Mooring System Calibration of the Intact Condition, Ultimate Limit State (ULS)

2017 ◽  
Author(s):  
Torfinn Hørte ◽  
Siril Okkenhaug ◽  
Øivind Paulshus
Keyword(s):  
Gulf Of Mexico ◽  
Continental Shelf ◽  
Reliability Analysis ◽  
Time Domain ◽  
Structural Reliability ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Reliability Level ◽  
Norwegian Continental Shelf

Structural reliability analysis (SRA) has been used to calculate the probability of mooring line failure in an intact mooring system as a function of the magnitude of the safety factor applied in design. A range of different units have been considered, comprising ship shaped units and semisubmersibles at different water depths from 100 m to 2200 m. Environmental conditions representative for the Norwegian continental shelf and the Gulf of Mexico are used in the analyses, and the characteristics of the results in the different environments are compared and discussed. Analyses for Brazilian environment are currently ongoing, but not included here. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the distribution parameters in order to describe the tension response in varying conditions. Joint probabilistic models of the metocean environment corresponding to the different geographical locations have been applied, taking account of wind, wave and current and their respective heading angles. A continuous model is used for the metocean conditions at the Norwegian continental shelf, whereas a hurricane model is used in the Gulf of Mexico. The effects of uncertainties in the response calculation are included. The mooring line component strength is based on strength data from break load tests. Conventional catenary chain-wire chain systems as well as polyester moorings are considered. With the probability of failure as a function of the safety factor, it is shown that present regulations result in a significant scatter in reliability level between the cases. Safety factors have been calibrated considering all cases. Alternative design formats are proposed and tested including a format with 2 safety factors. Calibration results are provided as a function of the target reliability level. The final recommendation on target reliability level is given in an accompanying paper at OMAE 2017, comprising both the ULS and the ALS. It is demonstrated that alternative design formats can provide a more consistent safety level across the cases. A different design philosophy is needed for the Gulf of Mexico in order to achieve acceptable risk. Options for design are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used in combination with detailed response calculations in the time domain. The results provide a basis for calibration of mooring design for ULS and subsequently for regulators to update their rules. The work has been carried out as part of the NorMoor Joint Industry Project, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. This paper is the first one in a series of three at OMAE 2017, where the second deals with structural reliability analysis of the ALS and the third one provides summary and recommendations for safe mooring design in ULS and ALS.

Mooring System Calibration of the Damaged Condition, Accidental Limit State (ALS)

2017 ◽  
Author(s):  
Torfinn Hørte ◽  
Siril Okkenhaug ◽  
Øivind Paulshus
Keyword(s):  
Gulf Of Mexico ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Mooring Line ◽  
Mooring System ◽  
Second Line ◽  
Weak Line ◽  
Initial Failure ◽  
Structural Reliability Analysis ◽  
Ordinary Line

