ABSTRACT Internal turret moored floating body rotates naturally around a turret and consequently aligns itself along dominant load direction with significant yaw in this process. Since yawing is an interesting feature and an important design factor, it is necessary to carry out accurate prediction, especially in complex conditions such as multidirectional irregular waves. In this paper, the nonlinear yawing of the barge with internal turret mooring systems in bidirectional and bichromatic waves was investigated by a second-order time-domain method. The results indicate that large-amplitude, slowly varying offsets persisting in the second order yawing are evident. It is pointed out that second-order low frequency wave drift forces and incident wave heights have dominant influences on the dynamic responses of the internal turret moored barge. INTRODUCTION Floating Production Storage and Offloading (FPSO) is deployed for oil and gas production in farther and deeper waters offshore. The FPSO is moored by a set of mooring lines connected to a freely rotating internal turret. The turret enables the vessel to rotate freely and consequently yaw to minimize the combined wind, current and wave-induced moment. The yawing of such vessels is one of factors of affecting motion behaviors, mooring loads and offloading operation. The prediction of the yaw for the internal turret moored vessels is a complicated problem. Many researches have been done up to now. Bernitsas and Papoulias (1986) developed a mathematical model for the horizontal plane slow motions of single point moored ships and studied yaw motions and nonlinear stability. O'Donoghue and Linfoot (1992) performed model tests in a range of long-crested and shot-crested sea states and the highlighted effects of turret position and wave spreading on motions and line tensions. The stability in yaw of a turret- moored monohull in head sea regular waves was investigated by Liu et al. (1999), and large steady yaw offsets occurring at specific wave periods was indicated. Yadav et al. (2007) conducted the parametric study of an internal turret moored FPSO to evaluate the effects of turret position and hull length in yaw instability using the AQWA suite of hydrodynamic software. Recently, Cho (2012) and Cho et al. (2013) conducted model tests and stability analysis for an internal turret moored Floating Storage Regasification Unit (FSRU). Paton et al. (2005) observed high sway yaw coupled motions and the inefficiency of mathematical tools to predict such motions. Based on the review of these literatures, we found that more attentions about internal turret moored vessels have been payed to stability analysis and experiments in head sea regular wave or irregular waves. Little dynamic analysis has been done for such vessels in complex wave conditions.

ABSTRACT The next large scale exploitation of wind energy will gradually move to the seas with the depth of 30-100m, in which only the semi- submersible and barge type foundation are suitable. Compared with the semi-submersible foundation, the barge type has simpler structure and is more adaptable to water depth, however, suffers larger seakeeping motions in waves. In order to improve the seakeeping performance of the barge foundation for offshore wind turbines, the present work proposes a concept of Air-cushion Supported Floating Platform (ASFP), and integrates the air cushion into barge foundations, which can buffer the wave loads acting on the foundation and reduce the motions. The air cushion makes the new floating foundation very different, and this paper presents a method to estimate the initial stability of the air- cushioned floating offshore wind turbine foundation NOMENCLATURE Optional, but do not use unless it is absolutely necessary. If used, place in alphabetical order, followed by any Greek symbols. INTRODUCTION The next generation of wind energy exploitation in China will move to the seas with the depth of 30-100m. Generally, the fixed offshore wind turbine is used in shallow water, and the cost increases very quickly with the increase of water depth. It is considered that the fixed one is not suitable for the water of depth more than 30m (Zhou, 2013), in which the floating one should be considered. Besides, the floating one could be built and assembled in shipyard, which is very useful to reduce the cost. So the floating offshore wind turbine should be used when the water depth is within 30-100m. Some types of platforms have been employed for floating offshore wind turbines (Ewea, et al, 2013), which can mainly be classified into four types (Wang, et al, 2010): Spar-buoy type, Tension-leg platform (TLP) type, Semi-submersible type and Pontoon type (Barge type). The Spar-buoy type needs a long body to lower the center of gravity and the required water depth should be more than 100m. The TLP type needs a certain water depth to adapt the tidal range and the required water depth should be more than 70m. So only the Semi-submersible and Barge type platforms are suitable for the seas with depth of 30-100m. Compared with the Semi-submersible platforms, the Barge type is more adaptable to water depth, and the simpler structure makes it possible to be built by concrete, which can reduce the cost and overcome the seawater corrosion effectively. But it suffers larger seakeeping motions in waves. So if the motion response of Barge type platforms in waves can be reduced, it will be very desirable to be used in the seas with depth of 30-100m.

