Tanker and SPM Motion Response Analysis due to Environmental Load

The use of tankers for oil and gas distribution requires auxiliary structures such as Single Point Mooring (SPM). In the process of loading cargo, tankers must maintain a level of stability so that the load transfer process runs smoothly. Stability needs to be checked using inputs in the form of structural responses in free-floating conditions to get structural responses when moored. Analysis of the motion response of tankers and SPM in free-floating conditions showed that ballast tanker loads had a greater motion response than full loads with the largest movement on roll at 3.59 deg/m. The moored motion response of the tanker has the greatest amplitude in the surge movement towards the AP with a value of -0.2251 m.


Introduction
Indonesia is known as a country rich in valuable natural resources, especially oil and natural gas.These resources continue to be exploited to meet basic oil and gas needs.In offshore oil and gas extraction, there are two main types of facilities, namely fixed and floating structures.Apart from that, the construction of offshore platforms can be categorized into several types [1] [2], namely: 1) Floating structures: semi-submersibles, drilling vessels, tension leg platforms (TLP), and jack-ups.2) Fixed structures: jacket platform, concrete gravity, and tripod.3) Flexible or suitable structures, namely articulated and shielded towers.
Floating structures have extraordinary adaptability and are more efficient and effective, so they are suitable for operation in deeper waters compared to fixed structures [3].However, floating structures are vulnerable to waves, which cause movement that can disrupt the comfort of the working crew's operations.Waves will cause dynamic responses in floating structures [4][5][6][7][8][9][10][11][12][13][14][15], so these movements need to be predicted early in the design.
Floating structures use Floating Storage and Offloading (FSO) or storage tankers as the primary means of transportation for oil and gas distribution.Among other things, shuttle tankers are essential in transporting oil to refineries or sending refined oil products for further distribution.This method of transportation offers greater flexibility and allows efficient oil distribution to different regions.To facilitate cargo transfer from shuttle tankers, a mooring system known as Single Point Mooring (SPM) can be used, simplifying and improving the transfer process.
SPM is an offshore floating structure designed as a mooring and interconnection point for loading and unloading gas and liquid products from tankers.Its effectiveness in the oil mining process makes it a component that is widely used in conjunction with FSOs or tanker units [16].Overall, these structures 1298 (2024) 012005 IOP Publishing doi:10.1088/1755-1315/1298/1/012005 2 are essential in facilitating efficient and smooth operations in the oil mining industry.Stability analysis of tankers during loading and unloading via SPM must be carried out to ensure the operation process runs smoothly.The analysis was carried out to obtain the response characteristics of the tanker ship and SPM motion, which then became the input for analysis of the tanker ship's motion response when moored at the SPM.

Study of Literature and Data Collection
Study and collection of literature as a reference or study material and theoretical sources needed in research.The data used in the studies are as follows: 1) Shuttle tanker and SPM principal dimensions 2) Material properties of mooring lines and mooring hawsers used.
3) Environmental data is in the form of wind, waves, and currents.

Shuttle Tanker and SPM Modelling
The modeling of shuttle tankers and SPM is supported by response motion basis software, utilizing principal dimension data provided in Table 1

Model Validation
The modeled structure's validation will follow the ABS Rules for Building and Classing Mobile Offshore Units (MOU).

Tanker and SPM Motion Response Analysis
Upon successfully meeting the validation criteria, motion response analysis is performed as a Response Amplitude Operator (RAO) to facilitate graphic processing and interpretation.Mooring system modeling is done with mooring system analysis-based software.The tanker load condition scenario is applied to the analysis, which will be described in the next chapter.After the modeling is complete, proceed with analyzing the motion response of the tanker when tethered to the SPM.

Model validation
Validation involves comparing the model's results obtained from the software with existing data.By the ABS Rules for Building and Classing MOU [18], validation is conducted with a maximum allowable error of 2% for displacement and 1% for other parameters.The validation results for the tanker and SPM can be observed in Table 3 and Table 4, respectively.Because the validation results have been met, it can be continued in the following analysis.

