Optimal mooring system deployment in line breakage condition

The design and analysis of a quayside mooring system for safe mooring of Prelude FLNG under extreme environmental conditions were carried out. The design of the mooring system considered the yard operation conditions and maximum wind speed during a typhoon. In order to secure the mooring safety of Prelude FLNG under an extreme environment, a special steel structure was designed between the quay and Prelude FLNG to maintain the distance from the quay to a certain extent to avoid a collision with the inclined base. The mooring safety was also ensured by installing additional new parts on the quay. A mooring analysis and mooring safety review were performed with more rigorous modeling considering the nonlinearity of the mooring rope and fender. In order to secure additional safety of the mooring system under extreme environmental conditions, a safety assessment was conducted on the failures of the mooring components proposed in the marine mooring guidelines. Based on the results of the mooring analysis, it was confirmed that the Prelude FLNG can be safely moored even under the extreme conditions of typhoons, and a worst case scenario analysis verified that the mooring system design was robust enough. The proposed mooring analysis and design method will provide a basis for the safe mooring of ultra-large floating offshore structures of similar size in the future.

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.

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.

With the global oil and gas resource development accelerating its expansion into deepwater and ultra-deepwater areas, taut mooring systems have gradually become key technical equipment. However, the complex coupling mechanism between the material parameters of polyester cables and their dynamic responses has not been fully revealed, and existing design methods mostly rely on simplified models, lacking systematic verification with real environmental data and dynamic responses. Based on on-site platform data, this study proposes a multi-dimensional integrated collaborative optimization design scheme for deepwater tension leg mooring systems, aiming to provide guidance for mooring design projects in similar sea areas and enhance the safety and economy of deepwater mooring systems. To address the nonlinear optimization challenges of multi-body coupled systems, this study establishes a time-domain multi-body coupled model of the platform-riser-taut mooring system and conducts systematic research in combination with the NSGA-II multi-objective genetic algorithm. First, a multi-body coupled time-domain simulation model is constructed based on the actual geometric dimensions of the platform structure and large-scale measured environmental data, covering wave-frequency and low-frequency dynamic response characteristics. Second, statistically representative working conditions are adopted to verify the accuracy of the model's dynamic response. Third, a global sensitivity analysis is conducted to quantify the influence weights of polyester cable parameters and reveal the main control factors of the mooring cable's dynamic response. Fourth, with API-RP-2SM as the constraint, a multi-objective optimization function of the standard deviation of mooring tension and the extreme value of platform displacement is established to solve the Pareto frontier of parameter combinations. Finally, the optimization effect is verified based on typical working conditions, forming a full-process analysis method for mooring optimization. This study provides a digital solution with practical engineering value for the design of deepwater mooring systems. The research results show that: (1) The environmental loads in the target sea area exhibit significant quarterly periodic characteristics, with extreme working conditions in the first quarter accounting for 1%. (2) The length and diameter of the polyester cable have a significant impact on mooring tension. When the length of the polyester cable increases from 1850m to 2000m, a single variable of 2.5% causes the mooring tension to change by approximately 60%. The innovation of this paper lies in proposing an optimization scheme for the mooring system based on the real data from the platform. This method combines environmental data with multi-objective optimization algorithms, breaking through the limitations of traditional empirical design. At the same time, based on the actual data from the site, representative working conditions are selected to support model verification, revealing the quantitative influence laws of polyester cable material parameters on the non-linearity of mooring tension, so as to provide real guidance for similar engineering design and construction in similar sea areas.

Time-domain numerical simulation is generally considered an accurate method to predict the mooring system performance, but it is also time and resource-consuming. This paper attempts to completely replace the time-domain numerical simulation with machine learning approaches, using a catenary anchor leg mooring (CALM) system design as an example. An adaptive sampling method is proposed to determine the dataset of various parameters in the CALM mooring system in order to train and validate the generated machine learning models. Reasonable prediction accuracy is achieved by the five assessed machine learning algorithms, namely random forest, extremely randomized trees, K-nearest neighbor, decision tree, and gradient boosting decision tree, among which random forest is found to perform the best if the sampling density is high enough.

