At present, one of the keys to develop deepwater natural gas hydrate resources is to control the downhole pressure during drilling of hydrate reservoirs. This paper analyzes the effects of slow drift motion of mobile offshore drilling equipment on the heave motion of the drill string and downhole pressure in a riserless dual gradient drilling (DGD) system. A coupling model of platform motion-drill string vibrationdownhole pressure in a hydrate reservoir was established. The effects of drilling platform positioning, environmental factors, drilling fluid density and mud pump parameters on wellbore pressure fluctuations were considered. INTRODUCTION The riserless drilling method is a type of dual-gradient drilling system. Compared with the conventional riser drilling system, it directly exposes the drill string to the seawater and separates the annulus of the wellbore from the seawater by a subsea suction device. When drilling without a riser, the drilling fluid is pumped into the drill pipe by the sea surface pump, and then reaches the bottom of the well from the drill pipe, and returns to the mud line from the annulus, and is diverted into the subsea lifting pump unit through the subsea suction device. The small-diameter up return pipeline is returned to the drilling platform. After the returned drilling mud is processed, it enters the mud circulation system again. The aim of controlling the wellbore pressure during the drilling of Marine hydrate reservoir is achieved by controlling the speed and flow of the submarine lift pump and controlling the mud slurry height in the rotary bop. Deepwater riserless drilling fluid recovery drilling technology (RMR) was first proposed and developed by Norwegian AGR Subsea Company, which is mainly used to solve various problems caused by conventional riser systems and is currently used in industrial applications was discovered (Alford and Asko and Campbell, 2005; Scott and Ledbetter and Chester, 2006; Stave and Farestveit and Yland and Rolland, 2005). Xu Qun (Qun and Guoming and Guodong and Changjing and Zhiming, 2011) summarized the development status of riserless offshore drilling technology and analyzed the application characteristics of RMR technology. Gao Deli (Deli and Hui, 2012) established a model of lateral and longitudinal vibration generated by external current load under the condition of riserless drilling and provide a new guiding method for the process design and strength check of the pipe string feeding process of deep water riser drilling system.However, the influence of the platform motion on the string vibration was not considered. Su Kanhua (Kanhua and Wei and Jilin and Chengwei, 2013) established a mechanical calculation model of the drill string by analyzing the motion boundary conditions of the upper part of the drill string in a deepwater without riser environment, and comprehensively analyzing the motion of the platform and the effect of external wave loads. However, the influence of the positioning mode of the platform on the heave motion of the drill string was not considered.

A tension-leg platform (TLP)-type floating offshore wind turbine (FOWT) has deteriorated stability and experiences transient responses under tendon failure. This should be considered as part of accidental limit state checks at the design stage. The WindStar TLP FOWT is considered in this investigation of the transient effects of tendon failure on its system responses. Tendon failure is first modeled using the fully coupled simulation tool FAST. Subsequent numerical simulations consider operational and parked conditions, and the response statistics of key design parameters are analyzed. The numerical results show several responses of the FOWT under operational conditions are higher than those under 50-year extreme condition. The design check shows that the remaining tendons have adequate redundancy to withstand the maximum tensions encountered during the transient responses following the failure of one tendon. These findings are relevant for the accidental limit state design of the considered TLP FOWT. INTRODUCTION Currently, the design and manufacture of floating offshore wind turbines (FOWTs) adopt practices and technology employed in the offshore oil and gas industry (Kaldellis et al., 2016). There are three major types of support platforms for FOWTs namely, spar, semisubmersible, and tension-leg platform (TLP) support platforms. A series of numerical and experimental studies of TLP-type FOWTs has demonstrated their excellent hydrodynamic performance (Vijay et al., 2018; Zhao et al., 2018; Hsu et al., 2019). The mooring system keeps support-platform response within a safe operational zone to ensure it maintains a proper position and orientation. A TLP is stabilized by vertical mooring cables called tendons, which are essential to the safety of the whole system. Maritime history and industrial practice show that the mooring system may fail in a wide range of regions and environmental conditions (mild to severe sea states) (Vijayaraghavan, 2018). During its lifecycle, a mooring system may fail as a result of fatigue, accumulated corrosion, accidental collision, or extreme storms (Ma et al., 2013; Gordon et al., 2014; Prislin and Maroju, 2017). In 2005, hundreds of oil-drilling and production platforms in the Gulf of Mexico were hit by hurricanes Katrina and Rita, with some TLPs' tendons failing (Cruz and Krausmann, 2008). After mooring failure, platform position and orientation change substantially, possibly causing the capsize or sink of the fault platform. Additional risks include loose floating structures colliding with other nearby structures.

