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Auv launch and recovery from us navy ships wpics compressed - color


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a paper that shows the genesis if launch and recovery systems at sea

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Auv launch and recovery from us navy ships wpics compressed - color

  1. 1. AUV LAUNCH AND RECOVERY FROM US NAVY SHIPS: PROBLEMS AND SOLUTIONS Jim Walton Ocean Systems Division Space and Naval Warfare Systems Center San Diego, California, U.S.A. ABSTRACT The Space and Naval Warfare Systems Center San Diego (SSC SD) has developed over thirty manned and unmanned submersibles and their launch and recovery (L&R) systems over the past four decades. The launch and recovery systems have ranged from motion compensated cranes for remotely operated vehicles to sea-state tolerant cocoons for autonomous underwater vehicles (AUVs). In recent years the techniques developed by SSC SD to launch and recover AUVs have begun to be adapted by commercial firms. Last year, SSC SD developed, installed and successfully tested an AUV L&R system on the T-AGS 64 for the Naval Oceanographic Office (NAVO). The NAVO ship, USNS Bruce. C. Heezen, will be using large AUVs for oceanographic applications. To successfully develop the L&R system for the ship, the SSC SD design team was required to overcome obstacles such as the high freeboard of the T-AGS. This paper will review applicable L&R technology, the problems associated with AUV L&R, and in particular, the design drivers and solutions in the operational L&R of AUVs from the Navy’s TAG 60 class of ships. INTRODUCTION Open ocean launches and recoveries of autonomous systems from research vessels and Navy ships have been accomplished over the years in many fashions. Free floating buoys, spars, pop- up devices, and weapons have been dropped overboard, slid down chutes, released from cables, ejected, grappled for, netted, scooped up, and even grabbed by robots. Much of the launch and recovery of developmental AUVs in calmer waters has been accomplished with simple harnesses and fixtures and has been assisted with small boats, swimmers, and divers. The earliest developmental ROVs were handled in the same fashion. The use of the simple connection on / off harnesses and fixtures became less feasible as ROVs and AUVs ventured offshore into higher sea states. The use of this type of equipment in the higher sea states results in snap loading with a high probability of damage to equipment and personnel. The use of small boats, swimmers, and divers for attaching and detaching rigging for these situations also increases the risk of harm. In response to these issues many of the solutions to the open ocean launch and recovery of vehicles have employed sophisticated compensators based upon following or matching motion of the systems and controlling the tension in their handling cables.
  2. 2. The Ocean Systems Division at Space and Naval Warfare Systems Center San Diego (SSC-SD) has a long history of launching and recovering developmental as well as open ocean buoys, autonomous oceanographic equipment, ROVs, and AUVs. SSC-SD has developed its share of simple low sea state equipment as well as sophisticated open ocean compensation equipment but has settled in on approaches for open ocean AUVs which are simple and robust. SOME LESSONS WERE LEARNED WITH ROVS SSC-SD developed a series of Cable Controlled Underwater Recovery Vehicles (CURVs) starting in the 1960s, and several small ROVs in the 1970s and 1980s. The CURV systems were open ocean ROVS, and eventually the small ROVs ventured offshore as well. Most of these systems were launched and recovered with harnesses which were hooked and unhooked at the ROV while it was at the ocean surface. SSC-SD archives retain a film clip of a CURV vehicle thrashing and banging the side of a ship during recovery while line handlers were yanked to and fro blurting expletives as they went. A significant amount of experience was gained on proper timing of the launches and recoveries and on how to use lines and humans as motion compensation systems. These human and line systems were replaced with automated motion/ tension compensation systems on the SSC-SD fleet delivered systems Mine Neutralization Vehicle (MNV) and Advanced Tethered Vehicle (ATV) ROVs as well as many other systems. A complex automated launch and recovery system used early on in the ATV project evolution was a large crane system which required a large support ship. The ultimate system for the fleet delivered ATV evolved to a simpler U-frame with a recovery cable which is tension compensated on a ram-tensioning device. This allows more rapid installation on smaller ships (Figure 1). The MNV system (of which there are nearly 60 in the fleet today) is launched and recovered with a saddle. The MNV and saddle are both beneath the surface of the ocean when they are mated and unmated with each other. One method in which the MNV is launched and recovered in fleet applications is a system where the MNV recovery saddle is suspended with a line pulling it inboard and two lines from booms pulling it outboard (Figure 2). The lowering and rising of the saddle is accomplished by coordination of the three lines being paid in or out. The use of sub-surface launch and recovery has been widely used in the ROV industry and is an attractive option for the launch and recovery of AUVs. Figure 2. MNV Launch and RecoveryFigure 1. ATV Launch and Recovery
