SHIP

The size of the shipping industry can be measured by the number of miles that ships sail and the tons of cargo they haul. Corrosion of ships involves several different types of corrosion. The most common form of corrosion is general corrosion or wall thinning of the hull because of seawater attack. Studies have shown that this form of corrosion is approximately 0.1 mm (4 miles) per year (21). At this corrosion rate, it would take approximately 62 years to have a reduction of 6.4 mm (0.25 in.). Because of this slow rate, general corrosion is normally not a consideration in a ship s design life. 

Galvanic corrosion occurs between two metals with dissimilar electrochemical potentials. In this form of corrosion, one of the metals is more electrochemically active and corrodes, while the second metal is protected by the corroding metal. The metals can even be of the same material if the electtochemical potential of one of the materials has been charged because of stresses or differential aeration. Previous studies have indicated that most hull corrosion is galvanic in nature (22). 

Salt spray and atmospheric corrosion can severely attack external ship components. Coatings provide the primary corrosion conttol, and maintenance of these coatings is required at regular intervals. 

Direct chemical corrosion attack occurs when certain chemicals are present in the internal holds and tanks of transport ships. Elements such as chlorine and sulfur can readily attack the steel and cause accelerated corrosion and pitting. 

Corrosion in ships can also be caused by MIC. In this type of corrosion, microbial organisms present in the environment can accelerate corrosion. For example, SRB, which are present in stagnant water of many harbors, can build up on the hulls of ships. Other corrosion-causing bacteria, such as acid-producing and anaerobic bacteria, are also present in ballast tanks as well as in the liquid products that some tankers carry. The microbes cause a localized change in the environment, which can promote aggressive pitting and other types of corrosion. 

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With respect to fuels utilized as heating fuels for industrial furnaces, or as motor fuels for large diesel engines such as those in ships or power generation sets, the characteristics of primary importance are viscosity, sulfur content and the content of extremely heavy materials (asphaltenes) whose combustion can cause high emissions of particulates which are incompatible with antipollution legislation. 

Heavy fuels are used for two kinds of applications industrial combustion in power plants and furnaces, and fueling large ships having low-speed powerful diesel engines (Clark, 1988). 

Drill ships are used in deep water and remote areas, and these vessels are equipped with a drilling unit positioned in the middle of the ship. Positioning is achieved dynamically by computer controlled thrusters. The ample storage space allows operation for long periods of time without re-supply. 

Where the distance to the customer is very large, or where a gas pipeline would have to cross too many countries, gas may be shipped as a liquid. Gas has to be chilled to -160°C in a LNG plant to keep it in liquid form, and is shipped in refrigerated tankers. To condition the gas for liquefaction any COj, HjS, water and heavier hydrocarbons must be removed, by the methods already discussed. The choice of how much propane and butane to leave in the LNG depends upon the heating requirements negotiated with the customer. 

Ship-shaped FPSOs must be designed to weather vane i.e. must have the ability to rotate in the direction of wind or current. This requires complex mooring systems and the connections with the well heads must be able to accommodate the movement. The mooring systems can be via a single buoy or, in newer vessels designed for the harsh environments of the North Sea, via an internal or external turret. Figure 10.33 shows a schematic of the Shell-BP Foinaven FPSO. 

The metal vacuum envelope is grounded and can be made quite thick. Even radiation-tight designs are feasible. The tube ean be shipped ready-to-use with a bonded cable. 

Classification Societies exist to promote the safe construction of ships and to protect that condition throughout the life of the vessel. One of the tools available to shipbuilders, shipowners and surveyors that can be used to achieve these aims is Non-Destructive Examination (NDE). The intent of this presentation is to describe the application of NDE to hull structure during construction and also during periodic surveys as seen from the viewpoint of the Classification Surveyor. 

The extent and manner of NDE applied in different shipyards has been the subject of a recently completed exercise within LR. The exercise involved randomly selected shipyards building ship types which included oil tankers, bulk carriers, gas carriers, container ships, ro-ro and general cargo ships. The variation in extent of applied NDE that was observed is summarised in Table 1. 