To make the dimension reduction for optimization possible, the sensitivity of long-term dynamic performance to the substructure configuration parameters for a V-shaped semi-submersible floating wind turbine is measured by using a statistical test based sensitivity analysis technique. It is found that platform draft, column spacing, pontoon angle and width have a significant influence on nacelle acceleration and tower base fatigue, while the pontoon height and the wall thickness are less important. INTRODUCTION The deep ocean contains abundant wind energy resources. The bottomfixed offshore wind turbines cannot meet the economic requirement of deep sea wind exploitation (larger than 50 m of water depth) and the floating offshore wind turbine (FOWT) technology has become the most promising solution to this issue. One of the big challenges for FOWT development lies in the substructure design, which is not only closely correlated to the capital expenditure, but also to the dynamic performance of the whole system. To obtain a reasonable design that can well trade off the economic and dynamic performance indicators, optimization techniques can be applied to determine the configuration dimensions of substructures (Karimi et al., 2016). The selection of design variables is a priori consideration for a standard optimization procedure. The most straightforward approach is to fully parameterize the design space. However, this will lead to a high dimensional optimization problem and considerable computational cost has to pay to ensure the optimization is well converged. Therefore, before the optimization starts, it is wise to make a general view on which design parameters are critical to the objective functions (cost functions) of concern. The less significant parameters can be regarded as constant values throughout the optimization. In this way, the design space is narrowed down, which will be beneficial to accelerate the optimization process. To achieve this goal, the sensitivity analysis (SA) technique is applied, aiming at measuring the importance of platform configuration/mooring layout parameters to the long-term dynamic performance.

Offshore renewable energies such as the wind and wave energies have attracted vast interests worldwide, due to their abundance in quantity and easy applicability in coastal areas. Numerous devices have been proposed to extract offshore wind and wave energies, and some of them have even achieved the commercial development stage. This is particular true for the development of offshore wind power, which has experienced rapid development in the last two decades. In addition, with intensive research work and technology development, offshore wind power generation has exhibited the trend for developing floating wind turbines in deep waters where the wind energy is more stable. Compared with the offshore wind power generation, the development of wave energy is, however, only at the demonstration stage. The most important inhibitor to the wave energy convertors is its survivability and reliability subjected to the complex environment as well as their high maintenance costs. Therefore, many researchers have considered the idea of integrating the wave energy convertor with an offshore wind turbine since offshore wind turbines have been successfully operating for decades. This study proposes a novel concept by integrating a heaving PAWEC (Point Absorber Wave Energy Converter) with a semi-submersible type floating wind turbine, to achieve the combined wind and wave power generation. The PAWEC is enclosed by the three legs of the semi-submersible foundation and its motions may be enhanced due to the complex hydrodynamic interactions between the PAWEC and the semi-submersible. In addition, the PTO device is mounted onto the floating wind turbine foundation such that the relative heaving motion between the PAWEC and floating wind turbine can be used for wave power generation. In this way, the PTO device will be placed above the water to prevent damage caused by the waves and to facilitate control, maintenance, and inspection work during its lifetime services. Three models are numerically simulated by using the professional hydrodynamics software ANSYS-AQWA, i.e., a single PAWEC, a single semi-submersible floating wind turbine and a PAWEC integrated with the semi-submersible floating wind turbine. For a preliminary study, only the heaving motions are analyzed in the numerical simulations. The mean wave power generations from a single PAWEC and a PAWEC integrated with the floating wind turbine are compared. Moreover, the influence of the PAWEC on the dynamics of the floating wind turbine is also investigated.

Ice-Mas is a simulator used for design of offshore structures interacting with ice combining a variety of models and approaches. It was initiated by TechnipFMC with a joint-cooperation with Cervval and Bureau Veritas. This paper present overall functionalities of Ice-Mas. It will then focus on the modelling approach used for a semi-submersible platform, the calculation of the different ice load and ‘shielding effect’ according to the ice incidence angle. The time series of load will be applied to moored submersible to calculate the motion response and tension force. The result can be used as a reference for design of semi-submersible. INTRODUCTION In the Arctic ocean, significant part of resources remains to be recovered especially in deep-water area. For further exploitation in Arctic area, the design of floating units is necessary. The design of floating structure in ice covered ocean faces a number of challenges due to complex phenomenon of ice failure and interaction with structures. The interaction between a structure and sea ice is a complex process. Ice failure caused by interacting with offshore structure has been investigated by many scientists using analytical and empirical techniques and numerical methods (FEM, DEM, CEM, etc). T.D.Ralston (1980) proposed the plastic limit analysis method to investigate the forces imposed on conical-shaped structures by moving ice sheets. D.E.Nevel (1992) obtained the analytical solutions for the failure of an infinite ice wedge beam on an elastic foundation. The solution then finds application in the calculations of ice breaking loads for various types of sloping structure. Lubbad and Løset (2011). and Konuk, Gürtner et al. (2009) combing different numerical models to predict the ice failure due to the interaction between sea ice and structures. These methods mentioned above are widely used in modelling ice behavior and calculating ice loads exerted on structures in ice covered ocean. However, failure modes of ice can be quite different depending on the shape of structure, ice type, ice thickness and interaction velocity etc.. Even with methods mentioned above, the prediction of ice loads is still a challenge. Especially in the case of semi-submersible units, the geometry of the interacting structure with ice is widely variable. The number of columns and the intruding angle of ice sheet and also size and depth of the pontoon are important parameters in the calculations. The broken ice from ice sheet will accumulate around the legs and contributes a non-negligible part of ice load on structure. This force is also affected by parameters mentioned above.

