LNG Tank Research Articles

Operational Experiences with LNG Storage Tanks

British Gas has 20 above-ground LNG tanks in service, representing some 250 tank-years of experience. No major incidents have occurred with any of these tanks. Operational problems have been experienced with tanks, bunds, LNG valves and pumps; these have all been overcome without recourse to warming up or decommissioning the tanks. This paper summarizes British Gas's operational experiences, describing, in particular, how problems have been overcome. One above-ground tank has been decommissioned recently, for operational rather than technical reasons. This provided an excellent opportunity for detailed study of the condition of an LNG tank after 14 years of operational service. In addition mention is made of the (now decommissioned) in-ground LNG storage tanks, formerly used by British Gas.

An Empirical Study on Reliability of Strength for Independent Prismatic Tank Type B

It has been reported that a contract was signed for building an LNG carrier of independent (self supporting) prismatic tank Type B system. This news must be received with a great interest as this is the first case of adopting the independent prismatic tank Type B system for a large LNG carrier.The idea of the independent prismatic tank Type B system was crystallized in 1970's and a group of ship builders started the development of its design standards in which Nippon Kaiji Kyokai (a Classification Society-NK) played an active role. With the design standards thus developed, number of LPG carriers and Ethylene carriers of this system were built with the NK class in the beginning of 1980's, and nothing has been reported of serious deficiencies and troubles.Nevertheless, on the occasion of adopting this system for the large LNG carrier, the authors attempted to review the design standards to verify their reliability. For this purpose, the authors reviewed the tank strength analysis, tank test procedure, design and inspection records of ships in service, and made an empirical research on probability of failure of the tanks.The study concluded that the prevailing design standards for the system were considered quite reasonable and this independent prismatic tank Type B system could be one of the LNG tank systems of high reliability.

STUDY ON LIQUID-FILLING BEHAVIOR OF STORAGE TANKS USING A BIG MODEL OF PERFECT SIMILITUDE

In 1985 the authors carried out a series of static tilt tests with a very big tank model in order to study the aseismic safety of large-scaled cylindrical liquid storage tanks. The model has about 10 meter diameter and 8 meter height and is made of aluminum alloy, satisfying the similitude requirements to actual large-scaled LNG, LPG and oil tanks. Prior to the static tilt tests, several cases of water-filling test of the model tank were conducted. It is very important from the point of view of tank design to grasp the tank behaviors in these conditions correctly. This paper is dealing with the water-filling behaviors of the tank which has not been clarified so far. The behaviors are discussed in relation with the initial deflections of tank shell and base plate and from comparison with the finite element analysis.

Free vibration analysis of partially liquid-filled shells of revolution.

The linear free vibration of partially liquid-filled shells of revolution is analyzed by means of the finite element method. Isoparametric thick shell-elements are used for the axisymmetric shell, and isoparametric quadrilateral elementes and line elements are used for the liquid interior and surface, respectively. The liquid is assumed to be incompressible and invicid. The initial axisymmetric deformation of the shell due to the static liquid pressure as well as the boundary condition on the free liquid surface are fully taken into consideration. It is founded that the static liquid pressure has significant on the natural frequency of the LNG tank and the minimum natural frequency of the fast breeder reactor containment has a circumferential wave number of N=5∼10.

A predictive model for rollover in stratified LNG tanks

AbstractThe incubation period preceding “rollover” within a stratified LNG tank involves intensive heat and mass transfers between layers. Numerical integration of equations describing these processes leads to predicted time‐history and boil‐off characteristics which are in excellent agreement with Sarsten's (1972) documentation of the LaSpezia rollover incident.

Development of Cargo Tank Insulation System for LNG Ship with Spherical Tanks

The LNG tank insulation technology is one of the key technologies in LNG ship building. We developed our own insulation system, called “Kawasaki Panel System”, for a large LNG ship with spherical tanks, and applied this system to our first built LNG ship “GOLAR SPIRIT”. In her unprecedented cooling test using LN2 and gas trial, it was verified that the performance required for LNG tank insulation was satisfactory and that “No Cold Spot” criterion we aimed at was fulfilled.This paper summarizes the development of our insulation system, describing the outline of the major large scaled model tests carried out to select insulation materials and insulation system and to grasp the thermal efficiency, strength and workability of the system as a whole, and also referring to the specifications of the system determined through these tests and to the outline of the quality control established.

