A Study on the Structural Impact of FLNG Topside Piperack Module Enlargement

Article information

J. Ocean Eng. Technol. 2024;38(5):307-314

Publication date (electronic) : 2024 October 25

doi : https://doi.org/10.26748/KSOE.2024.059

Eun-Hak Lee¹, Tak-Kee Lee^2,

¹Graduate Student, Department of Ocean Engineering, Gyeongsang National University, Tongyeong, Korea

²Professor, Department of Naval Architecture & Ocean Engineering, Gyeongsang National University, Tongyeong, Korea

Corresponding author Tak-Kee Lee: +82-55-772-9193, tklee@gnu.ac.kr

Received 2024 July 1; Revised 2024 August 7; Accepted 2024 September 13.

Abstract

1. Introduction

The floating liquefied natural gas (FLNG) unit is an offshore facility designed to extract, refine, and store natural gas as liquefied natural gas (LNG). Serving as a floating production base, FLNG offers several advantages over traditional onshore plants, including lower investment costs, increased eco-friendliness, and greater mobility. Often referred to as “an LNG factory at sea,” FLNG handles all processes involved in offshore gas field development, making it ideal for deep-sea gas fields located far from land (Choi, 2022).

Traditionally, natural gas extracted from subsea fields was transported to land via pipelines, where it was liquefied and stored before being shipped to demand centers using LNG carriers. In contrast, FLNG is a self-contained facility that performs all stages of production—extraction, refinement, liquefaction, storage, and loading/unloading—directly at sea. By developing subsea gas fields with FLNG, the need for expensive onshore liquefaction and storage facilities—typically costing around KRW 2 trillion—is eliminated. Additionally, this method helps preserve underwater ecosystems by avoiding the installation of subsea pipelines (SHI, 2013).

Both onshore and offshore plants are steel frame structures, but their structural designs differ significantly. In onshore plants, heavy equipment is installed directly on the ground, while lighter equipment and fittings are placed within the structure. As the assembly takes place on-site, transportation is not a major concern. However, factors such as ground conditions and seismic activity are critical considerations.

Offshore plants, on the other hand, operate at sea, making structural stability a top priority. Therefore, environmental factors such as waves, wind, algae, and saltwater corrosion must be considered in the design. Additionally, transportation plays a crucial role since the structure is assembled on land and then transported to the site (Oksk, 2019).

FLNG has various functions to liquefy the natural gas produced under the sea without transporting it to the ground. To this end, modules with various functions are installed on the topside of this equipment. Examples include a gas treatment module to refine natural gas and remove impurities, a liquefaction module to liquefy treated gas, a power generation module to supply power to equipment, a safety and emergency module equipped with safety equipment and emergency response systems, a living quarters module equipped with accommodation and living convenience facilities for workers, and a flare tower to incinerate waste gas (Fig. 1).

Fig. 1.

A typical FLNG arrangement

The piperack supports pipelines to transport treated gas between modules as well as power supply cables, and provides sufficient space for efficient movement of workers and equipment. It is generally arranged from the bow to the stern in a limited space between different modules in the middle of the upper deck. Therefore, the piperack has a significantly narrow width compared to the height and length (height/width = 1 to 6) (Fig. 2).

Fig. 2.

Example of FLNG piperack (Petronas FLNG)

For typical topside modules, a large structure referred to as topside module support (TMS) is installed on the hull deck, and topside modules are placed on it (Fig. 3). A rigid rubber-based pad, referred to as elastomeric bearing pad (EBP), is inserted between TMS and a topside module to reduce the impact between them.

Fig. 3.

Example of FLNG section and topside module support

The piperack is directly connected to the hull deck through welding without using the aforementioned method. This prevents falling by securely fixing and integrating the hull and piperack; however, the impact of hull motion is directly transferred to the piperack. To minimize the impact of the hull motion, the piperack is generally divided into four to ten sections and installed on top of the hull deck (Fig. 4).

Fig. 4.

Piperack layout plan (piperack: PR 01–05)

For these reasons, piperacks are typically built and installed in relatively small sizes, leading to several additional tasks compared to other modules. First, the installation of multiple piperacks requires the use of marine cranes for each one. Large marine cranes with capacities ranging from 3,000 to 8,000 tons are typically used to install piperacks weighing several hundred tons, resulting in significant inefficiency. Second, because the piperack is divided into sections, additional pipes and electrical components are needed at the connection points, which require further assembly after installation. Third, certain tests cannot be conducted on land prior to installation; they must be performed at the quay or at sea, which delays other work at the quay and impacts the overall construction schedule. This, in turn, reduces the manufacturer’s profitability. To address these inefficiencies and shorten the construction period, enlarging the piperack is necessary.

