Motion and Dynamic Responses of A Semisubmersible in Freak Waves

The present research aims at clarifying the effects of freak wave on the motion and dynamic responses of a semisubmersible. To reveal the effects of mooring stiffness, two mooring systems were employed in the model tests and time-domain simulations. The 6-DOF motion responses and mooring tensions have been measured and the 3-DOF motions of fairleads were calculated as well. From the time series, trajectories and statistics information, the interactions between the freak wave and the semisubmersible have been demonstrated and the effects of mooring stiffness have been identified. The shortage of numerical simulations based on 3D potential flow theory is presented. Results show that the freak wave is likely to cause large horizontal motions for soft mooring system and to result in extremely large mooring tensions for tight mooring system. Therefore, the freak wave is a real threat for the marine structure, which needs to be carefully considered at design stage.     

一种新型半潜式平台概念设计研究

在详细研究影响平台综合性能的诸多因素基础上,克服现有半潜式平台存在的不足,结合自升式平台甲板可以自由升降的优点,开发了具有自主知识产权的自升式半潜多功能深海浮式（JCSM）平台。JCSM平台结合了自升式平台与半潜式平台的优势,并以平台综合性能的提升为目标进行了细节上的优化,稳性裕度大且运动性能优良,可适应干式采油、湿式采油和干湿组合式采油等不同采油模式,并同时具有钻井、早期生产、修井、油气处理与油气储运等多种功能。在详述JCSM平台的组成及性能特点的基础上,对JCSM平台进行了稳性与运动性能数值分析,并与传统半潜式平台相比较,验证了所研发新型平台性能的优越性。研究为未来海洋油气田开发提供了一种新的选择,并对全新概念的浮式平台开发有借鉴和指导意义。     

A survey on semisubmersible wind loads

The methods and assumptions of calculating wind drags and overturning moments on semisubmersible offshore drilling rigs are investigated. The empirical methods and the wind tunnel testing are compared.     

On-line identification and convergence analysis of excitation-force and drag-force models for moored floating structures

This work presents an approach to identify hydrodynamic models for incident, diffraction and viscous forces acting on a moored floating structure. An important aspect treated here is the analysis of the unknown initial condition of the hydrodynamic state for the potential-radiation force. There is established its influence on the parameter convergence and the long-term effects. Afterwards the persistency of excitation of the regressor is analyzed in the case of both poor and rich excitation conditions. Theoretical results show that asymptotic convergence of the estimates takes place under arbitrary conditions of the wave excitation. A case study consisting in the identification of a moored semisubmersible is carried out to exemplify the application of the approach.     

Low-frequency damping of a moored semisubmersible obtained from simulated extinction tests and mean wave drift forces

Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective.     

CODAR wave measurements from a North Sea semisubmersible

CODAR, a high-frequency (HF) compact radar system, was operated continuously over several weeks aboard the semisubmersible oil platform Treasure Saga for the purpose of wave-height directional measurement and comparison. During North Sea winter storm conditions, the system operated at two different frequencies, depending on the sea state. Wave data are extracted from the second-order backscatter Doppler spectrum produced by nonlinearities in the hydrodynamic wave/wave and electromagnetic wave/scatter interactions. Because the floating oil rig itself moves in response to long waves, a technique has been developed and successfully demonstrated to eliminate to second order the resulting phase-modulation contamination of the echo, using separate accelerometer measurement of the platform's lateral motions. CODAR wave height, mean direction, and period are compared with data from a Norwegian directional wave buoy; in storm seas with wave heights that exceeded 9 m, the two height measurements agreed to within 20 cm RMS, and the mean direction to better than 15° RMS     

Multi-Objective Optimization of A Semisubmersible for Ultra-Deep Water

Semisubmersible will work well when oil exploitation goes to ultra-deep water because of its variable load capacities, and good motion performance in extreme waves. It is considered to be a main type of platform while the water depth is more than 3000 meters. This paper establishes a multi-objective optimization model of semisubmersible for ultra-deep water, and it is solved by a multi-objective genetic algorithm--NSGA-Ⅱ. The model is applied to a practical design, and Pareto results are obtained. The effectiveness of the method is verified by hydrodynamic analysis.     