  3. 3. WHAT DOES ALL OF THIS HAVE TO DO WITH AUVS? There are certainly both similarities and differences between the launch and recovery of ROVs and AUVs. Some of the differences fall out of the basic differences in the characteristics of the two. To date, ROVs are commonly box-like and AUVs are cylindrical although we see both of them crossing these lines as technology is advanced. ROVs are by nature attached to the support ship with a cable which can be used for retrieval, and ROVs can more likely afford to be heavier and strengthened to accommodate the rigors of launch and recovery. It is also more likely that the shape of an AUV (once attached to a line or cable to the ship) will allow it to be taken in tow behind the ship providing potential improvement in the launch and recovery possibilities. ROV experience has given us design insight applicable to the design of AUV launch and recovery systems: Divers, swimmers and small boats are not attractive for and are usually unsafe for open ocean launch and recovery; motion compensation and tension compensation approaches can be large, costly, and complex; and ship decoupled launch and recovery such as submerged mating or mating to surface following retrieval equipment provides an excellent way for a vehicle to stepwise safely transition through a recovery sequence without damage. IT REALLY STARTED WITH SONODIVER In the early 1970s while SSC-SD and others were occasionally banging ROVs against the hull of ships and just beginning to evolve the modern day open ocean ROV launch and recovery systems, SSC-SD was fielding a system which employed many of the features which have become standards of modern high sea state AUV launches and recoveries. The system was called Sonodiver (Figure 3). The Sonodiver buoy was an unmanned, untethered deep diving vehicle (an AUV of sorts). It provided a quiet platform for gathering acoustic data at predetermined depths. Sonodiver was launched and recovered employing a “garbage chute” which is the predecessor to AUV ramps developed at SSC-SD. Once Sonodiver left the garbage chute it descended, released its descent weight, hovered, took data, released its ascent weight, and returned to the surface. A floating line was released near the nose of the Sonodiver. The line was grappled for using a line throwing gun. The grappled line was used to pull the Sonodiver on-board the ship using the garbage chute. Figure 2, Sonodiver with Deployed Recovery Line
  4. 4. ADVANCED UNMANNED SEARCH SYSTEM RAMPS SET A STANDARD There are two SSC SD Advanced Unmanned Search System (AUSS) launch and recovery systems, each associated with an AUSS (prototype AUSS (late 1980s) and the fleet delivery AUSS (1990s)). The Sonodiver approach was studied and analyzed along with other approaches, and a launch and recovery ramp system was devised for the prototype AUSS and evolved for the fleet delivery. The tail of the ramp floats on the surface of the ocean as the support ship steams forward (Figure 4). The vehicle is simply released to slide down the ramp for launch. Both AUSS vehicles deploy floating lines upon surfacing (ala Sonodiver) which are grappled and brought on-board where they are attached to a line pre fed into the ramp. The vehicle is taken into tow and recovered up the ramp. The AUSS approach satisfies most of the optimum AUV launch and recovery criteria: no swimmers, divers, or small boats are required; the vehicle transitions to a ramp who’s motion is matched first with the surface of the water that the vehicle is riding upon and then is transitioned to the ships deck motion as it comes on board; and the complexities and size of automated compensation systems are avoided. Both of the AUSS ramp systems are focused on simplicity for ease of use, durability, reliability and low cost. These systems are completely mechanical except for electric motors which drive winches, capstans, HPUs, and compressors. The values of these simple/inexpensive features are verified in the fact that 134 AUSS launch and recovery evolutions have been accomplished with no vehicle damage and no loss of operational time associated with the ramp systems. The prototype ramp design employs a cart held in place at the aft end of the ramp which provides docking and centering of the vehicle as it approaches from astern. Once the vehicle contacts the cart the cart is released allowing the centered vehicle and cart to ascend to the top (front) of the ramp. Air hoses are inflated against the vehicle from the sides of the ramp holding the vehicle in place while the ramp is brought onboard. The fleet delivery ramp operates similarly to the prototype ramp except that the vehicle mechanically latches into the cart and the cart is winched up the ramp pulling the vehicle with it. Both AUSS ramps are brought on to the deck using a cable winch system, and once on board, the vehicles are moved forward out of the ramp into a maintenance shelter. These two ramps are models of simplicity and are all that is required for high sea state launch and recovery as long as the vehicle structural design is adequate to transfer the load at the nose attachment through its body. AUVs FIND SHELTER IN A COCOONFigure 4. AUSS Ramp
