Submarine Collisions - USS San Francisco SSN-711

This is in response to a loyal reader's request.  Happy to oblige :)

 Below:  USS San Francisco SSN-711

USS San Francisco is a Los Angeles (688) Class nuclear-powered attack submarine.  She was laid down in May 1977, launched October 1979, and commissioned in April of 1981.

688 Class boats were produced in three variants, or "flights".  San Francisco is one of the first "flight" of 688 class boats.  Later flights incorporated vertical launch tomahawk missile tubes, retractable bow planes, and more advanced sensors, electronics, and noise reduction technology.

San Francisco displaces about 6100 tons, has a maximum speed of approximately 35 knots, and a maximum operating depth estimated at 950-1450 feet.  I lean toward a maximum operating depth of 850-900 feet.  This is due to the large surface area of the pressure hull, the overall weight of the vessel, and the ferrous alloy that the hull is made from.  The ship is powered by an S6G reactor that is estimated to have a thermal output of 150-165 MW.  I know the actual value, but I'm not telling :)

The ship's primary sonar is the BQQ-5 with a bow-mounted spherical sonar array.  Secondary sonar systems include a towed array, and some other specialized sensors that probably shouldn't be discussed.

San Francisco's primary weapon is the MK 48 anti-ship torpedo, but she is also capable of launching other weapons from her four torpedo tubes.  These other weapons could include undersea mines, as well as Tomahawk subsonic cruise missiles.  The Tomahawk missiles can be equipped with either conventional or nuclear warheads, depending on the tactical needs of the moment.

USS San Francisco was assigned to Submarine Squadron 7, Home ported at Pearl Harbor, Hawaii.  She made successful deployments in '82, '83, '85 and 1986, and earned several operational excellence awards:  Two Battle "E" awards, a Navy Unit Commendation, and a Navy Expeditionary Medal.

In 1989-1990, USS San Francisco underwent a "Depot Modernization Period" at Pearl Harbor (This is Navy new-speak - for centuries these have been called refits or overhauls), where she would have received upgrades to the sonar electronics, targeting computer systems, as well as refurbishing propulsion equipment.

Following the refit, she again deployed in '92 and '94, earning a Meritorious Unit Commendation and a "T" tactical excellence award within Sub Squadron 7.  To sum up, USS San Francisco appears to have had a busy schedule and several significant accomplishments and awards throughout a lengthy career.

After nearly twenty years at sea, her nuclear fuel was spent.  From 2000-2002, she underwent a refueling and overhaul.  This refueling overhaul becomes significant, because it ties into her fate following the unfortunate collision.

Below, refueling a civilian commercial power reactor.  The spent fuel element is glowing with Cerenkov radiation. Submarine refueling is a much longer and far more tedious process.

Following refueling, in December 2002, USS San Francisco was assigned to a new home base, the submarine base in Guam.  She operated out of here for about two years until the collision occurred.

The collision:
On the 8th of January 2005, while operating at ahead flank (full speed - approximately 38 MPH), and at a depth of 525 feet, USS San Francisco struck an uncharted shoal or undersea mountain.  The mountain was located about 425 miles south-southeast of Guam. (click image to enlarge and clarify)

The collision caused a great deal of damage to the ship, while at the same time causing injuries to most of her crew.  98 of her crew were injured.  One man, Machinist's Mate 2nd class Joseph Ashley, age 24 died from head injuries on the following day.   Rest in peace, young man.

The damage to the ship was severe.  The port and starboard forward main ballast tanks were ruptured.  The sonar dome was sheared off, and the sonar sphere was smashed against the pressure hull.  Fortunately the sonar sphere access hatch and the pressure hull held, but there was a struggle to achieve positive buoyancy and bring the ship to the surface.  Fortunately, they succeeded.

One shortcoming of US submarine designs is the lack of reserve buoyancy.   Buoyancy is the ability of an object to float in water.  US boats have relatively small ballast tanks relative to their overall weight, so if the ship takes on water, an emergency ballast tank blow fails, or a ballast tank is compromised, you can be in serious trouble very quickly.  With both forward main ballast tanks damaged, it would have been very challenging to surface the ship.

Below:  A cutaway of an older Sturgeon Class submarine.  The inner pressure hull is smaller in diameter than the outer hull in a couple of places, and these are the locations of the main ballast tanks.  More of these would be better in my opinion.

In any event, San Francisco's injured crew was was able to surface her, and she arrived in Guam two days after the collision, on January 10, escorted by several other Navy ships and planes.  A helicopter transferred several medical personnel to the ship as she transited on the surface to Guam.

The commanding officer was reassigned to shore duty in Guam, and was later reprimanded and removed from command of the ship, even though the sea-mount was not marked on any charts present aboard the ship.  The US Navy is absolutely unforgiving about running a ship aground, and the punishment is consistent and certain.  It seems harsh and unfair - and it probably is, but it also serves a purpose:  Don't be that skipper.  A handful of others in the control room at the time of the event were also disciplined.

Let's now look at some photos and discuss.

Reserve buoyancy:  USS San Francisco normal vs damaged.  Look how the bow sits in the water in each photo. In the second photo it appears that air is continuously being pumped into the forward main ballast tanks.

Now look how high out of the water a Russian submarine sits.  This is a double-hull design where air can fill all the space between the inner and outer hull, allows a great deal more reserve buoyancy.  These ships are designed to take a few licks in battle.  The additional buoyancy also allows access into shallower harbors than a sub with less buoyancy, because a ship with less buoyancy sits deeper in the water.