SDPSO platform is a deep-draft floating platform integrating the critical functions of drilling, dry-tree production, oil storage, and offloading capabilities, and improves the flexibility of the technical solution and reduces the overall cost of deepwater oil field development. Most of FPSOs in the world, which are vessel-shaped and moored by single-point mooring systems, can be positioned weathervane. Their offload tankers are moored by tandem or side-by-side method, which allow the tanker to be rotated around the single point with the FPSO. SDPSO is positioned by a spread mooring, so the tandem or side-by-side mooring pattern cannot be used, because it may lead to a collision between tanker and FPSO. There are two feasible options. One is using a dynamic position tanker to offload oil, and the other is adding a CALM buoy between FPSO and tanker. Both two options are high costs, so it is significant to develop a new low-cost offloading method or system. This paper proposed a ring mooring system for a shuttle tanker to offload oil from SDPSO. The ring mooring offloading system (RMOS) can allow the tanker to be rotated around SPAR under the environment loads. Meanwhile, a smart setup of the floating hose is applied to avoid using an expensive oil transfer swivel. Besides, numerical analyses are performed to demonstrate the performance of this concept. Numerical simulations were conducted using a fully coupled nonlinear time-domain analysis. The distance between the shuttle tanker and the SDPSO is optimized based on the relative motions of the two floaters and the hawser tension requirement of the shuttle tanker. The ring mooring system demonstrates robust performance and cost-effectiveness for the SDPSO. INTRODUCTION As the demand for oil/gas increases rapidly, offshore oil/gas production is continuously extended to deeper waters. Spar Drilling Production Storage Offloading (SDPSO) platform, based on Classic Spar geometry and designed to be applied to deepwater oil field, such as South China Sea, West Africa, etc., is a deep-draft floating platform integrating the critical functions of drilling, dry-tree production, oil storage and offloading capabilities (Liu, 2017). This new concept SDPSO platform improves the flexibility of the technical solution and reduces the overall cost of deepwater oil field development. The SDPSO platform looks like a Classic Spar from the outside appearance, as shown in Fig. 1.

Under extreme met-ocean environmental loads, floating platforms may exhibit complex dynamic states. This paper provides a novel method to predict the extreme motions of floating platform based on deep learning algorithm which can provide some guidance for workers in platform. The distribution disciplinarians of ocean environmental loads are analyzed by the fractal theory and statistical analysis methods. The extreme values of 6-DoF-motions are selected as the characteristic parameters of the output layer. A Long-Short-Term Memory Network (LSTM) deep learning network is established between the ocean environment loads and the extreme values of 6 DoFs response. The predicted results indicated that the present LSTM neural network method could provide the higher accuracy with the lower computational cost. INTRODUCTION Due to their large operating area, high stability, and low cost, offshore floating platforms have been widely used for deep water oil exploitation. A floating platform typically consists of the superstructure, buoys, moorings, riser system, etc. Under extreme met-ocean environmental loads, floating platforms may exhibit complex dynamic states, including strong nonlinear and non-stationary characteristics. For this reason, it is of practical significance to analyze and predict the motions and dynamic responses of semi-submersible platforms and provide accurate early warning evaluations (Du, 2016). To date, many researchers have focused on the environmental loads from ocean waves and wind and the dynamic responses of marine structures. Physical methods which take into account meteorological parameters and physical laws have been widely applied to wave and wind forecasting problems. For example, it has been generally assumed that the generation of waves can be described as a function of meteorological parameters such as the fetch length and wind speed (Kazeminezhad et al., 2005). The most popular of such models are the SMB (Bretschneider, 1970), Wilson (1965), JONSWAP (Hasselmann et al., 1973) and Donelan (1980) models. While those empirically-based models are fast and accurate, they can only be applied to a limited number of scenarios (Bishop, 1983; Kamranzad et al., 2011). However, in the last two decades, numerical models (e.g. Deo, 2001) have been employed to forecast wave and wind characteristics, which has been made possible by rapid developments in computing technology. Wu et al. (2019) studied predictions of short-term wind and wave conditions for marine operations using a multi-step-ahead decomposition-ANFIS model, which accounts for the uncertainty brought about by the prediction horizon. These methods have proven useful in predicting environmental conditions for marine operations. Takbash (2019) used an extreme value analysis method, with long-term satellite records (30 years) of global wind speeds and significant wave heights as input, to obtain extreme values of global wind speeds and wave heights, set to analyze the peaks that exceeded thresholds. In the process, Takbash proved that the extreme value analysis method is limited to the initial distributions and averaged conditions.