Model for non-instantaneous LNG and gasoline spills

Predictions are made of the development and decay of the dimensions of liquid fuel spills on water resulting from collisional rupture of a single fuel tank on large tankers of LNG and of gasoline for the case in which ignition of the spills occurs immediately on rupture. Two sizes of rupture holes are considered, and the shape and duration of the LNG spills are compared with those of an ‘instantaneous’ spill. In contrast to the predicted circular shape of an instantaneous spill, the predicted shapes of the hole spills are long and narrow with aspect ratio, at maximum area, of about 10 to 1 for LNG and 5 to 1 for gasoline. The maximum spill area from a 9.3 m 2 (100 ft 2) hole in an LNG tank is predicted to be attained in half the time predicted for an instantaneous spill of the same fuel volume and is one-third the maximum area of the latter.

Statistical Estimation of Tank Stress by Total System Analysis for a Spherical LNG Ship Tank

Stresses in a LNG tank, which are random variables caused by various types of random wave induced loadings, should be treated statistically.The Square Root Method (SRM) is a conventional method for the estimation of maximum expected values of the tank stress resulting from these types of loadings. This method gives an accurate solution in a special case where the loadings are statistically independent and stress response is linear with regard to the loadings. However, there are, in general, certain correlations between wave induced loadings and stresses, such as Von Mises' equivalent stress and principal stresses, which are non-linear quantities with regard to the loadings.This paper explains and discusses an analysis procedure called “Total System Analysis” (TSA) procedure, which has been developed to accurately estimate longterm statistical values of tank stresses. This procedure is an extension of the concept of the commonly-used statistical analysis of the long-term prediction of response of ship's motions and wave induced loadings; the procedure treats non-linear stresses as well as linear ordinary stress components.In establishing the procedure specific items such as the introduction of stress response functions, the problems of non-linear stresses and the approximation of probability density functions of tank stress local maxima (or minima) to Rayleigh distributions, etc. are discussed. These items are verified by numerical experiments by a computer.A Hitachi Zosen CBI type spherical tank on a 130, 000 cubic meter LNG carrier has been analyzed by the TSA procedure ; the results show that the correlations between types of wave induced loadings have a large influence on the tank stress, tending to reduce it, and that the non-linear stresses including the influence of correlations between the stress components are smaller than those without the above influence.

A Study on Low Cycle Fatigue Strength of Aluminum Alloy A 5083 P-O Butt Welded Joints

Low-temperature, low-cycle fatigue strength of aluminum alloy welded joints were investigated, carring out deflection controlled repeated bending fatigue tests at room temperature and at -196°C on the following four materials : 5083-O plate, 5083/5183 butt weldments with and without reinforcement, and Tee-fillet weldments made by MIG welding process.The results showed the fatigue properties at -196°C to be superior to those at room tem..' perature, except for the butt weldment with reinforcement machined off in the fatigue life range less than 100 cycles. The fatigue strength ratio at both temperatures increased with increasing fatigue life up to about the ultimate tensile strength ratio at both temperatures.The strain-basis fatigue strength reduction factor due to the reinforcement of the weld Kf., as difined by the ratio of the fatigue strength of the weldment to that of the 'plate, was found to be essentially the same at both temperatures and to become greater than or equal to the elastic stress concentration factor at the weld toe. The above mentioned factor Kf., was reasonably evaluated by Stowell's equation.From these observations it is concluded that the application of the fatigue strength at'room temperature as an allowable stress in the fatigue design of LNG tanks will be on the conservative side.

Giant precast prestressed LNG storage tanks at Staten Island

Precast prestressed segmental construction was used effectively to build two of the world's largest LNG storage tanks for Distrigas Corporation. The author, who was a consultant on this project, discusses the design and construction highlights of these 900,000-bbl LNG tanks. In particular, he describes the fabrication and erection of the precast wall panels and the circumferential post-tensioning of the walls.