This paper discusses key technical issues for classing deepwater mooring systems, Special applications of new mooring components such as synthetic fiber ropes, suction pile anchors and Vertically Loaded Anchors (VLA) have been presented. This paper establishes the design criteria for synthetic fiber ropes, suction pile anchors, and VLAs. The rationale as presented is based on recent projects and research and development results from joint industry studies. Testing and installation considerations for synthetic fiber ropes and field test requirements for suction pile anchors and VLAs are reviewed and discussed. Additional design and analysis requirements for the use of synthetic fiber ropes are also proposed for a taut leg mooring system. This paper represents a summary of experiences gained through the application of these innovative mooring technologies in mooring components and anchoring devices. The guidelines set by ABS as presented in this paper are intended to enhance safety standards in marine operations while meeting the new challenge to moor floating units in deep water. Introduction With the recent explosive growth in deepwater drilling and production operations worldwide. American Bureau of Shipping (ABS) is actively involved in classification of several conversions to deepwater and new-built floating platforms including semi-submersibles, ship-shaped drilling units and tanker-based FPSOs (Floating Production. Storage and Oftloading), Most of these floating units will be moored in the ever increasing water depths. In deep water, the conventional catenary mooring systems become less stiff and have a larger horizontal spread, with much higher mooring component costs and payload penalties. To cope with these challenges, new mooring systems for stationkeeping of floating units have been developed for ongoing projects. such as those offshore of Brazil installed or to be installed. by Petrobras in 1997 and 1998. These include polyester synthetic fiber ropes. VLA and suction pile anchors applied in the taut leg mooring systems. Classification of Deepwater Mooring Systems General. ABS has established the position that the scope of classification of an FPSO system shall include three major items, Le., the mooring system, the vessel, and the production facilities [Ref. 1]. Other equipment and systems such as import and/or export risers may also be classed, if requested by the owner. Since the early 1970s, ABS has provided numerous classification services for floating production systems. Table I lists some recent projects in water depths greater than 500 meters. There are special considerations to be addressed for the design and analysis of mooring systems in deepwater applications. The key element, when it comes to novel systems involving new components such as synthetic fiber ropes, suction anchors, vertically loaded anchors, etc., is to provide a comparable and compatible safety standt1rd to that of the traditional catenary mooring with chain wire and drag embedment anchors. It is well recognized that the mooring line loads tend to favorably adjust themselves when movements of drag embedded anchors occur in the traditional catenary mooring system, This is the rationale for the safety factors of the anchor holding capacity being much lower than those of the mooring line tension,

A shallow water disconnectable STL turret mooring and riser system has been developed for water depth between 30 and 50 m. This technology is based on APL’s disconnectable STL (Submerged Turret Loading) and STP (Submerged Turret Production) technologies which had been widely applied for water depth between 70 m to 2600 m for FPSOs and LNG offshore terminals. The advantage of disconnectable system is that the mooring and riser system can be designed to a preferred sea state. When the sea state is higher than design sea state (like hurricane), the vessel can be disconnected and sail away. The shallow water STL system consists of STL buoy, mooring lines, riser system and landing pad. The interface with vessel is the same as traditional STL system. The mooring and riser system are connected to the vessel through STL buoy and can be pulled into vessel by using ship winch. Unlike traditional STP and STL buoys, the shallow STL buoy has a net weight and will stay on the landing pad when disconnected from vessel. The landing pad is designed to support the impact load from STL buoy and supply enough friction for the STL buoy to stay in position during 100-year storm. The mooring system design has taken the advantage of directionality of weather when close to the shore by using different mooring line length in different directions. Further an innovative Hold-Back-Wave riser configuration has been developed for shallow water system. The riser configuration has a larger flexibility compared to traditional wave configuration and has proved to be feasible for significant wave height at least 7 m when connected to the vessel and 10+ m when disconnected from the vessel. Model test for the disconnectable shallow water turret mooring and riser system had been performed in MARINTEK, Trondheim with a LNG re-gasification vessel model at 30 m water depth. For connected system, significant wave height Hs = 6 m and 8 m has been tested. The mooring and riser system perform well, as predicted. For disconnected system (when the buoy sitting on the landing pad), significant wave height Hs = 10 m has been tested. The STL buoy is sitting on the landing pad without significant movement and the riser system performs well. SIMO program has been used to calibrate the model test results with numerical simulations. By adjusting surge, sway, yaw damping and 2nd order wave drift force, the calibrated SIMO model agrees well with model test results and can be used for similar development.

