Detecting Noise in the Fleet
Wavelengths: An Employee's Digest of Events and Issues (NAVSEA Carderock)
By William Palmer
WEST BETHESDA-The Division's Signatures Directorate (70) is comprised of several components. One of these, the Signatures Characterization and Analysis Department (71), has the responsibility of measuring and assessing the acoustic signature of virtually every Navy ship and submarine. While submarines are a prime Code 71 focus, the acoustic signatures of surface ships are also evaluated, primarily to decrease their susceptibility to submarine threats and to reduce the susceptibility to mine threats. Additionally, the acoustic signature is evaluated to determine the effect the ship's own "noise" has on its sonar systems.
Three Code 71 branches involved in acoustic trials are Trials Planning and Coordination (Code 711), Radiated Signature Assessment (Code 712), and Sonar Self- and Structureborne-Noise Signature Assessment (Code 713). Each code plays a different role during the surface ship trials, with some personnel physically onboard the ship being tested, while others remain at facilities on shore. Code 711 and 713 personnel ride the ship being tested at sea. Code 711 "riders" serve as liaison between the ship's crew and NSWCCD technical codes working with the trial. Additionally, the 711 riders navigate the ship while on the acoustic range.
The Code 713 personnel acquire data during the transit and while the ship is "on-range." The Sonar Self-Noise personnel use the ship's own sensors to acquire data, while the Structureborne-Noise personnel use accelerometers and microphones to acquire data on machinery and on the hull frames.
Finally, the Code 712 personnel monitor the data from the hydrophone array and are in communication with the ship to advise them of the test results. The Code 712 engineers remain ashore in a facility called the "Beach House," a cinder block building with no windows. A typical trial involves 10 to 12 people, with three of them working at the Beach House. The acoustic trials may be conducted on either the East or West Coast.
The Trials Group conducts tests on new construction surface ships, gathering data during the ship's first time at sea. Subsequent trials of Fleet ships are conducted during Surface Ship Radiated Noise Measurements (SSRNMs). These are typically done for deploying vessels. Tactically significant tonals or broadband noise are highlighted during these trials, as well as any noise reduction system problems with recommended fixes. Most anti-submarine warfare (ASW) combatants have Masker air systems, which emit a layer of bubbles between the hull and the seawater. This has the effect of de-coupling the ship's noise from the sea. Prairie air systems emit air bubbles from the leading edge of the propeller and reduce propeller noise. Both systems are somewhat of a maintenance headache for the ship and can even increase the ship's signature if they are not working correctly. Again, these noise sources are measured because they affect the ship's ASW capabilities and the ship's susceptibility to influence mines.
Navy ships can be evaluated for different reasons. For example, ships have to meet radiated noise requirements and sonar self-noise goals. "We verify per NAVSEA instruction, that the ship meets the requirements and goals," explained Mike Miller, an engineer in Code 713, who has been performing these trials for 11 years, and who was in the Navy prior to working at Carderock Division. "If the ship doesn't (meet the noise goals), we have to explain why and recommend fixes."
Before, During and After the Tests
To prepare for a trial, the first item on the list is to devise a test agenda. For each ship, the agenda will have different content. For example, guided missile destroyers, or DDGs, have a preset agenda that the trials team follows. This agenda can be altered if special tests are required. Sections of the agenda are either added or deleted to accommodate sponsor requirements. Class history data are a crucial part of the trial, and are compared against the ship currently in testing to see how closely that ship will duplicate class-average data gathered from past trials on ships of the same class. The analysts, at trial's end, can brief the commanding officer about findings. "When we leave the ship, we . tell the captain about our findings and give pointers on what needs attention," explained Miller, adding that, if possible, the trials team will try to correct deficiencies before leaving the ship.
Miller places a lot of value in working with the crews on the ships. "You get to go out and talk to the crew and ride around in the Navy's latest and greatest," he explained. He usually finds ship crews to be very cooperative and helpful. "We take our data and provide them with a post shakedown availability (PSA) speedletter, as well as a trial report." The PSA speedletter lists items which can be altered or repaired during the ship's scheduled availability. The trial report has additional details regarding conditions discovered during the acoustic trial. Recommenda-tions and any other relevant information are also included.
Range trials are generally 48 hours long, with three days spent dockside in preparation for the at-sea trial. Then, the ship transits to Atlantic Undersea Test and Evaluation Center (AUTEC), located at Andros Island in the Bahamas for weapon systems testing. The Atlantic Force Weapons Test Facility (AFWTF), in St. Croix, is another test site, featuring a deep-water listening array. This array can add significant value to trial data, because it is fixed in one place and provides higher quality data than is typically gathered with a portable system (used in AUTEC).
