be coupled to the forward end of the corresponding one of the sensor streamers45a,45b. The rigid-stem lead-in assembly60may be formed by connecting a longitudinal end of one of the rigid stems65to a corresponding longitudinal end of an adjacent one of the rigid stems65. A linear-tensioning machine (not shown) disposed on the survey vessel10may deploy the rigid-stem assembly60into the body of water15. Additional rigid stems65may be coupled as the linear-tensioning machine (not shown) deploys the rigid-stem lead-in assembly60into the body of water15. In some embodiments, the liner-tensioning machine may comprise one or more wheel pairs that hold the rigid-stem assembly65in tension as it is deployed. Other suitable linear-tensioning machines may be used that are capable of holding the rigid-stem assembly65, including clamps that engage shoulders on the rigid stems65or grooves or chamfers on the rigid stems65clamps in belts or in pistons or other linear machines that apply force to the rigid stems65; a hook or other attachment device on a rope coupled to an attachment on the rigid stems65, or pins that enter holes on the rigid stems65and which may be spring driven. The liner-tensioning machine may also be used for retrieval of the rigid-stem assembly65.

Referring now toFIG. 4, a rigid-stem lead-in assembly60is illustrated in more detail in accordance with embodiments of the present invention. The rigid-stem lead-in assembly60may be a structure for a number of items, including feed lines, gas lines, optical and/or electrical signals, power, external devices, geophysical sensors, tension sensors, and geophysical sources. The rigid-stem lead-in assembly60is shown in a de-coupled configuration. In some embodiments, the rigid-stem lead-in assembly60may be stored on the survey vessel15in the de-coupled configuration and assembled prior to deployment into the body of water15.

As illustrated, the rigid-stem lead-in assembly60may comprise a plurality of rigid stems65. The rigid-stem lead-in assembly60(when assembled) is characterized as being rigid in that it has as bending, torsion, and/or inline stiffness than can be maintained for considerable lengths, for example, up to about 10 meters, about 50 meters, about 100 meters, or even longer. Unlike cables and structures that have been used previously as lead-ins, the rigid-stem assembly60should not exhibit catenary behavior over at least portions of the length, but should rather exhibit elastic behavior with deformation according to deformation of beams and not sinus hyperbolic or parabolic as for cables and the like. Accordingly, the rigid-stem lead-in assembly60when assembled cannot be stored and deployed from a drum, but rather may utilize a movable or fixed attachment point (e.g., such as a detensioning apparatus that comprises wheel pairs) for deployment from the survey vessel10(e.g., shown onFIG. 1). The attachment point can hold the rigid-stem lead-in assembly60by friction (e.g., as wheel pair) or a ring, for example. In some embodiments, the rigid-stem lead-in assembly60may be characterized as being rigid for a length of about 25 meters or longer wherein the rigid stems105have a smallest width or height of about 1 meter or less.

In some embodiments, the rigid-stem lead-in assembly60may have a bending stiffness of 700 Newton-square meters (“Nm2”) or greater over considerable lengths (e.g., over about 25 meters or more). For example, the rigid-stem lead-in assembly60may have a bending stiffness of 700 Nm2over substantially its entire length. Each of the rigid stems105may also have a bending stiffness of 700 Nm2. The stiffness of 700 Nm2corresponds to a stiffness in a cantilever beam of 1-meter length fixed in one end with a load of 1 Newton in the other, deforming roughly 0.5 mm under the load. This corresponds to an aluminum (with Young's modulus of 70 GPa) tube with a 2-inch outer diameter and a thickness of 0.2 millimeters, a steel (with Young's modulus of 210 GPa) tube with a 2-inch outer diameter with a thickness of 0.03 millimeters or a circular rod with a Young's modulus of 2 GPa. Each of these items, i.e., the aluminum tube, the steel tube, and the circular rod, are examples of items with a bending, stiffness of 700 Nm2. A 2-inch outer diameter typically requires 5% deformation to be wound on a 2-meter drum, which is difficult for most materials. Most rigid materials can deform a maximum of 0.1% or, in extreme cases, 1% so they cannot be wound on a drum without being wound in a wire or umbilical. Lower strength materials may be able to deform but will then be soft to enable bending.