ABSTRACT The vortex-induced motions (VIM) of semi-submersibles depends on the shape of the submerged structure, small changes in column geometries may have a significant impact on the VIM response of the platform. The study on the effect of column rounded ratios on the VIM of semi-submersible platforms has important guiding significance for the design of platforms. In the present study, numerical simulations of the VIM characteristics of a semi-submersible platform with different column comer radius at 45° current heading have been carried out by naoe-FOAM-SJTU. SST-DDES was used for modeling the massively separated turbulent flows. The displacements and hydrodynamic forces information in three DOFs including sway, surge and yaw motion are obtained. Based on the advantages of CFD method, the mechanism of the influence of the column rounded comer on the VIM of the semi-submersible platform is studied. Vortex shedding patterns and other flow field information are given for further analysis. INTRODUCTION As the increase of the depth of offshore oil and gas exploitation, semi-submersible platforms have been widely used in the field of ocean engineering because of their strong loading capacity. The heave motion performance is improved by increasing the platform draft. However, the Vortex-Induced Motions (VIM) response increases significantly because the increase of the effective excitation length of columns will lead to higher fluctuating pressure caused by the vortex shedding. As a kind of multi-column floating platform, the interaction of vortex shedding between the multiple columns makes the VIMs of the semi-submersible platforms more complex than those of the single-column floating platforms such as Spars. The VIM not only affect the operation of platforms, but also severely reduce the fatigue lives of risers and mooring systems. (Volnei et al , 2009) Experimental works on the VIM of semi-submersible platforms were carried out in recent years. (Waals et al , 2007; Magee et al , 2011; Goncalves et al , 2012; Liu et al , 2016). With the deep development of studies, many researchers transferred focus on the geometric parameters of the platforms effect on the VIM of the semi-submersible platforms. Studies have shown that the VIM response depends on the shape of the submerged structure and the columns have the most significant impact. (Gonçalves et al , 2015) At present, most of the columns of the semisubmersible platforms are square section with corner rounding, but the studies on the effect of the column rounded corner on the response of the platform are still very few. The study on the effect of column rounded corner on the VIMs of semi-submersibles platform has important guiding significance for the design of platforms.