Ship Type Length,m Extent of Testing - Number of Checkpoints ... 

Intersections of butts and seams of fabrication and section welds Throughout hull envelope, longitudinal and transverse bulkheads, inner bottom and hopper bottom The summation of checkpoint lengths (see note 2) examined at intersections is to be L where L is the overall length of the ship in metres... 

The amendments shown in Table 2 above should not increase the amount of NDE applied in quality conscious yards and in some cases may give a reduction by eliminating the double checking that can sometimes occur. Other yards may find that a more extensive NDE programme will be required. The benefits obtained from an enhanced inspection scheme should be apparent in the improved fatigue life of the ships. 

During the construction of a ship the Surveyor monitors NDE results for completeness of testing and overall quality of welding. This role includes the evaluation of checkpoints taken specifically for classification purposes but also includes the monitoring of additional checkpoints taken for quality control purposes by the builder. In the case of tankers built to LR Class since 1994 and for bulk carriers built to Class since January 1996 the Surveyor will witness the actions listed as part of the ShipRight Construction Monitoring procedure that is now mandatory. 

In order to maintain Class and to comply with international legislation a ship must undergo surveys at prescribed intervals. Ships such as tankers and bulk carriers must comply with International Maritime Organisation resolutions which require a Class Enhanced Survey Programme. This programme includes Special Surveys at five yearly intervals with close-up visual inspection and thickness measurements to be made at specified locations, the number of which increases with the age of the ship. 

There are no classification requirements for routine NDE beyond thickness testing and visual inspection except for repairs, modifications or where service history has identified a specific problem in which case the Surveyors will request NDE at the same locations in similar ships. Under circumstances where visual inspection has found evidence of fatigue cracking the Surveyor can also call for NDE to assess the full extent... 

The onus for the organisation of any inspection programme beyond that required by Class rests with the owner or operator of the ship. Under certain circumstances, for example, when the propagation of cracks could lead to pollution through the loss of cargo, an owner of an oil tanker operating in US coasted waters may spend a considerable amount of money on preventative inspections. 

As stated above the life of a ship will be determined by corrosion and fatigue damage. The assessment of corrosion and fatigue damage is discussed below. 

For existing ships the only NDE method nominated by classification for the detection of fatigue cracks is close-up visual inspection - although all Surveyors have the option of requesting additional NDE when warranted. The sensitivity of visual inspection is influenced by the degree of surface preparation and the level of lighting at the inspected surface - which may not always meet the level of 500 lux nominated by some NDE specifications. 

It is possible that much of the inspection in future years will be focused on areas identified from databases which collate corrosion and damage history in similar ships. The validity of any conclusions derived from assessment programmes or databases can only be accepted if the quality of the original product is known. This would suggest that the NDE inspection programmes applied by some yards will have to be expanded. 

Regardless of the amount of NDE applied no ship will last forever. Only when the costs of maintaining a vessel to the condition required by Class become unacceptable will a ship be scrapped. Through the more rigorous applicaton of NDE and the stringent imposition of repair procedures it is possible that the dangers posed by an aging fleet can be reduced. 

Bulk Carriers - an update", March 1998, Lloyds Register of Shipping. 

The Electronic Future for Managing Ship Condition Data", 1997, Dr K Brook... 

The New Ship Division Database and Its Impact on Survey Reporting", 1993, LRTA, A Buckland and DW Woodcraft. 

An important and expensive problem in surface science occurs in the prevention of the attachment of marine animals such as barnacles to ship surfaces, a process known as biofouling. Baier and Meyer [159] have shown that the Zisman plot can be used to predict biofouling, thus avoiding costly field tests to find a successful coating to prevent biofouling. 

As an example of the effect that corrosion can have on connnercial industries, consider the corrosive effects of salt water on a seagoing vessel. Corrosion can drastically affect a ship s perfonnance and fiiel consumption over a period of time. As the hull of a steel boat becomes corroded and fouled by marine growths, the... 

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