Embodiments of the present technique are for use with materials having a stiffness that make them difficult to take the rigid-stem lead-in assembly60on or off a drum. The rigidity will create a bending arm for the tension under which it is taken in or out. This distance multiplied with the tension, creates the load which the rigid-stem lead-in assembly60has to carry in the cross section of the first point of contact with the drum and is a critical load. The point of contact can be at or before the tangential point between the drum and the rigid-stem lead-in assembly60wherein the tangential point corresponds to no stiffness in the rigid-stem lead-in assembly60and bending arm of zero. Instruments used previously in marine surveying typically have several contributors to the bending arm. For example, sensors streamers may have repeaters, connectors, sensor housings and the like that can add to the bending arm. In addition, bend restrictors may also be placed in the ends to protect the wires inside which can add to the bending arm. Lead-Ins may also have several different contributors to increased bending arm, including reinforced umbilicals, solids such as gel filled, soft rigidified or the like, and true solids such as nylons, polyurethane, or compositions. For previous instruments used in marine surveying, the bending arm has been less than 0.3 m under a load, of 3 kiloNewtons (“kN”). Some types will have almost the same bending arm for different loads (typically hinged joints and rigid bodies), other will deform much under increasing load and hence reduce bending arm (while load goes up), but all materials are limited in stillness and have a certain deformation, even though the deformation can be very difficult to detect. Embodiments of the present technique may be used with a rigid-stem lead-in assembly60more rigid than 700 Nm2. This is more rigid than other cable or streamer-based instruments that have been used hereto for and, thus, the bending arm can become larger than 0.3 m. The rigid-stem lead-in assembly60is then in danger of damage or permanent deformation if subjected to 3 kN or more, hence winching is not a good handling method.

The rigid stems65may each comprise a stem body67. A variety of different materials and composites may be suitable for use in the stem body67. In some embodiments, the stem body67may be made from a material comprising aluminum, stainless steel, or titanium. In some embodiments, the stem body67may be made from a material comprising a composite, such as glass- or carbon-reinforced plastics, such as glass or carbon fibers in combination with epoxy or other resins (e.g., polyester, vinyl ester, nylon, etc.). In some embodiments, the glass fibers may include e-glass fibers. In some embodiments, the stem body67may be made from a material comprising a plastic, such as polyethylene, polybutylene terephthalate, polysulphone, or another suitable thermoplastic polymer. Combinations of suitable materials may also be used. One of ordinary skill in the art, with the benefit of this disclosure, should be able to select an appropriate material for the stem body67based on a number of factors, including selection of an appropriate stiffness-to-weight while maintaining cost and bonding ability to available resins.

In some embodiments, the stem body67be in the form of a pipe or other conduit that has a tubular portion that defines at least one interior chamber (e.g., interior chamber105shown onFIGS. 6A through 6C). In some embodiments, a buoyant filler material may be used to fill the interior chamber. One example of a suitable buoyant filler material comprises air or other suitable gas. However, other buoyant filler materials may also be used that can provide some degree of positive buoyancy for ballasting as well as electrical insulation, including foams, gelled hydrocarbon-based oil, hydrocarbon-based oil, visco-elastic polymer or other suitable electrically insulating, acoustically transparent materials, for example. In some embodiments, surface treatments may be applied to the exterior surface85of the stem body67, for example, to reduce drag and antifouling. For example, one or more anti-foulant agents may be applied to the exterior surface85. By way of further example, one or more drag-reduction treatments may be applied the exterior surface85. WhileFIG. 4illustrates the rigid stem lead-in assembly60having three rigid stems65, it should be understood that embodiments of the rigid-stem lead-in assembly60may include more or less than three rigid stems65, as desired for as particular application.

