New Developments in Prediction of Mooring System Damping

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.

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.

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 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.

Floating wind turbine vibration controlling becomes more and more important with the increase in wind turbine size. Thus, a novel three-bifurcated mooring system is proposed for Spar-type floating wind turbines. Compared with the original mooring system using three mooring lines, three-bifurcated sub-mooring-lines are added into the novel mooring system. Specifically, each three-bifurcated sub-mooring-line is first connected to a Spar-type platform using three fairleads, then it is connected to the anchor using the main mooring line. Six fairleads are involved in the proposed mooring system, theoretically resulting in larger overturning and torsional stiffness. For further improvement, a clump mass is attached onto the main mooring lines of the proposed mooring system. The wind turbine surge, pitch, and yaw movements under regular and irregular waves are calculated to quantitatively examine the mooring system performances. A recommended configuration for the proposed mooring system is presented: the three-bifurcated sub-mooring-line and main mooring line lengths should be (0.0166, 0.0111, 0.0166) and 0.9723 times the total mooring line length in the traditional mooring system. The proposed mooring system can at most reduce the wind turbine surge movement 37.15% and 54.5% when under regular and irregular waves, respectively, and can at most reduce the yaw movement 30.1% and 40% when under regular and irregular waves, respectively.

As the offshore industry looks beyond water depths of 3,000 ft for future oil and gas production, the industry standard steel catenary spread mooring (CSM) system used on all floating production systems installed to date becomes increasingly inefficient and costly, An alternative to the CSM is the Taut Leg Spread Mooring (TLSM) system with its characteristic short scope legs and vertically loaded anchors. In water depths greater than 3,000 ft, it has been shown that TLSMS comprised of synthetic mooring lines have performance advantages over systems utilizing steel wire rope and demonstrated potential for significant cost reductions. Early studies investigated the use of aramid fiber rope due to its high strength, lower in-water weight and lower axial stiffness as compared to ropes made from steel. Later studies indicated that the material properties of polyester fiber, primarily it's lower Young's modulus, made polyester mooring lines more suitable for use in TLSMS2'S. As the TLSM knowledge base expanded, aramid rope construction evolved. Earlier efforts to match the characteristics of steel wire rope have given way to new generation 'soft' aramid constructions. This paper compares the mooring performance and total installed cost of a "soft? aram id TLSM to a typical polyester configuration. Both systems were designed for use in the Gulf of Mexico (GOM) with an FPS based on the Aker P45. Introduction This study was performed with the objective of investigating and comparing two taut leg spread mooring systems comprised of differing types of synthetic rope. The two types of synthetic mooring line constructions investigated were:Whitehill 36-strand aramid rope.Marlow Superline parallel-strand polyester rope. Mooring systems using both constructions were designed for the new-build Aker F'45 Semi-submersible Drilling and Production platform (FDPU), The first P45 is currently being constructed for Norsk Hydro for their Njord field development. The vessel's displacement will be 49,600 tons when completed; production facilities will be capable of 70,000 BOPD of crude throughput. The particulars of the Aker P45 are presented in the Appendix to this document. The study this paper is based upon has been planned and organized such that it would be a suitable extension to work previously presented13 A deep water (3,000 ft) GOM location was chosen as the field location for this work. Three environmental design scenarios were investigated for this site. They are:Maximum Wave Hurricane Event with associated wind and currentMaximum Loop Current Event with associated wave and windMaximum Wave Case -5 year return The Maximum Wave Hurricane condition noted above was the controlling case and, therefore, was used in the design effort. The goal of this study is to evaluate whether mooring lines constructed of aramid fiber are cost and performance competitive with polyester for deep water taut leg spread moorings. As part of the scope of the study, installation costs and schedules have been developed for each system. Preliminary component specifications were developed to a level of detail where costs could be estimated for each mooring case.