  5. 5. Another evolution in launch and recovery of AUVs has been the SSC SD cocoon approach. The cocoon replaces the ramp and provides a shelter for the AUV as it is brought on board. The cocoon may in some cases be used with standard handling equipment onboard the ship, and the cocoon lessons the structural design requirements on the AUV. The cocoon sequence is similar to the ramp sequence in that a pre fed line goes through the cocoon and out to the vehicle in tow. The vehicle is pulled into the cocoon using a capstan. The cocoon was first conceived of at SSC SD when an AUV requiring launch and recovery was not compatible with the ramp approach. The vehicle’s outer finish was not amenable to the rigors of mating and sliding up a ramp, and more importantly, the vehicle could not structurally tolerate the loads of being pulled up the ramp by its nose. It was decided to pull the vehicle into a floating cylinder and the cylinder would be brought on board the ship using a U frame. Unfortunately it was not possible to provide a line permanently attached to the nose of the vehicle. A nose cage with a pre fed line back to and through the cocoon was the best solution to this shortcoming. The nose cage installation at sea required the use of a small boat and swimmers. NAVOCEANO and SSC-SD have recently cooperated in development of a cocoon system for launch and recovery of vehicles associated with the NAVO AUV program. Planning meetings between the two organizations and a design tradeoff study resulted in a system approach which employs the AUSS type deployment of a floating line from the AUV which is grappled for and attached to a line which is pre fed through the cocoon. This requires a deployable line stored in the vehicle but eliminates the need for a small boat and / or swimmers (Figure 5). As for AUSS, the vehicle is taken in tow but in this case it is in tow behind the deployed cocoon. The vehicle is pulled into the cocoon where it is captured by large air hoses (similar to those used by the AUSS prototype ramp). Once the vehicle is secure in the cocoon the cocoon is two blocked into the U frame and rotated on board the surface craft This was first accomplished with a low freeboard coastal ship operated by NAVO (converted landing craft, Figures 6 &7). A greater challenge has been developing the cocoon capability for the NAVO T-AGS 60 Naval ships which have about 10 foot of freeboard astern.
  6. 6. COCOON MATCHES UP TO US NAVAL SHIP T-AGS 60 Figure 5. Cocoon Recovery Sequence 1. Ship moves into position to grapple vehicle floating line 2. Line is grappled, brought on board, and attached to a line pre fed through the cocoon. 3. Attached lines are placed back into the ocean and the vehicle is taken in tow 4. Cocoon is overboarded using the U frame and cocoon line 5. Vehicle pulled into the cocoon and captured by inflation of air hoses inside the cocoon. 6. Cocoon line is taken up on its winch until the cocoon is two blocked to the U frame. Figure 7. Low Freeboard Cocoon Recovery and Storage (note submerged cocoon tail) Figure 6. Low Freeboard Small Ship Cocoon Launch and Recovery
  7. 7. The basic operational criteria associated with a good AUV launch and recovery system is that it gracefully transitions the AUV from the motion it experiences floating in or on the ocean to the motion it experiences resting on the deck of the ship. This criteria is met with ramps such as the AUSS ramps since they are attached to the ship on a pivot allowing the ramp to float and ride with the surface of the ocean at the point where the vehicle first matches up with it. The vehicle gracefully begins to match the ramp pitch, yaw, surge, and heave as it is pulled into and up the ramp. The ramp / vehicle system transitions nicely to all motions of the ship as the ramp is brought fully on board. The motion of the nose of a cocoon-recovered vehicle will first be matched to the motion of the cocoon’s tail and will gracefully transition to match all motions of the cocoon when pulled into the system and captured. The cocoon with vehicle will gracefully transition to the motion of the ship by first being two blocked up to the U frame and then laid against a stern roller. It is important that the tail of the cocoon remains submerged and the nose of the cocoon remains two blocked to keep the cocoon from swinging. Finally, the cocoon is laid over center where it is placed upon a saddle or cart while still two blocked to the U