The rigid stems65may each have a length, for example, in a range of from about 1.5 meters to about 50 meters or, alternatively, from about 3 meters to about 12.5 meters. In specific embodiments, the rigid stems65may each have a length of about 3.125 meters, about 6.125 meters, or about 12.5 meters. The rigid stems65may each have an outer diameter (e.g., D1onFIG. 6A) in a range of from about 0.02 meters to about 0.2 meters or, in alternative embodiments, of about 0.04 meters to about 0.08 meters, for embodiments with a circular-shaped cross-section, for example. The rigid stems65may each have a width (W1onFIG. 6C) in a range of from about 0.1 meters to about 0.5 meters and a height (H1onFIG. 6C) up to about 0.4 meters, for embodiments with a wing-shaped cross-section, for example. In some embodiments, rigid stems65may an aspect ratio (ratio of width to height) of about 1 to about 20, about 2 to about 20, or about 1 to about 8. When assembled, the rigid-stem lead-in assembly60may have a length, for example, in a range of from about 50 meters to about 1000 meters. If more than one rigid-stem lead-in assembly60is joined end-to-end, the combined assembly may have a length in a range of from about 200 meters to about 2000 meters or longer, for example. In some embodiments, the combined assembly may have as length of up to about 8000 meters, which may be used, for example, with towing depths of a few to several hundred meters.

In some embodiments, the rigid-stem lead-in assembly60may further comprise end connector elements, at either end of rigid-stem lead-in assembly60. In the illustrated embodiment, the rigid-stem lead-in assembly60comprises a male-type end connector element90at one end and a female-type end connector element95at the opposite end. The end connector elements should be configured for connection to corresponding connector elements (not shown) disposed at the longitudinal ends of adjacent rigid-stem lead-in assemblies. Each of the end connector elements can make mechanical and electrical connection to the corresponding end connector elements on the adjacent rigid-stem lead-in assembly (not shown).

In some embodiments, a flexible cable100, which may be an electrical or optical conductor, for example, extends between the rigid stems65. In some embodiments, the flexible cable100may conduct a gas, such as air, for maintenance of air volumes, ballasting, and recover, as well as supply to air guns, which may be on the rigid-stem lead-in assembly60, for example. As illustrated, the flexible cable100may extend from either end of the rigid-stem assembly65between the connector elements (e.g., from the male-type connector element90to the female-type connector element95). The flexible cable100may extend through the interior chamber (e.g., interior chamber105shown onFIG. 5) in the rigid stems65. In some embodiments, the flexible cable100may comprise multiple cables extending through the passageway.

While not shown onFIG. 4, sensors, actuators, transducers, and other electronics (e.g., tanks, batteries, etc.) may also be incorporated into the rigid stems65. Example sensors that may be incorporated include sound/pressure sensors, motion sensors (speed, velocity, and/or acceleration), EM sensors, magnetism (e.g., compass), pressure/depth sensors, tension sensors, surface or bottom echosounders/mappers. Examples of transducers include sound/pressure for acoustic positions, lateral (e.g., to maintain network of positions for several instruments, inline (e.g., bending/water properties), bottom (height) or surface (depth), and electro-magnetic. In some embodiments, one or more actuators may be incorporated into the rigid, stems105. Example actuators may include control surfaces, ballast tanks, openings, covers/lids, and connection points, among others. For example, control surfaces such as wings) for steering or rotational position may be used. The control surfaces may act to provide depth and/or lateral control for the rigid stems65. Moreover, the control surfaces may allow the rigid stems65to perform a desired move while in the water, such as an undulation, surfacing, diving, rescue, or recovery. Ballast tanks may be also be incorporated that can allow the rigid stems65to maintain depth, surface, or compensate for water intrusion, such as by gassing a flooded chamber in a particular rigid stem65. Openings may also be provided for access to sensor surfaces, ballast, and/or weight/mass center manipulation. Connection points that are openable and/or closable may also be provided in the rigid stems65, such as valves or ports for feed or transmission lines. Covers/lids that are openable and/or closable may also be provided, which may enable cleaning and/or streamlined handling, for example.