Structural reliability analysis (SRA) has been used to calculate the probability of two adjacent mooring line failures. The initial failure is caused by some exceptional causes which most likely is related to substandard strength, but could also be exceptionally high tension caused by mal operation. Empirical failure data are used to assess the probability of initial failure. The ALS in the context here should control the probability a second mooring line failure with ordinary strength, adjacent to the initial failure of a weak substandard line. This check is also called the ULS redundancy check in ISO 19901-7. A range of different units have been considered, comprising ship shaped units and semisubmersibles at different water depths from 100 m to 2200 m. Environmental conditions representative for the Norwegian continental shelf and the Gulf of Mexico are used in the analyses, and the characteristics of the results in the different environments are compared and discussed. Analyses for Brazilian environment are currently ongoing, but not included here. Considerations for when the initial failure occurs have been made, and three different time intervals are considered: i) Failure of the second line during the transient motion after first failure, ii) failure of both lines in the same storm and iii) failure of the second line during stationary conditions after the initial failure. Time interval ii) is identified as most critical, when there is practically no time to implement mitigating actions. Detailed SRA analyses have been carried out, and include the concept of having a weak line in the system that represents the first failure. It was found that weak lines, with strength distributions that are consistent with the empirical probability of line failure, are too weak to contribute significantly to reduce the probability of the 2nd line failure of an ordinary line. The probability of the combined event of 2 line failure can therefore be simplified and set equal to the product of the probability of the presence of a weak line and the probability of 2nd line failure of an ordinary line in a system with one line missing. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension in the most loaded line after one line is removed from the mooring system. A large number of different metocean conditions are considered. A response surface is used to interpolate on the distribution parameters to describe the tension response in varying conditions. Joint probabilistic models of the metocean environment corresponding to the different geographical locations have been applied, taking account of wind, wave and current and their respective heading angles. The mooring line component strength is based on strength data from break load tests. Conventional catenary chain-wire chain systems as well as polyester moorings are considered. The probability of failure, conditional on the initial failure, is calculated using SRA. Calibration of safety factors are provided for different reliability levels, and for different assumptions for the probability of the presence of a weak line. It is demonstrated how the ALS criterion can be relaxed if the frequency of initial line failures due to exceptional causes is reduced. The final recommendations on target reliability level and on the probability of having a weak line in the mooring system are given in a companion paper at OMAE 2017, which comprises both the ULS and the ALS. It is demonstrated that alternative design formats can provide a more consistent safety level across the cases. A different design philosophy is needed the Gulf of Mexico to achieve acceptable risk, and options are discussed. This paper is the second one in a series of three at OMAE 2017, where the first one deals with structural reliability analysis of the ULS and the third one provides summary and recommendations for safe mooring design in ULS and ALS.

Summary and Recommendations for Safe Mooring System Design in ULS and ALS

2017 ◽  
Author(s):  
Siril Okkenhaug ◽  
Torfinn Hørte ◽  
Øivind Paulshus
Keyword(s):  
Gulf Of Mexico ◽  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Mooring Line ◽  
Safety Factors ◽  
Design Code ◽  
Oil Companies ◽  
Reliability Level

DNV GL is currently running a Joint Industry Project, “NorMoor JIP”, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. It is a global study covering Gulf of Mexico, Northern Europe and Brazil waters. Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV GL, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent. The reliability level implied by these regulations is not known, and we also see that the mooring standards are interpreted and applied differently. Thus, there is a need for a mooring design code with a consistent analysis methodology and with safety factors that are in line with this methodology and calibrated at an appropriate target reliability level. This is achieved through reliability-based calibration for a range of different units, mooring systems, water depths and geographical locations. The focus in the present paper is the calibration of safety factors and selection of target reliability level. The underlying probabilistic analysis results used for the calibration are reported in two accompanying papers at OMAE 2017, [1] and [2], dealing with structural reliability analyses for the ULS and ALS respectively. For mobile units frequency domain analyses are common, and although the main attention in the JIP is towards time domain analyses, it is part of the JIP to calibrate safety factors for frequency domain analyses as well. The annual extreme value distribution of line tension for all cases is calculated in time domain and is applied both in the calibration of safety factors for time domain and frequency domain analyses. It is seen that characteristic tensions from time domain analyses are likely to be higher than those from frequency domain analyses. The dilemma of not being penalized when using more refined time domain analyses is discussed, and different safety factors have been suggested for use with time domain and frequency domain analyses. A discussion about target reliability level is included, and the target levels are proposed with basis in the existing mooring design practice for mobile units, where frequency domain analysis is prevalent. Different targets are proposed depending on consequences of failure. Calibration for different design formats are carried out. The current format using a single safety factor is challenged with a format with two safety factors. The objective is to arrive as close as possible to the target reliability for all cases analyzed. A different design philosophy is needed in the Gulf of Mexico in order to achieve acceptable risk, and options are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used to calibrate a mooring design code where the mooring line tensions are calculated in the time domain. The results provide a basis for regulators, such as ISO, to update their rules. ULS and ALS are covered here, and a potential phase 3 of the JIP will cover the fatigue limit state. When the NorMoor JIP is completed the plan is to implement the results into DNVGL-OS-E301, [5].

