AmbroseReportOCC.pdf

Ocean and Coastal Consultants, Inc.

a COWI US Company

35 Corporate Drive, Suite 1200 Trumbull, CT 06611

PH 203-268-5007 FX 203-268-8821

www.ocean-coastal.com

VIA ELECTRONIC & OVERNIGHT MAIL

November 21, 2007

US Coast Guard – First District Civil Engineering Unit Providence 300 Metro Center Boulevard Warwick, RI 02886

Attention: Mr. Claudio Polselli, Project Engineer

Reference: Contract DTCGG1-04-D-3WK054 Task Order HSCGG1-08-J-3WK027 Ambrose Light Tower – Inspection and Damage Assessment

Dear Mr. Polselli:

Between November 6 and 7, 2007, Ocean and Coastal Consultants, Inc. (OCC) performed an above and underwater investigation of the United Stated Coast Guard Ambrose Light Tower. The investigation was performed at the request of the United Stated Coast Guard Civil Engineering Unit (CEU) Providence in response to an allision which occurred between the vessel AXEL SPIRIT and the Ambrose Light Tower.

The allision reportedly occurred between November 3 and 4, 2007 (Photograph 1).

In accordance with the Scope of Work dated November 6, 2007, the purpose of the investigation was to 1.) assess the structural integrity of the structure, above and below water. 2.) document extent of damage to the structure; 3.) recommend immediate measures to be taken to stabilize the structure and 4.)

recommend what long term measures should be taken, based on a life-cycle cost analysis. This letter report provides the deliverables for items 1 through 3 above. The deliverable for item 4 will be incorporated into this letter report at a later date.

A 6-person inspection team led by a Professional Engineer-Diver conducted the above and underwater investigation. The investigation included Level I inspection of the Jacket, Boat Landings & Access Deck, Superstructure, Main Deck & Equipment Building, and Lantern Deck(s). Due to water depths exceeding approximately 100 feet, a Remotely Operated Vehicle (ROV) was utilized to investigate the Jacket below elevation -54.0 feet. The condition of the structural elements was recorded by topside personnel and above and underwater photographs and video were obtained to provide documentation of the findings.

All underwater operations were conducted in accordance with OSHA requirements and Association of Diving Contractor International recommendations.

Observations

OCC’s previously completed an above and underwater investigation of the Ambrose Light Tower in July 2007. Please refer to the Waterfront Facilities Inspection and Assessments report for the July 2007 inspection for additional detail on the structures and components inspected.

Jacket

The substructure (Jacket) of the Ambrose Light Tower is comprised of a prefabricated steel Jacket with multiple elevations of horizontal and vertical-diagonal steel bracing and three (3) driven steel pipe piles

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(Figures 1 and 2). The Jacket is approximately 110 feet high with top and bottom of jacket elevations of -

95.0 feet and +14.0 feet, respectively, referenced to Mean Low Water (MLW).

The southeast pile (A2) of the Jacket has severe impact damage. The jacket leg is deflected and buckled on the southeast face of the member. The area of direct contact from the vessel extends from approximate El. -30.0 feet to El. 0.0 feet. The deflection due to impact below El. -18.0 feet is approximately 15 inches deep (Photographs 2 and 3).

The horizontal and vertical-diagonal braces which are attached to the A2 leg, from the seabed to El. +12.0 (twelve (12) members), are failed due to severe impact damage. The impact has caused structural defects which include extensive buckling, deflection and shearing of the members at the jacket node (Photographs 4 and 5).

Moderate damage was observed on the horizontal and vertical diagonal braces between the A1 and B legs at elevation +12.0. Lack of marine growth, which is typically heavy on undisturbed members, and coating loss/cracking, was observed (Photograph 6). Additionally, both the A1 and B legs appear to be deflected in the southwest direction, possibly due to overstressing.