FIG. 5illustrates two adjacent rigid stems65coupled together by a stem joint70in accordance with embodiments of the present invention. To maintain rigidity of the rigid-stem lead-in assembly60(e.g., shown onFIGS. 1 and 2), the stem joint70may form a rigid connection between the adjacent rigid stems65. As illustrated, the adjacent rigid stems65may each comprise a stem body67having an interior chamber105with flexible cables100extending between the adjacent rigid stems65by way of the interior chamber105. In accordance with present embodiments, sleeves10may be used for holding the stem joint70in clamping position to couple the adjacent rigid stems65. The stem body67of each of the adjacent rigid stems65may have a longitudinal end portion115over which the sleeves110may be disposed. The sleeves110may each be slidably moveable on the corresponding longitudinal end portion115to cover the stem joint70and hold it in place. While not illustrated, a locking element may be provided for securing the sleeves110in locking position. For example, the sleeves110may each be spring loaded by a corresponding spring.

The stem joint70may comprise two clamp portions120. The clamp portions120should cooperate with one another so that, when the stem joint70is assembled, the clamp portions120define a rigid-stem passage that receives at least a portion of the longitudinal end portion115of each of the adjacent rigid stems65. In some embodiments, each of the clamp portions120may generally have a bent- or C-shaped cross-section. It should be understood that the cross-section of the clamp portions120may vary, for example, based on the particular configuration of the adjacent rigid stems65. The clamp portions120may each have an interior surface125. The interior surfaces125may each have axially extending recesses130for receiving the end portions115of the adjacent rigid stems65. As illustrated, the clamp portions120may have holes135for receiving bolts (not shown) to hold the clamp portions120in place. In some embodiments, the sleeves110may also slide over the ends of the clamp portions120to fasten the clamp portions120in clamping position to couple the adjacent rigid stems65. In other embodiments (not illustrated), the clamp portions120might go over not only the flexible cables, but also hinges or some other mechanism that could connect the rigid stems65while keeping one axis of the flexibility open for folding the stems to be closed by the clamp portions120.

Embodiments of the present invention are not limited to the stem joint70illustrated byFIG. 5. It should be understood that other types of connectors may be used to couple the adjacent rigid stems65to one another. Examples of connectors that may be used for the stem joint70include, without limitation, a locking nut with inline pin, socket connections, face fibers.

It should be understood that the shape of the cross-section of the rigid stems65may vary as desired for as particular application. The rigid stems65may have, for example, an oval-, circular-, triangular-, square-, pentagonal-, other polygonal-, wing-, or non-symmetrical-shaped cross-section.FIGS. 6A through 6Cillustrate rigid stems65having differently shaped cross-sections.FIG. 6Aillustrates as rigid stem65A, having a circular-shaped cross-section.FIG. 6Billustrates a rigid stem65B having a rectangular-shaped cross-section.FIG. 6Cillustrates a rigid stem65C having a flat or wing-shaped cross-section. The wing-shaped cross-section may be desirable, for example, to reduce the drag, coefficient for the rigid-stem lead-in assembly60. A reduced drag coefficient may particularly beneficial, for example, where substantial cross-flow may be encountered, such as when coupling a sensor streamer or other towed body in marine surveying. In some embodiments (not illustrated), the wing-shaped cross-section may have an asymmetric wing profile, which may be beneficial, for example, to provide one-side lift. The wing-shaped cross section may have ratio of width W1to height H1of greater than about and, alternatively, greater than about 1.5. In some embodiments, the wing-shaped cross section may have a ratio of width W1to height H1in a range of from about 1 to about 10.FIGS. 6Athrough6C further illustrate the rigid stems65having an interior chamber105, which may include various cables100, such as electrical or optical cables, for example.

