Case Study VI

Beam: 68.13 Feet

Depth: 39.5 Feet

Full Load Draft: Tf = 10'-7.9'', Ta = 15'-7.4''

Displacement: 6,758 LT (w/o cargo)

Vessel Type: US Navy LST 1179 Class

Flag: Leased by US Navy to Chile

The Chilean government requested salvage assistance from SUPSALV through the State Department. SUPSALV Salvage Engineer arrived 30 May. Initial observations were that the ship had 7 feet of trim by the stern and 2.5 degrees of list to starboard. Most of the bottom of the hull suffered some structural damage, but the presence of diesel and heavy surf limited diver accessibility for inspection. Initial salvage actions included pulling the stern seaward with a combination of Chilean Navy and commercial vessels.

A 16-man U.S. MDSU Detachment and beach gear from the ESSM system arrived 16 June. Due to low tides and small waves, renewed extraction attempts were not made until 22 June, when 4 legs of USN beach gear with hydraulic pullers had been deployed. Final extraction took place on 10 July, after redeployment of beach gear and ship and shore assets to facilitate pivoting of the ship to reduce coefficient of friction. The total pulling power available was as much as 700 short tons, and included 3 FFGs, 2 DDGs, 1 Icebreaker, 5 large ocean going tugs, 4 legs of beach gear, and beach-based pulling assets to facilitate twisting.

POSSE was used to model grounding reaction, freeing force, stability and residual hull strength.

A detailed hull model containing all compartments was on file at NAVSEA. Only insufficent and inconsistent information on loadings and stranded conditions was available prior to being on scene. Once on scene, an accurate listing of tanks and damage was obtained. This data allowed the first estimates to be completed in DETAILED Analysis. A RAPID model was not completed.

Back to Quick Link Index

Structural modeling included removal of damaged or buckled structure from the section modulus calculations, and reduction in effective section modulus of some members which partially buckled. This partial damage was modeled using a 50% corrosion factor.

The detailed analysis included estimates of the longitudinal strength, ground reaction, afloat stability, and forces to free. The ground reaction was also used as input to estimates of forces required to turn the ship from its broached position.

The effect of structural damage was a 46% loss in section modulus of the upper hull flange (deck), and a 79% loss in section modulus of the lower flange (keel). In extreme conditions, this corresponded to max. predicted stresses near yield levels. With a constantly changing buoyancy profile, it was difficult to find a maximum condition. The ship was in a hogging condition and waves impacting the stern lifted the stern, reducing hull deflection and relieving the deck stress condition. Continuous inspections were used to watch for signs of fatigue failure.

The ground reaction changed dramatically with tides and waves. With the stern nearly floating, the forward draft was the critical measure. The tidal rise was outweighed by the surge due to heavy weather. The center of ground reaction was a critical factor in determining pulling and twisting forces as this determines moment arms, as well as the swing area of the bow as it pushes further onto the beach. Earth moving machinery was used to clear way for the bow to swing. The friction factor was also somewhat unknown as the bottom was a combination of sand and small rocks. Eventually, the changing tides washed out most of the sand from around the ship and deposited significant amounts of sand in the ship. This sand was removed through eductors and trash pumps. The seafloor was mostly rock, although the rocks were not rigid. There was some fluidity to the small rocks (1 foot diameter) and they often were moved around by wave action. The recommended friction factor for rock varies from 0.8 to 1.5. If the high estimate is used vice the low estimate, force to free is altered by a factor of two, equating to several hundred tons of pulling force in this case, which is significant.

With all cargo being removed and flooding contained well below the vessel's KG, afloat stability was within acceptable limits.

Back to Quick Link Index

Crack propagation into the shear strake and continued buckling of strength members near amidships required continued inspections. Drill stopping and use of strongbacks provided a measure of added strength, but that was not included in the conservative analysis maintained in POSSE. K shoring and column shores were used in a 4 square foot center pattern to obtain additional strength to tank top and deck members. Continuos hull inspection and structural evaluation were essential to ensuring a safe extraction.

POSSE was a valuable tool in evaluating this complicated and involved salvage case. The results from the analysis were used as additional information in a very comprehensive salvage plan. However, results are only as good as the inputs. Careful hull inspection and continuos updates are essential in arriving at the right solution.

Back to Quick Link Index