The authors address the design issues and procedures required to enable the construction of safe and economical above-ground pre-stressed concrete LNG tanks of 200,000 m3 capacity - larger then the maximum size in existence.
 

The construction costs of the tanks occupy the major portion of the LNG terminal's total construction costs. Therefore, various efforts have been made to increase the cost efficiency of the LNG tanks, which can be largely divided into two parts:

(1) There has been a clear tendency toward LNG tanks with higher capacity during the past decades. This can achieve a decrease in the relative cost per unit of stored capacity and enable a more effective land use. It also matches well with the increasing capacity of LNG ships.

(2) The cost efficiency as well as the shortening of the construction period have been pursued through the technological development in material and structural aspects.

Table 1 shows the record capacity in each type of LNG tank. For the in-ground or underground tanks, 200,000 m3 capacity has been already attempted. As a comparison, the maximum capacity of above-ground tanks so far is 160,000-180,000 m3. For the full containment type above-ground tanks, which are popular these days and are the main concern of this article, 160,000 m3 capacity is no longer unfamiliar. In Korea, there are plans for further expansion of storage capacity of LNG tanks until 2015 as shown in Table 2. Aboveground tanks larger than 140,000 m3 capacity may be considered for future expansions.

As the capacity increases, special attention should be paid to the existing code provisions and the efficient procedure be established to design LNG tanks with structural and cost efficiency. Encouraged by the above-mentioned trend and with accumulated experiences of LNG tank construction for 15 years (Table 3), Daewoo E&C has investigated the design procedure of above-ground prestressed concrete LNG tanks with 200,000 m3 capacity. Some noteworthy points related to the large above-ground LNG tanks with concrete roof are reviewed in this article, focusing on the design aspects.

Design of 9% Ni Inner Tank

Recent documents have repeatedly reported that some revisions of the codes are required, especially for the inner tank, to design full containment type above-ground LNG tanks of higher capacity with economical proportions.

The most commonly referenced codes for the design of inner tanks have been API 620 and BS 7777. Also, many of the domestic constructions in Japan have been based on Japan's own codes. However, the Eurocode (EN 265002) now in preparation will reflect some important points required to design more reasonable dimensions of an inner tank with higher capacity; some parts of EN 265002 have already been published in PD 7777 (amendment of BS 7777).

Included in EN 265002 are two noteworthy features, i.e. an increased maximum thickness of 50 mm of the inner tank plate and partial height (60% of H.H.L. (maximum design liquid level)) hydrostatic testing. These substantially relax some of the very conservative provisions specified in BS 7777, i.e. a maximum thickness of 30 mm and full height (100% of H.H.L.) hydrostatic testing, by which it is expected that the way toward higher capacities will be wide open. The revised code actually reflects the state-of-the-art such as advanced manufacturing and welding techniques of thicker 9% Ni plates.

The height to diameter ratio of the inner tank has a primary significance since the overall proportion of the LNG tank is determined from the inner tank rather than outer tank in many cases. The following aspects can be discussed in relation to the sizing problem. As for the inner tank, provided that the strength of the inner tank plate is given, the only load-resisting factor is the thickness of the plate. On the other hand, there are some more load-resisting factors in the outer tank including concrete strength, thickness, and the amounts of reinforcing bars and prestressing tendons. When we consider that both the inner and outer tanks should possess the strength to resist design loadings independently, it is plausible that the overall proportion of the tank is more dependent upon the size of the inner tank, which has less redundancies in the load-resisting mechanisms.

Thus, it is recommended that in a preliminary design stage several alternatives of the inner tank dimensions (height, diameter, thickness, etc.) are compared together to select a candidate that minimizes the material cost of the outer tank as well as the inner tank.

Design of Roof Dome

Many of the existing concrete roof domes of above-ground LNG tanks have a radius of curvature equal to d (diameter of outer wall). It corresponds to a rise to diameter ratio of 1/8, which is often recommended as an efficient structural shape for the roof domes where self-weight or externally distributed load is dominant. On the other hand, a higher rise is advantageous when the roof is subjected to internal pressure, which is one of the main design loadings in LNG tanks, since the higher curvature can endure the internal pressure with less tensile stresses.

Therefore, it can be mentioned that the conventional rise of the above-ground LNG tank domes is not optimal, at least from the structural aspects. When designing large LNG tanks with increased dome span, it is recommended that several alternatives of the dome dimensions (radius of curvature, thickness, etc.) are compared to determine the structurally safe yet efficient shape with a minimum amount of concrete.

No special code-related restriction is imposed on the shape of the concrete dome; however the codes for the carbon steel liner that is attached inside the concrete dome should be followed also. For example, API 650 specifies that the radius of curvature of the liner should range from 0.8d to 1.2d. Some of the large in-ground LNG tanks where the roof dome is also exposed above the ground level have the radius of curvature close to 0.8d. Buckling safety of the carbon steel liner is also important and varies sensitively depending on the shape and placing method of the concrete dome.