The limit state of a newly proposed floating wind turbine concept, in terms of ultimate structural load and fatigue damage load, is studied in the present work. This novel floating structure utilizes a triple-column spar platform to support the wind turbine. An aero-hydro-servo-elastic numerical model is developed to simulate its coupled dynamics. The probability distribution of the ultimate structural load is estimated based on the Monte-Carlo method. In order to reduce computation endeavor, a statistic model is used to extrapolate the sampled-based distribution function. The S-N curve method, a state-of-art fatigue analysis approach, is used to assess the fatigue damage load. The limit states of tower base and fairlead are investigated. It is shown that the newly proposed triplecolumn spar concept suffers less fatigue damage and the ultimate structural loads are also reduced, resulting in the enhancement of safety level of the floating wind turbine system. INTRODUCTION Powered by the increasing global pursuit of sustainable energy, the traditional wind industry is moving to deeper water to exploit highquality offshore wind resources. Since the successful deployment of the world’s first floating wind turbine concept, Hywind demo (Equinor, 2017), various floating structures have been developed for offshore floating wind turbine application. Principle Power proposed the WindFloat (Principle Power, 2015), a semisubmersible floating wind turbine. Three types of floating wind turbine concept are designed in the OC4 DeepCwind project (Koo et al., 2014). More recently, Li and Wang (2019) investigated the long-term extreme loads of a new concept triplecolumn spar-type floating wind turbine, or TC Spar. The TC Spar is designed to be self-installed, thus no need for costly large DP crane vessels to perform offshore installation. Another advantage of the TC Spar is that the smaller diameter hull column makes it easier for efficient standardized fabrication. In advance of the practical application of a new floating wind turbine concept, the structural integrity and reliability must be carefully investigated. Usually, the ultimate limit state and fatigue limit state are two critical parameters to assess the reliability and safety of the structure.

The soft yoke mooring system is considered to be one of the best mooring methods for Floating Production Storage and Offloading (FPSO) on shallow seas, which is widely used in the Bohai Sea, China. Ice load is the main environmental load in this area. The effect of sea ice on single-point mooring is the key problem to the design and secure operation of the whole mooring system. In this paper, the main failure mode of FPSO single point mooring is defined based on the site monitoring of an FPSO single point mooring in Bohai Sea. Combined with the interaction between sea ice and FPSO, the influence of sea ice on the mooring system is analyzed. Finally, taking a typical FPSO single point mooring system as an example, the safety of performance of an FPSO under extreme ice conditions is analyzed using ANSYS numerical simulation. INTRODUCTION During the development of marine oil and gas, an FPSO is a very important piece of engineering equipment. A mooring system will let the FPSO stay in a fixed marine oil and gas position, which can ensure the FPSO's continuous production. Unlike using the ship's anchor or taking measures to avoid sailing in the bad marine environment, an FPSO is usually used in the form of permanent mooring. Under stress from wind, wave and current, an FPSO mooring system needs to resist certain environmental loads; it is a key component of the FPSO, which characterizes its high technology content (Yuan et al., 2005; Allen et al., 2006). The water depth in the Bohai Sea is relatively shallow, with mainly small and medium sized oil fields, which are more distributed, and belongs to the typical marginal oil field. The depth range is usually 15∼20 m. For shallow water marginal oilfields, the approach of single point mooring FPSO combined jacket platform was developed. This model, through which oil or gas transportation is carried out regularly, is economical and practical (Wu, 2012). It consists of a mooring frame, mooring leg, yoke and single point, and has 13 hinge points such as universal joint and thrust bearing, which are located at floating production, storage and unloading (Lyu et al., 2019). In China's Bohai Sea, an FPSO situated by soft yoke mooring is a commonly used combination.

During twin-barge float-over installation of topsides, there are two mating phases with load transferring. In this study, the load transfer process of the first mating phase is carried out continuously by approximately simulating ballast tanks with time-dependent point masses in the commercial software SIMA. Simplistically, linear stiffnesses are applied to simulate the characteristics of deck support units (DSUs) and leg mating units (LMUs). According to the numerical results, variations of the impact loads acting on the DSUs and LMUs are clearly recorded. The variations are captured similarly in the model test, indicating the feasibility of this continuous simulation method. INTRODUCTION As the topside of offshore platform becomes larger and larger, twin-barge float-over installation is widely used as a cost-effective alternative for installing large-scaled topside, compared with heavy lifting installation and single-barge float-over installation. In marine engineering, the entire float-over process is generally divided into six phases: standby, pre-docking, docking, pre-mating, mating, undocking (Liu, and Li, 2017). In these phases, the mating phase has always attracted wide attention because of the load transfer. During the mating phase, the stabbing cones of the topside will gradually contact the receptors of the LMUs on the top of the substructure to transfer the weight of the topside. Thus, there will be impact loads between the stabbing cones and the receptors, which will be absorbed by elastomers inside the LMUs. Different from the other two installation methods mentioned above, as shown in Fig. 1, there are two crucial mating phases with load transferring during the twin-barge float-over installation. The first mating phase refers to the load transfer process of the topside from the transportation vessel to the two installation vessels, while the second mating phase from the two installation vessels to the substructure. Float-over installation is widely applied to install topsides of fixed and floating platforms, which depends on the type of substructure to choose single-barge or twin-barge float-over method. Generally, the single barge method is used for jacket platforms with slots designed and semi-submersible platforms, and the twin-barge method for jacket platforms without slots designed and Spar platforms.

