dimarts, 24 de desembre de 2013

El submarí científic ICTINEU 3 supera amb èxit les primeres proves sota l’aigua*

Les maniobres s’han realitzat a la depuradora de Sant Feliu de Llobregat gràcies a la col·laboració d’Aigües de Barcelona i l’Ajuntament

El submarí científic ICTINEU 3 ha superat amb èxit les primeres proves d’immersió realitzades aquest dijous dia 19 de desembre davant dels mitjans de comunicació en una piscina de la depuradora de Sant Feliu de Llobregat. A casa nostra no hi ha instal·lacions específiques prou grans per poder fer aquestes proves amb un submarí tripulat i per això ha estat important la col·laboració de l’Ajuntament de Sant Feliu de Llobregat i d’Aigües de Barcelona, que han facilitat l’ús de les instal·lacions. Per aquesta raó, a banda de la presència del director general d’ICTINEU Submarins, Pere Forès, i de la directora d’operacions, Carme Parareda, les proves han comptat amb la participació de l’Alcalde de Sant Feliu de Llobregat, Jordi San José i Buenaventura, la regidora de Promoció Econòmica, Maria Cinta Daudé, el director d’Explotació de Sanejament d’Aigües de Barcelona, Pere Aguiló, així com la cap de planta de la depuradora, Rut Estany.

El submarí s’ha submergit dues vegades en una piscina per comprovar el bon funcionament dels sistemes de flotabilitat, navegació, comunicació i seguretat, com a pas previ a les proves de port i de mar previstes per l’any que ve, l’última fase abans d’obtenir la certificació i classificació final. Segons ha explicat la directora d’operacions d’Ictineu Submarins, Carme Parareda, “la immersió ha consistit en submergir dues vegades el submarí, amb 3 persones a dins, a una profunditat de 4,5 metres”. Parareda ha matisat que “és poca profunditat” però “suficient” per comprovar que l’aparell “funciona de manera correcta”.

Es tracta de la primera aparició sota l’aigua del submarí científic. L’última vegada que es va poder veure va ser en el marc del cinquè congrés de tecnologia marina MARTECH celebrat a Girona del 9 a l’11 d’octubre, al Parc Científic i Tecnològic de la Universitat de Girona.

* Notícia publicada al web del projecte Ictineu. Des d'aquest bloc no podem sinó alegrar-nos de la nova fita assolida per l'equip del Projecte Ictineu. Renovem la crida a totes aquelles persones i empreses que, en la mesura de les seves possibilitats, els ajudin.