Design of Ring Beam

The ring beam is located at the upper end of the wall. The primary role of the ring beam is to cope with the major portion of the thrust transmitted from the roof dome, thus reducing excessive deformation of the upper part of the wall as well as the roof dome. Therefore, the dimensions of the ring beam and the amount of prestressing tendons inside the ring beam have a close relationship with the shape of the roof dome. From the geometrical consideration, a higher rise dome induces less thrust to the ring beam, and is therefore advantageous for the ring beam as well as the dome itself.

The construction sequence of roof dome placing and ring beam prestressing has a primary importance because the stress state of the ring beam is much affected, depending on the sequence. The procedure should be so planned as to avoid excessive compression or tension in the ring beam throughout all the phases.

Design of Outer Wall

The safety and serviceability of the outer wall should be checked in various modes of operation: (1) The construction stage, (2) Normal operation, (3) LNG leakage from the inner tank. It may be an important design issue to maintain a slender wall thickness and proportion when a tank size gets bigger with the wall height increased. To achieve this purpose, it is expected that the conventional design practice should be revisited and improvements be made as necessary. Some of the possible improvements are introduced here.

High strength concrete contributes to raising the section strength often result-ing in a thinner section. As an example, a high strength of 600 kgf/cm2 over conventional 400 kgf/cm2 has been adopted for the wall of an LNG tank in Senboku, Japan. It is reported that the wall thickness was actually reduced by 30 cm by the strength increase.

Prestressing is a main design load in the construction stage, where the section is often subjected to a more critical stress state in the construction stage than the operation stage. During the prestressing sequence of horizontal hoop tendons, a large amount of section force (moment) and corresponding tensile stress is produced at the lower part of the wall from restraint of the wall deformation by the rigid bottom slab. When the wall becomes higher with increased capacity, the hoop tendons become too congested in the lower part of the wall according to the conventional design practice of tendons (Figure 1(a)). This would produce excessive or sometimes unallowable stresses at the joint of wall and bottom slab in the construction stage.

One straightforward countermeasure to reduce the stress is to increase the thickness in the lower part of the wall. Other than the thickness increase, one conceivable and powerful alternative is to revise the hoop tendon arrangement. According to recent researches, it is effective to modify the conventional tendon-induced trapezoidal pressure distribution into other types of distribution, an example of which is shown in Figure 1(b). This strategy can be achieved by adjusting the spacing of the hoop tendons in the lower part of the wall through some trial and error or optimization technique. The advanced pressure distribution produces moderate magnitude of the base moment during the construction stage as well as the normal operation. Of course, the required number of tendons can be reduced as an additional advantage.

Advanced Structural Analysis

During the past decades, computer-based structural analysis tools such as FEM (Finite Element Method) have achieved such great progress that the structural behavior can be estimated with high accuracy nowadays. It should be remembered that accurate and realistic structural analysis is always a key factor to design an LNG tank with optimized sections no matter what capacity it has. There are various structural analyses involved in the design of a LNG tank, some of which require more intensive and advanced techniques. Among them are heat transfer analysis of a structure subjected to the cryogenic temperature of LNG (Figure 2(a)), internal or external fire analysis, and seismic analysis of the structure containing the fluid (Figure 2(b)).

In the temperature-related analyses attention should be paid to establishing proper boundary conditions, since a LNG tank has complicated structural details inside including a suspended deck and various insulation layers. Especially, the favorable effect of the suspended deck which isolates the inner surface of the roof from direct exposure to the cryogenic atmosphere should not be overlooked.

In a detailed seismic analysis of an LNG tank, fluid-structure-soil-interaction should be considered. BS 7777 specifies that dynamic analysis should be carried out for the outer tank of double and full containment systems for areas with high seismicity. However, combined dynamic analysis considering the inner and outer tank as a whole could be required, e.g. at the client's request. In that case, the dynamic effect of the insulation layers between the inner and outer tank on the overall seismic behavior of the LNG tank makes the actual situation more complicated and needs more investigation.

Se-Jin Jeon works for the Institute of Construction Technology, Daewoo E&C Co., Ltd., as a senior researcher. He holds a PhD in civil engineering from Seoul National University, Korea. He has been involved in design and analysis of containment structures such as LNG tanks and nuclear containment buildings. His main concern is the prestressing analysis and heat transfer analysis under the cryogenic temperature of LNG.Eui-Seung Park is an electrical engineering graduate from Seoul National University, Korea. He has worked for over 25 years on world-wide plant and LNG terminal projects. He is a managing director of the Plant Division of Daewoo E&C Co., Ltd., and is a member of PGC B (Program Committee B) of IGU.