A Review of Mooring Analysis Methodologies for Permanent Offshore Installations

2020 ◽  
Author(s):  
Spiro J. Pahos ◽  
Georgina Maldonado ◽  
Paul C. Westlake
Keyword(s):  
Time Domain ◽  
Line Strength ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
System Response ◽  
Mooring System ◽  
Deterministic Approach ◽  
Safety Factors ◽  
Analytical Approaches

Abstract Traditionally mooring line strength assessment is based on a deterministic approach, where the mooring system is evaluated for a design environment defined by a return period. The mooring system response is then checked against the mooring strength to ensure a required factor of safety. Some codes adopt a deterministic approach [1], [2], [3]. Other codes like [4] adopt a partial safety factor format where uncertainties are addressed through load factors for load components and material factors for line strength. Industry practices give guidance on mooring analysis methodology together with analysis options like coupled, de-coupled, time domain, frequency domain and the associated line tension safety factors. Prior work has demonstrated that discrepancies in mooring line tensions are observed when different analytical approaches are used [5]. Namely, the mooring line tensions of a semi-submersible unit in a coupled time domain analysis, were found to be non-compliant, whereas those calculated using a decoupled time domain analysis returned compliant tensions. This work focuses on a coupled dynamic analysis where all inertial, hydrodynamic and mechanical forces are assessed to determine the subsequent motions. Despite being considered the most accurate to capture the true dynamic response, a coupled analysis is also the least efficient in terms of the required computer resources and engineering effort [1]. This paper presents further discussion on the above observation in mooring tensions and also considers differences in the installation’s excursion. All responses are evaluated in the time domain where the nonlinear dynamic behavior of the mooring lines, slowly varying wave drift forces and coupling effects are captured. Agreement is found in the present computations, carried out with two renowned hydrodynamic codes, which validate former results and reiterate the need to distinguish between time domain methods and recommended appropriate safety factors accordingly.

Investigation on the Use of Different Approaches to Mooring Analysis and Appropriate Safety Factors

2012 ◽  
Author(s):  
Sojan Vasudevan ◽  
Paul Westlake
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Dynamic Time ◽  
The Time Domain

This paper presents the results of the analyses of a twelve line catenary mooring system using a quasi-static method in the frequency domain, and uncoupled and coupled dynamic methods in the time domain. The latter is found to produce significantly higher tensions. The reasons for these differences are investigated. The minimum line tension safety factors required by design codes do not distinguish between uncoupled and coupled dynamic analyses and some codes use the same factors even for quasi-static analyses. Consequently, the present mooring system passes the acceptance criteria based on quasistatic frequency domain and uncoupled dynamic time domain analyses but does not meet the same criteria when a coupled dynamic time domain analysis is employed. It is understood that because the coupled time domain analysis determines the vessel motions using all forces the accuracy of mooring line tension estimation will be improved over other methods. Hence the application of less conservative safety factors is proposed.

North Sea Mooring Systems: How Reliable Are They?

2014 ◽  
Author(s):  
Will Brindley ◽  
Andrew P. Comley
Keyword(s):  
Continental Shelf ◽  
North Sea ◽  
Recent Decade ◽  
Mooring Line ◽  
Mooring System ◽  
Failure Rates ◽  
Floating Structure ◽  
Strength Capacity ◽  
Failure Statistics ◽  
Overall Reliability

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system? This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system. The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

