Marine geophysical surveys are often used for oil and gas exploration in marine environments. Marine environments may include saltwater, freshwater, brackish water, and other similar environments. Various types of signal sources and sensors may be used in different types of geophysical surveys. For example, one type of marine geophysical survey is based on the use of pressure waves. In such a survey, a vessel may tow an acoustic source (e.g., an air gun or a marine vibrator) and a plurality of streamers along which a number of pressure sensors (e.g., hydrophones) are located. Pressure waves generated by the source may then be transmitted to the Earth's crust and then reflected back and captured at the sensors. Pressure waves received during a marine seismic survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located. As another example, marine electromagnetic (EM) surveys may be conducted using EM signals transmitted by a submerged antenna and detected by EM receivers.
In a typical marine survey, the streamers on which the sensors are located are very long, typically multiple kilometers in length. Some surveys may be conducted with a single streamer, while some surveys utilize multiple streamer systems including one or more arrays of streamers. The individual streamers in such arrays are generally affected by the same forces that affect a single streamer. Equipment used to connect streamers to the towing vessel generally maintains the depth of the forward end of the streamers and maintains the forward ends of the streamers at selected lateral distances from each other as they are towed through the water.
Each streamer of the streamer array may include a tail buoy at the distal end of the streamer. Tail buoy may typically include geodetic position receiver such as a GPS receiver that may determine the geodetic position of the tail buoy. The geodetic position receiver may be in signal communication with other relevant survey equipment.
A typical streamer can extend behind the seismic vessel for several kilometers. Because of the great length of the typical streamer, the streamer may not travel entirely in a straight line (or other planned configuration) behind the towing vessel at every point along its length due to interaction of the streamer with the water and currents in the water, among other factors. As such, the streamers in the array may have a tendency to cross and tangle, resulting in operational downtime. During deployment or retrieval of the array of streamers, entanglement may be common. Generally, streamer positioning devices may be employed to prevent the entanglement of and detangle streamers. However, when the ropes or chains connecting the tail buoys become entangled, they may require manual untangling, because they are generally not equipped with such positioning device. Manually untangling the ropes or chains and the tail buoys to which they are attached can be time consuming and costly. Unless a nearby repair vessel has the capability and availability to untangle the ropes or chains and the tail buoys, the survey operation typically must be suspended so that the array of streamers and the attached ropes or chains and tail buoys can be retrieved to be untangled by the survey vessel crew.
Another instance of entanglement may arise when the array of streamers is being towed near an offshore structure or obstacle (such as ice floes). As the wind and current may push the array of streamers and the respective tail buoys into the offshore structure or the obstacle, the streamer or the array of streamers and the respective tail buoys may hook onto or cross the structure or the obstacle resulting in entanglement, and in some cases, damage to the streamers and the sensors attached to the streamers.
In addition to being a hazard to streamers and the sensors attached thereto, tail buoy entanglement can also be hazardous to the survey crew because untangling the tail buoys often requires manual operation. Particularly in deep sea survey operations, such manual operation can be dangerous and is thus highly undesirable.
Accordingly, in marine seismic, electromagnetic, and other types of surveying, the need exists for an apparatus in place of the tail buoys but without being physically attached to the trailing end(s) of the streamer or the array of streamers. The efficiency of a survey operation is likely to increase as the above mentioned entanglement and downtime may be curtailed. The efficiency may additionally increase as the quantity of the survey equipment and the complexity of operation may be reduced. Streamers without tail buoys may also reduce towing load, resulting in further cost savings of the entire survey operation. Moreover, streamers free of tail buoys may result in reduced tug noise thereby increasing survey data accuracy. Additional advantages may include less hazardous working environment for the survey crew.
This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.
This specification may use phrase such as “based on.” As used herein, this term is used to describe one or more factors that affect a determination. This term does not foreclose additional factors that may affect a determination. That is, a determination may be solely based on those factors or based only in part on those factors. Consider the phrase “determine A based on B.” This phrase connotes that B is a factor that affects the determination of A, but does not foreclose the determination of A from also being based on C. In other instances, A may be determined based solely on B.
Various devices, units, circuits, or other components may be described or claimed as “configured to”, “usable to”, or “operable to” perform a task or tasks. In such contexts, “configured to”, “usable to” and “operable to” is each used to connote structure by indicating that the devices/units/circuits/components include structure that performs the task or tasks during an operation. As such, the device/unit/circuit/component can be said to be configured to, usable to, or usable to perform the task even when the specified device/unit/circuit/component is not currently operational (e.g., is not on or in operation). The devices/units/circuits/components used with the “configured to”, “usable to”, or “operable to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a device/unit/circuit/component is “configured to”, “usable to”, or “operable to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112(f), for that device/unit/circuit/component.
While at least a portion of the explanation of the need provided herein refers to seismic surveying, it is important to recognize that the survey system here is not limited to seismic survey but rather any survey system which includes a plurality of laterally spaced-apart sensor streamers towed by a vessel. Such other types of streamers may include, without limitation, electrodes, magnetometers and temperature sensors. Accordingly, the references to seismic streamers are provided as non-limiting examples.