By controlling the ballast water to adjust the buoyancy of the platform to achieve automatic float. Designing a fuzzy PID controller for the characteristics of strong nonlinearity, time-varying and susceptibility to the process of blowing off ballast water. In order to simplify the calculation of the characteristics of the high pressure blowing system, making assumptions that the gas and water have water-liquid interface, and the change of gas parameters within the unit time step is considered as a static process. According to the law of fluid flow, the displacement of the ballast tank is calculated by volumetric method and Bernoulli formula, and the platform model is established by using Archimedes' law and Newton's second law. Secondly, in view of the maturity and extensive use of PID control, this paper designs a fuzzy PID control to reduce the influence of the control object model and the uncertainty of gas flow state. Finally, the depth requirement of the floating process of the platform is verified by Matlab simulation. INTRODUCTION Nowadays, the ocean has become a platform for modern military powers to display their high-tech equipment and show off technology. The focus of attention at home and abroad has focused on the development of high-tech in the ocean (Blidberg, 1991). Underwater lifting platform is widely used in marine resources exploration, marine and underwater environment test. The control system of the underwater lifting platform is mainly composed of three parts: the sensor system, the integrated control system and the actuator. This paper focuses on the establishment of the platform model and the design of the control system in the integrated control system. In the aspect of underwater lifting platform construction in China, most of the operating conditions have been basically realized automatic control. However, because the research of aerodynamic control is in the initial stage, most of the current underwater lifting platform operation drills rely on manual operation (Sun, 2001). Because only relying on human judgment and accuracy can not complete the analysis under complex conditions in a limited time. It is also difficult to carry out the specified program consistently and ensure the accuracy of the system control like the automatic control system, so there is a big potential safety hazard in the control, and the timeliness and accuracy of the platform lifting operation are not satisfactory.

Marginal oilfield is becoming the primary alternative oil and gas resources in Bohai Oilfield as most of the existing oilfields are entering the middle-late development phase. In general, the development of marginal oilfield relies on other oilfields or facilities due to low IRR (internal rate of return) if developed independently. However, the existing pipe networks of Bohai Oilfield are rather complicated. A number of problems would need to be resolved if marginal oilfields were connected to these networks. These problems are illustrated with a given marginal oilfield in Bohai Bay of China, which would be developed by three production wells and two water injection wells. The nearest existing platform is about 16.1 km away from the marginal oilfield and the existing pipe network contains five pipelines. In order to realize the development of this marginal oilfield, the problems requiring attention include the pressure change caused by the change of fluid properties of the marginal field, and the influence on flow assurance of the original pipeline and existing platform. Feasible and safe solutions are proposed in this paper to resolve these problems, which could help provide useful references to other marginal oilfields. INTRODUCTION Marginal oilfield is becoming the primary alternative oil and gas resources in Bohai Oilfield as most of the existing oilfields are entering the middle-late development phase. The proved reserves of marginal oilfield in Bohai can be rather large (Wan, Qian and Wang, 2018). However, the degree of difficulty in developing these oilfields is rather high due to many factors, such as environment, higher technology requirement and low IRR. In order to develop marginal oilfield economically, a strong reliance on existing infrastructure is often required. However, the existing pipe networks of Bohai Oilfield are rather complicated. A number of problems will need to be resolved if a marginal oilfield were to connect to these networks. A comprehensive assessment of these pipe network issues is required before proceeding with the development of a marginal oilfield.

Big Data is coming. Big Oil and Gas might be hidden behind Big Data. It also brings new opportunities and challenges in the development and management of oil and gas fields. Intelligent development and management is the starting point of efficient oil and gas development. At present, offshore oil and gas field development is facing many challenges, including increasing development cost, human resource cost, operating cost as well as pressure from stricter environment protection requirements. At the same time, development of new technologies such as Big Data and artificial intelligence (AI) has brought new opportunities to deal with these problems. Faced with problems and opportunities, how to develop offshore oil and gas resources efficiently is an urgent problem to be solved. In this paper, by studying the promotion of digitalization and intelligence to the development of oil and gas fields, a preliminary scheme of intelligent oil and gas fields is proposed. INTRODUCTION Big Data is coming. The essence of the world is data. The world presents itself to the human beings with data, so a better data means better decision making. The key of Big Oil and Gas hides behind Big Data. It also brings new opportunities and challenges in the development and management of oil and gas fields (Li, Liu, Bi and Li, 2018). Intelligent development and management of oil and gas fields is the starting point of efficient oil and gas development. At present, offshore oil and gas field development is facing many challenges, including increasing development cost, human resource cost, operating cost as well as pressure from stricter environment protection requirements. The formation conditions continue being more complex which rises the exploration cost; development and production management is increasingly difficult; the number of marginal oil and gas fields is increasing; drilling, completion and engineering operation resource are limited. All result the development cost increases. QHSE requirements are more and more strict. With oil company grows, more training and safety production issues need to be addressed. Meanwhile, the corresponding regulations are more and more strict, which requires efforts to reduce emission and enhance energy saving. The increase of human resource cost lead to an increase in operating cost. At the same time, development of new technologies such as Big Data and artificial intelligence (AI) has brought new opportunities to deal with these problems. Faced with problems and opportunities, how to develop offshore oil and gas resources efficiently is an urgent problem to be solved.