FIG. 7illustrates a rigid-steam lead-in assembly60in which the rigid stem65comprises wings140a,140bthat extend from the stem body67, in accordance with embodiments of the present invention. As illustrated, the rigid-steam lead-in assembly60may be towed in or close to the horizontal plane, for example. The flow direction is illustrated onFIG. 5by arrow145. The lateral angle α of the rigid stem65compared to the flow direction145may be small close to the path of the survey vessel10and larger for the outermost of the lead-ins50with a lateral angle α of up to about 60° or greater, in some embodiments.

To provide lateral force and place the forward ends of the respective sensor streamer45(e.g., sensor streamers45a,45bonFIG. 3) in a selected lateral position, the rigid stem65may comprise wings140a,140bmounted to the stem body67. As illustrated, one of the wings140a,140bmay extend upward from the stem body67and one of the wings140a,140bmay extends downward from the stem body67. In some embodiments, the wings140a,140bmay be foldable or retractable. By being able to unfold the wings140a,140binto an open position, the rigid-stem assembly60may be lift-activated after deployment. In other words, the wings140a,140bmay be unfolded after deployment into the body of water15to move into the selected lateral position. In some embodiments, the stem body67may have a rigid-stem cavity150for receiving the wings140a,140b. In a closed configuration, the wings140a,140bmay be folded and stored in the rigid-stem cavity150. To open and close the wings140a,140bany of a variety of different suitable techniques may be used. In some embodiments, a wing-covering stem sleeve155may cover the wings140a,140bretaining them in the rigid-stem cavity150. In alternative embodiments, the wings140a,140bmay be opened using hinges or joints (not shown), which may be automated or driven manually, in combination with springs (not shown) for biasing the wings140a,140b.

The wing-covering stem sleeve155may be disposed over at least a portion of the stem body67and be slidably moveable on the stem body67. For example, the wing-covering stem sleeve155may be configured to move on the stem body67and uncover the wings140a,140b. A thread screw or other suitable mechanism (not shown) may be used to drive the wing-covering stem sleeve155. :In some embodiments, the wings140a,140bmay be biased, for example, by a spring (not shown) so that uncovering the wings140a,140bshould cause the wings140a,140bto open. To close the wings140a,140b, the wing-covering stem sleeve155may be slid back over the wings140a,140bto cause the wings to fold back into the rigid-stem cavity150. The rigid stem65may further comprise a wing-cavity stem sleeve160disposed over at least a portion of the stem body67and slidably moveable on the stem body67. The wing-cavity stem sleeve160may be moved to cover the wing cavity150, for example, to prevent drag caused by having an opening in the rigid stem65. The wing-covering stem sleeve155and the wing-cavity stem sleeve160may have the same shape as the stem body67, for example, to reduce drag on the rigid-stem lead-in assembly60.

In alternative embodiments (not shown), the wings140a,140bmay be mounted on the stem body67at deployment from the survey vessel10and removed from the stem body67at retrieval from the body of water15. For example, the wings140a,140bmay be mounted on the stem body67by way of a snap-on connection not shown) or other suitable connection mechanism.

The wings140a,140bmay be mounted on the stem body67such that the wings140a,140bextend at an angle β from vertical with respect to flow direction145. In this manner, the wings may provide lateral lift as they are moved through the body of water15. For example, the wings may be at angle β of less about 90°, alternatively, less than about 45°, and alternatively, less than about 10°. As illustrated, the wings140a,140bmay be considered to be vertical as they extend vertically or an angle β from vertical with respect to the flow direction145. In some embodiments, the wings140a,140bmay be fixed at the angle β. In alternative embodiments, the wings140a,140bmay be coupled to the stem body67such that the wings140a,140bmay be moved, for example, to any angle β. For example, the wings140a,140bmay be mounted to the stem body67by an axle (e.g., axle165onFIG. 8) that can be actuated to move the wings140a,140bto the angle β. In other embodiments, the wings140a,140bmay be mounted on an axle that freely rotates. A freely rotating axle should achieve greater lateral-force-to-drag ratios and, thus, be more efficient in spreading the sensor streamers45. In addition, a freely rotating axle may enable the same wings140a,140bto be used in different locations of the rigid-stem assembly60and in any one of the lead-ins50, as they can be used in various angles β. The angle β can in other embodiments be actuated or driven by the opening/closing mechanism to change the angle β continuously or in steps to enable active steering of lift and, thereby, depth and offset of the entire towed assembly dynamically.