ABSTRACT Currently, the development of renewable energy has become a trend with the increasing demand for energy. Wind energy, as a renewable source of energy, is also getting more attention. Increasing effort is devoted to developing floating offshore wind turbines in deep water. In this paper, a V-shaped semisubmersible floating wind turbine was adopted to investigate the dynamic response of the system. Numerical simulations are conducted using aero-hydro coupled analysis in a time domain. The performance of the V-shaped semisubmersible floating wind turbine with respect to global platform motion, mooring line tensions and tower base moment is evaluated in this study. It turns out that the V-shaped semisubmersible offshore wind turbine is a promising concept that provides a good practice for the application of wind energy in deep water in the future. INTRODUCTION Currently, due to energy deficiency, many countries are devoted to developing renewable energy to meet energy demands. According to the Chinese 13 th renewable energy development five year plan, by 2020, total electric from renewable energy will grow to up to 27% of the total electricity generated(NDRC, 2017). Wind energy, one of the promising renewable energies, has attracted more and more attention because of its low environmental pollution. Compared with onshore wind energy, offshore wind energy has better wind condition, unlimited sites and negligible environmental impact. Especially in China, the area with rich onshore wind resource is far from the energy consumption center, which is located near the eastern coastline (Li et al., 2012). A number of studies have been carried out for offshore wind turbine analysis (Jiang et al., 2015; Shi et al., 2016; Shi et al., 2014). The bottom-fixed wind turbine is not suitable for deep water due to increase in cost (Shi, 2015). Therefore, the floating offshore wind turbine (FOWT) is becoming one of the promising solutions. According to the offshore oil and gas industry, several different foundations are suitable for FOWT: spar buoy, tension leg platform (TLP), semi-submersible platform and barge. In particular, the semisubmersible platform, compared with spar buoy and TLP, has more feasibility in various water depth, seabed conditions and low installation costs due to the simpler installation (it is fully constructed onshore). The semi-submersible platform can also avoid the main energy range of the waves because of its relatively large natural period. The OC4 semi-submersible offshore wind turbine was simulated by Bayati (2014) to focus on the impact of second-order hydrodynamics on semi-submersible platforms. Moreover, the second-order hydrodynamic force can stimulate the oscillation of the platform and further cause fatigue damage to the structure. How the mooring systems influence the motion of the FOWT (Masciola and Robertson, 2013) determined by using coupled and uncoupled model on DeepCwind semi-submersible FOWT. Luan et al. (2016) employed a braceless semi-submersible platform to establish a numerical model and performed extreme sea states analysis on a braceless semi-submersible platform. The results showed that the platform has good stability under extreme sea and is a good design concept. A 5 WM wind turbine was employed by Kim et al. (2017), and WindFloat and OC4 floating platform were carried out to focus on the motion of FOWT and evaluating the mooring system force by using FAST (Jonkman, 2005) code.

ABSTRACT Floating Production Storage and Offloading (FPSO) vessels for offshore operations use a Dynamic Positioning system (DP), which includes a controller to correct the position variation of the FPSO subject to internal and external disturbances. Most of these positioning systems use a classic Proportional Integral Derivative controllers (PID) and their deferent variants, where the control law is determined adjusting three control gains: proportional, integral and derivative, usually heuristic techniques are employed to determine the control gains. In this study we propose a theoretical tuning procedure in order to determine the control gains in a simple way, analyzing the boundary conditions in the matrices of the FPSO dynamic model, as well as the relation they have with the control gains for the FPSO motion in an adjust domain. In order to guarantee the semi-global stability of the closed-loop system, a stability proof in the Lyapunov sense is carried out. The theoretical results were validated in numerical simulations using Matlab. These results show that the methodology presented in this work is highly satisfactory for the control gain selection in the trajectory tracking control problem for FPSO motion. INTRODUCTION A Floating Production, Storage and Offloading (FPSO) system is considered in this paper. In the last two decades FPSOs have been the dominant offshore platforms used in oil and gas fields. Fig. 1 shows the Ta'Kuntah FPSO, which was the first FPSO operated in the Cantarell Field in the Gulf of Mexico. A FPSO can be operated in deep water and sometimes must perform maneuvering movements and marine activities with other vessels as shown in Fig. 2 (Tamuri, 2009), these activities involve huge risks due to the environmental random forces presence, affecting the normal operation and sometimes causing severe accidents (Moan, 2002) and lack stability (Chen, 2008). In offshore basically there are two ways to maintain the FPSO position, the first is by Mooring Positioning (MP) and the second by Dynamic Positioning (DP) (Sorensen, 1996; Sorensen, 1997). In DP the principal advantage is the immediate positioning on a required set point, in other hand the MP systems are limited to operate about 500m (Veksler, 2016).