diumenge, 22 de desembre de 2013

Plumbing the Depths: Unmanned Submersibles Come of Age*

Unmanned underwater vehicles (UUV) have been slow to attract attention in military circles – until now, that is. Today, Jon Rosamond looks at the technological advances that are enabling the development of unmanned submersibles and which countries are leading the way.
By Jon Rosamond for ISN
Two high-profile events from the past three years have served to highlight the growing importance of the unmanned underwater vehicle (UUV) in naval operations. The first incident saw the Royal Navy minehunter HMS Brocklesby, operating off Libya in May 2011 as part of the NATO mission to enforce UN Security Council resolutions, deploy a SeaFox UUV to destroy a buoyant mine laid by pro-Gaddafi forces outside Misrata harbour. Packed with 100kg of explosives, the mine was one of three placed by Gaddafi loyalists seeking to halt the flow of humanitarian aid into the port.
Subsequently, in August 2012, the US Navy sent dozens of SeaFox UUVs to the Persian Gulf after the Iranian government threatened to use its arsenal of Soviet-era mines to blockade the Strait of Hormuz, a move that would have effectively shut down a major proportion of the world's oil supplies. Both events validated the efforts that are being made by Western navies to reshape their post-Cold War mine countermeasures (MCM) capabilities. Instead of placing bespoke timber- or GRP-hulled minesweepers and ordnance disposal divers in harm's way, navies are taking advantage of developing technology to 'keep the man out of the minefield' by allowing unmanned platforms – on the surface as well as underwater – to perform critical tasks.
Naval hydrographic and oceanographic units are using UUVs equipped with a wide array of sensors to chart the seabed and/or determine the characteristics of a given body of water, providing essential data for planning submarine and amphibious operations. Unmanned submersibles are also employed on search-and-rescue and intelligence, surveillance and reconnaissance missions. Meanwhile, trials have commenced in the US that will result in the introduction of a new class of deep-diving UUV designed to stalk hostile submarines. The development of a time-critical strike capability is a real possibility too.
ROV, AUV and…
There are two broad categories of UUV: the remotely operated vehicle (ROV) and the autonomous underwater vehicle (AUV). The first is tethered to the host vessel and controlled continuously by a human operator, while the second is programmed to swim to one or more waypoints and operate independently for a predetermined period of time. Both types are routinely equipped with payloads that include sonars, cameras, environmental sensors, manipulator arms and (in the case of MCM assets) some form of mine-destruction device.
German company Atlas Elektronik, however, describes its ubiquitous SeaFox ROV as a 'semi-autonomous' vehicle. Although it is controlled via an optical fiber cable from the host ship, SeaFox can use its sonar to automatically relocate the previously acquired positions of mines or mine-like objects. These objects are identified using the onboard CCTV camera and, if necessary, destroyed by the one-shot mine neutralization system (a large caliber shaped charge).
At the Association for Unmanned Vehicle Systems International (AUVSI) 2013 exhibition in Washington, DC, Thales UK released details of an innovative military/commercial technology demonstrator for MCM operations using a Saab Seaeye ROV fitted with a Hydra multi-shot mine neutralization system. The ROV is deployed from an optionally-manned 11m boat; demonstration trials for the Royal Navy were scheduled for October.
Containerized solutions are becoming increasingly popular with navies. In 2012, for example, the Royal Danish Navy achieved full operational capability of a modular, containerized MCM capability featuring Saab Double Eagle ROVs equipped with mine destructor charges.
Meanwhile, the Royal Canadian Navy recently acquired eight SeaBotix vLBV950 ROVs to survey ships' bottoms and conduct deep water tasks such as locating lost aircraft and retrieving their 'black box' data recorders. These ROVs may also be used to retrieve objects jettisoned by pirates or smugglers, as well as in MCM roles.
Enter Knifefish