Work with Other Organizations
The trials team has also worked with such organizations as the Coastal Systems Station (CSS) in Panama City, Fla., on mine vulnerability analysis. At CSS, a prime responsibility is to dismantle mines from foreign countries. The goals are to learn how the mines work and to determine how sensitive they are to a ship's signature (acoustic and magnetic).
"Then they can use our information which shows how much noise the ship is making," explained Miller. "This allows them to determine the vulnerability of the ship to a particular type of mine. That information is extremely helpful." Recently, there was a joint NSWCCD/CSS test of the USS Roosevelt (DDG 80), which was the second ship in her class (DDG 51 Flight IIA). CSS laid out a minefield, using foreign mines with triggers, but with all explosives removed. NSWCCD personnel placed hydrophones next to the mines. Data was gathered about the mine regarding its type, orientation, and distance from the ship when triggering occurred. The data from this exercise yielded the ship's real time acoustic signature, as well as mine vulnerability information.
Trials have also been conducted with the Coast Guard in the past. And, more recently, the Division has been heavily involved with acoustic evaluations performed on the National Oceanic and Atmospheric Administration's research vessel, the FRV 40.
While Miller speaks of the surface ship side of the code, acting branch head Fred Anderson (713) makes some observations about the code's dealings with submarines. The Sonar Self- and Platform-Noise personnel who deal with submarine trials are using own-ship sensors, such as platform and towed array hydrophones, to gather information about the noise a submarine makes external to the hull when running submerged and its impact on the acoustic signature and sonar performance. Inside the boat-the "dry side"-accelerometers measure structureborne vibrations on hull frames and potential problem areas to determine the internal contributors to the acoustic signature. Together, these two onboard acoustical viewpoints of a submarine make up the data comprising the boat's signature. Couple this information with hydrophone array data gathered by the Radiated Noise personnel from off the test vessel, and you have an excellent diagnostic tool by which to measure the submarine's acoustic posture.
Anderson says the scope of the code is growing at a steady pace. "We've gone well beyond trials. We're conducting 60% to 70% full-scale trials, with propulsion system testing, system design support, specification testing, and vibration monitoring. We also support special R&D programs when they piggyback with our trials. We're basically the noise T&E [testing and evaluation] guys," he explains. The propulsion system testing is sponsored by NAVSEA 08, while specification testing supports the new Virginia Class submarines and other research/ development efforts in general. Anderson says the value of the kind of testing the Code does lies in the fact that it is what he calls "full-scale testing," which means that testing is done onboard the ship, with all components of the various shipboard systems in place and operating to capacities expected of them by design criteria.
The code performs valuable shipboard testing and also Fleet support. "When the Virginia Class comes into existence," says Anderson, "we'll have a very junior workforce, with four years or less of experience. On-the-job training is what it takes." Training slots on a trial are limited or non- existent because of budget constraints, making it hard to train new hires, and increasing the difficulty of meeting the code's mission. Simulators or static ship visits could probably be used, but going to sea on a trial is the best way to accomplish two important tasks: keep the code's workforce together as a team, and provide training and experience for code personnel.
Anderson points out that increased computing power available to analysts is helping to make the Code 713 team more efficient. "There are new digital acquisition systems, and smart systems with sophisticated data screening and filtering," he says. "We also have a large historical trial database to help our analysts." Another tool currently coming into usage is the Total Ship Monitoring System (TSMS), which will be installed on all Seawolf and Virginia Class submarines, and is planned for backfit on all Los Angeles and Ohio Class boats.
The TSMS systems have improved testing onboard data acquisition hardware and processing algorithms for ship's force acoustic signature monitoring use, and can easily support testing and evaluation exercises. Also, when TSMS is combined with the assets of the SIPRNET computer network, which connects the Navy's sea-going units together and provides secure data transfer capability, it may be possible for Code 713 to reduce or eliminate the presence of their personnel onboard a submarine. With proper training of the ship's crew, a trial could be conducted without onboard personnel being present.
Anderson says, "Our major cost is logistics. You have to pay for overtime, travel, equipment, and instrumenting a boat for an at-sea trial. Using TSMS is definitely a cheaper option and ultimately could be technically adequate to meet onboard measurement requirements for trials."
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