Fatigue Life Analysis of Mooring System: The Effect of Asymmetry Mooring System Configuration on Single Point Mooring

This journal explains the fatigue life analysis of anchor chain in the stand-alone single point mooring by comparing the 4x1 asymmetric and symmetrical mooring system configurations to determine whether the effect of the asymmetrical mooring system configuration on the fatigue life of the anchor chain is related to the length of the mooring lines, the pretensions of the mooring lines, and the angle of spread. The analysis was reviewed on the condition of ULS and FLS environmental loading based on API RP 2 SK code using Orcaflex with 3 hours of time- domain simulation. In the ULS condition, the symmetrical configuration can withstand environmental loads better in the direction of 0o and 180o with the generated maximum tension and maximum offset value smaller than the result from asymmetrical configuration. While the asymmetrical configuration can withstand environmental loads better in the direction of 90o and 270o with the generated maximum tension and maximum offset value smaller than the result from symmetrical configuration. In the FLS condition, the asymmetrical configuration has a longer minimum fatigue life and design life of anchor chain than the symmetrical configuration. This happens because of the spread angle of the mooring line, the length of the mooring line, and the pretension of the mooring lin

Fatigue Life Comparison of Modified and Conventional 3 Leg Jacket Offshore Structure

The jacket structure must be adapted to the conditions of the production field to support economic factors. So, the concept of a modular platform for minimal, low-cost facilities is adopted. However, the design differences will affect the performance of the jacket itself, in other words a modular jacket can withstand the same load as a conventional jacket model but has a different structural performance. Therefore, this research discusses the performance comparison, which includes the fatigue life and the natural period, between conventional and modular jacket structures, which in this study are referred to as modified jackets. Conventional jacket as a comparison structure takes the design basis of the modified structure, including the same structural profiles, and environmental loads. In this study, the two jackets will only be modeled on the jacket part and the superstructure will be modeled as a joint load on the three upper ends of the jacket legs. Fatigue life analysis in this study used the full spectral analysis method. By using SACS software, the natural period of modified jacket is 1.756 s and conventional jacket is 1.472 s. While the lowest fatigue life on modified jacket is 44.98 years and conventional jacket is 9125.79 years

Analisis Dampak Scouring Pada Integritas Jacket Structure Dengan Pendekatan Statis Berbasis Keandalan

Integritas jacket structure sangat terpengaruh dengan kondisi lingkungan yang dapat menimbulkan masalah tertentu, seperti masalah scouring pada jacket. Jacket APN-A adalah salah satu platform yang mengalami scouring dengan rentan masalah yang cukup dikatakan paling parah. Analisa In-place yang harus dilakukan untuk mengetahui kekuatan struktur dengan masalah scouring adalah dengan menghitung unity check pada member dan safety factor pada bearing capacity. Variasi skenario yang dilakukan untuk menganalisa kekuatan jacket APN-A adalah dengan menvariasikan kedalaman scouring hingga 5 meter dan dengan menvariasikan penambahan beban dead load dan live load hingga 30% dari beban awal. Hasil analisa didapatkan bahwa Jacket APN-A tidak terjadi tegangan yang melebihi batas ketentuan ketika mengalami skenario scouring hingga 5 meter, akan tetapi mulai terjadi tegangan yang melebihi batas ketentuan ketika mengalami penambahan beban 10% yaitu pada member 3 yang termasuk Jacket Brace dengan angka mencapai 1,0031. Bearing capacity pada Jacket APN-A, hasil analisa menunjukkan tidak ada yang melebihi kapaitas Safety Factor yang diijinkan ketika mengalami skenario scouring hingga 5 meter, akan tetapi pondasi tidak memenuhi syarat ketika mengalami skenario penambahan beban hingga 20% dari beban awal dengan angka mencapai 1,7. Keandalan dari platform APN-A mengalami penurunan yang cukup siknifikan ketika mengalami penambahan dead load dan live load

Ultimate Strength of the Buoy Structure due to the Loads Arise from the Tanker and Mooring Lines

