Using a Simplified Smoothed Particle Hydrodynamics Model to Simulate Green Water on the Deck

Severe storms have gained more attention in recent years. Improved metocean data have led to new insight into severe wave conditions for marine design. Therefore, there exists an industrial demand for fast and accurate numerical tools to estimate the hydrodynamic loads during e.g. green water events. Model tests generally play an important role in these studies. In the recent past, several practical engineering tools have also been developed, based on the experience from the experimental data bases in combination with simplified but still theoretical formulations. One such tool is Kinema2, which is based on non-linear random wave modeling combined with 3D linear diffraction theory to initially identify green water events, and then finally apply a simplified water-on-deck and slamming load estimation. This forms the background for the work presented in this paper which shows the feasibility of a new technique based on the Smoothed Particle Hydrodynamics (SPH). This method can give more detailed forecast of the hydrodynamics on the deck than the simplified water-on-deck estimation. SPH uses a Lagrangian framework (particles) to describe the fluid dynamics. The water propagation and kinematics of the green water events are, in this introductory stage of the study, reproduced by using a SPH inlet condition where particles are injected with given velocity from a curved rectangular area against the deck and the deckhouse. The relative wave height and water particle velocities found from KINEMA2. Numerical results for water elevation and velocity on deck are compared against model test time series and previous results from other numerical simulation methods. The present Lagrangian nature (compared to traditional Eulerian-VOF methods) can in principe significantly reduce the CPU demand and increase the simulation speed. Slamming pressures can then be calculated e.g. from simple slamming formula calculations. In principle, pressures can also be found directly from the SPH calculations, while this would demand a significantly larger number of particles which increases CPU demand of the SPH method.

Optimization of FPSO’s Main Dimensions Using Genetic Algorithm

An efficient concept design of FPSO is supported by a wide variety of tools and methods. Some use parametric solutions for a variety of projects, while others deal with project-specific issues. One of the early challenges is to establish main dimensions of the “buoyancy element” for Floating Production System. The size should provide appropriate load-carrying capacity at minimum cost within limits of a given field development project. Since the financial implications of the hull size are significant, the choice should be made by means of a proper engineering study. One of the most promising approaches to this task is to utilize Multi Objective Genetic Algorithm (MOGA) for optimization of main particulars. A method was developed that aims at finding optimum main dimensions of the FPSO unit of given storage capacity with respect to steel weight and strength. This was achieved by utilizing MOGA optimization tool available in NAPA stability software package. The paper presents procedures, challenges and samples of results of such studies. The method was recently utilized in two different FPSO concept projects. First comments and conclusions that were drawn from the results of the procedure will be presented. Some ideas for future development will also be given.

First and Second Order Wave-Current Interactions for Floating Bodies

The influence of current in sea-keeping problems is felt not only for first order quantities such as wave run-ups in front of the structure, but also mainly for second order quantities. In particular, the wave drift damping (which is expressed as the derivative of drift force with respect to the current) is of special interest for mooring systems. The interaction effects of a double-body steady flow on wave diffraction-radiation is studied through a decomposition of the time-harmonic potential into linear and interaction components. A boundary integral method is used to solve the first order problem. Ultimately, a far-field method is proposed to get access to second order drift forces.

Stochastic Analysis of Impact Loads on Marine Structures

An efficient Monte Carlo method is developed to perform the stochastic analysis of slamming loads on marine structures. The probability distribution of the maximum impact force during slamming is established for a given stationary sea state. The method is demonstrated by using a uniform wedge and Wagner’s flat plate theory. Stochastic effects related to deadrise angle, flow separation from the knuckle, vertical position of the structure, vertical motion and roll motion are investigated. The statistics of other parameters such as the duration of the slamming event and the maximum penetration of the structure into water are also assessed.

Support Vector Regression-Based Multidisciplinary Design Optimization for Ship Design

The design of ship is related to several disciplines such as hydrostatic, resistance, propulsion and economic. The traditional design process of ship only involves independent design optimization within each discipline. With such an approach, there is no guarantee to achieve the optimum design. And at the same time improving the efficiency of ship optimization is also crucial for modem ship design. In this paper, an introduction of both the traditional ship design process and the fundamentals of Multidisciplinary Design Optimization (MDO) theory are presented and a comparison between the two methods is carried out. As one of the most frequently applied MDO methods, Collaborative Optimization (CO) promotes autonomy of disciplines while providing a coordinating mechanism guaranteeing progress toward an optimum and maintaining interdisciplinary compatibility. However there are some difficulties in applying the conventional CO method, such as difficulties in choosing an initial point and tremendous computational requirements. For the purpose of overcoming these problems, Design Of Experiment (DOE) and a new support vector regression algorithm are applied to CO to construct statistical approximation model in this paper. The support vector regression algorithm approximates the optimization model and is updated during the optimization process to improve accuracy. It is shown by examples that the computing efficiency and robustness of this CO method are higher than with the conventional CO method. Then this new Collaborative Optimization (CO) method using approximate technology is discussed in detail and applied in ship design which considers hydrostatic, propulsion, weight and volume, performance and cost. It indicates that CO method combined with approximate technology can effectively solve complex engineering design optimization problem. Finally, some suggestions on the future improvements are proposed.

