Fatigue Analysis of Pedestal-mounted Crane on Offshore Fixed Platform Using Finite Element Method

The crane is one of the production support facilities on the offshore platform. During operation, it receives a large load, both from the environment and the crane’s operational load, so the crane’s strength and operational safety need to be maintained. Cyclic loading can cause the crane structure to experience fatigue. This evidence shows the importance of analyzing the crane pedestal. How much the load affects the fatigue life of the structure. A previous study on the crane loading effect on the Floating Production Storage and Ofﬂoading (FPSO) was provided as a comparison. Local fatigue analysis was performed on the pedestal crane using the Finite Element Method (MEH) and ANSYS Workbench 19.1 software. The validation stage was carried out by comparing the value of the reaction force, moment, and maximum equivalent stress (Von-Mises) on the crane pedestal between the simulation results and hand-calculation. Furthermore, a static simulation was carried out to obtain the stress value as the basis for cyclic loading. A dynamic simulation was carried out based on the stress obtained from the static simulation to determine the structure’s critical point and fatigue life. The fatigue analysis was carried out based on the Palmgren-Miner Theory. It is found that the load that has the most signiﬁcant inﬂuence on crane pedestal fatigue is the operational load with a contribution of 80.7%, wind load of 19.3%, and wave load of 0%. The minimum estimated fatigue life of the crane pedestal structure is 96.5 years or close to 5 times the design life. Compared to the crane in the FPSO in previous studies, the most signiﬁcant difference lies in the contribution of wave loads, where the wave load on the FPSO has the most considerable contribution to crane structure fatigue (97.8%).


Introduction
Offshore structures have developed a lot with different development objectives [1]. One of the offshore structures currently operating in Indonesia is a wellhead type fixed offshore platform in the Madura Strait, which is operated by Husky-CNOOC Madura Limited (HCML). This platform produces crude oil using wells as the outlet for crude oil and gas and injection wells to inject water. This pavilion is located offshore in the Madura Strait, East Java. The platform was installed at a depth of about 316.4 feet measured from the mean sea level.
In operation, fixed offshore platforms are subject to significant loads from the surrounding environment. Therefore, the fatigue analysis of offshore buildings and the supporting equipment components therein is essential to be carried out. The crane that is used as a material transfer tool is one of the facilities on the offshore platform that has a significant enough chance of experiencing fatigue, so it is necessary to maintain its structural strength so that it is safe during operation. The crane used on the platform can be seen in Figure 1.
In this study, a crane pedestal fatigue due to environmental and operational loads on a fixed offshore platform was carried out and the influence of each load on the structure's fatigue life. In addition, the effect of loading on the fatigue life of offshore platform cranes is also compared to that of previously studied FPSO cranes.
Structural analysis was carried out on the crane pedestal structure with the help of ANSYS Workbench 19.1 software based on FEM [2] and Palmgren-Miner Theory for the calculation of fatigue life.

Structural Model
The specifications and configurations of the analyzed crane pedestal cylinders [3], can be seen in Table 1 and  Table 2. Figure 2 shows the pedestal crane configuration. The crane and the beam on the deck that acts as a clamp for the crane pedestal are modeled as in Figure 3. Only pedestal and top ring modeling were used, ignoring the five existing beams. The boundary condition given to the object being analyzed was fixed support on the top of the crane pedestal's top ring. The boundary conditions in modeling can be seen in Figure 4.

Material
Steel and other structural materials were determined according to standard Structural Materials Specifications or from the data contained in the relevant drawing [3,4]. The properties of the materials used for the design are shown in Table 3 and Table 4.

Number of Loading Cycles Allowable Stress Range (MPa) From
To 20,000 100,000 193 100,000 500,000 110 500,000 2,000,000 69 Over 2,000,000 48  The convergence test was done by comparing the meshing results using the span angle coarse and medium. After the results of each test were obtained, the two were compared by sorting the number of nodes. Then, the meshing with the smallest number of nodes was selected at the point where the object's equivalent stress began to converge. The method used is Hex Dominant with Adaptive size function and Fine relevance center. The results of the tests can be seen in Figure 5. The final meshing method used medium span angle and element size of 50 mm, with the total number of nodes of 53,607 or total number of cells of 10,236 and the average mesh metrics was 0.68039. Calculation of the effect of cumulative damage on platforms using the S-N curve for tubular joints [5], regardless of the corrosion effects.

Loading Condition
The input of wind loading in ANSYS Workbench 19.1 software is the axial force due to self-weight, wind force, and moment due to wind force. Input to the software can be seen in Figure 6. The wind loading conditions with variations in wind speed based on crane pedestal elevation and boom is obtained as in Table 5.
The operational loading input on ANSYS Workbench 19.1 software is an axial force due to self-weight and moment due to crane lifting force. The input parameter to the software can be seen in Figure 7. The operational loading conditions with variations in the working radius of the boom in conditions without and with crane lifting loads is shown in Table 6.