A study was conducted to evaluate the ultimate strength of a 40 ton buoy induced by the dynamic loads arising from a 120,000 DWT Aframax tanker and mooring lines. The buoy is operated at Bangka Strait offshore oil terminal. The eva­luation was commen­ced by analyzing the motion characteristics of the buoy and tanker due to environmental excitations, both in free floating conditions. This is continued by the simulation and time-domain analysis of con­nected buoy and tanker to observe the hawsers and mooring line tension intensities. The correspon­ding results show the largest tension occurs in the in-line configu­ration with the tanker in ballast condi­tion, where hawsers tension reaches 1282.58 kN with a safety factor of 2.23 and mooring line tension 1974.18 kN with a safety factor of 3.20. The resulting tensions were further applied as input data for structural modeling using FEM to find out the stresses develop on the buoy structure. Results of this modeling reveal the maximum value of stress experienced by the buoy structure is approaching 184.28 MPa, which is below allowable stress of 200 MPa. Following this, the ultimate stress of 450 MPa will be violated by 143% incremental load above the maximum, namely 3,116.67 kN and 4,797.26 kN due to the hawsers and mooring line. This fact suggests that the structure is unlikely to experience ultimate failure if merely operated in the current operational site

Risk Analysis of Decommissioning Process: Case Studies of Lima-Compresor Platform

The process of petroleum exploration and exploitation is a crucial activity in the fulfillment of energy needs in the world. The process of petroleum exploration and exploitation is currently centred on shallow water regions in the continental shelf. In the process of petroleum exploration and exploitation in shallow waters, the structure of which is commonly used is the jacket structure. In Southeast Asia there are about 1300 platforms, of which 80% is over 20 years old. When the platform has reached its operational limit, according to the ministerial regulation of ESDM number 1 year 2011 The platform must be decommissioning in accordance with existing technical standards. In the process of demolition, there are certainly risks and hazards that can interfere with the process of decommissioning. It is necessary to do a risk analysis to map the risks that can occur while preparing the mitigation steps. In this study, risk analysis arel conducted by determining the activity that has a significant degree of hazard where it is concluded that activities that have a significant risk level are activities related to construction, lifting, maintenance, well service, and maintenance. From each activity that has a significant hazard level, there was a process of determining the cause of risk using the Fault Tree Analysis (FTA) method while determining the barrier that serves to prevent a risk occurring. The next is to determine the impact that can be inflicted from risk by using the Event Tree Analyis (ETA) method while determining the barrier that serves to prevent and reduce the impact that occurs when there is a peril. These two FTA and ETA diagrams are combined to create a Bowtie diagram to explain in detail the risk management performed at each stage of the decommisiioning process

The Study of Tandem Offloading Performance and Operability on The Cylindrical Hull FPSO Sevan Stabilized Platform with Variation in Mooring System Configuration

This study has been carried out to evaluate the dynamic behavior of the Cylindrical FPSO Sevan Stabilized Platform (SSP) and the LNG Carrier (LNGC) during the process of tandem offloading. The study includes hydrodynamics modellings, computations, and simulations of both cases SSP and LNGC operated individually and in combination for offloading operations. The SSP is moored with two variations of mooring, namely taut and catenary. Environmental loads are waves with the incorporated winds and currents propagating 90o, 210o, and 330o relative to the SSP headings. Excitation of random waves up to Hs = 4.50 m instigates the relatively low SSP motions in standalone condition. In offloading condition, when LNGC is connected, the SSP motion could magnify as much as 2.0 up to 5.0 times higher than that in standalone condition, but still considered in an acceptable level. The motion quality of LNGC in offloading operation is comparable with the SSP. For various random wave headings with Hs = 4.50 m during offloading operation may generate maximum tensions between 1,600 kN up to 2,600 kN in the casse of catenary mooring, and between 4,700 kN up to 7,000 kN in the case of taut mooring. Even then, this largest tension preserves a safety factor of 2.05 which is well above the limit of 1.67 as required by the governing standards. Finally, the study conclude an operability of as much as 90% could be achieved on SSP and LNGC offloading operation in the Masela Block of the Abadi Gas Field

Local Stress Analysis in the Chain Link of Mooring Line That Had Diameter Degradation