The main factor affecting the operating conditions and the reliability of offshore structures in the production of exploratory drilling in the Russian Arctic is the ice regime of the water area. The problems of exploration drilling in the Arctic are among the most pressing issues in the development of offshore hydrocarbon deposits in the Arctic. The article considers a new approach to assessing the exploration time of offshore drilling rigs in ice conditions and the possibility of their extension in a mode that ensures their normal (safe) operation in such conditions. As an example, the article considers the interaction of ice formations in the form of flat ice fields and their fragments with the legs of the Jack- Up platform, and their geometric parameters (area and thickness) are determined at which the ultimate state of the marine engineering structure does not occur. NOMENCLATURE ODP – offshore drilling platform JUP - Jack-Up platform INTRODUCTION Issues of reconnaissance drilling in water areas with ice regimes remain relevant for economic activities in the Arctic, as traditional drilling platforms such as Jack-Up, Semisubmersible and Drilling Rig, which are widely used in world practice for drilling offshore wells, are wave-resistant, i.e. not designed to work in ice conditions. In this regard, experts are developing new types of ice-resistant offshore drilling platforms to work in difficult ice conditions (For example, Urycheva and Gudmestad, 2014; Toropov, Mokhov, Semenov and Livshits, 2013). One of the most important indicators for assessing the effectiveness of the use of such drilling platforms in the freezing sea is the duration of the navigation period. In the Arctic, the duration of the navigation period is usually insufficient for the construction of one well. Therefore, an urgent problem is the creation of a technology for managing the timing of drilling exploration work in ice conditions based on the management of the geometric parameters of ice formations in the water area using icebreakers. Such technology should include the destruction of drifting ice fields into fragments of controlled sizes and methods for assessing the reliability of a drilling platform depending on the size of ice formations. This technique is applicable to estimating the duration of the drilling season in ice conditions for both existing drilling platforms and newly developed drilling platforms. A description of the methodology for assessing the duration of drilling operations in order to ensure the minimum required drilling time under given ice conditions is given by the example of a Jack-Up platform.

The performance of the overset meshing tool implemented in the opensource Computational Fluid Dynamics (CFD) software OpenFOAM© is investigated in this work. The method is validated against experimental results of free decay tests of a freely-floating vertical cylinder and a Floating Offshore Wind Turbine (FOWT) available in the literature. The meshes, the numerical schemes and options, and the boundary conditions of the model are detailed. The numerical results are in good agreement with the experimental measurements, in particular for the vertical cylinder. Discussions of the discrepancies between numerical and experimental results of the FOWT case are presented. INTRODUCTION Floating Offshore Wind Turbines (FOWTs) have a great potential in deep water areas, where the metocean conditions, such as waves, wind and current, are challenging. In order to assess the hydrodynamic forces on the structures, most engineering tools use models based on linear potential theory and/or the Morison equation. Both approaches present the advantage to require a low computation time. For these models to be accurate, the following assumptions must be valid: low wave steepness (linear waves) as required by the linear diffraction problem in potential flow theory and small structure dimensions compared to the wavelength – i.e. ‘hydrodynamic transparent’ structures as required by Morison equation. These models are appropriate for predicting behavior of FOWT in ‘operational’ conditions. But extreme conditions can lead to highly nonlinear phenomena such as run-up or wave slamming. In these cases, design standards suggest simplified approaches, but their applicability is limited, especially when considering complex geometries. CFD computations may be necessary to better estimate the wave loads. It is also these conditions that drive certain aspects of the floater design. However, such methods still need to be validated before to be used in engineering for the design of a FOWT. This paper presents a method to model a freely-floating body motion using the open-source CFD code OpenFOAM©. Large displacements of the structure may also occur. To deal with these displacements, an "overset mesh" method is used based on an integrated tool in OpenFOAM©. In this method, two meshes are generated. One mesh represents the global domain and one mesh surrounds the floating body and is body-fixed (so that it moves together with the body in case of a moving body). At each time step, an interpolation is done between these two meshes to provide a single solution. This meshing technique represents an alternative to remeshing or moving mesh techniques in the cases of large body displacements. It maintains the mesh quality throughout the simulation and avoids mesh distortions.

