Numerical Studies Fatigue Damage From Material Variations Catenary Mooring System FPSO Al

When operating, the FPSO receives environmental loads in the form of currents, wind and waves so mooring ropes are required (mooring line). Mooring line is a way to dampen the movement of the Al Zaafarana FPSO which allows it to move freely in the direction of the load. However, several cases of mooring rope accidents occurred including material fatigue of 20%, deployment 17%, and corrosion 11%. So the aim of this research is to analyze motion trajectory (surge and sway), mooring line stress, and deterministic fatigue on mooring line so you get it fatigue damage material catenary mooring line for FPSO Al Zaafarana. Thus, it is hoped that this research can make a scientific contribution in improving the quality and safety of materials mooring line for the Al Zaafarana FPSO, as well as increasing the efficiency of the oil drilling process. This research method uses a numerical study approach as a secondary data source with modeling and simulation using software as well as literature analysis that will support the numerical simulation results. From the results of numerical studies obtained fatigue damage The highest is the material type chain for FPSO Al Zaafarana. One of the reasons is that the mass of the rope is not proportional to the displacement of the ship, so a larger mass of rope is needed or an increase in the number of ropes. In addition, the numerical study simulation only uses 100 seconds so that the movement process only reaches movement transient namely temporary changes that occur in the transition phase before reaching a stable condition.


Introduction
The Ministry of Energy and Mineral Resources stated that Indonesia's oil and gas exploration was shifting from onshore (mainland) to offshore (off/deep sea) due to decreased production results due to the old age of the well.Moreover, around 70% of oil and gas reserves are in waters.In this case it is used floating platform in the form of an FPSO (Floating Production Storage and Offloading) Al Zaafarana because of its advantages, namely good operations in deep sea drilling, receiving and processing oil and gas from wells or from fixed platform, and economical because the FPSO can be used at another location when production time has run out [1].When operating, the FPSO receives environmental loads in the form of currents, wind and waves so mooring ropes are required (mooring line) [2].
Mooring line is a way to reduce the movement of the Al Zaafarana FPSO which allows it to move freely in the direction of environmental loads and mooring line help the process weathervaning so that the operation can be carried out safely.But in reality it failed mooring system quite high, Drori (2015) noted that from 1997 to 2012, there were 107 mooring rope accidents at 73 facilities from all industries.According to a survey conducted by Carra et al (2015), several causes of mooring rope accidents, including material fatigue 20%,deployment 17%, and corrosion 11% [3].
To maintain the operational reliability of the Al Zaafarana FPSO and reduce the risk of accidents in the deep sea, fatigue damage material mooring line is very important because it experiences dynamic loads continuously, causing the material to become fatigued which can reduce service life and increase the risk of operational failure [4].
Therefore, knowledge of types is required fatigue damage material mooring line for FPSO Al Zaafarana.So the aim of this research is to analyse motion trajectory (surge and sway), mooring line stress, and deterministic fatigue on mooring line so you get it fatigue damage material catenary mooring line for FPSO Al Zaafarana.Thus, it is hoped that this research can make a scientific contribution in improving the quality and safety of materials mooring line for the Al Zaafarana FPSO, as well as increasing the efficiency of the oil drilling process.

Research Methods
The research method used is a numerical study approach as a secondary data source with modeling and simulation using software.The first step is to choose 5 types of material mooring line for the Al Zaafarana FPSO, which was modeled using software to determine the material response to various dynamic load conditions.Apart from the numerical study approach, literature analysis was also carried out by accessing relevant library documents, scientific articles, books and research reports.This literature study will support the results of numerical simulations.This combination of methods will make it easier to determine the best material for the Al Zaafarana FPSO, with the aim of increasing the operational reliability of the FPSO and reducing the risk of failure.

Fig. 1. FPSO Al Zaafarana
The Al Zafarana FPSO is one of the FPSOs used in the exploitation of oil and gas in deep waters, so maintaining its operational reliability is very important.FPSO Al Zaafarana is located off the coast of Egypt in the Red Sea and is operated by Gemsa Petroleum Company (Gempetco).The dimensions of the Al Zaafarana FPSO are in table 1.The type of mooring FPSO Al Zaafarana uses catenary mooring system widely used in shallow water to deep water where the system catenary influenced by horizontal forces.Catenary moored at both ends, one on the seabed and the other at FPSO Al Zaafarana.This is due to the weight of the rope, which causes the stretch of the floating rope from the floating structure (FPSO) to the anchor (seabed) not tense but tense.In this research, focusing on the type of material catenary mooring system then the number of mooring ropes used is 4 ropes with an angle of 45 0 and a rope length of 3000 m at a depth of 1000 m as shown in Figure 2.  On FPSO Al Zaafarana need coordinates fixed point namely the coordinates of the mooring lines on the ship and connection point namely the coordinates of the mooring ropes on the seabed for each cable in the x, y and z directions as in table 2. In addition, 5 types of materials are used in table 3 to find out the best type of material to be used by the Al Zaafarana FPSO based on operational material reliability.