Abstract Two-dimensional gap resonance between oppositely leaning floating twin barges in proximity subjected to normal incident waves is investigated by both model test and numerical simulation. The experimental data and numerical results of the gap resonance between twin oppositely leaning barges are in good agreement generally, and both of which show the resonant frequency of the fluid oscillation in the narrow gap decreases with the increase of the leaning angle relative to the vertical and the resonant wave height in the narrow gap decreases with the increase of the leaning angle. Introduction Side-by-side arrangement of floating structures in proximity is often applied in marine field operations, for example, the offloading from an FPSO (Floating Production Storage and Offloading) to an oil tank or LNG (Liquid Natural Gas) ship. As the incident wave frequency is close to the natural frequency of the confined fluid bulk between floating structures, large amplitudes of fluid oscillation in the narrow gap can be observed. With a two-dimension assumption, the wave-induced gap resonance between fixed floating barges has been extensively studied by theoretical analysis, laboratory test and numerical simulation. Miao et al. (2000 and 2001) studied the wave interaction with two floating caissons with a small gap between them and examined the resonant phenomenon of the oscillation inside the gap theoretically. Saitoh et al. (2006) and Iwata et al. (2007) conducted two-dimensional experiment to investigate the gap resonance between two and three fixed boxes, respectively. Their experimental results indicated that the maximal amplitude of resonant wave motion excited in the narrow gap can approach up to about five times of the incident wave amplitude. As for the numerical simulation employed for the gap resonance, there are two main numerical models. One is the potential flow model and the other is the viscous flow model. Based on the potential flow theory, various numerical codes (Zhu et al., 2005; Li et al., 2005; Teng et al., 2006; He et al., 2006; Zhu et al., 2008; Sun et al., 2010) have been developed. The potential models can predict the resonance frequency at the gap between boxes accurately, and run very fast. It is widely used to find the resonant frequencies at the gaps of various floating bodies in practices. However, for predicting the wave amplitude in the narrow gap accurately, a viscous model must be used, as the physical energy dissipation due to fluid viscosity, vortex shedding and even turbulence plays a dominate role for the amplitude of resonant oscillation. Employing the viscous flow theory, Computational Fluid Dynamics (CFD) methods have also been utilized to investigate the fluid resonance in the narrow gaps between fixed bodies due to incident waves (Lu et al., 2008; Lu et al., 2010a; Lu et al, 2010b; Lu et al., 2011). The CFD models have a good performance in predicting both resonant frequency and wave height in the narrow gap. Though the fluid viscosity and vortex shedding are considered in those models, the application for the interaction between the incident waves and the moving floating barges are rarely seen due to the complex in meshing for moving and inclined bodies.

Abstract In this paper, a new controller is designed for OFWTs on the basis of the LPV control with the grid technique. It may cover both Region 2 and Region 3 instead of switching between the generator-torque controller and the blade-pitch controller. In addition, it may ensure the performance and stability at any point in the control region. Also, the additional motions induced by wind and waves are considered. Finally, the new controller is tested by a simulator in MATLAB/Simulink, and the model combining "NREL offshore 5-MW baseline wind turbine" with "OC3-Hywind platform" is chosen as an example for simulation.

Abstract This paper presents a numerical study on motion characteristics of a Tension Leg Platform (TLP) in waves. Time-domain simulation using convolution integral is applied to investigate the wave-induced motion response of the TLP. Hydrodynamic coefficients and wave forces are calculated using higher-order boundary element method (HOBEM) based on potential flow model. Viscous drag force acting on hull and tendon is included by using Morison's formula. Firstly, simulation results of wave-induced motion are validated by comparing with experimental data. Focus is made on nonlinear effect of viscous force on the TLP motion such as springing and ringing responses. Discussion is also made on surge drift motion of the TLP due to viscous drag force and Froude-Krylov forces.

Abstract This paper proposes optimization methods for the large cranes in the construction process of functional modules, jackets and steel piles. Functional modules are partially constructed in the way of jacking up instead of the traditional lifting way by cranes, and jackets are installed by the way of roll-up that substitutes for traditional lifting way, while the load-out method for steel piles at yard is changed from lifting by floating crane to rolling on by crawler crane and winch. Thus, the optimization methods largely reduce the quantities of large cranes that are used in the construction, and decrease the costs and schedules of construction process.