As far as autonomous types are concerned, General Dynamics Advanced Information Systems announced in August that it had successfully completed a comprehensive risk reduction phase for what is arguably the world's most high-profile UUV program: the US Navy’s Knifefish venture. Based on Bluefin Robotics' Bluefin-21 vehicle, the Knifefish AUV is scheduled to enter service as part of the Littoral Combat Ships' MCM mission package from 2017. Knifefish is GPS-guided and can operate with full autonomy for about 16 hours in total. Each vehicle is about 20ft (6m) in length, weighs about 3,000lb (1,360 kg) and can run at 6kt, carrying a novel Low Frequency Broadband sonar designed to detect and identify mines in highly-cluttered sea floor environments.
The high-fidelity sonar was tested during the risk reduction phase along with Knifefish's ultra-high density data storage/recording system, propulsion system (with the emphasis on noise reduction) and software interfaces.
Although Knifefish is probably the world's largest UUV development program, US-based manufacturer Hydroid (now owned by Norwegian company Kongsberg) is the global market leader, having supplied more than 200 AUVs to 13 navies. In February, Hydroid announced that its contract to provide Littoral Battlespace Sensing (LBS) AUVs to the US Navy's Space and Naval Warfare Systems Command (SPAWAR) had moved into full rate production. SPAWAR ordered three LBS AUVs – a variant of Hydroid's heavyweight Remus 600 AUV – for oceanographic and meteorological data-gathering duties.
In June 2012, the company said that Japan's Ministry of Defence had purchased a single Remus 600 system for MCM missions and also to investigate and map the sea floor dispersion of contaminants following the previous year's magnitude 9.0 earthquake and tsunami. Also in 2012, Germany's Federal Office of Defence Technology & Procurement ordered six of the smaller (man portable) Remus 100 AUVs to enhance the Deutsche Marine’s MCM capabilities in very shallow waters, while the Royal Norwegian Navy acquired four additional Remus 100 systems for MCM tasks.
Works in Progress
The technical challenges involved in developing UUVs for naval service are numerous, particularly in relation to speed and endurance, geospatial accuracy, sensor performance and data transfer. The small size of many vehicles, particularly the man-portable types, puts severe limits on the space available for batteries, propulsion and guidance systems, sonars and other sensors.
One solution to the problem of limited endurance is the underwater glider, which uses wings and small changes in buoyancy to convert vertical motion to horizontal, allowing it to travel thousands of kilometers (in a sawtooth-like pattern) over many months. Kongsberg unveiled its Seaglider model at AUVSI 2013 and will begin full production in December. Developed initially by the University of Washington with funding from the US Navy, Seaglider is designed for missions lasting up to 10 months. Heading, depth and altitude sensors allow the vehicle to navigate while submerged but it also surfaces frequently to fix its position by GPS.
The problems of data transfer are more intractable. Radio waves do not travel happily through salt water, making communication with a submerged AUV (lacking an umbilical link to the host vessel) extremely difficult. The need to come to the surface, or at least raise an antenna above the water, in order to transmit or receive data increases the risk of detection by hostile forces. With real time communication not an option, data is normally uploaded by personnel onboard the host vessel after the AUV has been retrieved from the water.
Looking to the future, one cutting edge initiative is the Deep Sea Operations (DSOP) technology and system development program run by the US Defense Advanced Research Projects Agency (DARPA), which is intended to create a deep-diving AUV able to operate at depths of 6,000m and covertly trail submarines operating overhead.
Bluefin Robotics completed six days of deep-water testing of one of its DSOP vehicles in April, including two 4,450m dives lasting 11 hours. Further tests will focus on sonar system integration while a second vehicle is produced to demonstrate networked operations. Effective power and noise management will be crucial if the required levels of endurance and silence are to be achieved.
While UUVs have been slow to gain traction in military service compared to unmanned air vehicles, the functionality and reliability of modern ROVs and AUVs mean they are now more than capable of taking on a wide variety of naval tasks that were traditionally regarded as 'dull, dirty or dangerous'. The fact that de-mining, hydrographic survey and other vital missions can now be completed in a fraction of the time that was previously required, and at a fraction of the cost, suggests that prospects for the naval UUV are bright indeed.