Superstructure

The Superstructure of the Ambrose Light Tower is comprised of prefabricated steel pipe legs, horizontal and vertical-diagonal steel braces and a steel pipe stairway. The Superstructure extends from El. +12.0 feet to El. +65.0 feet with two (2) Boat Landings and Access Deck located between El. +5.5 feet and El.

+12.0 feet.

The boat landing between A1 and A2 legs is failed due to severe impact damage. The impact which occurred at the A2 leg has caused extensive deflection and buckling of the lower horizontal support member.

Due to the severe deflection and buckling of the A2 leg, the elevation of the Access Deck is not consistent. The elevation of the Access Deck at the A2 leg is approximately 3 feet lower then the deck elevation at the A1 and B legs.

Due to the severe impact damage, the A2 Superstructure leg has sheared from at the node at El. +14.0 feet (Photograph 7). The three (3) Superstructure legs and spiral stairway have severe buckling between El.

+24.0 feet and El. +48.0 feet (Photograph 8). Pile inclinometer readings taken within the spiral staircase confirmed that the superstructure is approximately 5 degrees out of plumb and is leaning towards the southeast.

Main Deck & Lantern Decks

The Main Deck of the Ambrose Light Tower is comprised of steel pile caps and steel framing, fiberglass decking and a steel utility building. The Main Deck is located at El. +67.0 feet. The Lantern Deck is constructed atop the utility building at El. +75.0 feet and supports the Back-up Lantern Deck (El. +83.0 feet).

Due to the deflection and displacement of the superstructure, the Main Deck and Lantern deck are not level. The decks slope down to the southeast at approximately 5 degrees. Two (2) sections of fiberglass decking have been displaced on the southeast corner of the Main Deck (Photograph 9). Additionally, approximately 50 percent of the tie-down hardware for the decking has failed due to impact. Two (2) of

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the four (4) steel columns which support the Back-up Lantern Deck have failed due to shearing at the base (Photograph 10).

Structural Condition Assessment

Due to the level of damage to the structure, including the severe buckling of braces and the main legs, a precise structural condition assessment is difficult. In order to provide an initial assessment, OCC calculated the original strength of the main structural members along with dead loads and wind loads.

The ratios of these loads to the original structure capacity provide a qualitative assessment of the structure’s condition.

Based on our analysis, it appears that the Ambrose Light Tower structure was designed with some redundancy in main structural members. This has allowed the structure to remain standing despite the severe damage to the main load bearing members.

To evaluate the Ambrose Light Tower superstructure, above El. +12.0, for strength and stability in its current damaged condition an analysis of the superstructure in its pre-damaged condition was performed using STAAD2006 structural analysis software. The pre-damage condition analysis included only the existing dead load selfweight, and an anticipated wind load in the southeast direction, which is the approximate direction the structure is listing. For each damaged member, the maximum percentage of its undamaged combined axial and bending stress capacity was computed.

From field measurements, the buckles in each of the tower legs as a result of the impact were measured to be approximately 5 degrees in the southeast direction towards Leg A2.

A result of the 5 degree list is a load eccentricity of the Main Deck of 4.5 feet with respect to the base of the tower superstructure. The estimated dead load of all elements of the main deck is 40.0 kips, which includes the steel framing and decking, the building enclosure, the solar arrays, the light, and all other appurtenances. This load eccentricity developes an additional 180 kip-feet of bending moment in the legs of the structure.

The additional moment resulting from the eccentric load is approximately 10 percent of the original design bending capacity of the tower leg sections. Based on our field observations and engineering judgment, we believe the legs have sufficient remaining capacity to support the dead loads from the 5 degree list in the structure.

OCC calculated wind loads on the structure based on ASCE Standard 7-05 (Minimum Design Loads for Buildings and Other Structures). Wind speeds up to 120 mph were considered. Wind loads can increase the bending moment in the legs up to approximately 30 percent of their original design capacity. Graphs attached as Figures 9 and 10 show the increase in load with wind speed. Due to the observed bucking, it is likely that the superstructure tower legs do not currently have 30 percent of their tension or compression design capacity and could become over-stressed and unstable at higher wind speeds. Wave loads would add to the stress in the piles, but the influence of wave loading has not been assessed at this time.