ABSTRACT: This paper presents a comprehensive study on the hydrodynamic performance of a new dry tree semisubmersible concept, Deepwater Tumbler Platform (DTP). The DTP concept is developed to suit the environmental conditions of South China Sea (SCS) and to tackle the challenges in deepwater development of oil and gas resources in SCS. A life-time feasibility study has been conducted, and this paper mainly focuses on the hydrodynamics of such a concept. Both frequency domain analysis and nonlinear time-domain dynamic analysis have been conducted, which include: hydrodynamic characteristics; load response; frequency domain analysis; nonlinear timedomain dynamic analysis. INTRODUCTION With the great development of China's economy and the improvement of people's living conditions, China's demand for oil and gas has been increasing during recent years. The South China Sea (SCS) is one of the four world-renowned offshore oil and gas resources. A preliminary estimate of the geological oil resources in the SCS's deep water totals 8.686 billion tons, with the expected recoverable resources at 2.733 billion tons; the geological natural gas resources are expected to total 5.96 trillion cubic m, with recoverable resources reaching 3.68 trillion cubic meters (Chen, 2009). However, the complexities of extreme environmental conditions, distribution of deepwater oilfields, and the status quo of China's deepwater equipment and infrastructures in SCS bring huge challenges for the development of deepwater resources. In such circumstances, when selecting a deepwater floating production platform used for deepwater oilfields in SCS, following characteristics should be in pursuit of. From the viewpoint of improving safety, platforms with better stability should be preferred. From the viewpoint of reducing offshore installation time and cost, it would be better that the topsides and hull can be integrated at quayside and towed to site in a whole piece.

ABSTRACT : A box-type floating structure is considered for a candidate of floating wind turbine structures. The platform is consisted of a box structure and large damping plates for enhancing global performance characteristics. Numerical analysis is conducted by using higher-order boundary element method. The numerical results were validated through a series of convergence test and model tests data. The global performance of the model structure is compared with same class spar-type structure. Finally technical feasibility of the structure is discussed considering construction cost and installation procedure. INTRODUCTION Offshore floating wind turbine has been paid attention due to better quality of offshore wind and a large amount of potential source in deeper water region. Due to more powerful and high quality offshore wind resources than those on lands and less environmental issues associated with noise and view, floating offshore wind farm will be more popular and dominant solutions for utilizing wind energy(Hong et al., 2012). There is three types of floating structures being considered for floating wind tower substructures; a spar type structure, TLP type and semisubmersible platforms(Justin Wilkes et al., 2012). Blue H which has 80 kW floating wind turbine installed at 113 km off the coast of Italy was the first pilot utilizing tension-leg platform design. The first large-capacity, 2.3 megawatt floating wind turbine is Hywind spar, which was installed in the North Sea off of Norway. In October 2011, Principle Power" s WindFloat Prototype was installed 4km offshore of Aguçadoura, Portugal fitted with a 2.0MW offshore wind turbine is the first offshore wind turbine installed in open Atlantic waters and make use of a semi-submersible type floating foundation. Those three concepts have long been adopted for deepwater oil production platform successfully due to their excellent global performance in waves.

ABSTRACT: Based on the 5MW offshore wind turbine model developed by National Renewable Energy Laboratory (NREL), two new conceptual offshore wind turbine floating platforms with combined Tension legs- Mooring lines system were proposed in this paper. Taking the coupled dynamic responses of the top wind turbine, tower support structure and lower mooring system into consideration, the time domain hydrodynamic analysis coupled with wind loads has been done for the two offshore floating platforms: the single offshore wind turbine (SOWT) platform and the multiple offshore wind turbines (Multi-OWT) platform. According to the sea statistical historical data for China East Sea and the typical design cases in IEC61400-3 code, the motion performances of the two OWT floating platforms have been studied by numerical simulation technique, respectively. Furthermore, during the irregular wave analysis, the effects of the current force and the slow drift force on the motion responses of the two OWT platforms were also clarified. As a result, the performance of the new combined Tension legs-Mooring lines system on controlling the motion responses of the OWT floating platforms has been verified in both views of theoretical analysis and numerical simulations. In a word, the two new conceptual OWT floating platforms would play an active and instructive role in the future design of OWT floating platforms. INTRODUCTION At present, as the continued increase of energy demand of the whole world, more and more nonrenewable fossil fuels are consumed. Considering the supply limitation and the price volatility of conventional fossil fuels, many countries are paying more and more attentions to the exploitation of the offshore wind energy resource. Comparing with the onshore wind resource, the offshore wind resource has the advantages of high wind speed, low turbulence intensity, land saving, no noise annoyances and near the coastal developed areas.