Hydrodynamic Analysis of Tension Based Tension Leg Platform

Tension Leg Platforms (TLPs) are one of the best options for offshore industry in deep waters due to proven motion response characteristics. These are water depth sensitive structures and the motion responses in vertical plane motions (heave, roll and pitch) are critical for a TLP. Tension Based TLP (TBTLP) is a new concept and finds application in much deeper waters. A provision of a tension base at mid-depth results in an economical design of TLP. In fact, the TLP installed at a certain depth without any modifications can be made to be deployed in much deeper water depths by means of a tension base. In this paper, the concept of TBTLP is highlighted and hydrodynamic analysis of the chosen platform has been carried out using ANSYS AQWA package. The motion responses in terms of Response Amplitude Operators (RAOs) of TBTLP with one Tension Base in surge, heave and pitch have been obtained and compared with a TLP without a tension base.

Third Party HIL Testing of Safety Critical Control System Software on Ships and Rigs

The drilling industry is characterized by a rapid and up front technology development to conquer larger water and drilling depths. The level of automation has been steadily increasing over several decades, growing from manually operated sledge-hammer technology to space-age computer-based integrated systems. Most of the automation systems on today’s vessels are put into operation without independent testing. This is a paradox considering that a single control system may be more complex than all the mechanical systems onboard. It is also a paradox that the automation systems often contain safety-critical failure handling functionality that may be difficult or dangerous to test onboard the real vessel, and therefore is not properly tested until it is activated during an emergency situation. These automation systems are essential for the safety, reliability, and performance of the vessels. Examples are the Dynamic Positioning (DP) systems, Power Management systems, Drilling Control Systems, BOP control systems, Managed Pressure Drilling (MPD) systems, and crane control systems. Hardware-In-the-Loop (HIL) testing is a well proven test methodology from automotive, avionics, and space industries, and is now also gaining recognition in the marine and offshore industries. The aim of this paper is to clarify what HIL testing is, how third party HIL testing can be applied to safety critical control system software on drilling ships and rigs, and why this is an important contribution to technical safety, reliability and profitability of offshore operations.

The Wind Tunnel Experiment Study of Wind Load on Jack-Up Drilling Unit

Owing to the characteristics of the Jack-up drilling unit, the environment loads are important factors to the structure safety of jack-up. The wind load takes up a much larger proportion in structure analysis, and it has been now paid more and more attention in the field of offshore engineering. According to the MODU rule, the projected area method is used to calculate wind load. The height coefficient Ch and shape coefficient Cs are chosen by experience listed in MODU rule. However, the aerodynamic interference between platform components, such as wind shielding effects and acceleration flow effects are ignored. So the calculation results in accordance with MODU rule are conservative, and not good for structure optimization designing to some degree. Therefore, the wind tunnel experiment is very important and necessary. In this paper, a 400ft jack-up, which is developed by CIMC, is studied as an example. Considering the Reynolds number effects, the wind loads of jack-up both in normal drilling condition and severe storm condition are obtained in the experiment. Compared with the results, which are calculated according to MODU rule, it indicates that the experiment results are less, and the effect of aerodynamic interference and updraft can’t be ignored.

Practical Structural Assessment of Offshore Structures for Wave Slap

By the combination of theoretical and empirical approach, the methodology for practical structural assessment of offshore structures for wave slap is proposed. It is developed for engineers in the sense that the precise design pressure is easily obtainable and quickly applicable in early and detail design stage. For impact load prediction, the Pressure-Impulse theory that was well developed and validated in coastal engineering field is applied. The impact pressures are classified into three types (traditional, sharp, and immersed slap) according to model tests and BP Schiehallion FPSO’s bow monitoring. The time histories of impact pressures for the classified impact types are generated with the pressure impulse predicted by the Pressure-Impulse theory. Nonlinear transient structural analyses are performed using the time series of impact pressures to obtain equivalent static pressure factors. Finally, the design pressure is determined by multiplying the maximum peak pressure by the equivalent static pressure factor. The results are validated through the comparison with model tests and dedicated reports.

Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue. The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations. By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure. This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system. The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty. It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.