Resonant heave motion of semisubmersible vessels

This paper is concerned with nonlinear resonant heave motion of a semisubmersible vessel at the survival draft. Due to the small potential of the hulls at deep draft the resonant motion is governed almost entirely by nonlinear drag forces on the hull and bracing members.Long period ocean swell following a storm can excite semisubmersiblesat resonance in heave and delay the resumption of drilling activities.The paper presents a simple closed form solution for determining the heave motion at the natural period which agrees well with other theoretical and experimental results.     

The tuna offshore unit: concept and operation

A radical new concept for delivering Bluefin Tuna to the marketplace is proposed offering offshore technologies to the world's fish industry. The Tuna Offshore Unit is a 190-m length and 56-m breadth vessel that resembles a semisubmersible with a catamaran hull and internal arrangement of fish-pools that can be enlarged in operation by deploying a rigid net for fish cultivation. Using the vessel, juvenile bluefin tuna are purchased at selling centers (e.g., hatcheries), fattened to a total bio-mass of 1200 tons over a period of nine months in warm-water climates and delivered to live fish markets during periods of high demand. This paper describes the main features, working principles, and design challenges of the Tuna Offshore Unit.     

A decision support system for floating platform design

This paper describes a methodology developed for the preliminary design of floating production platforms. The preliminary design approach interlinks the mathematical design model with global optimization techniques. To illustrate the design procedure, the initial intermediate and final results are presented. They provide a clear understanding of the mathematical model and the decision support system. The preliminary design approach described here is based on the commercial software which is interfaced to what is called the FPS system and linked to a SEARCH package for design solutions. The paper summarizes the preliminary design approach, the mathematical model optimization and presents semisubmersible and TLP platform examples.     

An Effective Control System of Thrust and Moment Modulation for a Dynamically Positioned Vessel

Dynamic positioning (DP) is an operation method whereby the position of a surface vessel is maintained in close proximity to a required position in the horizontal plane through the controlled application of forces and moments generated by purposely installed thrusters. When estimating thrust, this kind of conventional control system often uses many acceleration sensors, velocity sensors, environment sensors, and filters. Usually, these sensors have measured electrical errors. To reduce the number of sensors used and to decrease the measurement errors, this article presents an effective control system for estimating thrust and moment commands, which is based on energy and impulsemomentum principles. Donha and Brinati's example is followed to verify the feasibility of the present control system, which performs semisubmersible platform positioning using an LQG controller, and the results are feasible and economical. A simulated coring vessel marine positioning in southern Taiwan is presented, which can estimate the counterthrust and moment commands, and the complex environmental forces and moments are described. The results can provide a valuable control system for dynamically positioned vessels.     

新型浮式基础的海上风机系统动力响应研究

概念性地设计了一种新型半潜—Spar混合浮式基础,以5 MW水平轴风机为例,研究了该新型浮式基础支撑的浮式风力机系统的动力响应。基于三维势流理论和Morison公式,应用SESAM软件建立浮式基础模型,在频域内计算了该浮式基础的水动力参数和响应算子,分析了浮式基础的运动性能。考虑叶片气动载荷和浮式基础波浪载荷,应用FAST软件对风机—浮式基础系统进行时域计算,分析风力机系统的运动性能。结果显示,该浮式基础运动幅值较小,具有良好的运动性能。     

