Case Study and Assessment on Vessel Structure Safety

From the case study, the nature defects of single-hull vessel were discussed to compare the safety level between single-hull vessel and double-hull vessel. Data was chosen from both investigation and research from ABS, as well as simulation by similar kind of vessel in model calculation. The study unders-cores the main defects of single-hull vessel for prevention of their losses, mentioning the most important measures taken by IMO for improvement of vessel safety. In the work, a critical analysis of the methodology of the International Association of Classification Societies (IACS) for calculation of sectional efforts induced by waves in vessel’s hull is described. In the end, the advantage of double-hull vessel was determined and the reason to eliminate single-hull vessel was proved again.


Introduction
On November 13, 2002, the PRESTIGE was reported flooding preceded by a sudden vibration or severe shudder. With such significant list, the PRESTIGE lost propulsion and began to drift. After Spanish authorities refused to allow the PRESTIGE safe harbor, on the morning of November 19, 133 nautical miles off the coast of Spain in international waters, the PRESTIGE broke in two and sank.
The ecological result of the PRESTIGE casualty was a massive 70,000-ton oil spill that devastated the local fishing and tourism industries. Due to the toxic nature of the oil, it was predicted to cause pollution in the area for ten years after the initial spill, coating over one hundred miles of Spanish and French coastline with toxic oil. The spill had an estimated clean-up cost of nearly three billion US dollars.
As an ABS class vessel, ABS was sued by EU for the loss caused by PRESTIGE, claiming that they were responsible for the damage, because the flaw in the hull structure should have been caught by an ABS inspection. ABS made extensive technical analyses of the PRESTIGE after the incident, but the lack of physical evidence prohibited them from reaching a definite conclusion. The vessel was properly loaded and the as-built hull structure met the 1973 ABS Rule requirements as well as the 2003 ABS Rule and IACS Unified Rule. Some as-built structural details failed the 2003 ABS requirements for fatigue, but ABS maintains that this is not a probable cause of the hull failure, as the PRESTIGE operated in a gentler environment than the criteria were developed for, and "most of the side longitudinal having insufficient fatigue life was renewed at the 4th and 5th Special Hull Surveys". In its intact state, the PRESTIGE was under the allowed values for still-water bending moment and shear force. According to their technical analyses, ABS believes that "a weakened section in the side hull or supporting framing in the vicinity of frame 71" caused the initial damage and flooding. ABS considered lightering damage to be the most likely cause of the side hull weakening. According to the ABS analysis, with these lightering operations, "permanent deformations of side longitudinal Nos. 32 to 34 and the transverse frames and transverse bulkhead could have occurred in the vicinity of frame 71". In fact, a sister vessel of the PRESTIGE sustained side hull damage from lightering in 1989 [1].
In order to avoid such kind of tragedies again, in 2003, EU has introduced laws to restrict single-hull oil tankers operating in waters. This paper aims to analyze the nature defects of single-hull vessel and try to find an explanation on the reasons to hull breaking down.

Comparison between Single-Hull Tankers and Double-Hull Tankers on Oil Spill Risk
Compared with single-hull tankers, double-hull tankers have a lower oil spill risk. The two stranding accidents in Milford Haven, Wales, showed that even if the double hull vessel suffered stranding damage to the outer hull, if the internal medicine is intact, no oil leakage will occur, while the single layer hull vessel will be damaged if it is rubbed. It will cause a lot of oil leakage and pollution. According to statistics, the main causes of oil tanker accidents are hull and mechanical defects, collisions hit the rocks and run aground. The most likely areas for reef strike and stranding accidents are near-shore waters and waterways. In these areas, vessels generally travel at lower speeds. It is foreseeable that collisions in these areas are of low intensity. According to many accidents and research, double-hulled hulls can withstand low-intensity collision damage more than single-hulled hulls, and are therefore less prone to oil leakage pollution than single-hulled hulls.
In order to further explain the difference in vessel strength between the double-layer hull and the single-layer hull, the following will analyze the total