Mooring systems are used to moored ships at a particular area. One of its type is SPM by using a buoy. The system generally uses chains used to tie buoys to the seabed. However, chains that are used continuously can experience degradation in the diameter of the chain connection. The degradation experienced by the connections between the chains will certainly affect the local ( von Mises stress).  According to ABS rules, the amount of local stress or von Mises stress that occurs on an object must not exceed 90% of the yield strength of the material. Therefore, it is necessary to do a local stress analysis to determine the extent of degradation of the diameter of the chain connection so that the local stress value does not exceed that allowed. The largest mooring tension value that occurs in the mooring system is 1838,252 kN. The results show that if the chain connection is subject to a tension of 1838,252 kN, the diameter degradation that occurs in the chain connection must not exceed 15% of the initial diameter so that the local or von Mises stress does not exceed 369 MPa (90% of the yield strength of the material 410 MPa)

Effect of Mooring System Configuration to Flexible Riser Tension

Floating Storage and Offloading is a floating structure that functions as hydrocarbon temporary storage from wells before being distributed to tankers and carried to production facilities on shore. The important components used to support FSO operations are mooring systems and risers. The mooring system is used to limit FSO movement due to environmental loads. Riser was carried out hydrocarbons from the well to FSO This study aims to analyze the effect of mooring system configurations on flexible riser tension. These variations consist of spread mooring, conventional buoy mooring (4 buoy), and two other variations combine spread mooring and conventional buoy mooring. This analysis shows that the mooring system with 4 buoys has the largest flexible riser tension that is 248 kN, while spread mooring has the smallest flexible riser tension, 55 kN. FSO with conventional buoy mooring configuration has the largest offset and spread mooring configuration has the smallest offset. The lazy wave type has smaller riser tension than the free hanging type, which is 4 – 58%

Experimental Study and Numerical Analysis of Floating Crane Catamaran Mooring Tension in Intact and Damage Conditions Using Time-Domain Approach

Floating Crane Catamaran equipped with a mooring system to keep stable while operating. During operation, wave load causes tension on the mooring system. In this study, the tension of the mooring system was analyzed using experimental studies and numerical analysis with intact and damaged mooring conditions. Experimental studies were carried out by simulating a physical model in the Ocean Basin Maneuvering Laboratory, BTH-BPPT. While numerical using related software. Mooring tension analysis is carried out using the frequency domain approach which refers to the API RP 2SK rules. The sum of the average tension, significant low frequency tension and maximum wave frequency tension is the maximum tension of the mooring system. The low frequency tension and wave frequency tension is obtained by the low-band-pass filter process. The stochastic value is obtained by the FFT of low frequency and wave frequency tension. The results of maximum tension from experimental and numerical at intact conditions, wave headings 90°, Hs  2.5 m, are 373.7 kN and 441.6 kN and at Hs  6.37 m are 565.6 kN and 1741.5 kN. In the damaged condition, wave heading 90°, Hs 2.5 m, the maximum tension is 863.9 kN and 2113.3 kN

The Study of Mooring Buoy Operability to Support Offloading Operation of Shuttle Tankers with Various Capacities

This study was conducted to analyse of the operability of mooring buoy initially designed for offloading operation of 35,000 DWT shuttle tankers enhanced to serve the offloading operations of shuttle tankers with 50,000 DWT, 75,000 DWT, and 111,000 DWT capacities. Operability is reviewed in term of mooring line tensions induced by each new variation of tanker capacity under environmental conditions of 1-year, 10-year, and 100-year recurrence. The governing criteria is that the safety factor should meet the appropriate limit as stated in the API RP2SK. Tension on the mooring line increases in parallel with the increasing of tanker capacity. For the case of 35,000 DWT and 50,000 DWT shuttle tankers the operation can be performed in all environmental conditions. For the case of 75,000 DWT shuttle tanker with full load and 67% DWT capacity can fully operate in all environmental conditions, but with 47% DWT capacity could not be operated in the 100-year environmental condition with significant wave height 3.31 m for the direction of inline-L1, inline-L2, and between line-L1&L4. For the case of 111,000 DWT shuttle tanker at all capacity conditions can fully operate in the 1-year environmental condition with significant wave height up to 1.48 m