Experiences in Classification of Deepwater Mooring Systems for Floating Installations

The use of catenary moorings systems in deep and ultra deep water requires long and heavy mooring lines. As a consequence the floater has to support a large weight and the mooring footprint becomes large and may interfere with neighbouring infrastructure. Installation of these mooring systems becomes very expensive due to the vessels that are required. For deep and ultra deep water mooring systems the more appropriate moorings system is the taut leg mooring, using shorter and lighter mooring lines. The mooring loads arrive at an angle between 30 and 45 degrees at the bottom, requiring the anchor points to resist not only horizontal, but also considerable vertical loads. This paper discusses the use of synthetic ropes and anchors in deep water mooring systems. The characteristics of the ropes and anchors are presented and how they can be used in both catenary and taut leg mooring systems. When oil and gas exploration and production took place in shallow to deep water, the most common mooring line configuration was the catenary mooring line consisting of chain and wire rope (Fig. 1). For exploration and production in deep to ultra deep water, the weight of the mooring line starts to become a limiting factor in the design of the floater. To overcome this problem new solutions were devised consisting of synthetic ropes in the mooring line (less weight) and/or a taut leg mooring system (Fig. 2). The major difference between a catenary mooring and a taut leg mooring is that where the catenary mooring arrives at the seabed horizontally, the taut leg mooring arrives at the seabed at an angle. This means that in a taut leg mooring the anchor point has to be capable of resisting both horizontal and vertical forces, while in a catenary mooring the anchor point is only subjected to horizontal forces. In a catenary mooring, the restoring forces are generated by the weight of the mooring line. In a taut leg mooring, the restoring forces are generated by the elasticity of the mooring line.

This paper describes the experience of Petrobras in the design, installation and operation of deepwater mooring systems incorporating high efficiency polyester fiber ropes. These ropes have been used now for more than 12 years in catenary and taut configurations to moor drilling and production units as well as storage and offloading facilities. The paper provides a history of the development of this technology, highlighting the key issues involved in introducing in mooring systems a component that has non-linear visco-elastic behavior. Differences to all steel systems in terms of: behavior, failure modes and design criteria and procedures are discussed. The paper presents the life cycle management strategy adopted by Petrobras in the early years of the use of this technology and its evolution that have guaranteed that the only failure mode observed over all these years was mechanical damage. It is expected that this paper shows that the use of polyester (polyethylene terephthalate) ropes in deepwater moorings is a mature technology. Introduction In the mid 80's Petrobras was drilling in Campos Basin in water depths around 600m and producing in depths around 400m. Mooring systems were all based on chain, wire rope and drag embedment anchors. Increasing water depths indicated that all steel systems would become anti-economical due to the reduction in restoring efficiency. Initial studies were based in using synthetic fiber ropes as extension wires in the mid-catenary. Since compliance to wave induced motions would be given by the geometry of the steel components the fiber rope should have stiffness similar to a steel wire rope. Therefore aramid ropes were the first to be considered (Riewald, 1986). Cost considerations led Petrobras and the US Navy (Karnosky & Liu, 1988) to start studying the use of polyester fiber ropes for moorings. It was also devised that, in deepwater, the axial compliance of polyester ropes could provide enough compliance so that a long catenary might not be required (Baxter, 1987). The use of polyester ropes in permanent moorings was a big challenge for several reasons, the most obvious one being establishing design methodologies and criteria for all possible failure modes. Since most large diameter fiber ropes were not optimized for tension and axial fatigue performance (Parsey, 1983)), it was also necessary to further develop rope constructions and generate enough data on the performance of these ropes. Results of a comprehensive study on the use of fiber ropes for deepwater moorings were presented in 1992 (Chaplin & Del Vecchio, 1992). Full details of the study, including assessment of failure modes, load-elongation behavior, design methodologies and criteria were produced (Del Vecchio, 1992). Since the study was partially based in model ropes, confirmation of load-elongation behavior in full scale and additional data on fatigue performance were obtained in the following years and incorporated in the type approval process of the ropes. Field trials were conducted in 1995 to assess installation procedures.

