Current Developments: LNG
LNG safety research a t TNO
For many decades the marine sector around the world has been a bulkconsumer of heavy fuel oil and
marine diesel oil. This was and is used as the fuel of choice in the vast majority of ship designs. The times
change however, and we are overwhelmed with the message that we will soon run out of this type of
fossil fuel. We are eager to explore possibilities with other types of (fossil) fuel. Liquified Natural Gas (LNG)
is one of these types. This essay from Lex Vredeveldt is aimed to give us some information about the
current developments in the field of safety research with LNG, as conducted at TNO.
Door Lex Vredeveldt
Introduction
There are strong incentives to use liquefied natural gas (LNG) as bunker fuel on board ships. One is related to emissions after combustion; neither sulphur nor any particulate matter are present in the exhaust products. Another one, generally overlooked, is that there vv'ill be no pollution when LNG bunkers fuel is spilled because the LNG will evaporate, even in arctic areas. A third incentive is the huge global reserves of natural gas. It is however fair to note that natural gas, spilled through methane slip (engines are unable to combust all methane) or leaks, does contribute considerable to the greenhouse effect. Another downside of (L)NG is safety. In fact, IMG (International Maritime Grganisation, London) as well as CCNR (Central Committee for Navigation on the river Rhine, Strasbourg) regulations do not allow fuels on board with a flash point below 5S°C, which disqualifies natural gas. However, authorities are willing to consider lifting this ban on natural gas, provided ample safety is ensured. For example, at the moment seven inland waterway tankers have been granted permission to use LNG as bunker fuel, albeit on a temporary exemption basis. The first ship to sail on LNG was the Argonon operated by Deen Shipping (Figure 1). The exemptions are based on hazard identification studies which, for their technical evidence, largely d e p e n d on regulations pertinent to sea going LNG tankers, which may not be relevant to inland waterway ships in all aspects. An important reason
for granting these ships a temporar/ exemption is the fact that they operate as chemical tankers, which implies they comply with the ADN requirements for such ships, both in terms of hardware and operations. Should non-ADN ships be granted exemptions as well, more tangible technical evidence is required
Figure 1 MTS A r g o n o n , t l i e first L N G f u e l l e d I W W t o n k e r
instead of just relying on regulations. The challenges are to identify the measures which must be taken to achieve a satisfactory safety level and how to demonstrate the effectiveness of these measures. These measures refer to both hardware and operations. In order to do such an identification and demonstrations, knowledge and skills related to physics and materials are required which are being thought at universities and polytechnics. This article states which these ore and how they c a n be exploited in working towards safe application of natural gas. Also some boundary conditions pertinent to human behaviour and politics are discussed.
The physics
One of the hazards is uncontrolled pressure build up in the LNG system. Natural gas can only exist at ambient temperature in its gaseous phase. In order to understand this one needs to be aware about the pressure -specific volume diagram, as shown in Figure 2 below.
It shows that substances can only exist in either liquid or gaseous phase below a so called critical temperature (critical isotherm), which for natural gas (methane) lies at -82 °C. The pressure where both phases c a n exist is 46 bar (critical pressure). At ambient pressure (1 bar) the boiling temperature lies at -162 °C. This behaviour implies that when LNG becomes trapped in say a hose or a pipe, a pressure will build up driven by a heat influx, which will eventually cause a burst. For this reason LNG storage tanks are fitted with heat insulation and pressure relief valves.
V o l u m e , V F i g u r e 2 lypical p-v d i a g r a m
(source: l i i t p ; / / p l i i l s c h a t z . c o m / p h y s i c 5 - b o o l < / c o n f e n f s / m 4 2 2 1 8 . h t
ijskoud
The heat transfer is governed by the following formula.
d
with: q heat transfer [W], k thermal conductivity [W/m K], A heat trensfer area [m2], T temperature [K], d thickness of heat transfer barrier [m]. In case of LNG fuel tanks there is basically only one parameter, which can be 'designed', i.e. the thermal conductivity. For this reason, at least up to now, all bunker tanks are of the vacuum type, i.e. on inner tank inside an outer tank, with the annular space vacuumed.
When heat transfer becomes too high, e.g. because of a loss of vacuum, the heat influx will increase and a pressure will build up. Consecutively a pressure relief valve will open in order to reduce the tank pressure.
