Intact stability analysis of crane barge due to loading orientation effect during heavy lifting operation

Changes in the position of the lifting load on the crane barge during heavy lifting operation significantly affect the stability of the vessel. The changes in stability during this heavy lifting operation must be analysed to prevent operational failures. This study conducted an intact stability analysis of the crane barge during lifting of topside structures in the installation phase. The analysis was performed with variations in the position of the lifted load from starboard to bow for every 15° change in the crane angle. The stability results of the Crane Barge for all variations, using both analytical and numerical methods, have met the IMO requirements. The extreme condition occurred in load case 8, with a maximum GZ value of 2.904 m at a 22° angle and an area of 103,894 m.deg. The comparison of the stability results of the crane barge during the structural lifting operation using analytical calculation and numerical methods indicates that the overall validation error values have met the ±5% requirement according to the ABS Rules. However, when considering the corrections for changes in the lifting load moment in the analytical calculation, stability error values increase significantly as the heeling angle of the vessel increases.


Introduction
Floating crane is a floating structure equipped with accommodation facilities and one or more cranes used for lifting operations at sea.Various activities in offshore areas, such as transportation, structure assembly, installation, and decommissioning, typically involve heavy lifting and are commonly performed using floating crane vessels [1].
Heavy lifting is generally defined as lifting operations with a load weight ranging from 60 to 60,000 metric tons, primarily for transportation purposes [2].Due to the significant weight and lifting moment of the load on the vessel, heavy lifting operations by a crane barge can significantly impact the stability of the vessel.According to the previous research, the mass load percentage to displacement ratio during lifting operations of 0.66 (% Δ) above the lifting boom of a crane vessel can reduce the GZ value by approximately 40% in calm water conditions [3].
In a more detailed research study by Australian Marine Safety Authority (AMSA), calculations were conducted on the transverse stability of fishing vessels under the combination of fishing gear, wind, and 1298 (2024) 012020 IOP Publishing doi:10.1088/1755-1315/1298/1/012020 2 waves.The moments caused by the fishing gear and load, in many cases, are more critical than those generated by the prevailing sea conditions.With the catch lifted above the lifting boom, the ship's center of gravity shifts to the upper and lateral sides, resulting in a smaller metacentric height (GM).Additionally, when the lifting boom extends outward, the load leverage induces a torque that tends to cause the vessel to heel to one side [4].This condition can be observed in Figure 1.Therefore, the safe and effective operation of a floating crane in the open sea becomes a crucial requirement, especially with the current increase in demand for offshore structure lifting operations using crane barges [5].The International Maritime Organization (IMO) mandates that specific vessels during operation must comply with the applicable stability requirements [6].This study aims to analyze the intact stability of a floating crane barge in a free-floating condition during offshore structure lifting operations.The research's objective is to determine the vessel's stability during the lifting of offshore structures for each loading direction variation using both analytical and numerical methods.In the end, the study aims to understand the stability conditions of the crane barge when subjected to various loadings of the lifting structures using both analytical and numerical approaches.

Topside Structure Lifting Installation
The operation conducted involves lifting the installation of the topside structure from an offshore platform.The lifting operation is carried out from the side of the ship, and subsequently, the crane is rotated until the structural load is positioned at the front side of the crane barge.Load case variations are determined based on the lifting operations of the topside structure.The following topside structure weight data presented in Table 1 and lifting configuration model as shown in Figure 2 is used in the study.The lifting operation's configuration involves an intact stability analysis for every 15 degrees of crane rotation, starting from the starboard position to the bow area of the crane barge.This operation loadcase condition can be seen in Figure 3.The percentage of the load is determined based on the booklet, while ballasting is determined based on the lifting operation's load variations, so the final equilibrium is achieved with trim and heel at approximately close to 0 degrees.The ballasting configuration is as follows: 97% in ballast tank No.