Structure Motion Response Free Floating Condition
The structural analysis of the shuttle tanker and SPM in regular waves is performed using response motion-based software.The analysis is conducted under two tanker conditions: full load and ballast load.For the tanker, simulations are conducted with headings at 0˚, 45˚, 90˚, 135˚ and 180˚.However, due to the symmetrical shape, the SPM simulations for the SPM are carried out only at headings of 0˚, 45˚, and 90˚.Table 5 presents the maximum RAO values for each tanker's six degrees of freedom under full load conditions.On the other hand, Table 6 shows the maximum RAO values for each of the tanker's six degrees of freedom movements under ballast load conditions.Based on the analysis results under full load conditions, the highest value obtained in translational movement is in the heave motion at a heading of 90˚, with a value of 1.44 m/m.Regarding rotational movement, the roll motion at a heading of 90˚ has the highest value, measuring 1.85 deg/m.Under ballast load conditions, the highest value in translational movement is observed in the heave motion at a heading of 90˚, measuring 1.17 m/m.This value is smaller than the corresponding value in the fullload condition.However, in rotational movement, the roll motion at a heading of 90˚ exhibits the highest value, measuring 3.59 deg/m.This value is greater than the roll motion value in the full-load condition by 52%.
Despite the roll motion value increase in ballast load condition, it is relatively small compared to the ship's dimensions.These results indicate that the tanker possesses typical seakeeping characteristics.The RAO value for the primary vertical motion mode is not excessively high, which suggests that the ship has an excellent damping ability and stiffness factor, contributing to its stable behavior during motion at sea.The SPM analysis revealed that the highest value obtained for translational movement is in surge at 0˚ heading and sway at 90˚ heading, with a value of 1.658 m/m. on the other hand, in rotational movement, the highest value is observed in pitch motion at 0˚ heading, with a value of 3.323 deg/m.The higher response values for the SPM are attributed to its small geometry and free-floating condition.However, in real-world applications, this issue has been effectively addressed by implementing mooring lines capable of withstanding the SPM's movements.The SPM's response can be significantly reduced by employing these mooring lines, leading to improved stability and functionality.

Mooring System Modelling Scenario
After obtaining a structural response in free-floating conditions, motion analysis will continue when the structure is moored.The analysis utilizes various input data, including the response of motion structures, such as displacement RAO, load RAO, wave drift, hydrodynamic drag, wind drag, stiffness of the structures, added mass, and damping.Additionally, the material properties of the moorings used in this case, the hawser used in this analysis is Double Braided mooring hawser with 18" Circ: 70 m length and 759-ton minimum breaking load.The chain studless mooring R3 type with a 4-inch diameter has the Minimum Breaking Load of 1.960 kips selected for the mooring line.
Moreover, the analysis requires essential environmental data, which includes sea depth, wave spectrum, wind speed, current speed, and their respective headings.All this environmental data is presented in Table 8 for reference during analysis.In this analysis, the wind speed considered is the 3-second gusts, which allows for capturing the maximum wind load value.Additionally, the environmental load applied in the analysis is collinear, meaning that it acts in the same direction.The environmental load includes the combined effects of wave, current, and wind load.This load scenario is applied at a sea depth of 35 meters.Due to the collinearity and alignment of the environmental load, it results in the highest impact on the structures.The combined effect of these forces amplifies their impact, making them the dominant factor affecting the structural response in this analysis.The analysis is performed using a time domain analysis, enabling the modeling of a non-linear matrix system and accurately representing all external loads.The analysis encompasses both full load and ballast load conditions for the tanker.Full-load is when the tanker has 100% of the cargo, while ballast IOP Publishing doi:10.1088/1755-1315/1298/1/0120056 load is when the tanker has 20%.The configuration used in the analysis is an In-Line configuration where the tanker is aligned with one of the mooring lines on the SPM.Based on the ABS Guide for Position Mooring System, the minimum duration for time domain analysis is 3 hours (10800 s) [19].A cut-off method is applied, involving a selection of a 200-second time frame during which peak waves occur.It allows for the extraction of the maximum motion response of the structures.

Tanker Motion Response Moored to The SPM
From the analysis, the distribution of motion responses in the structure during the 2000s is produced in Figures 3 and 4, and Table 9 shows the maximum value of movements that occur in full-load and ballastload tankers, respectively, as follows.It can be seen in the table above that in the surge movement of ballast conditions and the resulting value is more significant than in full conditions.It is due to the cargo from the ship being lighter, with the damping and stiffness of the ship getting smaller, so the environmental load has a more significant impact.In movements other than surge, the resulting movement value is minimal due to the heading of the environmental load.

1.M otion response free-floating condition
In a tanker, the maximum response occurs under ballast load conditions with the value for translational motion being 1.165 m/m in heave motion.In comparison, the maximum rotational motion occurs in roll movement with a value of 3.592 deg / m.In SPM, the maximum translational motion response is 1.658 m/m in surge and sway motion.At the same time, the maximum rotational motion is 3.323 deg/m in pitch motion.

2.M otion response moored condition
In full-load tankers, the farthest amplitude in translational motion is surge movement with a value of -0.1538 m towards the after perpendicular (AP) tanker.In rotational motion, the farthest amplitude of the tanker occurs in pitch movement with a value of 0.0065 degrees when the tanker's bow is going up.For ballast load conditions, the farthest amplitude of translational motion occurs in the surge movement with a value of -0.2251 m towards the AP direction of the tanker.For maximum rotational motion, the farthest amplitude occurs in pitch movement with a value of 0.0224 degrees when the tanker's bow goes up.

Figure 1
Figure 1 displays the outcomes of the Elizabeth I. A. shuttle tanker in a lines plan model from body plan, breadth plan, and sheer plan.Figure 2 displays the SPM model from the top, side, and isometric view.

Table 5 .
Maximum RAO full load condition.

Table 6 .
Maximum RAO ballast load condition.

Table 7 .
Table 7 displays the maximum RAO values for each SPM's six degrees of freedom movement.Maximum RAO on SPM.