WhileFIG. 7illustrates only as single rigid stem65it should be understood that two or more rigid stems65each having wings140a,140bmay be employed, in a rigid-stem assembly60in accordance with embodiments of the present invention. To vary the lateral lift generated by the wings140a,140b, the wings140a,140bon as subset of the rigid stems65may be opened. In some embodiments, the rigid-stem assembly60may be become engaged with an undesired object, such as fishing gear, debris, or ropes that are in the body of water15. To disengage the object, the wings140a,140bon successive rigid stems65may be closed until the object has become disengaged. Depending on the proximity to the sensor streamer45and the desired lateral position, different angles β may be selected for the wings140a,140bon each of the rigid stems65. Accordingly, example embodiments may include different angles β used for the rigid stems65on the same one of the lead-ins while additional example embodiments may include different angles β for the rigid stems65on different lead-ins50. It should be understood that less wings140a,140bmay be needed on the particular rigid-stem lead-in assemblies60coupled to the innermost of the lead-ins50as less lateral lift may be needed, for example.

A cross-sectional view of a rigid stem65having wings140a,140bmounted to the stem body67is shown onFIG. 8in accordance with embodiments of the present invention. The wings140a,140bare each shown folded in the corresponding rigid-stem cavity150. As illustrated, the wing-covering sleeve155may be disposed over at least a portion of the stem body67and can cover the wings140a,140b, for example, to hold each of the wings140a,140bin the corresponding rigid-stem cavity150. In the illustrated embodiment, the wings140a,140bare mounted to stem body67by axle165. The axle165may be fixed or freely rotating, for example. The stem body67may also define one or more interior chambers105wherein various components may be installed, such as cables100. While not shown, sensors, actuators, transducers, and other devices (e.g., tanks, batteries, etc.) may also be incorporated into the interior chambers105.

Example embodiments of the rigid stem65may comprise one or more attachments or devices for depth control. For example, horizontal wings, ailerons, ballast tanks, or other devices known to those of ordinary skill in the art may be used for depth control. In some embodiments, the rigid stem65may comprise, substantially horizontal wings that extend from the stem body67. The substantially horizontal wings may be configured to provide vertical lift as the rigid stem65is moved through the body of water15.

FIG. 9illustrates a rigid stem65comprising at least one aileron170attached to the edge175of the stem body67in accordance with some embodiments. As illustrated, the aileron170may extend longitudinally in a direction that is generally parallel to longitudinal axis180of the rigid stem65. In addition to providing vertical lift, the aileron170may also be configured to control rotation of a rigid-stem assembly in which the rigid stem65may be incorporated. For example, the angle of the aileron170may be adjusted to control rotation.

FIG. 10illustrates an embodiment of a rigid stem65comprising at least one ballast tank185disposed in the stem body67. As illustrated, the ballast tank185has an interior volume190in fluid communication with first port195. In some embodiments, a piston200may also be disposed in the ballast tank185. The piston200may be operably coupled to a linear drive205and a motor210. The linear drive205may operate, for example, to convert mechanical energy generated by the motor210to produce a straight line force such that the piston200can move longitudinally within the ballast tank185. In some embodiments, the interior volume190of the ballast tank185may contain seawater. Water may be drawn into or expelled from the interior volume190, for example, to control depth. At a desired time, the seawater may be expelled from the ballast tank185via the first port195. To expel water from the ballast tank185, the motor210may be used to move the piston200, thus forcing water from the interior volume190through the first port195. Air from interior chamber215of the stem body67should fill the interior volume190as the seawater is expelled. The piston200may be moved in an opposite direction, for example, to draw water into the interior volume190. As illustrated, the interior volume190of the ballast tank185may be in fluid communication with the interior chamber215via second port220. In other embodiments (not illustrated), other types of ballasting using, for example, elastic membranes or other methods of changing volume or mass of chambers by pumping or actuation, may be used as will be appreciated by those of ordinary skill in the art.