Jon Rosamond is a freelance journalist and editorial consultant specializing in the global defense arena, with particular expertise in the maritime domain. He was previously the Editor of Jane’s Navy International and a defense correspondent for The News.

* Article publicat al web de l'ISN. Les tecnologies no tripulades, tot i ja ser presents en el camp de l'aviació militar, no són tant conegudes en l'aspecte naval. Vet aquí un interessant article sobre el tema.

dissabte, 7 de desembre de 2013

U.S. Navy Launches UAV from a Submarine*

Naval Research Laboratory XFC Sea Robin demonstration Atlantic Undersea Test & Evaluation Center in August 2013. US Navy Photo

The U.S. Navy has demonstrated the launch of a small unmanned aircraft from submarine for the first time, the service announced in a statement on Dec. 5. The successful test could herald the arrival of potentially revolutionary new intelligence, surveillance and reconnaissance capabilities for the Navy’ submarine force and special warfare communities.


The demonstration “represents an unprecedented paradigm shift in UAV [unmanned aerial vehicle] propulsion and launch systems,” Warren Schultz, program developer and manager for the program at the Naval Research Laboratory (NRL). “This six-year effort represents the best in collaboration of a Navy laboratory and industry to produce a technology that meets the needs of the special operations community.”

During the demonstration, the NRL developed eXperimental Fuel Cell Unmanned Aerial System (XFC UAS) was fired out of the USS Providence (SSN-719)’s torpedo tube using a ‘Sea Robin’ launch system, which is designed to fit inside an existing Tomahawk launch canister.