ABSTRACT: The second order wave exciting force on a floating ship considering tank liquid dynamic effect is examined. The second order drift forces are quadratic products of the first order quantities. The ship motions in wave considering tank liquid dynamic effect are calculated by linear potential theory. Then the second order drift forces are computed by near field method based on the pressure integration and validity is confirmed by far field method. For a numerical calculation example, the motions and drift forces of 250K LNG FPSO in wave condition are demonstrated. INTRODUCTION To estimate the ship motion in waves containing large liquid tanks are very complex problem. In most seakeeping computations, the liquid inside the tanks has been generally treated as a rigid mass. However, for LNG carriers and FPSO vessels during loading and unloading cargo operations the ship motion is very important topic. Coupled global ship motion and tank liquid dynamic effect have been studied numerically and experimentally by Chen, X. B. et al.(2007), Gaillarde, G. et al.(2004), Rognebakke, O. F.(2003) and Kim J. W. et al (2005). FPSO vessels are operated at a fixed station point by some kind of mooring systems. Therefore in addition to estimate the ship motion ability the evaluation of the drift force acting FPSO is required from the operational view point. Newman (2005) conducted the ship drift force calculation considering tank liquid dynamic effect based on the commercial program WAMIT. The result showed that the tank liquid dynamic effect on the drift force is significant at sloshing resonance wave period. This suggest that during loading and unloading cargo operations between FPSO and shuttle tanker which are considering the tank liquid dynamic effect on the ship motion and drift forces are essential problem.

ABSTRACT: According to the ABS Guide for Building and Classing Floating Production Installations (ABS, 2009), the design criteria to reflect the site-dependent nature of a ship-type Floating Production, Storage and Offloading (FPSO) system can be accomplished through an introduction of environmental severity factors. These factors are introduced to adjust the North Atlantic unrestricted service load and fatigue damage requirements that apply to a trading vessel to the siteand route-specific service conditions for the specific FPSO installation. The two types of environmental severity factors, labeled as the a- or b- type, are defined to account for the effects of wave condition on fatigue damage and dynamic loads, respectively. To accommodate this concept, ABS has developed the Sea Environment Assessment System (SEAS) as a part of the analysis modules in the ABS Eagle FPSO software. This paper provides technical background information about the SEAS concept and its criteria for determining the environmental severity factors that are applied for the evaluation of FPSO hull structural scantling strength. The criteria that can handle complicated wave conditions in a region where both sea and swells exist simultaneously and propagate in different directions are also addressed. INTRODUCTION Knowledge of winds, waves, currents, tides and other environmental factors is crucial for a reliable design of a floating structure such as an FPSO. The proper sea environment assessment should be made for several sea environmental conditions pertinent to FPSOs. Depending on the FPSO type, i.e., a new build FPSO, an existing FPSO expected to move to another site or an existing tanker converting to an FPSO, it may have four different sea environment situations, i.e., intended site for operation, transit from building location to the operating site, historical sites for past operation and historical routes for operation as a tanker.