The purpose of this paper is to describe the authors' recent experience with the analysis, design, and installation of polyester rope mooring systems for deep water applications. Among the latest research activities, this article describes a series of full-scale tests performed on polyester rope provided for several spread moored systems offshore Brazil and West Africa. One of the main purposes of the performed tests was to determine non-linear stiffness characteristics of the rope; but other parameters used in the design and optimization of the polyester rope mooring system were also investigated. Introduction The past decade has seen a steady increase in the number of polyester rope deep-water mooring system for oil and gas exploration, especially floating production facilities. Brazil has always been in the frontier of the application of synthetic mooring ropes. Industry forecasts point to a strong increase in the number of such systems, considering the latest deepwater discoveries made offshore Brazil. Due to the past and present history of success in the use of polyester rope for the mooring of deep water floating system offshore Brazil, it is a general consensus that this trend will continue into the future and expand to other producing regions such as The Gulf of Mexico and West Africa as it can already be observed. There are many obvious advantages in using polyester ropes on deep water mooring systems. We could mention a few of them such as: lighter weight, desirable elastic characteristics and superb fatigue performance; on the other hand when engineering and installing such a system there are specific issues to be appropriately dealt with by the designer and the installation personnel. This paper will discuss some of the peculiarities associated to the design; analysis and deployment of polyester rope mooring system for deep water applications. The main topic of discussion in this article is a practical way to model the elastic stiffness of polyester mooring ropes in order to have a simple and reliable model that produces reasonably conservative extreme mooring loads and vessel-offsets. Such numeric model describing the elastic behavior of the polyester ropes have a significant impact on mooring system for floating facilities for deep water applications, with a specific application to the mooring of FPSO's. Typical Polyester Rope Mooring System for Deep Water FPSO Applications Fig. 1 depicts a typical deep water FPSO spread moor system. Each anchor leg is constituted by two segments of chain, one located at the bottom and the second close to the surface; joined by a single or multiple segments of polyester rope. In case of the mooring system shown on Fig. 1 we can see three (3) segments of polyester rope joined by two (2) short segments of chain.

The evaluation of existing mooring system and the development of an appropriate optimization design became the key issues in the Liuhua 11–1 FPS dry-dock updating project because this optimized mooring system has to face new challenges, such as the update of site metocean conditions, the revision of wind force coefficients due to upgraded topsides, and the additional corrosion allowance for chain links due to extra 15-year service life extension compared with the original system design. A design team was formed to give the evaluation of the existing mooring system based on mooring line tensions and offset limits according to the latest industry standard for a permanent mooring system. Since the existing mooring system marginally failed the design requirements based on the evaluation, system optimization design focused on the adjustment of mooring line pretensions has been carried out to improve the mooring system performance and eventually to satisfy the classification requirements for the tension safety factors and the platform offset limits driven by the riser designs. The mooring system analysis was conducted in time domain. The Ariane program developed by BV was used in the detailed mooring system analyses. For the calibration and verification purpose, the system evaluation and the optimization design had also been independently verified with DNV's DeepC program. Introduction The LH 11–1 oil field is located about 200km southeast of Hong Kong in the South China Sea in approximately 310m depth of water. LH 11–1 FPS, which named ‘Nanhai TiaoZhan’, is a major component of LH 11–1 oil field development. The FPS was a modified drilling semi-submersible converted from a Sedco 700 series drilling rig built in 1975 and in 1995, it was converted again to a floating production unit in Singapore and then deployed at LH 11–1 oil field, permanently moored by 11 mooring legs. The Unit is classed by American Bureau of Shipping (ABS). It was originally designed for a service life of 10 years (from 1995) and has been continuously operating for more than 15 years. At the same time, a new oil field Liuhua 4–1 (OTC-24042), about 11 km away northwest LH 11–1, was in development planning with the utilization of LH 11–1 FPS facilities through a subsea tieback. Therefore, additional topsides capacities and the life extension of this aged semi-submersible FPS became necessary. CNOOC Ltd, Shenzhen is the owner of this LH 11–1 FPS. COOEC was selected as the contractor for the engineering and construction support of this FPS dry-docking upgrading and life extension project. CSSC Guanzhou Huangpu Shipbuilding Co., Ltd was contracted by the owner as the dry-docking shipyard for the upgrading construction.