After it was fired out of the Providence’s torpedo tube, the Sea Robin rose to the surface and launched the XFC UAS—but only after it received permission from the Los Angeles-class boat.
The NRL describes the XFC as a “fully autonomous, all electric fuel cell powered folding wing” unmanned air system. It has an endurance of greater than six hours, according to the NRL. The aircraft uses an electrically assisted take-off mechanism, which boosts the small UAV vertically out of its container.

Once in the air, the XFC “flew a successful several hour mission demonstrating live video capabilities streamed back to Providence, surface support vessels and Norfolk” read the NRL statement. The XFC eventually landed at the Naval Sea Systems Command Atlantic Undersea Test and Evaluation Center (AUTEC) in the Bahamas after the completion of the sortie.

According to the NRL, the lab received funding from the Office of Naval Research’s SwampWorks officer and the Department of Defense Rapid Reaction Technology Office (DoD/RRTO).

The Navy and industry have both explored launching unmanned aerial vehicles from submarines.
In late 2009, Boeing and Northrop Grumman proposed equipping the Multiple All Up Round Canisters (MAC) in the Block III Virginia-class (SSN-774) attack submarines and the Ohio-class guided missile submarines (SSGN) with the compressed carriage version of the ScanEagle UAV.

* Article publicat al US Naval Institute. Interessant esdeveniment, tot i que caldrà seguir-ne l'evolució, que obre tot un camp en les operacions de reconeixement clandestí.

divendres, 6 de desembre de 2013

diumenge, 1 de desembre de 2013

What is a Corvette? And What Next?*

Classification of surface warships as cruisers, destroyers, frigates, or corvettes, has become like pornography. There are no generally accepted definitions, but “I know it when I see it”–except that everyone sees it a little differently.
Since this is “Corvette Week” what are we really talking about?
(Note: unless otherwise specified, lengths are over all and displacements are full load)
My Combat Fleets of the World, 16th Edition, which I have used here extensively for reference, defines Corvettes as, “Surface Combatants of less than 1,500 tons but more than 1,000 full load displacement–essentially, fourth rate surface combatants.”  but goes on to note that “…the designation as used here essentially refers to smaller frigates and does not correspond to the European concept of corvettes as any warship larger than a patrol craft but smaller than a frigate.” I feel to confine the definition within a 500 ton range is too restrictive. in fact it would have excluded the Castle class corvettes of WWII as too large, and other corvettes as too small.

Royal Navy Photograph of Castle class corvette HMS Denbigh Castle (K696)
Pre-WWII
During the age of sail, corvettes were originally warships typically smaller than a frigate, but larger than a sloop, usually with guns on a single deck. Some ships continued to be called corvettes as steam was introduced, but in the Royal Navy, in 1877, corvettes along with sloops and frigates were subsumed under the new designation “cruisers.” Corvettes, as a type, essentially disappeared from the English naval lexicon until 1939. The term was kept alive in some navies (including the French, German, and Italian) as a rank that translated corvette-captain, a rank generally equal to Lieutenant Commander.
World War II:
Corvettes as a type reemerged just prior to WWII. As it became clear that U-boats would be a major threat, Britain saw the need for an escort vessel that could be built quickly and in large numbers, in yards that had not been considered capable of building warships. Just before WWII, they ordered the first of 267 “Flower Class” corvettes that were built in the UK and Canada. They modified the design for a whale catcher named Southern Pride, enlarging it to 205 feet overall and a displacement of 1245 to 1390 tons. They were terrible warships, weakly armed, cramped, uncomfortable, and slow. Single screw, reciprocating steam propulsion gave them a maximum speed of only 16.5 knots, a knot slower than a typical (Type VII) surfaced U-boat. They were originally intended only for coastal operations, but because of their long range, they were thrown into the Battle of the Atlantic, where they were by far the most numerous transatlantic convoy escorts for the critical early years, taking slow merchant convoys across the mid-Atlantic air gap, while the Home Fleet’s more capable, but shorter legged, fleet destroyers were generally held back to escort the battle fleet or met convoys only as they approached the British Isles.