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

With the increasing demand for key technologies in deepwater oil and gas exploration, the mooring system, a critical component for the positioning of deepwater floating platforms, plays a vital role in ensuring the stable operation of offshore oil and gas platforms under harsh working conditions. In this study, we propose an optimization method for mooring systems based on Enhanced Multi-task Genetic Optimization (EMGO). Taking a semi-submersible platform mooring engineering project in the South China Sea as an example, a multi-body coupled model of the semi-submersible platform and its mooring system was established in compliance with mooring system design specifications. A custom-developed program was utilized to generate 7,751 sets of dynamic mooring system data. Subsequently, an enhanced multi-task learning (MTL) neural network was designed to predict platform offsets and mooring line axial tensions under various working conditions with high accuracy. The optimal performance of the MTL model achieved a mean squared error (MSE) of approximately 0.044 and R-Square (R²) of 0.96. Combining the enhanced MTL model with a genetic algorithm, iterative optimization was performed to converge on an optimal mooring design parameter set tailored for the semi-submersible platform. The proposed method achieved a reduction of over 20% in maximum axial tension in mooring lines and a reduction of over 10% in maximum platform offset. The EMGO algorithm demonstrated excellent capability in high-dimensional nonlinear fitting and generalization. Validation of the optimized design was performed on taut mooring systems of three additional semi-submersible platforms, confirming the feasibility and effectiveness of the optimization method under extreme environmental conditions in the South China Sea. This study introduces an innovative intelligent design method for deepwater taut mooring systems, which significantly improves the safety and reliability of ultra-deepwater platforms. It provides valuable guidance for intelligent design in offshore and marine engineering fields.

The focus on alternative energy sources has increased significantly throughout the last few decades, leading to a considerable development in the wave energy sector. In spite of this, the sector cannot yet be considered commercialized, and many challenges still exist, in which mooring of floating wave energy converters is included. Different methods for assessment and design of mooring systems have been described by now, covering simple quasi-static analysis and more advanced and sophisticated dynamic analysis. Design standards for mooring systems already exist, and new ones are being developed specifically forwave energy converter moorings, which results in other requirements to the chosen tools, since these often have been aimed at other offshore sectors. The present analysis assesses a number of relevant commercial software packages for full dynamic mooring analysis in order to highlight the advantages and drawbacks. The focus of the assessment is to ensure that the software packages are capable of fulfilling the requirements of modeling, as defined in design standards and thereby ensuring that the analysis can be used to get a certified mooring system. Based on the initial assessment, the two software packages DeepC and OrcaFlex are found to best suit the requirements. They are therefore used in a case study in order to evaluate motion and mooring load response, and the results are compared in order to provide guidelines for which software package to choose. In the present study, the OrcaFlex code was found to satisfy all requirements.

As nearshore aquaculture spaces become saturated, the development of fisheries aquaculture for deep sea has become an inevitable trend. This paper proposes an innovative deep-sea aquaculture platform that incorporates a vessel-shaped main structure and a single-point mooring system. The potential flow theory and the Morison equation are utilized to calculate the hydrodynamic loads on the main structure and the netting and mooring systems, respectively. The deformation and force of the netting in current are simulated, and the accuracy of the analytical methods used is validated based on experimental results. The influences of the netting system on the hydrodynamic characteristics of the platform are analyzed. Optimization on the single-point mooring system is conducted under static and dynamic conditions, considering the influences of various mooring parameters, including mooring line length, buoyancy of buoys, and mass of sinkers. The patterns of changes in motion response, mooring line tension, and minimum touchdown length under different mooring parameters are calculated and analyzed. The results indicate that changes in mooring line length have minimal impact on the dynamic response of the platform and mooring system. The addition of appropriate buoys or sinkers can reduce the motion response of the platform and the tension in the mooring lines. Moreover, compared to adding buoys, incorporating sinkers more effectively enhances the overall safety and stability of the platform system.