Now let's look at the damage.

Below, moored in Guam shortly after return to port.  There is a continuous low pressure blow of the forward main ballast tanks, with air probably supplied from the pier.  We have wrinkles in the hull from the collision, and sections of the sonar towed array fairing (just left of the khaki dressed man's feet) along the starboard side of the hull have popped loose, probably due to the hull flexing.

Below, all that remained of the sonar dome - the nose cone of the sub.

Before continuing, let's briefly discuss bow-mounted sonar, since that is where the majority of the damage occurred in this accident.

Bow-mounted sonar typically has a golf-ball looking sphere mounted off the front of the pressure hull, with a small round access tunnel that is normally sealed off.  Sometimes the tunnel is open for in-port maintenance.  The purpose of the golf-ball shape is to place a hydrophone facing outwards in as many directions as possible.   Whichever hydrophone(s) have  the strongest sound signal is the direction the sound is coming from, and where the enemy is located.

Submarines do not normally use active sonar (ping a contact by emitting sound), because that gives away the sub's location.  Instead passive sonar is used, and the submarine simply listens to noises that other ships make.  An active ping might be used moments before shooting a torpedo to get a more accurate range/bearing on the target, but otherwise submarines try to be absolutely silent.

Below is a cutaway of a generic modern submarine with a bow-mounted spherical sonar array.  The composite nose cone is allowed to fill with water, and is not part of the ship's hull.  The purpose of the nose cone (called the "sonar dome") is to ensure the smooth flow of water over the front of the ship as it moves.  This smoothing of water flow serves two purposes.  It makes the ship more "aerodynamic" (actually hydrodynamic), and it reduces flow noise.  Reduction of noise on the ship platform helps the sonar to be more sensitive to outside sounds.

The sphere with the criss-cross pattern is the a sonar sphere.  Hydrophones cover the outside of it, but their output jacks pass through the sphere and are sealed against seawater pressure.  Therefore the outside of this sphere is wet, but the inside is dry.  The inside is where each hydrophone connects to a signal cable to be routed to the sonar computer, which is inside the ship's pressure hull.  Also note the location of the rectangular covers for the torpedo tube outer doors.

Importantly, the inside of the sonar sphere, while dry, is not part of the ship's pressure hull boundary.  The access hatch to the sonar sphere is the pressure boundary of the ship's hull.  So while the sonar sphere is not supposed to take on water, it could also be allowed to flood without seawater entering the ship proper.

Below is a sonar sphere on a lowboy trailer.  Each small hole would have a hydrophone mounted in it and sealed against sea pressure.  Obviously this has too many penetrations to be a part of the ship's pressure boundary. 

Below:  San Francisco in drydock, undergoing repairs.  The collision flattened the sonar sphere and pushed it off toward the starboard side of the ship.  The hydrophone array is in tatters.  Along the port side, it's impossible to see the mounting ring for the nose cone, or sonar dome.

Below: Obvious deformation damage extends back to the torpedo tube outer doors, just above the guy's hard hat.  The green framework is the forward pressure hull, coated with chromate-based paint to protect it from seawater corrosion.

Below, starboard side damage.  Again showing damage as far back as the torpedo tube outer doors.

Below:  USS San Francisco receiving a replacement sonar dome after completing temporary repairs in a Guam dry dock.

Repair:

San Francisco had been recently refueled.  This is a difficult, hazardous, and expensive process that cost about $170 million dollars, and she had been operating on this new reactor core for only 3 years.  Although the damage to her bow was pretty severe, she was still repairable, and the Navy had just spent *a lot* of money refueling her.

The Navy had another 688 class submarine that was about to be retired - the USS Honolulu.  Although the Honolulu was four years newer than San Francisco, she was ready for refueling, and the Navy had previously decided that it wasn't going to refuel her - instead Honolulu was bound for decommissioning.  The Navy determined that the least expensive route would be to cut the nose off Honolulu, and attach it to San Francisco.  The alternative being refueling Honolulu and scrapping San Francisco.  $170 million to refuel USS Honolulu and scrap San Francisco with her new reactor core, vs $79 million to swap noses.

Nothing like this had ever been attempted before, but apparently it worked out very well.  Three years and nine months to the day that she pulled into Guam following the accident, USS San Francisco undocked from Puget Sound Naval Shipyard.  On October 10, 2008, she was re-floated, wearing the front end of ex-USS Honolulu.  She completed sea trials in April 2009, and was then stationed in San Diego.

She returned from her final deployment in October 2016, and after that was moved to Charleston, SC, where she will be modified and converted into a "moored training ship" to train future Navy nuclear propulsion plant personnel.

Below: Moored Training Ships at Charleston, SC.  USS San Francisco will replace one of these vintage ballistic missile subs with their aging S5G reactor plants, and La Jolla will replace the other.

Interesting material!

By the way, there is an excellent investigative book about the design, procurement and construction of the 688 class submarines.  It ended the careers of three amazing men: Admiral Hyman Rickover, David Lewis - Chairman of General Dynamics, and Takis Veliotis - General Manager of Electric Boat.  In many ways, the 688 class was the 'Joint Strike Fighter' boondoggle of its own era.

For a brief discussion of the recent collision of USS Connecticut, SSN-22, see here.