Flower Class Corvette HMS Polyanthus, Source =www.oldships.org.uk, Author =Leidseplein Date =1943-09-
Reportedly Winston Churchill had a hand it designating this new class “corvettes,” probably in an attempt to make them appear more glamorous than the term “patrol vessels” which had been applied to similar vessels previously. Two years after the re-introduction of the term “corvette,” the term “frigate” was also resurrected to describe another war emergency escort program, this one more complex and more capable but still using reciprocating steam propulsion. Larger commercial yards converted to making frigates (301 to 307 ft, 1920 to 2420 ton), but smaller yards continued to make corvettes of the improved Castle class (252 ft, 1590 to 1630 tons), while naval yards continued to produce small numbers of sloops like the Black Swan class that were the true premier ASW escorts of the Royal Navy.
Australia also built corvettes, 60 ships of the similar but even smaller, slower Bathurst Class (186 ft). Initially they were classified as minesweepers, but found more employment as escorts, so were more frequently referred to as corvettes.

Bathurst-class corvette, HMAS Fremantle, State Library of Victoria
Japan, Germany, and Italy all made similar escort ships, but only the numerous Italian  Gabbiano class (193 foot, 728 tons, with combined diesel or electric propulsion no less),  were actually referred to as corvettes.
All of the WWII corvettes were primarily ASW escorts, but their were a number of classes of vessels, many built prior to the war, that share DNA with today’s missile armed corvettes. These were small, fast, torpedo armed vessels that resembled destroyers, but most had a standard displacement of 1000 tons or less. Usually they were referred to as “torpedo boats.”  Japan built twelve, The Germans built 48 (the last 15 were large enough to have been considered destroyers in other navies). The French Navy completed twelve. The Italians completed 69 (some of which were closer to frigates or destroyer escorts). The Italian Spica class (269 ft, 885 to 1,030 ton, 34 knots) may serve as an example.
Italian Spica Class torpedo boat
Generally, the war emergency programs had one thing in common. They were not the ships these navies would have chosen to build in peacetime. In wartime priorities change; planning horizons contract. Producibility may trump quality. They were all compromised in some fashion–in their speed, survivability, weapons, or economy of operation. Corvettes filled a need for large numbers of escorts, but after the war, most were quickly discarded.
The MCM Connection:
The Flower Class Corvettes were originally also equipped to sweep mines. As noted the Australian Bathurst Class began life as minesweepers. While the US built no “corvettes” during the war, the minesweepers of the Raven (220 foot/1040 tons), Auk (221 foot/1,250 tons), and Admirable ((180 foot) classes frequently functioned in this role. In fact, with minor modification Admirable class ships were redesignated PCEs (Patrol Craft, Escort). All these minesweepers were built with sonar. By the end of the war, most were equipped with hedgehogs, depth charge projectors (K-guns) and dual depth charge racks, having enjoyed priority for ASW equipment second only to destroyer escorts.

File:PS-74 Rizal.jpg

Former Auk class minesweeper still serving in the Philippine Navy as Corvette BRP Rizal (PS-74), US Government photo, 050822-N-6264C-145 Sulu Sea (Aug. 22, 2005)