FIG. 11illustrates an embodiment showing a segment of a rigid-stem lead-in assembly60having three rigid stems65a,65b,65c. As illustrated, rigid stem65bis disposed between the other two rigid stems65a,65c. In example embodiments, the rotation of the rigid stems65bmay be controlled using, for example, wings (e.g., wings140a,140bshown onFIG. 7) so only rigid stem65bis rotated to generate lift. In this manner, the middle rigid stem65bby a different angle than the outer rigid stems65a,65c, whereby lift may be generated to force down the rigid-stem lead-in assembly60, for example.

Accordingly, embodiments may include using a rigid-stem lead-in assembly comprising a plurality of interconnected rigid stems in a marine seismic survey. Some advantages of employing the rigid-stem lead-in assembly may include one or more of the following. One of the many potential advantages is that embodiments of the rigid-stem lead-in assemblies may be configured to have a stream-lined, low-drag profile for drag reduction as the rigid-stem lead-in assembly is towed, which may result in reduced tension in the lead-in assembly, reduced strumming, and lower fuel consumption for the survey vessel, for example. Another potential advantage is that exampled embodiments of the rigid-stem lead-in assemblies may have a larger interior volume as compared to the previously used cables, thus providing larger buoyancy while also making the rigid-stem lead-in assemblies particularly advantageous for embodiments where sensors, actuators, transducers, and other devices (e.g., tanks, batteries, etc.) may be incorporated into the rigid stems without needs for separate housings, seals, and penetrators, for example. Yet another one of the many potential advantages is that due to the layout of the cable inside the rigid stems and their handling, the inside cable can be positioned behind each other rather than around each other, thus leading to reduced height of the assembly with potentially lower drag. Yet another one of the many potential advantages is that due to their increased rigidity, embodiments of the rigid-stem assemblies should be less susceptible to rotation and tangling, which can be beneficial for lead-ins. Yet another one of the many potential advantages is that embodiments may not use a surface reference commonly used with spreading devices, such as trawl doors, thus reducing potential interference with other vessels. Yet another one of the many potential advantages is that the stiffness of the rigid-stem lead-in assembly should provide less position change as potential rotation of the lead-in may be reduced. Yet another one of the many potential advantages may come from flexibility of having many rigid-stem lead-in assemblies, each coupled to its own sensor streamer or other towed body combined with the flexibility of each with different wing settings of control of wings, ballast or other, able to selectively position each of the rigid-stem lead-in assemblies, thus potentially reducing the need for lateral ropes between each sensor streamer and allowing replacement of a single sensor streamer/lead-in without the need to cease operation of the others.

In contrast to systems that use rigid-stem lead-in assemblies comprising a plurality of interconnected rigid stems in a marine survey.FIG. 12illustrates a conventional seismic survey system300. As illustrated, the seismic survey system300may include a survey vessel305towing a plurality of sensor streamers310through a body of water315. Lead-in lines320may be used to couple the sensor streamers310to the survey vessel305. Each of the sensor streamers310may include sensors325. The sensor streamers305may also include lateral force and depth (“LFD”) control devices330(e.g., “birds”) and associated acoustic range sensing devices335, which can be disposed at selected positions along the sensor streamers305collocated with the LDF devices330or at separate positions. Spreading devices340, such as doors or paravanes, may be used to maintain lateral separation of the sensor streamers305. Unlike the system illustrated byFIG. 12, embodiments of the present invention that use rigid-stem lead-in assemblies in a survey may contain almost no buoys, doors, paravanes, chains or extra ropes, or LFD devices, such as birds. For example, a seismic survey may be performed that only has the desired lights and antennas above the water.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present 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. Although individual embodiments are discussed, the invention covers all combinations of all those embodiments. 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 illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted for the purposes of understanding this invention.