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D ET N ORSKE V ERITAS 10530832/DNV - Job.Id.: MEMO TO: Whom It May Concern MEMO NO. : MGGNO893/SEI/29-J-10376 FROM : Hull DATE : 2010-04-13 PREP. BY : CVE/SEI Low Temperature Cargos Introduction: Export of products, i.e. gas condensate, from ports exposed to low temperatures is increasing. For some ports, i.e. Vitino on the White Sea, the cargo is transported to the terminal by rail and stored in tanks exposed to the ambient air temperature. Due to the transport and storage time it can be anticipated that the cargo will be close to the ambient air temperature. The terminal has no means for heating the cargo. When the cargo is delivered to the ship the terminal can inform the vessel of the cargo temperature, but it is the vessels responsibility to decide how to handle it, for example by low filling rates and/or use of ballast water to partially warm the cargo. There can also be challenges at discharge ports. Based on information received from operators some discharge ports have a minimum acceptable cargo temperature. One operator indicated a minimum acceptable temperature for discharge of -4 o C, but this will certainly vary from terminal to terminal. Another indicated that after a voyage from Vitino the cargo was still at a temperature of approximately -15 o C upon arrival at Rotterdam and he was not allowed to discharge resulting in extended lay time. DNV regularly receives requests for guidance/acceptance for loading product cargos at low temperatures for vessels not specifically designed for this. Temperatures as low as -20 o C to -30 o C have been requested. The material in some parts of the cargo tank boundaries is often grade A. Grade A mild steel is not required to be charpy tested and its properties at low temperatures can not be guaranteed. This means that at these low temperatures there is a possibility that the steel is below the transition temperature and will be brittle with an increased risk of brittle fracture. Brittle Fracture: Generally steel is a ductile material – it can be deformed to some extent without cracking and takes a certain amount of energy to cause a crack to propagate or grow when it has first started. The resistance of a steel to cra ck propagation is its “fracture toughness”. Cracks often occur in steel structures, but since the material is tough the crack develops relatively slowly and it is possible to discover and repair the crack before it becomes critical for the safety of the vessel. Under certain combinations of crack length, stress/load level, and material toughness, a structure can experience unstable fracture. When this occurs a crack can propagate very rapidly, possibly leading to a serious structural failure. This can occur even in a material with good toughness provided the crack is large enough and/or the stress is high enough, but it is more likely to occur in a material with poor toughness. If low toughness is the primary cause of unstable fracture, then it is called brittle D ET N ORSKE V ERITAS Page 2 MGGNO893/SEI/29-J-103 10530832/DNV - Job.Id.: fracture. In summary, three factors have to come into play simultaneously in order to experience brittle fracture: - Initial crack/defect - Temperatures low enough such that the material is brittle - Loads on the structure Material Properties: All structural steels become brittle if the temperature is low enough. The properties of a steel are defined by its “grade” and yield strength. The grade is a measure of the steels toughness and is expressed by a letter, A, B, D or E corresponding to increasing toughness. The yield strength is expressed by a number, 24, 27, 32 or 36, corresponding to guaranteed yield strengths of 235, 265, 315 and 355 N/mm 2 respectively. Steel with a yield strength of 235 N/mm 2 is “mild steel”. Steels with a higher yield strength are “high tensile” steels. The toughness is normally measured using the Charpy V-notch impact test method. For mild steel (yield strength 235) the minimum impact energy is specified in the Rules for temperatures of +20, 0, -20 and -40 o C for A, B, D and E grade respectively. However, in accordance with IACS unified material requirements, it is not required to test grade A material less than 50mm thick or grade B material less than 25mm thick. It is sufficient that the mill guarantee the quality. High tensile steels are charpy tested at 0, -20 and -40 o C for A, D and E grade respectively (HTB grade is not used). Lloyds Register carried out a study of A grade steels. 