While various studies have been conducted on the structural effects of topside modules under different conditions, few have focused specifically on piperacks using welding methods. For example, Jang and Ko (2018) researched the structural behavior of FPSO topside modules based on TMS support conditions, and Seo et al. (2021) studied the effects of different topside and TMS interface implementations on the hull during global ship analysis of ship-type marine structures. In this study, a piperack twice the length of the conventional, verified piperack design was modeled. The structural behavior of both the original and enlarged piperacks was then compared and analyzed using static stability analysis (in-place analysis), assuming environmental loads such as hogging and sagging under identical conditions at the fore-body position.

2. Analysis

2.1 Analysis Modeling

The piperack analysis was performed using SACS (Structural Analysis Computer System; Bentley, 2019), a software commonly utilized in marine structure design. SACS can analyze a wide range of marine structures and is primarily used for the analysis of fixed structures, compliant towers, and floating topsides. It also supports various other marine applications. The software enables both static and dynamic structural analysis, pile-structure interaction analysis, collapse analysis, as well as marine transportation and installation analysis. Additionally, it can verify compliance with the codes of various classification societies, making it an invaluable tool for marine structure designers.

For this analysis, a standard piperack model was created based on the geometry of previously constructed modules to ensure reliability and facilitate comparison of results, as shown in Fig. 5. To compare the effects of size enlargement, the length of the piperack was doubled by connecting two identical piperacks, as illustrated in Fig. 6.

Fig. 5.

SACS Modeling of one piperack module (Case 1)

Fig. 6.

SACS Modeling of two piperack modules (Case 2)

2.2 Boundary Conditions

The piperack is directly connected to the hull via welding. Since the piperack support is welded to the top of the hull web, the stiffness of the hull web must be considered during the analysis. Additionally, the stiffness of the piperack stool is modeled as a spring support element. Fig. 7 illustrates the boundary conditions for the piperack support, which is welded to the hull in this analysis. These boundary conditions are typically determined early in the construction process through discussions, as different methods can be employed based on the shipowner’s experience and preferences.

Fig. 7.

Boundary condition of piperack module (0: free, 1: fixed)

2.3 Load Conditions

Table 1 shows the load cases considered in this study.

Table 1.

Summary of elementary load cases

2.5 Relative Displacement

The vertical displacement caused by the hull’s hogging and sagging is typically expressed in relation to the aft and fore ends of the body. To apply this displacement to the topside structure, relative displacement values for each module must be used.

The method of obtaining the relative displacement applied to each module is as follows.

Fig. 8 illustrates the relationship between absolute and relative displacements. In the figure, the dark horizontal solid line at the bottom represents the main deck before deformation, while the light solid line at the top represents the main deck after deformation.

• (1) Relative longitudinal deflection (L₀)

  (1) L0=l×Average(Strain_LegA,Strain_LegB)

• (2) Relative vertical deflection (V₀) and rotational deformation (θ₁, θ₂)

  • (2-1) Hull vertical deflection

    (2) d2ν(x)dx2=M(x)E(x)I(x)
    (3) ν(x)=∫0x∫0ξM(ξ)EI(ξ)dξdx+∫0L∫0ξM(ξ)EI(ξ)dξdxLx

  • (2-2) Rotated deformation (θ)

    (4) θ=dν(x)dx=∫0xM(x)EI(x)dx+∫0l∫0ξM(ξ)EI(ξ)dξdxL

  • (2-3) Relative vertical deflection (V₀) (V₀ ≈V₀ cos(θ₁))

    (5) V0=V2-(V1+L⋅tan(θ1))

  • (2-4) Relative rotational deflection (θ₀)

    (6) θ0=θ2−θ1

Fig. 8.

Relative displacement concept

The relative displacement data used in this study are presented in Tables 2, 3, and 4. Table 2 shows the maximum displacement in the X-direction, while Table 3 presents the maximum displacement in the Y direction. Table 4 provides the maximum rotation angle. The “operating condition” (Op.) refers to the normal operational state of all FLNG systems and equipment, whereas the “damaged condition” (Dam.) represents a state of impairment caused by accidents, human error, natural disasters, or aging, among other factors. Conditions 1 and 2 reflect the acceleration values corresponding to these operational conditions.

Table 2.

Relative longitudinal hull deflections (unit: mm)

Table 3.

Relative vertical hull deflections (unit: mm)

Table 4.

Relative rotational deflections (unit: radian)

3. Analysis Results

3.1 Case 1 : Single Module

This case involves a single module and includes all the data from Tables 2 to 4, covering Op. 1 to Dam. 2. As a result of the analysis, the maximum unity check (U.C.), displacement, reaction force (Z), and reaction moment (Y) values are presented in Table 5.