Strength Assessment of Single-Hall Vessels
For safe operation of vessels, they must perform the voyage loaded in accordance with loading manual or in accordance with a cargo-plan developed by the crew using loading instrument existing onboard, which was previously approved by the classification society. Loading and unloading of vessel at terminals shall be X. B. Yang World Journal of Engineering and Technology carried out in accordance with Manual on loading/unloading sequences existing onboard or a plan with loading/unloading sequences developed by the crew using loading instrument. Likewise, the general strength assessment of vessels in operation is conducted when periodical surveys at 5 years interval, of intermediate surveys carried out at 2.5 years interval and of annual surveys [2]. When periodical and intermediate surveys, thickness measurements of stiffeners are carried out, by companies authorized by classification society or competent naval authority, measurements which shall be registered on forms to be kept in a folder onboard.
Measured reduced plating thicknesses are compared with maximum reduced thicknesses accepted, and in case these accepted reduced thicknesses are exceeded, the stiffeners are replaced. Maximum decreases accepted are usually corrosion allowances adopted at design stage [3].
Greater reduced thicknesses could be accepted, but in this case the assessment process of general and local strength of design phase shall be recommenced, with the exception that initial thicknesses of stiffeners are those of the Report of thicknesses' measurements. Based on this assessment, restrictions in vessel operation could be imposed and the Loading Manual, Stability Booklet, the Loading Instrument and Manual of loading/unloading sequences in case of vessel shall be modified accordingly.
At annual survey, the followings shall be visually verified by sampling: the integrity of stiffeners, the presence onboard of Loading Manual and correct functioning of Loading Instrument. Oil tankers and vessel are subject of the enhanced program of inspections in conformity with 2011 ESP Code (IMO Rez. A.1049 (27)) and MSC.287 (87).
The Common Structural Rules for Vessel and Oil Tankers introduce the approach of stiffeners renewal criterion if the corrosion allowance was lost from initial thickness established by design. Fatigue strength assessment is realized through detailed inspection and non-destructive examination of structural joints most exposed at variable loads indicated by designer, in order to track down possible cracks specific to fatigue phenomenon.
If these tests were carried out after Y years since the putting into service of the vessel and in laboratory the residuary cumulative damage factor D r , which remained until the break, then the real operational life of the vessel L r can be found using equation: In case when cracks are discovered due to material fatigue before expected life of the vessel expires, the fatigue strength of structural joints affected by this phenomenon will be reassessed, these joints will be redrawn and the cracked and tired stiffeners will be replaced by new redrawn stiffeners, and the inspection of joints will be carried annually. Moreover, during the inspections, anticorrosive protection is verified and if it is found damaged, it will be reconditioned.

Discussions by Delphi Method
As discussed in last chapter, for environmental protection considerations, sin-World Journal of Engineering and Technology gle-hull vessels are no longer suitable for international oil transportation services, especially under the premise that major economies in the world have banned the use of single-hull hulls. The disposal of these single-hull vessels has become a new issue. In order to discuss the handling of single-hull vessels from a more objective point of view, the author considered the Delphi method to deal with this problem. This issue was discussed by selecting anonymous professionals from the shipping industry to issue questionnaires. For the sake of objectiveness and fairness, although the selected expert information is hidden in this article, the basic data of the experts are summarized as shown in Table 1.
The questionnaire was designed in two batches. In the first batch, there were two main questions.
Question 1: From 1 -10, whether to support the elimination of single-hull vessels to transport oil, 1 is strongly against and 10 is strongly agree.
Question 2: From 1 -10, whether to support the dismantling of all single hull vessels, 1 is strongly against and 10 is strongly agree.
After the retrieving of all questionnaires, the second questionnaire was designed containing two questions below: Question 3: From 1 -10, whether to support the modification of single-hull vessel to meet other maritime requirements, 1 is strongly against and 10 is strongly agree. Question 4: Potential options for single-hull vessel modification. Table 2 indicates the retrieving results for the questionnaires and Table 3 shows the scoring result for the two batch questionnaires.
According to the above results, all the questionnaires are effective so we can get the effective results in Table 3 as below.   According to the results of this scoring result in Table 3, although most experts agree with the elimination of single-hull transportation, they do not agree with direct dismantling, because it is considered that the act of dismantling vessels may also pollute the environment. Therefore, according to the opinions of most experts, instead of direct elimination, it is more appropriate to adopt ship modification to adapt to other forms of transportation for smaller vessels. The main choices are direct modification to double hull vessels and bulk vessel or offshore engineering.

Conclusions and Proposals
In the paper, the problems on the single-hull vessels safety have been treated.
The requirements for estimating the vessel hull general strength have been presented. The methods are illustrated by two examples of the sectional efforts' estimation. According to the results, certain inconveniencies after the great accidents occurring in the last decades have been expressed by the vessel builders