ABSTRACT: Off late, nontraditional structures and concepts are being explored for use in offshore industry. Experiments were carried out on a 1:45 nonship shaped FPSO model. The main objective of the model test was to study the response of the vessel (heave and pitch) under regular and random waves. The vessel was tested for three different mooring configurations. Tests were also conducted on models with and without damping plates. In the present paper, the effect of mooring configurations in the heave response of the FPSO and the effect of damping plates in the heave and pitch responses of the FPSO under regular waves have been presented. Salient features of the details of the experiments carried out are also included. INTRODUCTION Floating, Production, Storage and Offloading systems (FPSO) are increasingly competitive to the traditional deepwater production solutions, e.g., SPAR, TLP and semi submersible in the current offshore oil and gas environment. Traditional solutions do not have onsite storage and rely on transport of crude via pipelines, whereas FPSO with its in-house storage uses offloading of the crude oil to a shuttle tanker. It is a great benefit and truly a cost and time effective to use FPSO in remote areas when new oil is discovered. Some of the ship shaped FPSO have a turret buoy system. During inclement weather conditions the vessel is disconnected from the system and re-connected back during favorable weather conditions for production operations. These operations of disconnecting and reconnecting involve lots of time, and interrupts in the production of oil and gas. Several researchers have studied the dynamic characteristics of ship shaped FPSO in winds and currents. Fernandes and Rossi (2005) studied the response of FPSO moored using linear spring lines and distorted polyester lines under regular and random waves.

ABSTRACT: In this paper, the finite element analysis of collision between a supply vessel and semi-submersible platform is presented. LS-DYNA generalpurpose explicit finite element code, which is a product of ANSYS software, is used to model and analyze the non-linear response of the platform due to ship collision. Due to the importance of damage to the platform, the ship hull is assumed rigid. Most probable impact locations and impact geometries are studied based on the dimensions and geometry of the platform and vessel, operational sea-states and relative motions of the vessel and platform. Then, various impact scenarios are considered. The worst impact scenarios are modeled and analyzed. At the end, based on the results, the mechanics of energy absorption and the extent of damage is presented and discussed. INTRODUCTION Semi-submersibles are often employed in deep waters and hostile environments instead of drilling rigs as mobile drilling platforms. They can also be used as floating production systems (ABS, 2001). Ship collision is one of the major hazards to offshore platforms. Columns of floating platforms that play a crucial role in platform" s stability are more vulnerable to the collision. In recent years, the growing demand of offshore structures and increasing maritime traffic has increased the risk of collisions. In this regard, several incidents have been reported during the last few years. The most likely events are happened between supply vessels and captive boats servicing the platform during its installation and operation. For example, in United Kingdom Continental Shelf (UKCS ) , supply vessel collisions, have a rather high probability of occurrence, approximately 17 percent per platform in year (Serco Assurance, 2003). Most of these events have occurred during operational mode of platform. Local indentation is most likely to occur in members with large diameter/thickness ratio such as semi-sub columns.

ABSTRACT: This paper summarizes a number of different development models and three major development concepts for offshore oil field developments that may be applied throughout the world. Concept 1, considers offshore facilities connected to a floating production storage and offloading (FPSO) tanker; Concept 2 considers offshore facilities connected by a pipeline to an onshore receiving terminal; Concept 3 considers offshore facilities connected by a pipeline to existing infrastructure located in the surrounding area. Based upon the same oil and gas reserves and production profiles, the three concepts were compared in terms of investment cost and economic return on investment. The results show that Concept 3, a development concept using existing infrastructure, is the most economical when the existing infrastructure is located within a reasonable distance from the new developments. The relative merits and project economics of Concepts 1 and 2 are greatly influenced by factors including the size of the reserves, water depth, distance to shore etc. This paper is useful in determining the optimal development concept, and it provides guidance for the preparation of an overall development plan. INTRODUCTION The purpose of this paper is to study offshore oil field's development cost in South China Sea under different conditions. These conditions include water depths of 100m, 500m, 1000m, 1500m and 2000m; various sizes of the reserves and 13 development options. The paper then proposes an optimal development concept for the environment conditions in the South China Sea. South China Sea lies in the semitropical zone. Average temperatures about 24 ° C for a year, July or August experiences the highest temperature of 36 °C, and January or February experiences the lowest temperature of 7°C. Annual rainfall is 1500mm, and average humidity is over 80%.