半潜式平台动力定位系统控制力动态约束分配法

半潜式平台动力定位推力分配系统通过计算每个推力器的推力和推力角,将控制力分配到推力系统的各推力器上。基于系统耗能最小模式,根据推力器之间的损失模型,通过动态设置推力可行域,将推力分配问题转化为求解二次型优化模型。考虑推力器的物理性能,以推力变化率和转角变化速率为控制参数,利用序列二次规划法得到数值计算结果。该方法充分考虑推力器的物理性能,降低推力器的机械磨损,且可以避免处理可行域为非凸集问题,有效地提高推力器的推力效率。     

An Effective Control System of Thrust and Moment Modulation for a Dynamically Positioned Vessel

Dynamic positioning (DP) is an operation method whereby the position of a surface vessel is maintained in close proximity to a required position in the horizontal plane through the controlled application of forces and moments generated by purposely installed thrusters. When estimating thrust, this kind of conventional control system often uses many acceleration sensors, velocity sensors, environment sensors, and filters. Usually, these sensors have measured electrical errors. To reduce the number of sensors used and to decrease the measurement errors, this article presents an effective control system for estimating thrust and moment commands, which is based on energy and impulsemomentum principles. Donha and Brinati's example is followed to verify the feasibility of the present control system, which performs semisubmersible platform positioning using an LQG controller, and the results are feasible and economical. A simulated coring vessel marine positioning in southern Taiwan is presented, which can estimate the counterthrust and moment commands, and the complex environmental forces and moments are described. The results can provide a valuable control system for dynamically positioned vessels.     

海上风电半潜式基础初步选型Pareto-Optimal评价

针对半潜漂浮式风电基础初步选型,采用Pareto-Optimal评价方法对不同吃水、平台立柱直径、立柱间距和垂荡板直径四个参数的不同组合进行分析比较。基于浮体动力学频域计算方法,采用我国阳江某海域极限波浪条件计算得到叶轮中心水平加速度,同时考虑完整稳性的计算结果。对比分析表明平台吃水和立柱直径宜选择适中的取值,较大的排水量和立柱总体积并不会显著减小叶轮中心水平加速度。垂荡板对于改善平台整体性能是较为敏感的,垂荡板与立柱的直径比存在一定的最佳范围。平台立柱间距是影响平台运动性能最大的因素,增大立柱间距可以有效地降低叶轮中心水平加速度,但立柱间距的增大对立柱间的撑杆结构强度以及平台整体的建造和下水提出了较大的挑战。     

Optimal identification of potential-radiation hydrodynamics for moored floating structures—a new general approach in state space

In this paper, a new state-space model of the potential-radiation hydrodynamics in moored ocean engineering floating structures and its parameter identification are presented. The raw data for this goal are the added mass and potential-damping matrices in frequency-discrete domain.In a preliminary study of existing approaches in the literature, two mathematical models and their estimation are comparatively analysed in detail. These served in the development of the new approach that shares certain main advantages of the previous approaches.The model is identified in a least-squares sense using a weighted norm and a free control parameter to accomplish a trade-off between quality and stability. This reduces numerical instability problems and also keeps the analytical and computational benefits of a parametric state-space model. The model can be conveniently expressed in any usual canonical form in state space.The application of the model acceptably accurate reproduces the behaviour of the potential-radiation hydrodynamic forces in time. Case studies involving a semisubmersible and buoys are shown to demonstrate the features of the proposed approach.     

A new concept of a hull form to improve wave response characteristics for marine construction barges

The object of the new hull form is to provide a single hull which possesses long natural periods of roll and heave and has substantially reduced motion response amplitudes in very high sea states. Model tests and preliminary estimates indicated that the new hull form can be designed for roll and heave motions nearly equivalent to those of much larger semisubmersible units.All existing conventional marine construction barges have rectangular cross section hull. The new hull form consists of a system of upper side tanks and lower side tanks added onto a rectangular cross section hull. The upper tanks and lower tanks form longitudinal troughs on the port and starboard sides. Structural grillage of any open type is to connect the upper and lower tanks at the side of the vessel. Figure 1 indicates a profile and a typical transverse section of the new hull form. The new hull comprises the concept of reduced water plane area which is turn results in low transverse metacentric height and low tons per in. immersion. The novel features of combining low GM_T and low TPI with extremely heavy damping and added mass of the entrained water characteristics result in very long natural periods of roll and heave and considerably small rolling and heaving amplitudes in high sea states. The open side shell plating on the side of the vessel functions to dissipate wave energy at the side of the vessel which would have otherwise been transmitted to the vessel and caused the vessel to respond. This paper presents the conceptual foundation and outline of the new hull form. Model test results are presented and implemented. Also presented is the design philosophy.     