Wave Load
In operation, the platform is subjected to loads from sea waves. Several design criteria must be met to ensure the platform's structural integrity and serviceability, such as local and global deflections in the horizontal and vertical directions [3], as shown in Table 7. For the relevance of this study, the criterion that needs to be considered is the global deflection of the platform to the horizontal on the leg.
When a crane is placed on a mobile structure, it can experience inclination. The movement of the structure where the crane is anchored causes the crane base to experience two rotational motion modes, namely heel, and trim. Both of these movements cause a moment of inertia that acts on the crane base and can be calculated using the equation M = I × α, where α is the amount of acceleration at the crane base. The two modes of rotational motion are illustrated as in Figure 8.
Analysis of simulation results with the help of offshore structure analysis software (SACS) conducted by the company shows that the maximum deflection that occurs due to working loads, including environmental loads such as ocean waves, is 4.07" (= 102.16mm) [3]. The deflection that occurred in the B2 leg of the bridge resulted in a vertical inclination of 0.054. The deflection is described in Figure 9.
Meanwhile, according to API, the value of horizontal acceleration due to crane inclination varies depending on the placement of the crane. Horizontal crane acceleration as in [5], can be seen in Table 8. The platform is fixed with an inclination angle that does not exceed 0.5 • in the heel and trim direction, accelerated by 0.0 ft/s 2 , as required by API-2C. Thus, it can be concluded that there is no moment of inertia that occurs on the crane base due to the movement of the platform. This indicates that the wave load results in the possibility of movement on the offshore-fixed platform and the contribution of the loading to the reduction of the platform's jacket leg fatigue life but does not have a significant effect on the crane pedestal fatigue life. So that in this study, the simulated loading was only wind and operational loads.

Component
Deflection Limit

Legs/columns
The maximum horizontal/lateral deflection from the mudline to the top of the platform structure is the maximum sway with a height of H/200.

Crane Pedestal
The maximum tilt angle is not more than 0.5 • .

Wind Load
Static analysis was carried out by finding the Equivalent (Von-Mises) stress value at each wind speed according to load conditions 9 to 13, where one wind speed at four different elevations is considered to occur at the same time. An example of static simulation results with the greatest Equivalent (Von-Mises) stress value, namely at the highest wind speed (17.3 m/s), can be seen in Figure  10. The static simulation results at each wind speed are presented in Table 9.
Furthermore, a dynamic simulation was carried out to find the value of life and damage with a predetermined cycle according to the Table. The type of loading used is Zero-Based, and can be seen in Figure 11. An example of dynamic simulation results with the largest damage value and the smallest fatigue life, namely at a wind speed of 8 m/s, can be seen in Figure 12. The dynamic simulation results at each wind speed are presented in Table 10. Dynamic calculations showed that the total damage ratio due to crane wind loads at five different wind speeds is 0.039944.

Operational Load
In a static simulation of operational loading, a minimum and maximum stress value are required from two different loading conditions, namely maximum equivalent (Von-Mises) stress when the crane is not loaded as minimum stress, and maximum equivalent (Von-Mises) stress when the crane lifts the load of 10 Tons as the maximum stress, because the type of loading used is ratio.
The maximum equivalent (Von-Mises) value of the crane without load based on the static simulation results can be seen in Figure 13. Whereas an example of static simulation results with the largest Maximum Equivalent (Von-Mises) value, namely when lifting a maximum load of 10 tons with the largest radius of 13.7 m, is shown in Figure 14. The static simulation results for each radius are presented in Table 11. Furthermore, a dynamic simulation was carried out to find the value of life and damage with a predetermined cycle according to Table 10. The type of loading used is the ratio. The type of loading ratio (R = 0.8297) on the Fatigue Tool can be seen in Figure 15.
This ratio is then used in dynamic simulations to calculate life and damage using the crane loading conditions to lift the maximum load. Each ratio was used under four loading conditions with different radii, according to the minimum and maximum stresses in the radius. An example of dynamic simulation results with the smallest life value and the most significant damage, namely at the largest radius (13.7 m), can be seen in Figure 16. The dynamic simulation results for each radius are presented in Table 12. The dynamic simulation results showed that the total ratio of cumulative damage due to crane operating loads at four different radii is 0.16722.

Total Fatigue Analysis
After obtaining the damage ratio due to each wave, wind, and operational loading, the three damage ratios were added to get the total cumulative damage ratio. Then from the damage ratio, the fatigue life and safety factor were obtained.
The results of the total fatigue analysis are presented in Table 13. The fixed platform has a design life of 20 years. Based on API RP2A, the platform's design life taking into account the safety factor 5.0 is 100 years. Based on the calculation results obtained in this study, the crane is feasible to operate because it has a fatigue life approaching the fatigue life of a fixed platform, which is 96.54 years.

Contribution of Loading
The effect of each load on the fatigue life of the crane pedestal is known from the analysis. The magnitude of the effect of the load is presented in Table 14. Based on the simulation results, it was found that the fatigue that occurs in the crane pedestal structure due to cyclic loads are most influenced by operational loads, which is 80.7%, followed by wind loads of 19.3%, and wave loads have no significant effect on the fatigue of the crane pedestal structure at fixed platform. Table 15 shows the comparison of data and loading analysis between crane manufacturing design limitations on a fixed platform as the object of the current study, actual crane loading, and previous research with crane research objects in Floating Production, Storage, and Offloading (FPSO).

Comparison of Fatigue Analysis Results
While the comparison of the results of the analysis between the crane on a fixed platform as the object of the current study with the crane at the FPSO as the object of previous research is presented in Table 16.
The most significant difference between the results of the analysis between the current study and previous research lies in wave loads contribution. The last study shows that the wave load on the FPSO had the most significant contribution to the crane structure fatigue (97.8%). In wind and operational loading, the difference between the previous research and this study is due to the difference in wind speed in the fixed platform area and operating FPSO, the size and weight of each crane, and the lifting load and the number of cycles.

Conclusion
Based on the analysis that has been done, the following conclusions can be drawn. Loads that affect the fatigue life of crane pedestals on fixed platforms are sorted from those that have the most significant influence is operating loads and wind loads. The wave load only affects the fixed platform, but the movement that occurs on the platform does not significantly affect the fatigue of the crane pedestal structure.
The contribution of the effect of each loading to the fatigue that occurs in the crane pedestal structure is the operational load of 80.7%, the wind load of 19.3%, and the wave load of 0%. The minimum estimated fatigue life of the crane pedestal structure is 96.54 years or close to 5 times the design life, under the fixed platform's design with the safety factor (SF) = 5.0.