The Thunder Hawk semi-submersible platform is located in 6060 ft. water depth, Mississippi Canyon, Block 736, Gulf of Mexico. The mooring system is composed of chain-polyester rope-chain. On April 14, 2009 line #10 was installed on the platform. On May 15th, 2009 while attempting to complete construction stretch removal operations of the polyester mooring ropes, an equipment failure caused line #10 to fall completely to the seabed. The mooring line was subsequently recovered on June 11, 2009 and a spare polyester mooring line was run in its place. The mooring line was fitted with two long segments and a test insert, so all testing and evaluation was done on the recovered #10 test insert. In order to maintain the " used" rope as a spare, a qualification program was proposed to BOEMRE (now BSEE). The evaluation first required a complete visual inspection of the mooring line and components and then a detailed two phase evaluation of the test insert. Fatigue testing was completed June 14, 2012. The results and conclusions of this testing provide insight into the expected behavior of polyester mooring ropes and potential testing problems. Microscopic inspections using SEM were done on the rope fibers. The results show significant formation of salt crystals on the rope fibers. EDX examination was not possible on dissolvable solids as it was not possible to transfer the material to the EDX slide. Therefore the results were not conclusive. Break testing qualification consisted of three (3) subrope breaks and one break after a fatigue test representing 25% more than the API spiral strand equivalency. Regulatory approval required the mechanical testing. Test results indicate the polyester rope met the required break strength, was not affected by soils ingress, and had additional improvement in strength after the fatigue test. Installation of polyester moorings using a pre-lay approach can significantly reduce the mooring installation cost for deepwater floating platforms. This data presents real world test results and testing issues from a typical polyester mooring rope in a deepwater Gulf of Mexico application. Introduction The Thunder Hawk facility is operated by Murphy Exploration & Production Company and leased from SBM. The semi-submersible has twelve (12) mooring lines consisting of four (4) groups of three (3) lines each. The mooring lines consist of a chain, two (2) long segments of polyester, one or two test inserts depending on the corner, platform chain and messenger chain. Connections between the polyester and between the polyester and chain are made with plate style H-links. A fairlead is located on the hull and a chain stopper with load monitoring is located at the the top of column. Dedicated windlass units on each column provide pay in and pay out capability and mooring line tensioning. Each mooring line has a chain locker to store the messenger chain and extra length of the platform chain. The polester ropes used are 254mm diameter with a qualified minimum breaking strength of 1930 te. The long segments are 1261 m each, and test inserts are 15.74 m long. One complete spare rope was purchased with the original order, consisting of two (2) long segments, two (2) test inserts, and connectors. On April 14, 2009 the mooring line was installed and tensioned to approximately 204 te (450 kips). This tension is slightly above the target tension for this line. On May 12, 2009 construction stretch operations were undertaken for the polyester. On May15, 2009 construction stretch for line #4, the Southeast corner of the platform, had line tensions of about 408 te on the Northwest corner of the platform. While adjusting line #10 the load released suddenly and the complete mooring line fell to the seabed. The top end of the mooring line was carried deep into the mud due to the weight of the platform chain.

The definition of the mooring systems is one of the most important stages on the design any offshore unit. Some effects associated with it, on the responses of the floating body, are still under investigation. In this context, the full nonlinear high-order hierarchical modeling and the numerical integration of the resulting equations of motion might not be the most cost effective approach for the evaluation of those effects during the early design process. Thus, an expedite analytical formulation to assess the mooring system stiffness, a tool that could help the initial design and analysis. Using classic approaches from Analytical Mechanics, the nonlinear generalized restoring forces associated with the mooring acting on the vessel due to the mooring lines are formulated. The six-degree-of-freedom (DoF) problem is herein addressed. The stiffness matrix is obtained from the linearization of the generalized forces around a generic position. Mooring line characteristic tension function is an input of the method. The closed formulation does not requires a specific line model, although the formulation for a multi-segment mooring line is also derived. The methodology is applied taking the OC4-DeepCwind Floating Wind Turbine semi-submersible platform as a case study. Two Spread Mooring Systems arrangements are studied, in order to demonstrate the use of the presented formulation as a design tool. The calculated mooring system stiffness matrix, evaluated at the trivial equilibrium position, exhibits good agreement with numerical results found in the literature by high hierarchy models. Additionally, the stiffness coefficients are evaluated for other positions than the trivial equilibrium one in the form of colored maps. The natural periods of the motions on the horizontal plane are also mapped. These maps help to understand the effects of the static vessel mean position on the mooring system stiffness and, consequently, on the natural periods associated with the motions on the horizontal plane. Considering the original OC4 mooring system, the effects of the mooring line pre-tensioning are also investigated. Some conclusions on the axial stiffness of catenary cables are also made. The main contributions of the present master dissertation are: (i) the stiffness matrix analytical closed formulation and (ii) the use of colored maps to evaluate the stiffness and the natural periods as functions of the mean offset position. The present master dissertation brings then an innovative closed-form formulation with important practical applications.

Synthetic fibre mooring lines are used as an alternative to traditional steel wire ropes due to their higher strength to weight ratio. Benefits are also found in relative ease of handling, and therefore the marine industry has largely accepted this type of mooring line. By rules and regulations, the design of mooring lines should be based on a coupled dynamic analysis of a particular mooring system and moored vessel. This approach incorporates damping and inertial forces (i.e., hydrodynamic reactions) acting directly on the mooring lines due to their motion through the seawater. On the basis of the outer diameter of the synthetic fibre rope, the Morison equation gives estimations of the mooring line hydrodynamic reactions. In comparison to the traditional steel wire ropes, the synthetic mooring lines usually have relatively larger elongations and consequently larger reductions of the outer diameter. Furthermore, the lower diameter certainly leads to reduced values of damping and added mass (of mooring lines) that should be considered in the coupled model. Therefore, the aim of this study was to develop a new numerical model that includes diameter changes and axial deformations when estimating the hydrodynamic reactions. The development of the model is carried out with a nonlinear finite element method for mooring lines with the assumption of large three-dimensional motions. The obtained results show the effectiveness of the newly developed model as a more accurate approach in calculation of hydrodynamic reactions.

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.

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,

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.

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.

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.

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.