This paper presents a probabilistic study of various Gulf of Mexico (GoM) MODU mooring systems that was performed as part of the MODU Mooring JIP (Joint Industry Project). The intent was to determine the relative changes in reliability between different types of MODUs, with different mooring systems, in different water depths, designed for different hurricane conditions and located at different regions in the Gulf. In addition, the effects of assuming reduced mooring line strength were investigated. Twenty cases were selected which cover four different classes of MODUs, four different types of mooring systems, three different numbers of mooring lines, as well as three different water depths. These base cases were chosen so that trends or differences between different hull shapes and mooring systems could be identified. These twenty base cases are further expanded into sixty cases according to location in the Gulf as well as the return period used to design the mooring system. The results indicate that on average the " notional?? (i.e., relative) annual failure probability of mooring line failure decreases by one order of magnitude when increasing the MODU mooring system design return period from 10-yr to 50-yr. Two orders of magnitude decrease are observed if the design return period is increased from 10-yr to 100-yr. In addition to the above findings, reliability sensitivity to four different GoM Metocean regions is also provided. Redundancy of the mooring system for 8, 12 and 16-lines is also examined in terms of the additional reliability gained by the extra lines. Introduction The probabilistic reliability approach was performed using an updated procedure that was originally developed in prior work by DeepStar [1]. The results can be used by operators, drilling contractors, API work groups and regulators alike to understand, on a comparative basis, the performance of various MODUs and mooring systems. A base set of 20 separate MODU and associated mooring cases were evaluated as shown in the first twenty cases of the matrix in Table 1. These selected twenty base cases cover four MODUs (or MODU classes), four types of mooring systems, three numbers of mooring lines and three water depths. The footnotes of Table 1 provide definitions of these cases. These base cases were chosen so that trends or differences between different hull shapes and mooring systems can be identified. This base set of cases was then further defined into a larger set of 60 according to location in the Gulf as well as the return period used to design the mooring system. The Gulf locations were defined as the Central, West Central and West according to MODU JIP metocean study [5] and the latest API guideline [6], and the design return periods were defined as 10, 25, 50 and 100 year. The remainder of Table 1 shows the resulting matrix of cases studied. The full set of 60 cases was initially evaluated for reliability using 100% CBS (Catalogue Break Strength) for the mooring line. The entire set of 60 was then re-evaluated using 80% CBS to determine the sensitivity of the results to lower than expected mooring line strength. For the 8-10 mooring line cases, the first and second line failures were determined. The second line failure was used to represent the mooring system failure. For the 12 and 16 line cases, the third line failure was also determined, with the third line failure representing the mooring system failure. This allowed a comparison of the effect of mooring line redundancy in terms of the first line failure, versus the mooring system failure.

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 offshore industry has spent the last several years developing semisubmersible platforms capable of supporting both drilling and production activities. The production trees are located on top of the top tensioned risers (TTRs) on a Dry Tree Semisubmersible. A key challenge in the design of these vessels is to reduce the heave motion as much as possible to enable the use of state-of-the-art riser tensioners. A model test campaign was executed as part of the developmental program. The primary objective of this campaign was to improve the accuracy of the numerical tools to be used in the design process. Riser tensioners are typically hydro-pneumatic devices, with a nonlinear tension-stroke relationship. A riser tensioner was developed at model scale which had a similar nonlinear behavior to the prototype. Examining the effect of this tensioner on the global motions was an additional objective of this test campaign. The techniques used to model this nonlinear spring is described, and its effect on global motions investigated. A key challenge in model testing platforms intended for ultra-deep water (e.g. greater than 7000 ft) is the modeling of the mooring and riser system. The premise for the design of the model mooring system is 1.) maintain as best as possible the force-offset relationship of the mooring lines and 2.) be able to describe the model test configuration in the numerical tools to be used for global design. The near taut behavior of the prototype mooring system is modeled using heavy chain and a high-catenary mooring line.

In recent decades, the emphasis on renewable resources has grown considerably, leading to significant advances in the sector of wave energy. Nevertheless, the market cannot still be considered as commercialized, as there are still other obstacles in the mooring system for wave energy converters (WECs). The mooring system must be designed to not negatively impact the WEC’s efficiency and reduce the mooring loads. Firstly, the overview of the types of wave energy converters (WECs) are classified through operational principle, absorbing wave direction, location, and power take-off, respectively, and the power production analysis and design challenges of WECs are summarized. Then, the mooring materials, configurations, requirements, and the modeling approaches for WECs are introduced. Finally, the design of mooring systems, including the design considerations and standards, analysis models, software, current research focus, and challenges are discussed.