Tension leg platform (TLP) floating wind turbines (FWTs) experience high-amplitude, dynamic structural vibrations under complex loading conditions. Tuned mass dampers (TMDs) are a simple yet effective means to reduce such loadings. This work focuses on load reduction by TMDs for the WindStar TLP system, and optimizes the TMDs' parameters in a parametric study. Dynamic analysis then assesses FWTs with and without a TMD in a selected set of load cases. Results show that optimized TMD can reduce nacelle acceleration by up to 20%, especially in extreme conditions. INTRODUCTION Wind power is a promising form of renewable energy, and many countries are developing it as a replacement for traditional energy sources. Commercial wind farms are currently mostly located on land using fixed-bottom wind turbines. However, locating a wind farm offshore could harness wind speeds that are, on average, 90% greater than those on land (Archer and Jacobson, 2005). The gradual shifting of global wind energy exploitation offshore (Kaldellis and Kapsali, 2013) can encounter problems such as wave and ice loading, more-expensive foundations, and higher operation and maintenance costs (Stewart and Lackner, 2013). Among these, the massive vibration responses due to the combined environmental and working loads can lead to serious structural safety problems. Therefore, load reduction is extremely important for offshore wind turbines, as it allows increased reliability and possibly lighter and cheaper structures (Musial, Butterfield and Ram, 2006). Offshore floating wind turbines (FWTs) for deep water are currently being researched. Their support structures are mostly based on the floating platforms used by the offshore oil and gas industry, with examples including barge, spar, tension leg, and semi-submersible platforms (Si, Karimi and Gao, 2014). The tension leg platform (TLP) is a vertically moored platform with small vertical motions and rotations, and it can serve as a stable foundation for wind turbines. Due to its excellent motion performance and economic feasibility in certain depths of water, TLPs are one of the most important classes of floating structure to support offshore wind energy (Stewart, 2012).

Minimal structures of jacket platforms are commonly applied in development of marginal offshore oil and gas fields. It is subjected to continuous cyclic wave loads during services, causing fatigue damage to the platform structures. In design of this kind of structures, assessments of fatigue damage become particularly important. Spectral fatigue analysis approach is a computationally efficient method to describe the random nature of environmental ocean wave conditions during calculating wave loads on the offshore jacket structures and subsequently structural responses. However, its fundamental theory is based on the assumption of linearity of both structural system and wave loading mechanism,but it has still been widely utilized for the design and assessment of shallow water jacket platforms with strong nonlinear mechanism. Therefore, the paper focuses on the studies on the improvement of the accuracies of the fatigue calculations by a new technical approach that can reduce the errors in the spectral fatigue analysis of shallow water platforms. This new approach can reasonably reflect the individually local sea state data by using wave height-period joint probability density function, the discussions of comparisons between the improved approach and the design code oriented method carry out by means of the analysis results. In addition, the wave probability density function which is employed for computing the fatigue damage in the existing design software is only effective for the narrow band spectra, and it causes additional errors for the broadband spectra during the computation of the fatigue damage. The appraisal on improving the calculation of the fatigue damage for the broadband spectra also carries out in the paper. INTRODUCTION Fatigue analysis plays an important role and has become compulsory in the design of offshore jacket platforms since fatigue failure is one of the major reasons causing the welding joint defects of offshore jacket platforms(Bishop, Sherratt, 1989). According to the international authoritative institutions in offshore platform, joint damage of the structures, about 70% above is caused by fatigue. The spectral fatigue analysis method has many advantages compared with other fatigue analysis methods, such as deterministic fatigue analysis and time history fatigue analysis etc(Pierson, Mosskowitz, 1964). The major reason that the spectral fatigue analysis method is recommended is computationally efficient, assessing structures where the response can be assumed to be linear, Gaussian, stationary and random.