Time-Domain Coupled Dynamic Analysis of Mooring Systems in Extreme Sea Condition

2018 ◽  
Author(s):  
Xuliang Han ◽  
ShiSheng Wang ◽  
Bin Xie ◽  
Wenhui Xie ◽  
Weiwei Zhou
Keyword(s):  
Dynamic Analysis ◽  
Body Surface ◽  
Time Domain ◽  
Mooring Line ◽  
Mooring System ◽  
Dynamic Coupling ◽  
Spar Platform ◽  
Coupled Dynamic Analysis ◽  
Motion Responses ◽  
The Time Domain

In order to predict the coupled motion and external wave load for the design of deepwater floating structure system, based on the three-dimensional time-domain potential flow theory, this paper present the indirect time-domain dynamic coupling method and the body nonlinear dynamic coupling method. The perturbation expansion theory is adopted to evaluate hydrodynamic on the fixed mean wetted body surface for the former method. The transient free surface Green function has been extended and applied to calculate the nonlinear hydrodynamic on the instantaneous wetted exact body surface for the latter method. The finite element model is employed to solve dynamic response of mooring line. Then asynchronous coupled method is adopted to achieve the coupled dynamic analysis of platform and mooring lines. The time-domain motion responses and spectrum analysis of Spar platform are verified and compared with the traditional indirect time-domain coupling dynamic method when the mooring system is completed. Also the time-domain motion responses and statistical characteristic of Spar platform are investigated with one mooring line broken in extreme sea condition. Some conclusions are obtained, that is, dynamic coupling effects are significant and transient position hydrodynamic calculation of platform has a great influence on the low frequency motion. The results also show that the influence on the global performance of mooring system is different when the broken line is in different place. A remarkable influence occurs when the broken mooring line is in the head-wave direction.

scholarly journals Study Optimasi Panjang Mooring Line Tipe Spread Mooring pada F(P)SO

2019 ◽  
Vol 5 (1) ◽  
pp. 13-19
Author(s):  
Budi Setyo Prasodjo
Keyword(s):  
Time Domain ◽  
Mooring Line ◽  
Mooring System

Dalam analisa mooring untuk F(P)SO (Floating (Production) Storage and Offloading, banyak hal yang harus dipertimbangkan agar mooring system bisa memenuhi standard kelayakan dan keamanan. Di paper ini akan dibahas mengenai optimasi panjang mooring line untuk mooring system tipe spread mooring pada kapal tanker dengan kapasitas sekitar 10,000 dan 20,000 DWT yang nantinya akan dikonversi menjadi F(P)SO. Jumlah mooring line diberi batasan untuk 8 spread mooring line (tipikal) dengan konfiguarsi mooring line dan pretension mooring line yang sama untuk kedua kapasitas kapal yang berbeda. Variasi hanya pada panjang mooring line untuk melihat secara jelas sejauh mana pengaruh perubahan panjang mooring line terhadap tension dari tiap mooring line dan offset gerakan dari FPSO saat di tambat dan menerima beban beban lingkungan laut (gelombang, angin, arus) dengan menggunakan full analisa dinamis dengan metode time domain. Dari studi ini bisa diketahui dengan penambahan panjang mooring line maka akan terjadi pengurangan maksimum tension pada mooring line, tapi sebaliknya terjadi penambahan offset gerakan kapal.

Dropped Polyester Mooring Line Qualification for Reuse

2021 ◽  
Author(s):  
Craig R Gage ◽  
Pierre F Liagre ◽  
Caspar N Heyl ◽  
Cesar Del Vecchio
Keyword(s):  
Gulf Of Mexico ◽  
Water Depth ◽  
Cost Savings ◽  
Mooring Line ◽  
Mooring System ◽  
Potential Cost ◽  
Mooring Lines ◽  
Minimal Damage ◽  
Recovery Effort ◽  
The Impact