Optimal Stiffness for Flexible Connectors on A Mobile Offshore Base at Rough Sea States

This paper investigates a simplified method to determine the optimal stiffness of flexible connectors on a mobile offshore base (MOB) during the preliminary design stage. A three-module numerical model of an MOB was used as a case study. Numerous constraint forces and relative displacements for the connectors at rough sea states with different wave angles were utilized to determine the optimized stiffness of the flexible connectors. The range of optimal stiffnesses for the connectors was obtained based on the combination and intersection of the optimized stiffness results, and the implementation steps were elaborated in detail. The percentage reductions of the optimized and optimal stiffness of the flexible connector were determined to quantitatively evaluate the decreases of the constraint force and relative displacement of the connectors compared with those calculated by using the original range of the connector stiffnesses. The results indicate the accuracy and feasibility of this method for determining the optimal stiffness of the flexible connectors and demonstrate the rationality and practicability of the optimal stiffness results. The research ideas, calculation process, and solutions for the optimal stiffness of the flexible connectors of an MOB in this paper can provide valuable technical support for the design of the connectors in similar semisubmersible floating structures.     

济阳坳陷探井地质风险评价方法——以陡坡带砂砾岩体为例

济阳坳陷圈闭已进入隐蔽油气藏勘探阶段,当前面临的主要地质风险是不同类型隐蔽油气藏识别及含油气性判断、同一类型隐蔽油气藏的规模等。此文针对勘探重点类型陡坡带砂砾岩体建立探井地质风险评价方法,主要是从成藏地质条件和失利探井两个方面对地质风险的成因进行分析,总结成藏的主控因素,进而确定地质风险评价的地质要素,并通过地质条件和统计分析,对地质要素进行分级,建立分级标准;利用勘探成功率影响程度法,确定权系数,从而建立地质风险评价标准。用2009年已钻探井对评价标准进行检验,评价结果与钻探结果吻合率为80％,取得了较好的应用效果。     

In situ tests for ice strength measurements

Ice effects have caused extensive damage. Enormous forces have been recorded. On the other hand, ice has been used as a structural material under various situations. To use ice beneficially and to assess the danger of ice forces, it is necessary to know the mechanical properties of ice.Results from the large number of ice tests carried out in laboratories show the difficulties of sampling and testing procedures. Most of the tests have not reproduced faithfully enough the conditions existing in nature. To obtain adequate results, laborious and slow samplings and preparations of test specimens have been necessary. Recently, extensive use has been made of in situ tests in order to reduce the number of factors whixh can induce errors in measurements. Large scale tests have simulated actual structures. General ice properties have become available from small-scale in situ tests. Since these tests give rapid results, a large number are already available with a good coverage of field conditions.A theoretical basis has been devised for these in situ tests which measure essentially the unconfined and confined compressive strengths of ice, and practical devices have been developed to execute these tests. A correlation of measurements from laboratory tests and in situ tests has established a link between natural and artificial environments. Brittle elastic and clastic theories have been applied, to find the best correlations for ice tests. Test results have been related to the mechanical properties as used in engineering practice.A very summary conclusion of results is that cold sea ice and cold fresh water ice are both very strong. However, depending on the load rate, ice can be quite brittle, which mitigates dangerous effects from ice action against massive structures.