A prototypal robotic platform that consists of a small autonomous surface vehicle (ASV) and of a micro-ROV, connected by an umbilical, is described. 4G/internet connection makes it possible to operate the platform from remote location. The operator can concentrate on guiding the micro-ROV by means of visual data, while the surface vehicle automatically coordinates its movements with those of the micro-ROV. Small dimensions, low costs and easiness of use make the platform an effective tool for low-budget survey in shallow water. The mechatronic characteristics of the platform are illustrated and discussed, together with the results of functional tests. INTRODUCTION Hybrid robotic platforms that consists of an Autonomous Surface Vehicle, or ASV, and a Remotely Operated underwater Vehicles, or ROV, are a very effective solution for inspecting underwater structures in e.g. harbor areas, rivers, natural and artificial lakes. The ASV acts as a bridge for communication with a remote station, that in this way can receive data from the ROV, avoiding the necessity of a supply vessel to operate the ROV (Scaradozzi, Conte and Sorbi, 2012; Conte et al., 2016). Examples of cooperation between autonomous surface vehicles and unmanned underwater vehicles (UUV) in hybrid robotic platforms have been considered in previous paper. In (Pascoal et al., 2000; Healey, Pascoal and Santos, 2002; Ferreira et al., 2006) an ASV and an UUV are deployed and managed independently and cooperation consists in communicating through an acoustic link to exchange information and commands. The ASV serves as acoustic relay between the AUV and a supply vessel. In (Djapic and Nad, 2010), an ASV is used to carry and deploy an AUV in a mine counter-measures applicative scenario. A robotic platform consisting of an ASV that is capable of deploying and recovering an ROV has been first considered in (Conte, De Capua and Scaradozzi, 2016) and more recently in (Gray, Schwartz and Anglerfish, 2016; Sarda and Dhanak, 2017; Jung et al., 2018; Lindsø, 2018). From a commercial point of view, the C-Worker platforms of L3 Technologies have the above-mentioned characteristics and OceanScan-MST developed a device called Manta Gateway that, mounted on a buoy or ASV, acts as a bridge between Wi-Fi, mobile broadband, Iridium communications and acoustic signals for long-range underwater transmission.

Compliant tower platform is a fixed rig structure normally used for the offshore production of oil or gas. The platform consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are typically used in water depths ranging from 1,000 to 2,000 feet, which exceeds the economic limit of the fixed platform. In this paper, Compliant tower platform working principle was fully discussed, in contrast to a conventional skirted fixed platform, a compliant tower does not have battered legs. The overall configuration is usually slender, tubular steel, and the spatial frame is relatively constant with the cross-section size in the direction of the whole structure height. Compared with the traditional fixed platform, the structure is designed to resist the environmental force and inertia force. In addition, the platform structure is the least sensitive to deck weight load among all platforms in deep waters, and the design stress level, steel tonnage and manufacturing cost are significantly lower than deep-water jacket platforms in water depth of more than 1,000 feet. Since the application of the first deep water compliant tower platform in the Gulf of Mexico in 1983, after more than 20 years of development, there are five compliant tower platforms in the world, three in the Gulf of Mexico and two in Angola, West Africa. Compliant tower platform is not a simple evolution and substitution, but a revolutionary technological development. Through the comparative analysis of the existing deep-water compliant tower platforms, the specific technical characteristics and application difficulties of the compliant tower are obtained. INTRODUCTION Compliant Piled Tower(CPT)platform has a more slender profile that requires less steel to construct. Due to its small base dimension, regular section geometry much more even distributed weight on skid beam, fabrication of CPT is relative easier. Unlike conventional platforms that are designed to resist forces, the compliant tower is designed to flex with the forces of waves, wind and current. The flex- legs are attached to the lower section of the tower with brace members and shear plates and connected to foundation pile with grout connections. The tower is secured to the seafloor with foundation piles. With flex-legs, resonance is reduced and wave forces are de-amplified. Compared with floating systems, such as tension-leg platforms and SPARs, the production risers are conventional and are subjected to less structural demands and flexing. However, because of cost, it becomes uneconomical to build compliant towers in depths greater than 1,000 meters.

SCRs have been found to have the advantages of relatively low manufacturing cost and good adaptability of floating platform's motion. For deepwater SCRs in extreme harsh environment, a steel lazy-wave riser has been adopted for reducing vessel payload and to decouple vessel motion in touch down zone. However, the challenge in installation and add on cost of buoyancy modules with their uncertain hydrodynamic response have motivated the use of a deep-draft semisubmersible to support conventional SCRs for Lingshui 17-2 field. This paper investigates the design of conventional SCRs to be used in South China Sea. As the application moves to deepwater and harsh environment, challenges of a conventional SCRs increase significantly in terms of the global performance, fabrication, and installation. This paper starts with SCR properties, coating, VIV suppression device, interface with the pipeline and the semi-submersible platform. This paper further explores the challenges and likely solutions to deepwater SCRs application for Lingshui 17-2 field. One of the challenges is the potential typhoon induced vessel heave motions that may cause significant compression loads at the touchdown zone (TDZ). It may be difficult in designing a SCR to meet working stress-based API RP 2RD requirement thus the newly released API STD 2RD is used to assess riser strength, especially under 1000-yr cyclone environmental conditions. The SCR design starts with wall thickness sizing for dealing with shut in tubing pressure, flowing temperature, and hang off tension. With the wall thickness sized, the global strength and fatigue assessment of the SCR are investigated to fit for the strength and fatigue requirements. The installation concerns come from the capacity of an installation vessel, and riser pipe size and the water depth. This paper studies the feasibility of S-lay installation for Lingshui 17-2 SCRs. It further investigates the SCR fatigue caused by Vortex-Induced Vibration (VIV) from ambient current. Moreover, Vortex-Induced Motion (VIM) of a semi-submersible platform can adversely affect the result of a SCR fatigue.