The success of this mechanism is controlled by Bernoulli's law;
-pv^ + p = constant
Another aspect to be dealt with is the quality of the construction materials to be used for containment and piping at cr/ogenic temperatures. Most materials lose their strength and b e c o m e brittle. Only a few materials are known to be cryogenic resistant; aluminium, austenitic steel 316, austenitic steel 304 and 9% Ni steel. It is Interesting to know how these materials behave when a tank is subjected to a mechanical impact, e.g. involved in a ship collision. Limited literature is available on material properties under crash at plate thicknesses used in actual designs. Therefore at TNO material tests were done with a drop tower (Figure 3) allowing tests at realistic deformation rates on specimens with realistic thicknesses (Figure 4).
F i g u r e s . 10 l o n n e s d r o p t o v / e r at T N O s s t r u c t u r a l d y n a m i c s l a b .
in one drop (depicted).
The curves clearly show that tests on 0 4 mm specimens tend to be conservative from a yield stress point of view. The fracture strain could be taken from test specimen observations after the tests (Figure 6, next page). It does not seem to be affected by material thickness and strain rate. It is noted that the tests shown relate to a uni-axial stress state where width reduction and thickness reduction of the specimen are half the elongation in loading direction. 2500
F i g u r e 4 t y p i c a l m a t e r i a l test s p e c i m e n (source: T N O )
Test results are shown in Figure 5. The green a n d the blue line refer to the drop tower tests on 100 x 8 mm specimens, while the other three curves refer to quasi static tests on 0 4 mm specimens, all at cryogenic temperature. It is noted that the energy available in de drop test was insufficient to fracture the specimens
With this data explicit finite element model (EFEM) calculations can be done in order to predict the vulnerability of cr/ogenic storage tanks with respect to mechanical impact. In doing so, another mechanism needs to be observed. When impacting a pressure tank, at some stage the volume decrease is larger than the initial vapour space and a 'liquid full' condition occurs.
0.2 0.3 Strain [-]
F i g u r e 5 stress strain curves ( T # stands f o r test s p e c i m e n n u m b e r , s o u r c e : T N O ]
Figure 6 s p e c i m e n o b s e r v a t i o n s
The question then is whether the tank will instantaneously rupture or the tank shell will only yield. The required mechanisms are the 2nd gas law pertinent to the vapour and more importantly the pressure volume decrease ot the liquid with the bulk-modulus (loosely: Young's bulk-modulus) as material parameter. These relations were implemented in an EFEM computer program and used in the simulations. In order to make sure that the simulations reflected reality sufficiently, small size tank crash tests were done at cryogenic temperatures. Figure 7 shows the test set up, where a 1.5 tonnes dropped object indents a small double walled tank pressurised tank (tank diameter 240 mm and shell thickness 0.7 mm).
The measured contact force -penetration curve matches the predicted one closely. Moreover measured and predicted deformation resembled satisfactory as shown in Figure 8(next p a g e ) . Having demonstrated the adequacy ot the EFEM crash calculation method, a realistic tank was analysed as shown in Figure 9(next page). For this purpose a 30 m3 cryogenic vacuum tank was chosen OS a typical example. The tank was specially designed for the purpose of the investigation since no literature is available on actual designs.
A push barge bow collides with an unprotected tank. Collision energy
absorbing capacities between 15 and 20 MJ were found, while the tank did not rupture. It is interesting to note that the inland waterway regulations for transport of hazardous cargos state a design value for crashworthiness of conventional gas tanks of 22 MJ. The crash analysis did not consider any internal piping, which is likely to tear loose from the inner tank during a crash. However it is expected that associated outflow openings will not exceed piping cross sectional areas, which will lead to outflow, evaporation and pressure build up scenarios which may be manageable.
Risk perception
The above mentioned analyses contribute to the task of conducting a safety assessment (risk assessment) on the use of LNG as bunker fuel. An initial 'what can go wrong' analysis, formally called a hazard identification analysis (HAZID), is carried out to identifiy the accident scenarios. Consecutively, technical analyses are carried out to determine the risks of the various scenarios. A ship colliding with a fuel tank is one such scenario. As mentioned such an event may lead to two different results; minor leaks due to torn piping and a prolonged loss of containment with low flow rates or a total rupture of the tank with an instantaneous loss of the entire tank contents. The first mechanism may be manageable and hence the consequences would remain in the a c c e p t a b l e region, i.e. a rating 3 or lower in the risk matrix (Table 1, next page). The consequences of a total rupture are hard to imagine and hence the worst must be feared, i.e. a consequence rating above 3. Yet this risk may be a c c e p t a b l e if it c a n be demonstrated that the probability of total tank rupture is sufficiently low, in the risk matrix this is indicated with a probability rating of B or preferably A.