The Perdido platform is a spar located in a water depth of 7,825 feet in the Alaminos Canyon Block 857in the Gulf of Mexico. The mooring system consists of nine mooring lines in three groups of three, spacedapproximately 120 degrees apart between each group. Each mooring line is composed of a platform chain,a multi-segment polyester rope including a 120 feet long test insert at the top, a ground chain, a pile chainand other associated connectors. The mooring lines are connected to suction piles. The Minimum BreakStrength for the Perdido polyester mooring line is 4,000 kips. Installation of the spar hull was completed inSeptember 2008 and the topsides was set in March 2009. The spar and its mooring system were originallydesigned for a twenty (20) year life. On May 4, 2019, mooring line # 6 (ML6) was contacted by a marine vessel down line and was severed.Contact occurred along the polyester test insert. A recovery effort was planned, and the mooring line wasreplaced in early June. The original ML6 was recovered from the seafloor on June 4, 2019 as a part of thatcampaign and submitted to an initial inspection. This paper is not intended to go into either the cause of the incident or the replacement of ML6 but willlook to the inspection of the recovered mooring line and explore its suitability for reuse. Initial inspection ofthe lines suggested minimal damage to the polyester rope segments and raised questions to the impacts of 10years of use. Testing was envisioned as a learning opportunity for the impact of service on polyester mooringand was reinforced by the potential cost savings that could be attained though reuse. A methodology wasdeveloped, supported by initial inspections and a suite of testing was performed. The results of these testsare presented in the following, along with a proposed process for assessing and considering reuse of a linefollowing a drop. Additionally, conclusions will be shared for the process, the results, and the potentialramifications for the industry.

Reliability-Based Design Criterion for TLP Tendons

2006 ◽  
Author(s):  
Federico Barranco Cicilia ◽  
Edison Castro Prates de Lima ◽  
Lui´s Volnei Sudati Sagrilo
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Limit State ◽  
Design Criterion ◽  
Ultimate Limit State ◽  
Safety Factors ◽  
Extreme Response ◽  
Load Effects ◽  
Partial Safety Factors ◽  
State Models

This paper presents a Load and Resistance Factor Design (LRFD) criterion applied to the design of Tension Leg Platform (TLP) tendons in their intact condition. The design criterion considers the Ultimate Limit State (ULS) of any tendon section along its whole length taking into account both dynamic interactions of load effects and the statistics of its associated extreme response. The partial safety factors are calibrated through a long-term reliability-based methodology for the storm environmental conditions, like hurricanes and winter storms, in deep waters of the Campeche Bay, Mexico. In the reliability analysis, the uncertainties in the definition of load effects and analytic limit state models for calculation of tendon strength and randomness of material properties are included. The results show that the partial safety factors reflect both uncertainty content and the importance of the random variables in structural reliability analysis. When tendons are designed according to the developed LRFD criterion, a less scattered variation of reliability indexes is obtained for different tendon sections across a single or various TLP designs.

Time Variant Reliability of Mooring System Considering Corrosion Deterioration

2005 ◽  
Author(s):  
Zhen Gao ◽  
Torgeir Moan ◽  
Svein E. Heggelund
Keyword(s):  
Reliability Analysis ◽  
Failure Probability ◽  
Breaking Strength ◽  
Mooring Line ◽  
Mooring System ◽  
Uniform Corrosion ◽  
Element Analysis ◽  
Weakest Link ◽  
Chain Link ◽  
Chain Links

This paper deals with time variant overload reliability analysis of a mooring system due to corrosion deterioration. A probabilistic model for uniform corrosion is adopted to predict the strength degradation. A simplified method and nonlinear finite element analysis are used to calculate the breaking strength of the chain link and comparison is made. The strength of one mooring line is modeled by a weakest link system. The effect of correlation in corrosion models for different chain links in one mooring line and the effect of higher corrosion rate in the splash zone are discussed. The annual failure probability of the most loaded mooring line of a semi-submersible is calculated. The first and second order motions of the semi-submersible and the corresponding line tensions are found by a simplified analysis. The time variant reliability analysis is performed by approximating the degraded strength by a piece-wise constant model. The annual failure probability is obtained for different years. It is found that the annual failure probability increases significantly as the chain is corroded.

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