Floating Production Systems (FPSS) can be either, custom built or a converted semi-submersible, tanker or barge. The paper describes the necessary steps to be taken for the Classification and Certification of FPSS and FPSOS. The paper outlines the latest Classification and certification requirements for both semi-submersible and ship type FPSS. Classification and regulatory requirements for the Hull Structure, Stability, Station Keeping, Shipboard and Production Systems are discussed. 1.0 INTRODUCTION Hydrocarbon Production from a floating platform was first used in the early 1970s, with the "TRANSWORLD 58" in the North Sea. As far as Classification and Certification are concerned this unit was treated similar to a mobile offshore drilling unit. In recent years many other types of floating production systems have been installed offshore in various parts of the world. For the purposes of Classification and Certification, American Bureau of Shipping (ABS) considers a Floating Production System an offshore system comprised of a floating structure with station keeping capabilities engaged in any combination of oil and gas production, processing, storage and/or offloading operations. The floating structure may be a semisubmersible, tanker, barge, spar buoy, or a TLP. It may be custom designed and built or an existing vessel converted for FPS service. Station keeping can be from a single point, spread mooring system or by dynamic positioning. This paper presents ABS' approach to Classing/Certifying semi-submersible and ship-type floating production vessels. The relationship, requirements and responsibilities of the Classification Societies and the relevant regulatory authorities are, in general, well defined. However, for FPS projects these relationships and requirements are often more complex and sometime not understood by all parties. The selection of which type of vessel is most appropriate depends on a number of parameters such as field size, environmental parameters, cost etc. Table 1 shows a representative number of a selection of FPSS and FPSOS classed and/or certified by the ABS. 2.0, CLASSIFICATION REQUIREMENTS 2.1. General During the past decade the use of FPSS has increased dramatically, some purpose built and others converted semi-submersibles or tankers. ABS Classification requirements are essentially the same for either new building or a conversion. In general ABS approach to FPS Classification is based on:treating the entire FPS as a site-specific systemcombining the requirements of ABS' various Rules and Guides, as relevant, to the type of floating system involved in each project,identifying and defining requirements for interfacing "marine system" practice with "production system" practice,utilizing existing and new industry standards and recommended practices wherever possible. The minimum requirements for Classification of an FPS are the Classification of:the floating hullthe production system and equipment,the station keeping system. Optional Classification can be obtained for the subseatemplate, production riser and export systems. After the design review, on site surveys, hook-up and commissioning are completed to ABS satisfaction.

Numerical design study of multipoint mooring systems for the floating wave energy converter in deep water with a sloping bottom