Crane Barge Intact Stability Criteria
In operational conditions, all loading scenarios must be calculated, and stability conditions must be verified according to the applicable regulations.The vessel must meet stability requirements to ensure safe operations.In this vessel's stability booklet, the IMO International Code on Intact Stability (2008) criteria is used, referring to MODU Code (2009) subsections 3.2 and 3.3, with stability requirements as follows: • For the surface and self-elevating unit categories, the area under the righting moment curve (  ) to the second intercept or downflooding, whichever is reached first, must not be less than 40% of the area under the wind heeling moment curve (  ).• For column-stabilized units, the area under the righting moment curve (  ) to the second intercept or downflooding, whichever is reached first, must not be less than 30% of the area under the wind heeling moment curve (  ).• The righting moment curve (GZ) must be positive throughout the range of angles from the upright condition to the second intercept.
The righting moment and wind heeling moment curves as shown in Figure 4, along with their supporting calculations, must cover the entire draft range of operational conditions, including transit conditions, while considering maximum loading at the most unfavorable position that can be applied.Additionally, the free surface effect of fluids in tanks must be taken into account.The wind heeling moment curve should be depicted for wind forces calculated using the formula as follows: Furthermore, wind forces should be considered from all directions relative to the unit, and the wind speed values should be : • Generally, a minimum wind speed of 36 m/s (70 knots) for offshore service should be used for normal operating conditions, and a minimum wind speed of 51.5 m/s (100 knots) should be used for extreme conditions.• If a unit will be restricted to operations in sheltered locations (inland waters such as lakes, bays, swamps, rivers, etc.), consideration should be given to reducing the wind speed to not less than 25.8 m/s (50 knots) for normal operating conditions.

Numerical Stability Analysis
The numerical simulations were carried out with Maxsurf Stability software.The surface model of the crane barge is modeled using Maxsurf Modeller software first, where the model is created based on the vessel's principal dimension presented in Table 2 and the general arrangement.Then, modeling of the tanks and compartments of the crane barge in Maxsurf Stability software is done regarding the General Arrangement (GA) and tank data from the Stability Booklet.The numerical model created using Maxsurf software has been validated against the hydrostatic data in the vessel's stability booklet to ensure alignment with the actual conditions.The error value limits are determined according to the ABS Rules for Building and Classing Mobile Offshore Units [7].The final numerical model for the vessel's hull, tanks, and compartments of the crane barge is presented in Figure 5 and subsequently utilized as input for further analysis of equilibrium and stability during operational conditions using Maxsurf Stability software.

Equilibrium and Hydrostatic
The total displacement of the crane barge, the load, and the total CoG are calculated according to equations (1)-( 4).

𝐷𝑖𝑠𝑝 = 𝐿𝑊𝑇 + 𝐷𝑊𝑇
(2) Where displacement () is the total weight of the ship, which is the sum of the lightship weight () and deadweight ().The values for the ship's center of gravity (, , ) are obtained by dividing the total longitudinal, vertical, and transversal moment by the total displacement value.Next, from the known displacement and total moment, the trim and draft values are calculated using the following formulas: / =   ±  *   (8) For the trim calculation, the values of LCB (Longitudinal Center of Buoyancy) and MTC (Midship Transverse Coefficient) are obtained by interpolating data from the stability booklet's even keel condition.Then, the KG (Vertical Center of Gravity) correction due to the free surface is calculated as follows: The value of the free surface moment () is the product of the tank's inertia (), obtained from the stability booklet, and the density of the fluid contained ().From this free surface moment, a correction for the vertical Center of Gravity (CoG) value is derived.Using the obtained CoG value along with the free surface, the ship's metacenter height (GM) can be calculated using equations ( 10) and (11).

Stability Analysis
With the initial stability data obtained from the stability booklet in the form of KN (Righting Arm Curve), the calculation of the restoring arm (GZ) from the KN data is done as follows: In the ideal condition, the lifted load is connected to the structure (crane pedestal) through a rope, making the position of the lifted load dynamic as shown in Figure 6, following the vessel's inclination.During heel conditions, the shifting of the load can cause a shift in the total CoG, and the TCG (Transverse Center of Gravity) position must be corrected as follows: The corrected TCG value is the sum of the initial TCG value and a correction factor, which is the product of the lifting load mass (  ) and the transverse arm shift (  × sin ) divided by the total displacement value.Figure 6.Lifting load on heel condition [8] This CoG shifting factor affects the final GZ value, so the calculation of the GZ value after the correction becomes: represents the final righting arm value with the correction for the change in lifting load moment.The value of  is the length from the keel to the vertical line when the ship heels,  is the vertical centre of gravity of the ship, and  is the transverse centre of gravity of the ship that has been corrected.

