Tail Buoy

Key Takeaways

• Tail buoy GPS and acoustic positioning provides the real-time position of the streamer's far end that, combined with the positions of the front (head) of the streamer from the vessel's navigation system and the intermediate streamer positions from in-line bird compasses and acoustic positioning nodes, allows the seismic contractor's positioning software to reconstruct the actual 3D geometry of the streamer array at every shot point during the survey: the tail buoy GPS receiver transmits its position via a radio link (UHF or satellite) to the navigation system on the vessel, providing the boundary condition for the far end of each streamer in the array positioning calculation; acoustic positioning systems complement the GPS tail buoy position by using short-baseline or long-baseline acoustic ranging between the tail buoy, in-cable acoustic nodes, and acoustic transponders on the vessel's hull, providing cross-track position information that is not captured by the along-streamer GPS positions alone; the positioning accuracy of the streamer array (needed to correctly assign each recorded seismic trace to its actual subsurface reflection midpoint) is improved when the tail buoy position is combined with multiple intermediate positions from in-cable SEGY birds (small hydrodynamic control devices with compasses and inclinometers that report their position within the cable) and acoustic ranging nodes distributed along the streamer length; the uncertainty in the feathering angle (the angular deflection of the streamer from the vessel track caused by crosscurrents) is smallest when the tail buoy position is known with high accuracy because the feathering angle determines the actual position of every midpoint along the entire streamer length.
• Tail buoy safety lighting and radar marking requirements are governed by maritime collision regulations (COLREGS) and seismic industry standard practices that specify the minimum visibility and radar detectability for tail buoys deployed in areas with vessel traffic: the IMO COLREGS require all objects towed or deployed from a vessel (including seismic streamers and their tail buoys) to be marked to avoid collision risk, with the tail buoy carrying all-round white lights that are visible for at least 2 nautical miles at night, and radar reflectors providing a radar cross-section equivalent to a vessel of at least 200 square meters to ensure detection by other vessels' marine radars in rain and sea clutter conditions; active radar beacons (racons) that transmit a coded radar response when interrogated by a passing vessel's radar are used in busy shipping lanes to provide positive identification of the tail buoy as a seismic equipment marker rather than a vessel; AIS Class B transponders on the tail buoy broadcast the buoy's GPS position, MMSI identification number, and vessel association to all AIS receivers within VHF range, providing other vessels with the information needed to avoid the seismic array even when the streamer itself (which runs several meters below the surface and is not visible) might foul a passing vessel's propeller if they approached without adequate standoff; the spacing between tail buoys in a wide-array 3D survey (which may span 600 to 1,200 meters of lateral width for a 12-streamer array at 100-meter streamer spacing) requires that each buoy be individually identifiable to prevent other vessels from attempting to pass between streamers, which would snag the cables.
• Tail buoy retrieval and management during survey operations requires coordinated deck procedures to prevent damage to the buoy and the streamer during line turns, equipment changes, and emergency recoveries in rough weather: at the end of each survey line, the vessel slows and the streamers are winched in, with the tail buoy of each streamer arriving last over the stern ramp or A-frame after all the seismic hydrophone sections have been retrieved; in calm conditions, the tail buoy is lifted manually by deck crew or by a dedicated tail buoy crane from the water, the tail line is disconnected, and the buoy is stowed in a purpose-built rack on the stern deck; in rough weather, the tail buoy retrieval is more hazardous because the buoy is being pulled toward a moving vessel in wave-driven sea conditions, with the potential for the buoy to be slammed against the vessel hull or for deck crew to be struck by the tail line during retrieval; automated tail buoy deployment and retrieval systems (mechanical hoists and guided launch rails) are used on modern seismic vessels to reduce the manual handling risks during rough-weather operations; streamer paravane positioning of the outermost streamers in a wide array places the tail buoys at large lateral offsets from the vessel where they are difficult to reach if the vessel must recover a damaged streamer, requiring the vessel to maneuver the damaged streamer's tail buoy into reach by adjusting the array configuration.
• Tail buoy design evolution has progressed from simple foam floats with passive radar reflectors to sophisticated multi-sensor electronic packages that transmit GPS position, water temperature, current velocity, and streamer tail tension data in real time to the vessel's positioning system: early tail buoys were essentially watertight fiberglass or polyethylene floats with a retroreflective radar reflector mounted on top and a strobe light powered by a sealed battery pack with a service life of 24 to 48 hours; modern tail buoys incorporate GPS receivers with sub-meter positioning accuracy, two-way radio links (UHF or satellite) for data transmission and remote monitoring, AIS Class B transponders, acoustic transponders for positioning in the sub-surface streamer geometry calculation, current meters (DVL or ADCP instruments) that measure the depth and direction of currents near the streamer tail that are input to the hydrodynamic model of the streamer's shape, and solar charging panels combined with high-capacity batteries that provide weeks of operational endurance without battery replacement; the data from the modern intelligent tail buoy is transmitted continuously to the vessel's seismic processing and navigation system, where it is integrated into the real-time positioning calculations that determine the actual midpoint geometry of every recorded seismic trace within seconds of each shot firing.
• Tail buoy loss and streamer damage events represent significant operational and financial risks for seismic contractors, as lost streamers and tail buoys create marine hazards (a sinking or submerged streamer can foul passing vessels' propellers), require costly replacement of the seismic equipment (a single 6-kilometer hydrophone streamer can cost USD 1 to 3 million to replace), and result in the loss of previously acquired seismic data from the repositioning and re-survey time required: the most common causes of tail buoy and streamer loss include fishing vessel or merchant ship crossings that cut the cable (a risk managed by maritime communications, exclusion zone enforcement, and AIS broadcasting), severe weather events that cause streamer breakage from excessive mechanical loading during retrieval or from storm-induced wave action on a deployed array, and equipment failures including winch hydraulic failure that prevents streamer retrieval and leads to the streamer being released to prevent it from dragging the vessel off course; seismic contractors maintain emergency procedures for controlled streamer release (releasing the front deck attachment and allowing the full streamer to float to surface when a winch failure prevents normal retrieval) that minimize the length of cable lost and the risk to other vessels while the recovery operation is organized; some modern seismic vessels use acoustic releases on the streamer-vessel connection that allow the streamer to be dropped and subsequently recovered by a standby vessel rather than dragged along the seafloor if a mechanical emergency prevents normal retrieval.