ABSTRACT: In this paper computations are presented to investigate hydrodynamic interaction for large multiple submerged bodies in shallow water near free surface. This includes hydrodynamic calculations for single submerged spheroid, while hydrodynamic interaction for two and three submerged spheroids is also investigated. Computations for all above mentioned cases are made for different wave headings and water depths. Moreover, for multi-body case, computations are made for different separation distances. Calculations include motion RAO's (response amplitude operators) linearized forces and moments as well as second order mean drift forces and moments, in regular waves. Analysis reveals that wave heading, water depth and separation distance between the bodies are very important factors, which significantly affect the results. INTRODUCTION Hydrodynamic analysis of wave action on multiple submerged bodies in shallow waters is one of the most demanding research fields in present offshore engineering. Presence of multiple submerged bodies in close proximity can cause forces of attraction or repulsion between the bodies. Moreover, hydrodynamic calculations become much more sensitive, when sea bed is close to the body. Therefore accurate prediction of hydrodynamic forces for offshore and ocean structures including multi-body interaction and shallow water effects is very important for a reliable structure design. However a relatively new and emerging concept is the use of three submerged pontoons for semisubmersibles and oil drilling platforms. Tri-hull ships are another practical example of this innovative concept. As far as hydrodynamic interaction for multiple submerged bodies in shallow water is concerned, presently, most existing works have not addressed this problem. For submerged bodies, some useful results are available in (Lee and Newman, 1991), and (Feng, Miao and Jiang, 1996), but these are mainly focused on single submerged body in infinite water depth case, and multi-body interaction effects or shallow water effects are not investigated.

ABSTRACT: The non-linear interaction of steep waves with an FPSO is discussed in the paper. The study of numerical analysis and experiments for a steep wave group interaction with a simplified FPSO has shown that the second-order free surface components at the bow of the ship are very significant, and cannot be neglected; otherwise a major underestimation of the wave impact on the structure could occur. By applying the same wave group to a few non-head on wave directions, the effect of the wave heading on the non-linear free surface elevations and run-up on the ship has been examined and discussed. INTRODUCTION The use of ship-shaped floating oil production systems is becoming commonplace around the world, even in areas with severe wave environments, with the offshore industry moving into deeper and deeper water. Non-linear wave interactions with such a ship-shaped body then become more of a concern to engineers and researchers. Fixed and floating offshore production units are very much prone to the effects of the weather, in particular as a result of their requirement to remain in the same position. FPSOs have a ship-like form with one axis of symmetry and with the longitudinal dimension much larger than the transverse one, which make them particularly sensitive to the direction of the incoming waves. A recent report of structural damage from green water on deck has shown that four of five production ships on the Norwegian continental shelf have experienced damage due to green water, and the incidents have occurred in the bow area, amidships and aft (Ersdal & Kvitrud, 2000). Recent research undertaken in Oxford has demonstrated the importance of non-linear effects for both wave loads and the surface run-up around the bow of an FPSO in random seas (Zang et al 2006).

ABSTRACT: The development of oil and gas field has reached a new stage with the substantial increase of oil price in recent years and with the increasing demand on the economical development of the field. The use of the combined tension leg platform with tender assisted drilling and fixed platforms has once again attracted the industry attention. It has been used in recent large oil field development in West Africa. This combined system has many advantages in developing China oil & gas fields in shelf area. For the areas lack of infrastructure, such as pipelines, this approach provides the flexibility in drilling, processing, production, oil storage, and future development in nearby areas. At shelf area, the water depth typically can change significantly from shallow water to relative deeper water. Fixed platform(s) can be used in shallow water area. For deepwater area, the cost of fixed platform can increase substantially. Tension leg platform can be used in the relative deeper water for drilling and initial processing, and then export the oil and gas to the fixed platform for production processing. This approach can produce substantial economic results in the shelf oil & gas field development. In South China Sea, there are many shelf areas with dramatic changes of water depth. This approach can be chosen to efficiently develop the field. If other support system is also used, such as FPSO exporting system, and tender vessel for assisting TLP drilling, it will provide more efficient field development means. This paper will explore the potential application of tension leg platform combined with fixed platform(s) in China oil field development. The advantages and technical difficulties for each system will be addressed. The paper will also address some successful examples of using the approach by industry and some technical issues encountered during the practice.