Equilibrium Condition
The equilibrium results for each loadcase condition with all loading directions using analytical and numerical methods have been obtained.Overall, the values of each hydrostatic property meet the planned operational conditions, as specified in the numerical and analytical equilibrium calculations.The designed equilibrium conditions are considered satisfactory, as the trim values range from 0 to 0.12 m, and the heeling values remain below 1 degree.From the analysis conducted, a graph illustrating the changes in trim and heel conditions during operational circumstances is presented in Figure 7.This finding is consistent with the research conducted by T. V. Tao [9], where the trim and heel angle of the vessel increase as the lifting load shifts to the transverse side of the vessel.Throughout the operation, with constant ballasting conditions and a load weighing 88 tons continuously shifting from the ship's starboard side to the bow will result in dynamic changes in trim and heel conditions during the operation.This needs to be specifically attended in order to prevent failures during the course of the operation.

Stability Analysis
The stability analysis results conducted using numerical and analytical methods with and without corrections are obtained.The results include the GZ values, wind heeling moment, and downflooding point (DF) of the vessel for heeling angles ranging from 0 to 70 degrees under each loadcase condition, and all loading directions are shown in Figure 8. -1.000 0.000 1.000The stability curve for all the loadcases in both the analytical and numerical methods is presented in Figure 9. From that graph, we can observe that the stability curve's value and area will decrease further as the load shifts to the ship's lateral side and the transverse arm of the lifting load increases.When the crane barge has the lowest stability value, the extreme condition occurs when the lifting load is positioned on the ship's side (portside).The extreme condition occurs in loadcase 8, with a maximum GZ value of 2.904 m at a 22° angle and an area of 103,894 m.deg using the numerical method.

IMO Criteria
The stability analysis must be verified against the stability parameters required by IMO, as specified in the booklet.The IMO criteria, specifically for the specific unit MODU category [6], apply to the overall results and are presented Table 3.This table shows the value of the wind heeling moment area, the comparison of the righting moment and heeling moment area, and the GZ value compared to the IMO criteria.

Analytical and Numerical Method Comparison
From the comparison of stability results of the crane barge during structural lifting operation using analytical and numerical methods, as shown in Table 4, it is known that the overall validation error values for analytical and numerical stability calculations are below 5%, meeting the requirement of ±5% according to the ABS Rules for Building and Classing Mobile Offshore Units [7].However, when considering the lifting moment correction in the analytical stability calculation, the error values increase significantly as the heeling angle of the vessel increases.It is because the correction of CoG and moment calculations for the lifted load led to a reduction in GZ value that cannot be validated using the Maxsurf numerical method.It can be observed in Table 5.From the research findings obtained, during heavy lifting operational conditions, the stability calculation results with correction for changes in lifting moment will yield a significant error compared to the results from analytical calculation using a fixed lifting load and the Maxsurf stability analysis.It can be observed in Table 5 for angles 50 degrees or above.It shows the potential for errors and the ineffectiveness of the Maxsurf Stability Software when used for heavy lifting analysis on ships or in conditions involving significant variable of lifted loads on the ship.The Maxsurf Stability Software cannot model conditions of variable loads that can dynamically change positions and input them as loads with fixed positions.

Conclusion
Based on the intact stability research on crane barge during the installation phase of lifting the topside structure, it is concluded that the stability of the Crane Barge for all variations of the rotational angle in the structural lifting operation, using both analytical and numerical methods, complies with IMO A.

Figure 2 .
Figure 2. Topside structure model and lifting configuration.

Figure 9 .
Figure 9. Stability curve for every method used.

Table 1 .
Summary weight data of the topside structure.

Table 4 .
Analytical method stability error validation.

Table 5 .
Analytical method stability with correction error validation.
749 Intact Stability Code (IS Code) 4.6 MODU with extreme condition occurs in loadcase 8, with a maximum GZ value of 2.904 m at a 22° angle and an area of 103,894 m.deg.For stability calculation