Fracture Toughness of Welded Joints of 9% Ni Steel for the Tank of LNG Carriers

The basic premise of the MOSS-Rosenberg LNG containment system and other spherical LNG tank system is “leak-before-failure”. In other words, the design concept applied to this type of pressure vessel is basically “fail-safe”.Therefore, to evaluate the structural safety of the spherical LNG tank, it is neccesary to obtain the fracture toughness of the tank shell plate and the welded joints, quantitatively. Considering the strength of brittle fracture initiation of 9% Ni steel LNG tank, the fracture toughness in the heat affected zone is the most important problem.Accordingly, the authors carried out the tension test with wide center-notched plate test specimens, notched in the heat affected zone of submerged arc welding with heat input values of about 1050 KJ/cm, in order to evaluate the fracture toughness of welded joints of 9% Ni steel.The following conclusions were obtained.(1) It is reasonable to calculate the critical crack length, using the experimental results obtained from the standard size specimens (about 400 mm wide) on the basis of Linear Elastic Fracture Mechanics.(2) For the butt-welded joints produced with the most practical heat input values of about 1040 KJ/cm, the lowest Kc values at the cryogenic temperature of LNG, -62°C, will be 775500 kg, √mm/mm2, and the critical crack length for brittle fracture initiation of 9% Ni steel LNG tank is satisfactorily long.

Observations on 1, 000m3 experimental LNG carrier at sea (1st report)

Observations on a ship at sea carried out to verify the usefulness of a method of analysis developed for the safety assessment of LNG carriers.The ship is 60 m in length between perpendiculars and has two LNG tanks (spherical and prismatic).In the observations, various kinds of dynamic responses such as ship motions, pressures on tanks, stresses on tanks and their supports etc. totaling about 60 channels are measured simultaneously. Besides them, wave is measured using a wave buoy with a pressure transducer.For the first report of the observations, this paper deals with measurments and analyses of wave data, and evaluation of wave spectra dfrom measured ship responses.The wave spectra evaluated from measured ship responses show better agreement with measured ones in head seas than in following seas. The errors of the evaluated wave heights and wave periods are within 20% mostly.

A Study on Fatigue Strength of Aluminum Alloy for LNG Tanks

In order to obtain useful and reliable data of fatigue strength of 5083 type aluminum alloy and its welded joint for designing of LNG tanks, the fatigue strength was studied experimentally and analyzed statistically including available data in literatures surveyed.Axial zero-to-tension fatigue tests were carried out of 25 mm thick base plate specimens and the transverse butt welded joint specimens whose reinforcements were ground partly, flushed out, and as welded. Rotating bending tests were also carried out to evaluate the fatigue strength of a heavy section aluminum plate, where the specimens were cut out of thickness direction of a 200 mm thick plate, and longitudinal of a 40 mm plate and crosswise to welding direction of the butt welded plate.Present test results show that the fatigue strength of the butt welded joint decreases remarkably due to the reinforcement of the weld, but increases up to about 1.3 times by grinding out partly the reinforcement toes, and reaches to the level of the base metal by flushing out the reinforcement. It is also noticed that the relative fatigue strength Sa/σu of the 200 mm thick plate along the short transverse direction decreases to about 72% of the strength of 40 mm thick plate at Nf = 2 ×106 mainly due to its porous structure.Statistical analysis made on the fatigue strength under axial load including the present data derived fatigue design curves presented as an exponential relation Sa/σu=C (Nf) -k, where the C and k values for the 99% survival were defined with 50% or 95% confidence.Finally fatigue design curves of the 99% survival with 95% confidence were proposed, of which reduction rates of the strength resulted in 2/3, 4/5, and 1/2 for the base metal, the butt joint without reinforcement and with reinforcement respectively. A Pohl diagram (Smax/ σuvs. R) of which Smax/σu is determined by a straight line from Smax/σu at R=0 to that for R=1 in a modified Goodman diagram (Smax/σuvs. Smin/σu) is presented as a design curve which shows the effect of the stress ratio.

On Marine Transportation of Liquefied Natural Gas

Notwithstanding the very few experience we have in construction and operations of LNG carriers throughout the world, recently, there is a remarkable tendency to rapidly increase the ship's size only from a view point to improve the; economical feasibility in transportation cost.The writer is afraid that this concept will not satisfy the expectation for a scale merit but rather lead to concentration of risks, whereas the LNG tank has a fatal restriction in unit tank volume due to technical problems, both in the case of the self-supporting type or the membrane type, and this boundary is quite different from the case of conventional oil tankers.