While floating offshore wind farms have predominantly concentrated on shallow water applications thus far, it is only a matter of time before they extend into deeper waters, especially in regions without large shallow continental shelves. This evolution is expected to follow the trend of the offshore oil and gas industries. Studies have been underway to explore the feasibility of offshore wind farms in deeper waters. Therefore, further research into Floating Offshore Wind (FOW) technologies for deep-water applications, including advancements in mooring technology, will significantly benefit the offshore wind farm industry. This study aims to further investigate the applications of taut leg mooring technology in mid to deep-water regions. The primary objective is to examine the fatigue behavior of this deep-water mooring system as it relates to water depths, inclination angles, and levels of pre-tension. To accomplish this, sixty scenarios are analyzed, encompassing a range of these parameters to address industry-relevant situations for the near future of floating offshore wind farms. Each scenario is rigorously assessed for both the Ultimate Limit State (ULS) and the Fatigue Limit State (FLS). A streamlined fatigue assessment methodology has been developed and implemented to efficiently evaluate a substantial number of cases while maintaining a high level of accuracy. The findings from these comprehensive case studies reveal general trends and provide valuable insights into the performance of taut leg mooring systems. The study was further enhanced by incorporating design constraints derived from the wind farm layout and spatial efficiency considerations. A typical floating offshore wind farm project utilizes multiple floating offshore wind turbines (FOWTs) to maximize wind energy generation. Increasing the number of turbines installed within the leased area enhances the spatial efficiency of the wind farm. However, optimizing this spatial efficiency is a complex task, as it involves navigating various disciplines and design parameters. This study builds upon the established minimum spacing requirements between turbines, which are dictated by wind turbulence, to introduce further constraints on the selection of mooring configurations and their associated footprints. Additionally, it explores the implications of these minimum spacing guidelines on the overall design of deepwater mooring systems. It was observed that most promising mooring configurations, in terms of spatial efficiency, are predominantly influenced by fatigue concerns. Certain selected cases exhibited significantly low fatigue life, requiring the implementation of mitigation strategies to address these concerns. Several approaches were discussed, including increased component size, the use of additional mooring lines to share loads, and the use of fatigue mitigation devices, such as polymer springs. Notably, since fatigue loading is primarily influenced by both the magnitude of dynamic tension variations and their frequency of occurrence, the findings highlight that mitigating line dynamics is significantly more crucial than simply lowering tension levels. Therefore, the polymer spring emerges as the most effective solution for addressing fatigue issues in mooring systems explored in this study. A high-level cost assessment was conducted based on key selected cases to evaluate the cost trends in deep water mooring systems across different alternative mooring configurations and their associated fatigue mitigation strategies. In particular, the study explores the potential cost savings offered by a three-line mooring configuration that incorporates polymer springs for fatigue mitigation. The key findings and recommendations from this analysis are summarized in the concluding section of this technical paper.

Mooring line snap loads occurs more frequently for floating offshore wind turbines (FOWTs) deployed in intermediate water depths (50–100 m) due to the use of shorter mooring lines compared to deep water applications (>100 m). Current design standards estimate maximum mooring line tension values based on the assumption that snap load occurrences are rare. However, this approach risks underestimating the maximum design tensions in systems where snap loads occur frequently. In this study, different mooring systems for a 15MW VolturnUS-S reference FOWT targeted for deep (200 m) and intermediate (60 m) water depths are investigated. The analysis focuses on the effect of water depth on the occurrence of snap loads and their influence on maximum mooring line tensions. The chain-catenary mooring (Cat-200) system, designed for 200 m water depths, serves as the baseline for comparison against two alternative mooring systems for intermediate waters, i.e. the chain-catenary (Cat-60) and taut (Taut-60) systems. Dynamic analysis results show that the mooring systems in the intermediate water depth result in higher tension standard deviations compared to that of the deep water mooring system, due to their stronger wave-frequency responses. Moreover, the maximum mooring line tensions under the 50-year storm environmental condition are analysed using the mean up-crossing rate (MUR) method with a threshold level determined experimentally. Among the systems analyzed, the Cat-60 mooring system experiences the highest maximum mooring line tension—348% and 95% greater than the Cat-200 and Taut-60 systems, respectively. In terms of the snap load occurrence, the Taut-60 mooring system is effective in reducing the occurrence of snap events, offering a more stable performance. Lastly, results in this study also indicate that the MUR method accurately predicts maximum mooring line tensions for systems with both high and low snap event occurrences.