Probability density functions of collision energies (l/2-m-v2) available on w a t e w a y s , c a n be determined
Figure 7 s m a l l scale crash test o n p r e s s u r i s e d cryogenic t a n k
ijskoud
Table 2 risk p e r c e p t i o n ( s o u r c e : k a n s e n in d e c i v i e l e t e c h n i e k D G r i i k w a t e r s l a a t ) yearly probability to get killed typical cause 1 / 1 0 0 mounteneering accident 1/1000 illnes 1 / 1 0 0 0 0:
car accident 1 / 1 0 0 000 aviation accident 1/1000 000 dyke failure F i g u r e 8 crash d e f o r m a t i o n o f t a n k , o b s e r v e d (top) a n d p r e d i c t e d ( b o t t o m ) mm F i g u r e 9 C o l l i s i o n d a m a g e 3 0 m 3 t a n k , c o n t o u r s r e f e r t o e q u i v a l e n t p l a s t i c s t r a i nthrough analysing AIS (Automatic Identification System for Ships) data, which gives ship displacements a n d speeds in a given sailing area in conjunction with either 'near misses' or collision statistics from the past. The acceptability of a given probability must be decided by society, which in practice means that politicians are responsible. Figures as shown in Table 2 may prove useful in this respect.
Future w^ork
As mentioned, pipe rupture in the annular space ot a vacuum tank still needs to be analysed with respect to its severity. It is believed that the probability of this event cannot be 'designed' to an acceptably low level because current vacuum tanks, once built, cannot be inspected internally, albeit that there exists one exception, where a vacuum tank is designed a n d built with a dome
which accommodates all tank piping and appendages. The other task ahead is determining the probability of tank rupture following a collision, as outlined earlier. This probably only needs to be determined for vulnerable navigation areas, i.e. areas with a high traffic density and general public nearby.
There are still many mechanisms which ore not mentioned in this article which also need attention, e.g. a pressure relief valve on a fuel tank which capsized with the ship, bringing the valve in contact with liquid gas rather than vapour. It is unknown whether valves will operate satisfactor/ in such coses, moreover questions could be raised on flow rates ot the liquid being sufficiently high to actually relief the pressure. Another event is an LNG tank engulfed by a Are. No doubt other mechanisms will be identified in future.
Table 1 t y p i c a l risk m a t r i x ( s o u r c e : ISO 1 7 7 7 6 )
Education of all who will b e c o m e involved with LNG is another task to be done.
None of these need to deter us from applying LNG as bunkerfuel, provided designers, manufacturers, operators, research institutes, classification societies a n d authorities are willing to invest in understanding LNG as a bunker fuel a n d a c t accordingly. The main incentive for writing this article is to show to the current generation of students that all the subjects with which they are being pestered at university are, in spite of this, worth-while.
A l e x W. V r e d e v e l d t s e n i o r scientist n a v a l a r c h i t e c t l e x . v r e d e v e l d t @ t n o . n l
P r o b a b i l l f v
J
Rating P e o p l e A s s e t s EnvironmenI Reputation
A B C D
J
Rating P e o p l e A s s e t s EnvironmenI Reputation H a s o c c w e d i n indusUy H35 o c c i i f i e d in a maritime c o m p a n y O c c m s at l e a s t cMice a y e a r in a maritime c o m p a n / O c c u r s a l l e a s t o n c e on a s h i p / plaltcrm J 0
No rr^'ury No d a m a g e Zero effect No impac I
J
I
Slight injury S b g h l damage Slight effect S b ^ l impact
J 2 Minor injury Minor d a m a g e Minor eRecl Limited impact J
3
Major injury L o c a l d a m a g e L o c a l elfecl C o n s i d e r a H e impact
J
4 S i n g l e blality Major d a m a g e Pitajor effect
Major notional impact J 5 r.lulliple btatily E x l e r u i i ^ d a m a g e Fil3S5i-.e elTecl Itlajof intemotiofial impact