Post WW II:
Since the end of WWII corvettes have generally fallen into two categories, with some designs attempting to incorporate elements both types. They tend to be either:
—Small, fast, missile armed vessels optimized for ASuW, like Sweden’s Visby Class (40 knots, 239 ft, 650 tons) usually expected to operate in groups, either with others of their kind or acting as flagships for even smaller missile boats, or
—Smaller versions of frigates with moderate speed optimized for patrol and presence in peacetime and escort during wartime like the Damen designed SIGMAs or  India’s Kamorta Class (25 knots, 358 foot oa, 3100 tons).
File:K33 HMS Haernosand Karlskrona Marindagen2008.jpg
Visby class Corvette, HMS Härnösand, Source: Xiziz at en.wikipedia
File:Kri-diponegoro-1600-1200.jpg
SIGMA class corvette
Largest Operators of Corvettes:
The largest operator of corvettes is Russia with approximately 53 (3 Buyan, 1 Buyan M, 7 Parchim II, 23 Grisha V, 4 Grisha III, 2 Dergach Project 1239, 13 Nanuchka) (80 if you count the 27 Tarantuls that fall slightly below the 500 ton threshold I have assumed).
India, China, South Korea, Indonesia, and Italy also maintain large numbers of corvettes.
File:Type 056 corvette 583 Ganzhou.jpg
Chinese Type 056 corvette 583 Ganzhou, by 樱井千一
Corvettes in the USN:
While the US Navy has never built corvettes for its own use, the type is not without precedence in the US.
In the early days of WWII, when the US navy was desperately short of escorts, 18 Flower class corvettes were transferred to the USN. Eight of those were manned by USCG crews.
File:USS Intensity (PG-93).jpg
Coast Guard manned Flower Class Corvette USS Intensity (PG-93), mid-1943. Former  HMCS Fennel (K194) [http://www.history.navy.mil/photos/images/h97000/h97406.jpg]
In the 50s the Navy was interested in experimenting with types that might be built hurriedly in an emergency. The result was the four ships of the Claude Jones class (DE-1033-1036) built by Avondale between 1956 and 1959. At 312 feet long and 2000 tons, they were essentially the same size as the preceding Dealey Class, but they were  simplified, diesel powered, slower, and more lightly armed. These ships were really a update of the corvette concept of a cheap simple escorts that lent itself to rapid construction. (Similarly about the same time the British were building 14 HMS Blackwood Class  (Type 14) that were “2nd Rate Frigates” of 1536 tons, powered by a single shaft steam turbine plant with no gun larger then 40mm.)
File:USS Claude Jones (DE-1033).jpg
USS Claude Jones (DE-1033), US Navy photo, www.navsource.org
In the late 1960s the US built four corvettes, given US hull numbers PF-103 to PF-106, that were immediately turned over to the Iranian Navy. They became the Bayandor Class (275 feet long, 1,135 tons).
In the early ’70s, two additional PF-103 class ships (PF-107 and 108), built to a modified design, were delivered to Thailand’s Navy. These were the Tapi Class.
Between 1977 and 1983 Tacoma Boat built a class of four CODOG powered “PCG” for Saudi Arabia, the Badr class, 245 feet, 1,038 tons, 30 knots.
Between 1983 and 1987 Tacoma Boat built two diesel powered “PFMMs” for the Thai Navy Ratanakosin class 252 foot, 960 tons, 26 knots.
Between 1989 and 1995 Northrop Grumman Litton built three CODOG Corvettes for the Israeli Navy, the Sa’ar 5 class, (281 foot, 1,275 tons, 33 knots).
File:Three Sa'ar 5 Class Missile Corvettes Going For a Cruise.jpg
American built Israeli SA’AR5 corvettes, http://www.flickr.com/photos/idfonline/6871983192/in/photostream
Between 2008 and 2013, VT Halter Marine has been building a class of four missile corvettes for the Egyptian Navy, the  Ambassador MkIII class (205 feet, 700 tons, 41 knots). The first has already been delivered.
An undated photo of the ENS S. Ezzat, an Egyptian Fast Missile Craft. VT Halter Marine Photo
An undated photo of the ENS S. Ezzat, an Egyptian Fast Missile Craft. VT Halter Marine Photo
While the Littoral Combat Ships are not normally considered corvettes, on June 10, 2013, Rear Admiral John F. Kirby, the Chief of Information for the Navy called them Corvettes. Without a mission module or aviation detachment, they are really more like OPVs. But when the Mine Warfare module is mounted they become MCM vessels. When an ASW or ASuW module is mounted, they start to look like corvettes.
The Claude Jones class ships were transferred to the Indonesian Navy and continued in service there until 2006. Of the six PF-103 class ships, two Iranian ships were lost in combat with Iraq, but the remaining four are still in service with the Iranian and Thai Navies and have been updated. The Badr class and the  Ratanakosin class are still in service with their respective navies, and the Sa’ar Vs are still the most advanced surface ships in the Israeli Navy. All but the two Thai Navy Ratanakosin class (PF-107 and 108) have been equipped to launch anti-ship cruise missiles.
The Coast Guard Connection:
During WWII Coast Guard Cutters were frequently used as ASW escorts, some quite successfully, filling corvette and frigate roles. After the war, new construction frequently included provision for ASW systems either as built or as planned upgrades in the case of a major conflict.
The 16 Reliance class Medium Endurance Cutters (210 feet, 1,050 tons, 18 knots) delivered 1964 to 1969, were built with provision for adding sonar, hedge hogs, and torpedo tubes. They were originally to have been designated PCs. a designation shared with the sub chasers of WWII.
The 12 Hamilton Class High Endurance cutters (378 feet, 3,050 tons, 29 knots) completed 1967 to 1972, were built with ASW systems installed and their systems were upgraded and provision for harpoon installed 1989 to 1992. As built, they were not the equal of contemporary Destroyer Escorts with their AN/SQS-26 sonars, but were comparable to those built only a few years before. An argument can be made that these ships, as built and later modified, could be considered, if not frigates, at least corvettes.

