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Tanker Spills: Prevention by Design APPENDIX C Large Tanker Structural Survey Experience* The major difficulty in adequately surveying VLCCs is the physical size of the task. Table 4-1 illustrated the scope of work required to give complete coverage to a typical 250,000 DWT vessel. Historically, owners inspected vessels during routine drydockings, thus limiting survey to structure that was readily accessible. Special staging rarely was used due to its cost and the expense of extending the drydock periods. Owners now generally feel that the only practical time for conducting complete inspection of a vessel is during ballast voyages. At that time, tanks can be flooded progressively to different heights, and structure can be accessed by raft. This method has limitations in that safety considerations do not permit placing inspectors in close proximity to the deckhead. However, significant areas of structure can be inspected successfully. The most critical aspect of the at-sea survey, and the principal limitation on its success, is adequate cleaning of tanks. Safe access for extended periods requires thorough removal of residual hydrocarbon, in order to ensure a gas-free environment. Heavy accumulations of wax, sludge, sediment, and scale have to be removed to expose bare steel for inspection and to ensure that structure can be climbed safely. The efficiency of this operation is dependent on the type and amount of tank cleaning equipment, its level of maintenance, and the previous services of the vessel (length of voyage, time spent in floating storage, and the type of crude oil carried). Cleaning requires extensive planning and effort by management and crew. In many cases, the process may begin several voyages prior to the survey, with schedulers positioning the vessel to maximize cleaning opportunities * Excerpts from paper: Exxon Corporation, 1982.
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Tanker Spills: Prevention by Design and the crew making preliminary inspections of tanks to ascertain where increased cleaning procedures are necessary. It is sometimes necessary to provide additional manpower to remove sediment from critical survey areas. The direct cost of a survey in 1981 was approximately $75,000 (1990 dollars),* including charges for a steel inspector, ultrasonic team, and analysis and reporting by a naval architect. Additional costs were incurred for incremental fuel consumed in shifting ballast water during the survey, as well as for cleaning the vessel to an adequate standard. Ballast shifting alone typically costs $12,000.* This level of expense emphasizes the need for thorough preparation by the vessel prior to embarking a survey team to minimize the necessity of re-inspection of inaccessible tanks. A normal ballast voyage provides about 20 days to complete an inspection. This is a tight schedule and requires careful coordination between the survey team, ship's staff and shore management. A planning meeting is held with ship's staff after embarkation to review safety procedures associated with tank entry, to acquaint the crew with the procedures used by the team, and to plan the sequence of full inspection. Emphasis is placed on conformance with approved company and industry safety standards. The survey team is required to reject any tank for inspection that does not meet these standards. An inspection sequence is selected that minimizes ballast shifts to conserve time and fuel. The first tank structure inspected is normally the bottom plating and adjoining structure in the tanks that the chief officer requires to be immediately available for minimal ballast movement. These tanks then are ballasted and bottom structure in remaining tanks is inspected. The team then returns to the partially ballasted tanks to complete the inspection of the upper portion of the tanks. Inflatable rubber rafts are used to move about the tank, although a rigid boat which can be split for access through a tank hatch and reassembled in the tank is undergoing operation evaluation. Within each bay, free climbing of the structure is employed to improve the speed and efficiency of the inspection. The remaining tanks are completed in a similar manner. The inspector checks the structure for corrosion wastage, buckling, and cracking. Crack detection is accomplished visually, as suitable equipment is not available. The side shell attachments, bottom fore and aft girder bracket attachments, and stringer platform to bulkhead attachments are among locations examined where experience has indicated a high probability of local failure. While the inspector is making his visual inspection, the ultrasonic techni- * Costs are reported in 1990 dollars, based on U.S. Department of Labor, Bureau of Labor Statistics, Employment Cost Index—Private Industry, total compensation. Costs are rounded to the nearest thousand dollars.
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Tanker Spills: Prevention by Design cians take readings at predetermined locations in the tanks. These locations are selected ship-by-ship prior to the survey to give representative readings of the structure and coverage of known suspect areas. Additional ultrasonic readings are taken as necessary by the inspector during the visual inspection of the tank. On average, about 8,000 readings are taken, although in excess of 11,000 readings have been taken in cases of severe corrosion. Due to the large areas to be covered, the inspector has to focus on historically suspect areas to optimize the effectiveness of the survey. In-house training, based on past experience with these ships, is necessary to ensure satisfactory results from the inspector. Complete coverage of the structure generally has not been achieved. However, on adequately cleaned vessels 85-90 percent of the tank section has been inspected. The most difficult area to inspect is the deckhead due to a lack of access other than by elaborate staging methods. Rafting is not used for the deckhead since they survey team would have to be temporarily trapped between deep transverse web frames as the water level was raised and lowered to inspect each bay. As a result, the deckhead is inspected visually from the highest practical water level, or upper walkways (if fitted) and ultrasonic readings are taken from the main deck of the plating and of any deck longitudinals available through deck openings. Although tank cleanliness has been the major factor limiting the effectiveness of the surveys, other factors have been pertinent. Heavy seas causing roll of 5° or more prohibit safe tank work. High temperatures and humidity encountered in places such as the Red Sea and the Arabian Gulf can result in extremely difficult work conditions. For instance, a tank ambient temperature of 35°C with 95 percent relative humidity restricts effective working time to as little as 15 minutes per hour. Higher tank temperatures and humidity can easily occur and very little can be done to reduce them. Upon completion of the survey all data is returned to the office for analysis and report preparation. The survey report includes estimates of necessary steel renewals, coatings, and anodes. The nature and extent of any structural defects are detailed and repairs, modifications or changes in operating procedure are specified.
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