The renewable energy industry through offshore wind has been growing rapidly with increasing project capacities. To accommodate this huge energy transition demand, the projects are moving towards open sea and deeper water. In line with this trend, various design development and innovation on Floating Offshore Wind Turbine (FOWT) technology are progressing to improve the feasibility as well as the economy of the projects. Mooring system is part of major costs components in Floating Offshore Wind (FOW). Reducing mooring loads is desirable to create more room for optimization of the mooring configuration. This could lead to opportunity for saving on the capital costs, more environmentally friendly solutions through reducing the mooring footprint, as well as better mooring integrity. The innovative polymer spring component has been introduced in the mooring industry that enables significant reduction on the mooring loads if it is properly designed. In this paper, the applicability of polymer spring component in FOW applications is further investigated through mooring case studies utilizing comprehensive design analysis. For this purpose, the VolturnUS-S semi-submersible floating platform design is selected. This reference floating offshore wind turbine is developed by the University of Maine – USA to support the International Energy Agency (IEA) 15-MW reference wind turbine. In total, there are eight individual mooring designs analyzed in this study, which are rooted from two very different base mooring systems. The first base mooring system is a ‘conventional’ steel wire with ground chain semi-taut mooring system. While the second one is based on a ‘more novel’ nylon-based taut mooring system with three different declination angles. For each base mooring configuration above, an alternative mooring configuration is proposed by having the polymer spring as part of the mooring component. Dynamic mooring analysis is performed for both the base and the alternative mooring configurations using the same design load cases, and the results are compared. The mooring assessment is performed using fully coupled dynamic time-domain analysis tools Orcaflex with the hydrodynamic database of the platform generated by the Orcawave. All the proposed mooring systems are designed in compliance with the applicable industrial design codes for the Ultimate Limit State (ULS), while meeting the target allowable platform offsets. It should be noted that this paper focuses on optimizing the mooring system for the ULS scenario only. It is acknowledged that Fatigue Limit States (FLS) may be governing in some projects. The expansion of this work to FLS, however, is outside the scope of this paper but will be explored in future papers. Offshore Ulsan, eastern sea of South Korea is selected as the basis for the operational location for all the case studies presented in this paper. Ulsan and its offshore region are being intensively promoted as one of the hubs for the FOW projects. Hence, the results of this mooring study may provide useful input for the developers of these wind farms to further optimize their mooring systems. The benefits of the polymer spring as a mooring component are demonstrated through the case studies. Promising prospects of combining the polymer spring with synthetic nylon rope for FOW mooring system applications are highlighted. At the end of this paper, conclusions and recommendations for future work are drawn.

A number of spars are being installed in deepwater areas in the Gulf of Mexico (GoM) which are subject to loop / eddy current conditions and require that their mooring system be designed for Vortex Induced Motion (VIM). This paper presents insights into fatigue and wear issues for mooring systems of spars subject to VIM, and illustrates the importance of reassessing industry practices for mooring strength and fatigue design for mooring systems that must resist VIM. During the GoM Millennium Eddy Current Event in April 2001, the ChevronTexaco Genesis spar platform, which is installed in Green Canyon Block 205 in the GoM, underwent VIM which was greater than anticipated during the original design of the mooring and riser systems. A key part of developing a permanent solution for ensuring integrity in future large VIM events was the assessment of fatigue and wear damage accumulated in past VIM events, for the purpose of ensuring fitness of the components of the mooring system for future service. The main elements of this assessment were: (a) The hindcast of the mooring line tensions from measured platform motions. (b) The evaluation of stress concentration on the mooring chain in the fairleads. (c) The assessment of fatigue damage for high amplitude/low cycle conditions, and (d) The assessment of accumulated wear. The results of this fatigue assessment led to a decision to replace the fairlead chain segments on 4 of the 14 mooring legs of Genesis, in order to ensure that they would have adequate fatigue endurance under the Stepped Line Tensioning (SLT) solution developed to mitigate the risk of VIM of the spar in the future. The project team’s unique understanding of mooring system response under VIM and resulting mooring component fatigue and wear issues led to the development of novel fatigue and wear hindcasting procedures that allowed the project to reliably assess the remaining life of the mooring components, and make decisions to selectively replace some of the components to ensure integrity for the remaining life of the platform under the SLT solution.