USCGC Mellon after upgrades including Harpoon, CIWS, and support for LAMPS

The thirteen Bear class cutters (270 feet, 1,780 tons, 19.5 knots) completed 1983 to 1990, were built without ASW systems, but had provision for adding a towed array and supporting a LAMPS I helicopter. If these systems had been provided, then the ships might have also been considered corvettes.
The Coast Guard’s National Security Cutters, of the Bertholf class (eight ships planned, 418 ft/4,500 tons) have no installed ASW systems or ASCMs, but they do have excellent aviation support facilities and the ship has been marketed as the basis for a frigate program. Aside from Exocets carried by the French ships, they are in most respects more capable warships than the Floreal “light surveillance frigates” (307 ft/2950 tons) and similar to the French Lafayette Class frigates (410 ft/3,600 tons) which also currently have no sonar.
File:USCGC Waesche by Yerba Buena Island.jpg
USCGC Waesche, U.S. Coast Guard photo ID: 100228-G-2129M-004
File:Ventose 1.jpg
French frigate Floréal-class Ventôse (F733)
The Coast Guard is in the process of procuring a new class to replace its Medium Endurance Cutters. The resulting ship is likely to be similar to the Floreal class (90 to 100 meters in length and 2500 to 3500 tons) but faster and will share sensors and some weapons with the Bertholf class and the Littoral Combat Ships. Addition of ASW or ASCM systems would result in ships many would classify as light frigates or corvettes.
Bottom Line–What is a Corvette?:
Corvettes slot under frigates but above patrol boats or missile boats as a classification of surface combatants. To me, this means that they are the smallest or perhaps least capable ocean-going warships. This is a bit of a stretch for Corvettes like the Visby, but in fact the Swedes have deployed even smaller warships to the Indian Ocean for counter piracy operations. That sets the low end of the the displacement range at about 500 tons, but when we look for an upper limit, it seems a moving target, with no similar performance based limit.
The US and Britain already build destroyers the size of WWII cruisers. Germany and in the near future Britain will build frigates over 6,000 tons full load. Japan’s Coast Guard has OPVs displacing 9,350 tons full load.  If we tripled the displacement of WWII corvettes as we have done with WWII Frigates and Destroyers, Corvettes could displace almost 5,000 tons, so I don’t think displacement is a reliable determinant.
Strict naval vessel construction standards don’t necessarily distinguish a corvette from an OPV either. They were not applied to the original “Flower” class, and they don’t apply to the Damen designed Sigma class, built or building for Indonesia, Morocco, and Vietnam, or to the French Lafayette class (also operated by Taiwan, Singapore, and Saudi Arabia) and Floreal class (also operated by the Moroccan Navy) which are rated as frigates but which it might be argued are actually corvettes.
The only metric that doesn’t seem to have changed much over the last 70 years is crew size. Corvettes generally have crews of 120 or less, frigates from 120 to perhaps a bit over 200, while destroyer crews begin slightly under 200 and go up to about 350, and cruiser crews are larger still. The DDG1000s will apparently have a frigate sized crew, but their final crew may be larger than currently planned. OPV crews tend to be corvette sized or smaller.
Just as the difference between Spruance Class Destroyers and Ticonderoga Class cruisers was mission and associated equipment, not displacement, the differentiation between the various types of warships and between Offshore Patrol Vessels (OPVs) and corvettes may simply comes down to their missions and equipment. OPVs include a wide range of ships, but the common thread, generally accepted, is that they have no ASW weapons, no heavy anti-ship cruise missiles, and only a self-defense AAW capability. Adding an ASW capability and/or cruise missiles would convert an OPV into a corvette. Perhaps they would not make very good warships, but then the original Corvettes weren’t very good warships either, but they served a vital role. Conversely an old frigate or corvette, stripped of all its weapons except a medium caliber gun and heavy machine guns would become an OPV, even if it nominally retained its frigate or corvette designation as in the case of Portugal’s Joao Coutinho and Baptista de Andrade class or some of Italy’s Minerva class.
If we had no history, and we could start ship designations on “a clean sheet of paper” we might define ships types based on their missions and equipment, saying destroyers are vessels designed with robust capacity to perform well in all three major surface combatant warfare areas, AAW, ASuW, and ASW. Frigates are designed to perform well in only two missions areas  (with possibly modest self defense capability in the third). Corvettes would be single mission specialists with only modest capability in the other two missions (if at all). OPVs would be vessels equipped for missions that did not require robust capabilities in any of these three mission areas. All four types might be called generically “cruisers” which would bring that designation back to its original meaning, a vessel smaller than a ship of the line that can operate independently.
The Future of Corvettes:
WWII corvettes were small ships packed with crew and weapons.They were small because there was an urgent need for many ships that could not be met by the shipyards that normally built warships. They were a way of making the small commercial yards serve the war effort. If we are ever engaged in a prolonged conflict against a near peer adversary we may again resort to a similar expedience. If so, the resulting corvette is more likely to be based on a petroleum industry offshore support vessel rather than a whaling or fishing vessel.
But when ships are built in peace time, for a 20 to 40 year life, other factors beside construction cost start to dominate. In the West, crew costs weigh heavily, while increasing hull size appears less important, provided we do not load up the larger hull with additional systems which will in turn drive up crew costs. Larger hulls are more seaworthy, allow greater endurance, and may be made quieter. They may even be more economical to operate and maintain because of easier access.
Some European Countries that formally operated a number of Corvettes seem to have abandoned the type in favor of ships with more range and better seakeeping including The Netherlands, Denmark, and Norway. Denmark has instead produced frigates and a novel class of ships, the Absalon Class “support ships,” (450 ft/6,600 tons) that include a relatively large hull of modest speed, with a relatively small crew of about 100, and a large reconfigurable spaces–an open one topside midships where missile systems can be placed and a “garage” area under the flight deck that can accommodate vehicles and containerized loads. These ships are perhaps too large to be considered corvettes, but they are not nearly so well armed as the frigates of the similarly sized Iver Huitfeldt-class. They do have characteristics I would expect to see on future corvettes, a relatively commodious hull (because “steel is cheap and air is free”), a relatively small crew (because that is the most expensive component over the life-cycle of the ship), and reconfigurable spaces and weapon systems, that allow the ships to be adapted to different missions (because that is allow us to hedge our bets regarding what capabilities will be needed, while allowing that minimal crew over most of the life of the ship).
Because Corvettes are always compromised, they are likely to be controversial. Many will not agree with the compromises accepted. That is certainly true of the new American Corvette, the Littoral Combat ship.
In some respects the LCSs may be the prototype of the future corvette, in that it is not particularly small, but they were made cheap to operate with a minimal crew, and they are single mission ships, but with the advantage that the mission can be changed over time, although not as quickly as once advertised. Other aspects of the ship were perhaps not as well thought out, but they will serve a purpose, and perhaps the next generation LCS  or convertible corvette will better meet our needs.

 * Article publicat al CIMSEC. Interessant i molt necessària reflexió al voltant de la definició de les corbetes. Lectura obligada per totes aquelles persones interessades en la futura configuració de les forces navals catalanes.