Table 5.

Results of Case 1

3.2 Case 2 : Enlarged Module with Double Length

This case refers to an enlarged module that connects two single modules and includes all the data from Tables 2 to 4, covering Op. 1 to Dam. 2. As a result of the analysis, the maximum U.C., displacement, reaction force (Z), and reaction moment (Y) values are presented in Table 6.

Table 6.

Results of Case 2

3.3 Comparison of the Results

3.3.1 U.C

U.C. is the ratio of the actual member stress (σ_act) to the allowable member stress (σallow). In this study, the U.C. value was evaluated based on the allowable load of AISC 9^th. In general, the value obtained by dividing the actual load by the allowable load should not exceed 1.0 in design (AISC, 1989).

(7) U.C=σact/σallow

In Case 1, the maximum U.C. value was 0.798. Overall, the U.C. values are relatively low, except for two braces (Fig. 9) between deck levels 1 and 2, where the U.C. values exceed 0.7. In Case 2, the number of locations where the U.C. exceeds 0.7 increased to five (Fig. 10). A comparison of Cases 1 and 2 revealed that changes in Z-direction relative displacement and the friction load of piping in the X-direction had the greatest impact. When these two factors were adjusted, the U.C. values showed relatively significant changes.

Fig. 9.

U.C location view (≥ 0.7) for Case 1

Fig. 10.

U.C location view (≥ 0.7) for Case 2

In Case 2, the maximum U.C. value occurred at the brace between the hull and the piperack main deck, rather than the member that showed the maximum U.C. value in Case 1. For this member, the U.C. value increased from 0.44 under the damaged condition in Case 1 to 1.05 under the operational condition in Case 2 (see Figs. 11 and 12). This increase is likely due to the additional axial stress caused by vertical deflection from the piperack enlargement. Fig. 9 also shows that the U.C. value of the main vertical column increased.

Fig. 11.

U.C of the brace under main deck for Case 1

Fig. 12.

U.C of the brace under main deck for Case 2

3.3.2 Displacement

The analysis results indicate that there is almost no difference in the maximum deformation (displacement) of the piperack between Cases 1 and 2, with values of 11.87 mm and 11.77 mm, respectively (Figs. 13 and 14). This demonstrates that the relative displacement of the piperack did not affect its overall deformation, suggesting that the differences in displacement are primarily due to changes in external forces.

Fig. 13.

Total maximum displacement for Case 1

Fig. 14.

Total maximum displacement for Case 2

3.3.3 Reaction Force Z

When comparing the reaction force Z values, which transfer the load to the hull, the total value in Case 2 slightly decreased compared to Case 1 (Figs. 15 and 16). The left side of Case 2 is 1.01 times that of Case 1, while the right side is 0.85 times. Overall, approximately 93% of the doubled sum of the reaction force Z in Case 1 became the sum in Case 2. Despite doubling the piperack’s length, no significant increase in reaction force was observed, with the center value being relatively high. Thus, comparing the reaction force Z values alone does not reveal any significant differences in impact.

Fig. 15.

Maximum reaction force Z: Case 1

Fig. 16.

Maximum reaction force Z: Case 2

3.3.4 Reaction Force Z

A comparison of the maximum reaction moment Y shows that the vertical column at the center of Case 2 increased by 1.3 times (Figs. 17 and 18). This increase is primarily due to the X-direction pipe friction load, which rose as the piperack’s length doubled. However, the total reaction moment in Case 2 is identical to the doubled sum of Case 1. In other words, although local loads on the vertical columns may change, the overall load increases proportionally to the X-direction load.

Fig. 17.

Maximum reaction moment Y: Case 1

Fig. 18.

Maximum reaction moment Y: Case 2

4. Conclusion

This study conducted in-place analysis assuming that the length of the piperack, one of the FLNG topside modules, was doubled, and the results were compared to the original design. The piperack’s length was doubled by simply connecting two units, and other load conditions were exactly doubled accordingly. It was anticipated that the enlargement would have significant structural effects, but contrary to expectations, no major differences were observed. The findings suggest that reinforcing specific members alone can sufficiently mitigate structural issues, indicating that the risk associated with weight increase is minimal.

However, stress values in some vertical columns directly connected to the hull increased, and the stress in the braces at the bottom of the piperack also rose. As stress increases, so does the potential for fatigue failure. Therefore, it is essential to conduct a fatigue analysis in addition to the in-place analysis and compare the results. Furthermore, issues related to maintenance and repair should be identified and verified.

Piperack enlargement offers potential cost savings and shortened construction times by reducing the number of marine crane installations and the quantity of fittings (steel structure, piping, electrical components, and instruments). However, careful consideration is required, examining potential problems from other perspectives before applying this approach.

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