Analysis Response Amplitude Operator (RAO)
The movement of a structure on regular waves is known as Response Amplitude Operator (RAO).RAO movement response to translational movement viz surge and sway is the ratio between the amplitude of the incident wave and the amplitude.Response Amplitude Operator as a response function that occurs when waves hit a structure in a frequency range that functions to transfer external loads in the form of a dynamic response of the structure [5].

Motion Trajectory
In research Degree of Freedom which is used is surge and sway.Surge is the movement of the ship on the X-axis translation, namely movement along the longitudinal axis of the ship where the ship moves forward (forward) or backward (backwards).Where as, sway is the movement of the ship on the Y-axis translation, namely the lateral movement of the ship where the ship moves to the side (left or right) [6] as seen in Figure 3.  Based on the table above it can be seen that the material wire segment have movement surge and sway largest while material chain on fairlead and chain on anchor have movement surge and sway smallest.This is influenced by the parameters in table 3, namely the mass of the rope (the greater the mass of the mooring rope, the greater the inertial force produced by the mooring rope when the ship is moving), rope diameter (the greater the diameter of the mooring rope, the stronger and stiffer the rope.the mooring), the cross-sectional area of the rope (the larger the cross-sectional area of the mooring rope, the stronger and stiffer the mooring rope), the stiffness of the rope (a mooring rope that is too stiff can limit the movement of the ship and require a more elastic mooring rope), and the tension of the rope (rope Moorings that are too tight can limit the movement of the vessel and require looser mooring lines).

Mooring Line Stress
Mooring line stress caused by an external force which causes a pull on the mooring rope [8].Stress is divided into two types, namely static stress and dynamic stress.Static tension is the weight of the mooring rope itself and the wet weight of the mooring rope.Meanwhile, dynamic stress arises as a result of dynamic loads (wind, current and waves).Additionally, resistance to fatigue and changing environmental conditions is another thing to consider when analyzing mooring rope tension.Therefore, to prevent failure, it is necessary to select the right material to ensure safety, stability and reliability during the mooring process.So, the calculation is carried out mooring line stress for each material to get the best material used by FPSO Al Zaafarana.

Hot Spot Stress (HSS)
Hot spot stress used to identify critical areas on mooring ropes that experience high stress or high fatigue.This area is the starting point for fatigue which can cause the mooring rope to break.Calculation hot spot stress mooring ropes help in proper material optimization.In calculation shot spot stress used data cable force at time 0 seconds to 100 seconds from the simulation results for each material and the diameter of each material is also needed to get the cross-sectional area of each material which is presented in table 9.
Where : F : pulling force (N) A : cross-sectional area (m 2 ) From the calculation result shot spot stress from 0 seconds to 100 seconds on 4 strings of each material, a graph is obtained hot spot stress for each type of mooring rope material as in Figure 5 above.Next, look for the maximum value for each mooring rope in 5 types of material which can be seen in table 10.

Table 10. Maximum Value Mooring Line Stress
In mooring rope fatigue analysis, stress range (S) is an important parameter that gives an idea of how much stress variation the material will experience during a particular load cycle.Therefore, calculations need to be made stress range to reduce the risk of mooring rope fatigue.Data used in calculations stress range is value mooring line stress in table 10 and the DAF values that have been obtained.

Fatigue Damage
Fatigue damage is the main factor in reducing the service life of mooring ropes.Where fatigue damage is damage that occurs to the mooring rope material as a result of repeated stress that continuously occurs so that the material will experience fatigue [9].Cyclic stress changes cause small cracks in the mooring rope material that can develop into more severe damage if not repaired.To ensure that mooring ropes operate safely and reliably, calculations are made fatigue damage very important.Following are the calculations fatigue life for 5 types of material.

Discussion
From the results of numerical studies obtained fatigue damage the highest is chain for FPSO Al Zaafarana.One of the reasons is that the mass of the rope is not proportional to the displacement of the ship, so a larger mass of rope is needed or an increase in the number of ropes.In addition, the numerical study simulation only uses 100 seconds so that the movement process only reaches movement transient namely temporary changes that occur in the transition phase before reaching a stable condition.It is hoped that the next researcher will carry out a numerical study simulation for 10,800 seconds or until movement is reached steady which is a stable state [10].

Conclusions
In operational processes catenary mooring system The Al Zaafarana FPSO experiences static loads and dynamic loads continuously, causing material damage which can reduce service life and increase the risk of operational failure.After carrying out numerical studies on several types of materials with calculations motion trajectory (surge and sway), mooring line stress, and deterministic fatigue on mooring line so you get it fatigue damage the highest is chain for the FPSO Al Zaafarana which is caused by the mass of the rope not being proportional to the ship's displacement and the numerical study simulation has only achieved the movement transient at 100 seconds so it has not yet reached a stable condition.

Table 3 .
Material Catenary Mooring System

Table 5 .
Maximum and Minimum Values Motion Trajectory

Table 6 .
Masela Block Wave Distribution Data

Table 7 .
Masela Block Wind and Current Data

Table 8 .
DAF Calculation Data

Table 12 .
S-N Fatigue Curve

Table 13 .
Number of Wave Events EachStress Range (S)