frame. This is easily accomplished on a ship with little freeboard and a standard U frame with reasonable angular throw as seen in Figure 7. Some new issues arise with a ship of significantly greater freeboard such as the T-AGS 60. It is tempting to violate the two block critera at either the aft extent or the forward extent of the U frame throw on the larger ship since this seems to allow the use of a standard U frame. Violation of the two block criteria means that the cocoon line has been paid out at some point in the process. The nose of the cocoon in this circumstance is allowed to swing rather freely resulting in a dangerously under restrained system. The SSC-SD/ NAVO team concluded that the two design criteria which best address the higher freeboard challenges are a U frame which rotates nearly a full 180 degrees (from horizontal aft to horizontal forward) and an extension in the length of the cocoon to assure its tail remains in the water until it is laid into the stern roller. An extension module was added to the cocoon (Figure 9). The U frame design employs rotary actuators to accomplish the large angular rotation without taking excessive amounts of deck space forward. SSC-SD designed and delivered the cocoon and the U frame (after undergoing the rigors of ABS approval) and assisted NAVO in the installation and early testing of the system on-board the T-AGS 60 U. S. Naval Ship Bruce C. Heezen (Figures 8-13). NAVO has taken the system to sea recently and reports safe / reliable operation during launch and recovery in sea state four.
  8. 8. COMMERCIAL APPLICATIONS Several AUV launch and recovery ramp systems have been developed by commercial firms to date. Some follow the simplicity, durability, reliability and low cost criteria. Some systems are quite involved and have therefore missed the mark. These systems might not be particularly simple and robust or in some cases even easy to use. The cocoon approach has not yet made its way into the commercial world despite the fact that this approach has been used for many years with hundreds of safe high sea state launch and recovery cycles to its credit. One system which is expected to see a lot action in the future is the Ocean Workers patented system. Ocean Workers states that their system benefits from features of two proven Navy systems (the ramp and the cocoon). The Ocean Workers system (Figure 14) relies upon a vehicle which deploys a float for grappling and which has the structural integrity to be pulled out of the water by its nose on to a small ramp with rollers. One of the major advantages is that the U frame, winch, and line used in the cocoon approach are replaced with a cable and trolley system integrated into the maintenance van itself. The only other piece of equipment which must be Figure 8. Vehicle in Tow for Recovery into Cocoon (note rotary actuator) Figure 9. U frame Horizontal Toward Aft for Haul In (note cocoon extension between fore and aft pontoon sections) Figure 12. Cocoon On Board, U frame Horizontal Forward Figure 11. Cocoon Near Two Blocked and Against Stern Roller Figure 13. Cocoon On Deck, U frame Horizontal Figure 10. Cocoon Two Blocked With Tail In Water
  9. 9. transported and installed at the stern of the ship is the small ramp which the vehicle rolls over for launch and lays into for recovery. Figure 14. Ocean Workers Patented System
  10. 10. CONCLUSIONS Robust AUV launch and recovery systems for the open ocean have been in use for the last two decades but we see that their evolution has been underway during most of the last four decades. AUV developers are now able to identify launch and recovery system features which are important to their specific AUV and pick a system which satisfies those features. A superior approach is to consider and design the launch and recovery as an integrated piece from the very start. An AUV which can be taken in tow with a nose line deployed upon surfacing and furthermore withstand the loading and rigors of being hauled out of the water can benefit from a simple easily transportable system such as the Ocean Workers’. A launch ramp smoothly transitions the AUV from the water to the deck of the boat by stepwise matching of the AUV motions (degrees of freedom) first to the ramp and then to the ship. A properly implemented ramp may decrease the amount of force which must be carried through the skin and body of the vehicle. Ramp installations can be quite involved if ship interface is not carefully considered during system design. A cocoon launch and recovery system can be the most protective of the AUV and least structurally demanding on the AUV. This may lead to a lighter AUV using lighter materials and having less structure-born weight. The cocoon approach can, in some cases, be accomplished with existing ship handling equipment but usually an integrated U frame design is required. In the case of the NAVO T-AGS 60 class ships, a design / trade study supported the cocoon approach when high sea state, high freeboard, and AUV vulnerability issues were taken into account.