Due to a recent mooring shackle failure within the offshore industry, focus has been directed toward existing in-service facilities that have similar manufactured shackles installed in the mooring systems. An in-service deep water facility was identified as having similar mooring components by the same manufacturer that failed in-service. A subsequent assessment of the material properties was undertaken on readily available spare shackles that identified Charpy impact toughness values below the original material specification. Consequently, a fitness-for-service assessment was carried out that included the assessment of the shackle fracture toughness, a finite element analysis of the shackle and associated mooring components, and a deterministic analysis procedure that assessed the structural capacity of the shackle by considering the response of the shackle to the presence of a crack or flaw located at a shackle stress hot spot coupled with a significant return period event. Finally, full-scale structural testing of spare shackles was performed to enable verification of actual shackle load capacity with a flaw located at the shackle stress hot spot. The focus of this paper is on presenting an overview of the fit-for-service methodology and the results of the structural integrity assessment. While a full quantitative risk assessment of the effect of a low fracture toughness shackle in the mooring system would have been ideal, due to the level of data available and the timeframe required for a probabilistic assessment, it was deemed appropriate to undertake a deterministic assessment of the mooring shackles under realistic scenarios representative of service and severe loads that may accompany the service. The results of the deterministic assessment on an actual in-service deepwater floating facility will be presented. The fit-for-service methodology presented in this paper provides a robust method to assess in-service floating facilities that have been identified or suspected of having below-specification mooring components. It has successfully been applied to a number of Chevron in-service floating facilities in different geographical locations. Introduction Following the early failure of a mooring shackle on a new deepwater floating facility, attention has been directed to other new and existing assets that have similar manufactured mooring shackles installed in the mooring system. Additionally, the release of Safety Alert No. 256 from the U.S. Department of the Interior (Minerals Management Service), provided recommendations to operators to review their current specification and testing requirements for mooring systems to ensure that the mooring components are adequate for the proposed or current usage.

Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

This paper presents a mooring system design featuring polymer springs for the VolturnUS-S 15-MW reference floating wind turbine in site conditions for the New York Bight at a 50-m water depth. Polymer springs have a nonlinear stress-strain curve that allows a stiffer response at low loads and a more flexible response at higher loads, potentially reducing peak mooring line tensions. The mooring dynamics model MoorDyn has been extended to model springs with nonlinear tension-strain curves from a user-inputted look-up table. This MoorDyn modeling advancement is verified against OrcaFlex simulation results. Using MoorDyn’s updated capabilities, a spring-equipped catenary mooring system is designed for the 15-MW floating system, along with a baseline catenary mooring system that does not use polymer springs. The floating wind turbine simulator OpenFAST is used to simulate the mooring systems in design-driving load cases to show the effect of polymer springs on key dynamic behaviours. The results show that the spring-equipped design reduces peak tensions by up to 60%, whereas the turbine offsets stay within a maximum of 7.2 m, which is still a reasonable offset limit for cable considerations. The reduction in peak tensions results in a significant decrease in damage equivalent loads — on the order of 50% for upwind lines in fully loaded conditions. These results show that polymer springs can effectively reduce peak tensions and fatigue loads in mooring systems at shallow water depths.

This paper presents the results of an experiment carried out on a semi-submersible model to measure the steady drift force and low frequency surge motions. In the experiments, the influence of mooring systems was also investigated in different combinations of current and sea state. The measurements were carried out with a 1/50 scale model which was moored using horizontal springs and catenary mooring lines. A comparative study of the mean values of steady drift motions and the standard deviation of the low frequency motion amplitudes is presented. In addition, the effect of current on the damping ratio is discussed. It is found that for both horizontal and catenary moorings, the presence of a current increases the damping ratio of the system. For the catenary mooring system, as expected, the presence of mooring lines and their interaction with waves and current increases the damping compared to the damping of the horizontal mooring system. The measured mean values of the surge motions in a wave–current field are compared to the superposed values of those obtained from waves and current separately. For the horizontal mooring, it is found that there is good agreement in moderate sea states, while in higher sea states the measured motion responses are larger. In the wave-current field, the standard deviation of the surge motion amplitudes is found to be less than that obtained in waves alone. This can be explained by the increased magnitude of the damping ratio. Only in the cases of high sea states with the horizontal mooring system, was it found that the standard deviation of the surge motions is slightly larger than those obtained for waves and current separately. This may be explained by the absence of catenary mooring line damping.

Reliability study of suction caissons for catenary and taut-leg mooring systems