39 plate samples were investigated and all the samples exhibited acceptable charpy test values down to -20 o C. However, 5 of the samples started exhibiting brittle behaviour at 0 o C and another 4 samples at -6 o C. Material from “Lake Carling”, a bulk carrier which experienced brittle fracture in its side plating, satisfied the requirements for grade A (a minimum impact energy of 27J at 20 o C). However, the steel started experiencing brittle behaviour at 32 o C and had a charpy test value of 10J at -10 o C. The conclusion is that while most A grade steel exhibits good ductility, there is no guarantee since it is not tested and there are examples of A grade steel with very low toughness, particularly at low temperatures. Summary: There ar e very few documented cases of failure due to brittle fracture. The “Lake Carling” is probably the best documented incident. However, the Transport Safety Board of Canada in their report on the “Lake Carling” ref ers to several vessels lost in cold climates where the root cause was never found. The “Lake Carling” experienced a crack in the ship side from the hopper tank to the top wing tank with subsequent water ingress and could easily have joined those statistics. Vessels trade frequently in cold climates, i.e. the Baltic Sea in winter. It could be argued that this documents through practice that low temperatures are not a problem. However, trading patterns are changing. With increasing export from northern Russia/White Sea, temperatures will be lower and frequency of exposure to low temperatures will increase, increasing the risk of brittle fracture. D ET N ORSKE V ERITAS Page 3 MGGNO893/SEI/29-J-103 10530832/DNV - Job.Id.: The Rules allow for an extreme ambient (air) temperature of -30 o C. It can be argued that internal structures should be accepted for the same temperature. There are two effects, however, that weaken this argument. The first is that a liquid cargo is much more effective at heat transfer than air. If a cargo tank is filled with cargo at -30 o C the cargo boundary is at that temperature. The second is that the cargo will remain cold for a significant length of time. One operator says that he was refused discharge in Rotterdam after transit from the White Sea because the cargo was too cold. This means that there will be a longer exposure time to cold temperatures and a greater probability of encountering the combination of initial defect/loads/brittle material necessary to cause brittle fracture. Arguments for allowing low temperature cargo: Few documented incidents of brittle fracture, none related to low temperature cargo. Grade A material is in general much better than minimum required by Rules. Tankers usually have HTS in deck and bottom – will act as crack arresting plate. Probability of having an initial crack, high loads and brittle material simultaneously is low. With double hull risk of pollution is low. Extreme ambient air temperature can be -30 o C, should allow same for internal structures. Arguments against allowing low temp cargo: Uncertainty in safety level. Zero tolerance for a pollution incident in arctic waters. Poor facilities for dealing with pollution in arctic waters. Liquid cargo more effective at cooling steel, steel temperature will be lower than if only subjected to low air temperature. Cold cargo stays cold – longer exposure time for steel to low temperature. Trading patterns are changing. White Sea more extreme than Northern Baltic. Colder cargos and higher frequency, so historic experience not relevant. From the above discussion it is clear that there are arguments both for and against accepting low cargo temperatures. However there is uncertainty when carrying extremely cold cargos and DNV can not confirm that the safety level is acceptable. Based on a technical evaluation of the material available cargo temperatures down to -10 o C can be accepted. Carrying cargos at lower temperatures can not be recommended. Operators, of course, can choose to carry colder cargos, but it is outside the scope of class. If a vessel is intended for regular service with cargos at lower temperatures then this should be taken into account at the design stage. D ET N ORSKE V ERITAS Page 4 MGGNO893/SEI/29-J-103 10530832/DNV - Job.Id.: If it is necessary to load cargos at temperatures lower than -10 o C then the operator should consider means to raise the cargo temperature during loading. Possible solutions could include the use of heat exchangers on deck, reduced loading/deballasting rates, warming of ballast water, and so on. Based on the above discussion the response to questions concerning cold cargos will be as follows: Quote: Our concern with respect to the carriage of low temperature cargos is the risk of brittle fracture. The latest Rules have as a design basis a minimum cargo temperature of 0 o C and we cannot recommend carrying cargos below that temperature. However, based on technical considerations, loading cargos down to a temperature of -10 o C should normally not present an unacceptable risk of brittle fracture although it is outside the criteria specified by class. Loading of cargos with a temperature below -10 o C is discouraged due to the increased risk of brittle fracture. Unquote. References: 1) IACS Technical Background, UR S6, Rev.5 (Sept. 2007). 2) IACS UR S6, “Use of steel grades for various hull members – ships of 90 m in length and above”, Rev.5, Sept. 2007. 3) Lloyds Register, “Review of the Fracture Properties of LR Grade A Ship Steel”, Materials and NDE Department, 1999. 4) “Marine Investigation Report, Hull Fracture, Bulk Carrier Lake Carling , Gulf of St. Lawrence, Quebec, 19 March 2002”, Transportation Safety Board of Canada, Marine Reports – 2002 – M02L0021. 5) “Toughness of Steel”, Transportation Safety Board of Canada, Marine Reflexions Magazine, Issue 22, July 2005. - o0o -