Below water, the damage to the braces also reduces the capacity of the jacket legs. If the braces had failed completely, the capacity of the leg could be reduced by 10 percent, due to the changes in unbraced length. Additional reduction of axial capacity of the jacket legs can be attributed to the change in shape of the damaged jacket section. We believe that the buckled and partially cracked braces retain some capacity, and will likely support the dead load in the near term. As with the above water portions of the structure, the jacket legs may become overstressed under storm conditions.

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Temporary Stabilization Recommendations

OCC considered temporary stabilization measures to restore the original capacity of the damaged main structural members above water. Two (2) stabilization concepts are attached. Figures 6 and 7 show a stabilization concept for the buckles in Legs A1, A2 and B. Figure 8 shows a stabilization concept for the failed connection of Leg A2.

The stabilization details were designed in accordance with AISC Steel Construction Manual 13th Edition, Allowable Stress Design Method. The stabilization details were developed to return the cross sections of the buckled and fractured legs back to their original full and complete tension, compression, and bending capacity.

The stabilization of the fracture on Leg A2 should be considered on a priority basis. The fracture does not allow for moment transfer through the joint and will cause movement of the superstructure under wave and higher wind loads. This will “work” the buckles in legs A1 and B and could lead to a fatigue failure.

Repairing this fracture will increase the loads to the buckled Leg A2, but we believe that increasing the load is preferable to allowing continued movement of the superstructure.

The stabilization of the buckles on Legs A1, B and the upper portion of A2 should be considered on a value engineering basis. The possibility of storm conditions overloading the structure versus the cost of the repairs will need to be considered.

A stabilization concept similar to that shown in Figure 8 could also be applied to the underwater buckle (elevation -30 feet) on Leg A2. However, due to the cost of underwater repairs, this repair is not anticipated to be economical.

The overall stability of the tower may be improved by adding an anchor system. However, this concept needs further evaluation.

The absolute and relative position of the structure should be monitored prior to, during, and following installation of any stabilization techniques. This monitoring will be used to determine if the list in the structure increases, signaling on-going instability of the structure at a location that has not been stabilized.

Temporary Stabilization Opinion of Probable Cost

Due to the short time frame for the damage assessment, OCC contacted several qualified marine contractors to discuss, in general, the type of repairs to be completed, the equipment and personnel required, and the costs for completing the stabilization. The Opinion of Probable Cost (OPC) assumes the stabilization will be completed utilizing a crane barge and tug boat. Project constraints related to working in the offshore environment and the current condition of the Ambrose Light tower are taken into consideration.

Repair Item Unit Opinion of Probable Cost

Mobilization / Demobilization Lump Sum $100,000

Stabilize Fracture in Leg A2 Lump Sum $150,000

Stabilize Above Water Buckles in Legs A1, B and A2

EA

$125,000

Weather Contingency of total 25%

SITE PHOTOGRAPHS

Photograph 1: Overall view of Ambrose Light Tower, looking northeast.

Photograph 2: Severe deflection/buckling on A2 leg (El. -30.0 feet) due to vessel impact.

Photograph 3: Severe deflection of A2 leg from vessel impact/abrasion.

Photograph 4: Severe buckling of horizontal braces at El. -18.0 feet.

Photograph 5: Sheared horizontal member.

Photograph 6: Loss of coating and overstressing of horizontal brace at El. +12.0 feet.

Photograph 7: Sheared A2 Superstructure leg (approx El. +14.0 feet).

Photograph 8: Severe buckling of Superstructure legs and spiral staircase.

Photograph 9: Displaced fiberglass decking on Main Deck (El. +67.0 feet).

Photograph 10: Sheared steel column support to Back-up Lantern Deck (El. +83.0 feet).

A2 Leg @ Separation (EL. + 16'-0")

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