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 PROJECT PROFILE PROJECT PROFILE 

Low tide access condition.

Reinstated cover concrete.

Concrete and steel condition after cover concrete was removed by hydro-demolition.

Old fender removal by barge.

Hybrid CP – Background Many reinforced concrete structures suffer attack from ingress of water soluble salts into the concrete and or carbonation of concrete due to atmospheric exposure leading to loss of passive alkaline conditions surrounding the embedded reinforcing steel. The associated corrosion of the reinforcing steel is an electrochemical process and the most effective means of arresting this process is to apply electrochemical treatments. One recently developed preventative treatment is called Hybrid Cathodic Protection. This technology is a combination of electrochemical treatments using both an impressed current and a galvanic anode cathodic protection phase. An impressed current is applied to the reinforced concrete for a short period, (7-10 days), during which propagating acid corrosion sites are neutralised to re-passivate the reinforcing steel. The system then reverts to a galvanic anode system providing a relatively low current to maintain the passive condition of the reinforcing steel and prevent long term further corrosion. This simple cost effective corrosion control process is robust, rapidly inhibits corrosion, can be applied in shutdown conditions,

requires no long term power supply, requires no long term maintenance and can easily be monitored. For these reasons the client chose to adopt this treatment for the Dampier Salt mooring dolphins.

The ContractThe initial contract works included the following on mooring dolphins 3, 4 and 5:

Remediation of the reinforced concrete including design supply, installation and commissioning of Hybrid CP;

Hawser rail replacement; and

Removal of existing fenders and the assembly and installation of replacement fenders.

In addition, the contract required the following works on all seven mooring dolphins:

Existing mooring hook removal and replacement; and

Installation of access bridge mounting plates.

A further contract was also awarded which involved the following refurbishment works on dolphins 3, 4 and 5:

Removal of ancillary steelwork and marine growth from all ten piles of each dolphin; and

Surface preparation and installation of a heavy duty marine pile protection system to the steel piles for 5.8m below the soffit.

Details of the WorksConcrete Remediation and Hybrid CP InstallationThe works commenced in June 2011 with the shutdown of all ship loading for a period of eight weeks. Work was performed around the clock commencing with removal of existing fenders using a barge mounted crane and installation of fully encapsulated scaffold access platforms on dolphins 3, 4 and 5. Delaminated cover concrete removal was performed using the hydro-demolition technique employing the use of a filter-cloth debris capturing system and waste water quality monitoring throughout to satisfy environmental requirements.

IntroductionThe Rio Tinto Dampier Salt Limited mooring dolphins were constructed in 1971 in conjunction with the load out wharf and ship-loader platform at Mistaken Island in the Burrup Peninsular Archipelago, 1250km north east of Perth and 21km northwest of Karratha in Western Australia. The dolphins or ship berthing stations are used for the shipping of up to 4.2 million tonnes of salt per annum. The dolphins comprise seven reinforced concrete platforms 7.5m

long x 4.57m wide by 1.55m to 2.15m deep supported by ten steel encased reinforced concrete driven piles.

Situated in a severe marine environment, the reinforced concrete dolphins have been subject to 40 years of chloride attack associated with saltwater windblown spray and splash. Some of the dolphins had previously been conventionally patched repaired in 1984 and 2009, but it was recognised in 2010 that dolphins 3, 4 & 5 were in need of further

remediation using cathodic protection (CP). Invitations to tender were called in March 2011 and following a detailed review of submissions, Duratec Australia was awarded the works in May 2011 on the basis of its alternative proposal which included design and installation of a Hybrid CP system, (the first in Western Australia), which satisfied the need for a simple, cost effective, robust, long-term reinforced concrete protection system in this environment.

Rio Tinto Dampier Salt Operations Mistaken Island Mooring Dolphin Repair Project

Dolphins on a moonlight nightshift.

Original dolphin condition. Original soffit condition. Original fender hook condition.

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 PROJECT PROFILE

The exposed reinforcing steel was replaced or augmented where greater than 20% loss of section occurred. All reinforcement was checked for electrical continuity, with isolated areas requiring welding in place of 6mm bar to make good. While the reinforcement was exposed, negative cable connections were made along with the installation of permanent reference cells. This was closely followed by dry spray gunite application of cover concrete to restore the original surface profile and when sufficiently cured, core drilling and installation of 20mm studs for fender cone and chain anchor points. Mooring hook removal and replacement works were performed concurrently during these works.

Progressive installation of the design approved Hybrid CP system followed involving the following procedures:

Drilling and initial soaking for 24hours of anode holes 30mm dia. x 150mm deep in vertical faces and soffit;

Installation of anodes in alkaline paste backfill;

Progressive powering of each zone using a temporary TR at 12 volts for 10-12 days with current recorded daily;

Passage of a minimum charge of 50KCoulombs/m2 rebar area in each zone with the aim of re-passivating the reinforcement;

Once the applied charge criteria had been met, the system was converted to galvanic mode with the rebar negative connection connected to the titanium wire connection in the junction box. The galvanic current achieved was between 2 and 3mA/m2 of reinforcement; and

Backfilling of titanium wire slots with cementitious repair mortar.

Evaluation of the system performance was assessed against a corrosion rate criterion based on the Stearn Geary

Equation with a targeted corrosion rate of less than 2mA/m2 of steel which is equivalent to a corrosion rate of 2mm section loss in 1000 years. This criterion has been recently adopted by the latest European Standard BSI EN ISO 12696:2012 Cathodic Protection of Steel in Concrete. This was measured by recording the galvanic current of each subzone and completing an instant-off and depolarisation test, (similar to ICCP), utilising these figures to calculate actual corrosion rates. Corrosion rates so obtained were all below the 2.0mA/m2 target.

Pile Protection ApplicationPile surface preparation and wrapping was undertaken using a specialist diving contractor and installation of the Denso Seashield Series 200FD heavy duty marine system. This was complemented by a conventional abrasive blast surface preparation and the application of Interzone 954 high-build epoxy protective coating system applied in the complex geometry zones.

PROJECT PROFILE 

New Fender InstallationOn completion of these works installation of the new pre-assembled fender units was performed using a 35m barge with 20 tonne lifting capacity to place the 10.5 tonne units comprising:

Front fender panel;

Fender rubber cone;

Fender cone flange plate; and

Support and link chains.

Project SummaryA project like this can only be delivered by sound project management and the commitment of the entire workforce including specialist subcontractors to working collaboratively and harmoniously in

New mooring hooks were installed.

Some piles were coated and partially wrapped where the geometry did not allow full wrapping.

Hybrid CP installation on vertical face. Hybrid CP installation on dolphin soffit.

New fender at work on dolphin 3.

Piles wrapped in the Denso Seashield system.

a team environment. There were many challenges which the project team had to face. These included:

Major logistics working on multiple dolphins separated by water;

Working over water;

Difficult weather conditions including wind and high temperatures;

High tidal range;

Underwater works;

Access restrictions;

Working 12 hour shifts 24/7 for 8 weeks;

Co-ordinating multiple tasks in limited space and access only by boat with a large workforce;

Precision works on concrete structures; and

Operating in a sensitive marine environment.

It was with a strong team approach that despite these challenges all of the works were completed safely, on time and within the eight week shutdown and without incident.

AcknowledgementsOur thanks go to the project management team at Rio Tinto DSL, Infracorr Consulting, and Bhagwan Marine, Seaforce, the Duratec Project Management team and work crew and subcontractors who all contributed to the successful delivery of this remediation project.

Completed Dolphin 5.