Motion Response Simulation of Damaged Floating Platforms

This thesis describes a computer based method and a procedure to simulate the motion response of a damaged platform under wave, wind and current effects. The aim of the study was to develop an analysis procedure which could be a useful tool to designers and certifying authorities in assessing the safety of mobile platforms in extreme environmental and damaged conditions. The thesis begins by explaining the benefits of using floating structures in developing oil fields. Basic stability requirements for floating production vessels are summarised. Recent and past damage simulation studies in the literature are reviewed. Some information about the number of accidents involving floating offshore platforms operated world-wide is presented. A few of the disasters occurring in recent years are given as examples to emphasise the importance of the subject. The Morison approach and 2D source-sink distribution technique are reviewed, and calculations of wave forces acting on a semi-submersible are carried out in order to make comparisons between the two methods. Theoretical derivations of wave forces in the frequency domain based on the Morison approach are carried out in detail for a twinhulled semi-submersible. The development of computer programs based on both methods is summarised. A general method for calculating wave forces and moments on circular cylinders of offshore structures is derived. By using the developed method one can calculate the wave loading on cylindrical members of fixed or floating offshore structures orientated randomly in waves. This method also provides a basic tool for determining the wave forces and moments that a floating structure is subjected to as it experiences large amplitude oscillations in six degrees of freedom. A general method is established in this chapter to calculate the hydrodynamic loading due to the rigid body motion of the platform. The calculation of restoring forces is discussed: a detailed description of the methods used to calculate hydrostatic forces, mooring stiffness coefficients and wind forces is given in the appendices. The calculation of inertia forces and moments defined from Newton's second law is introduced as part of a general calculation procedure. The derivation and the solution of motion equations in the time domain are presented. Details of model tests carried out to validate the non-linear large amplitude motion calculation procedures are presented. A description of a circular twin-hulled semi-submersible model and the loading conditions is given. The test setup and instrumentation are presented briefly. Test procedures for inclining, natural period and motion tests in waves are discussed. Methods of analysis of motion response measurements in six degrees of freedom in intact, transient and damaged conditions for head and beam seas are given. The results of motion response measurements are presented in time histories. In order to validate the numerical prediction procedures and the software based on these procedures, the physical test conditions are simulated numerically and a comparison of test results with numerical predictions is presented. Simulation studies based on the non-linear motion equations are presented with the aim of providing comparisons to illustrate the effects of non-linearities in wave and motion induced forces. A summary of the systematic study carried out to illustrate the effects of non-linear terms on the solution of the motion response equations is given. The results of the parametric studies to investigate the effects of flooding rate and of size of damaged compartment on motion response characteristics are also discussed. The other aspects of roll motion such as the effects of non-linear drag force, first order wave elevation, different wave heights and GM's, and non-linear added mass and damping forces on the motion behaviour and the steady tilt of semi-submersibles are investigated. The variations of GM and GZ values as a function of heel angle are also presented

STUDIES ON THE NONLINEAR INTERACTIONS ASSOCIATED WITH MOORED SEMI SUBMERSIBLE OFFSHORE PLATFORMS

The design of moored semi submersible systems constitutes a challenging engineering problem in which, the platform offset, stability, payload and system-optimized cost requirements are to be met simultaneously. This problem is complicated by the incomplete understanding of the nonlinearities associated with the multiple interactions such as wave to wave, wave to platform, platform to mooring, fluid to mooring and mooring to seabed. In this study, an attempt has been made to probe into these nonlinearities through numerical, experimental, and parametric studies. In the numerical study, moored semi submersibles were analyzed in the time domain. The dynamic equilibrium conditions were satisfied through a set of coupled nonlinear differential equations for the six DOF motions. For representing the platform to mooring nonlinear interactions, the 6x6 mooring stiffness matrix was derived based on the mooring stiffness and on the fairlead coordinates relative to the structure CG. For the evaluation of the slow frequency horizontal motions of the platform, the second order wave forces resulting from the second order temporal acceleration and the structural first order motions were formulated. For the assessment of the fluid to mooring and mooring to seabed nonlinear interactions, a deterministic approach for the dynamic analysis of a multi-component mooring line was formulated. The floater motion responses were considered as the mooring line upper boundary conditions. Lumped parameter approach was adopted for the mooring line modeling. Mooring to seabed nonlinear interactions were modeled assuming that the mooring line rested on an elastic dissipative foundation. A numerical dynamic analysis method in the time domain was developed and results for various mooring lines partially lying on different soils were validated by conducting a comparative study against published results. The contribution of the soil characteristics of the seabed to the dynamic behavior of mooring line was investigated for different types of soil. Two phases of experimental studies were conducted to provide benchmark data for validating the numerical methods. In the first phase, the seakeeping performance of a semi submersible with eight circular columns was studied. The model was built to scale of 1:100 using Froud’s law of similitude. The tests were conducted for head, beam and quartering seas. In the second phase, a semi submersible with six circular columns was modeled using the same scale as for the first semi submersible. Linear mass-spring system was arranged to facilitate measurements of the horizontal drift forces. The system natural periods, still water damping, nonlinear viscous damping, drag coefficient and inertia coefficient information were evaluated from the free decay tests. Seakeeping tests were conducted for head and beam model orientations. The measured drift forces were compared to available formulae in the literature to assess the available semi-empirical methods for evaluation these forces. In both experimental phases, twin-hulled conventional semi submersibles were considered. By comparing the results of the numerical and experimental models, the validity of the numerical method was established. Based on the validated numerical algorithm, a number of parametric studies were conducted for investigating the contributions of various design parameters on the dynamics of moored semi submersibles. The effects of pretension, mooring line configuration, clump weight, cable unit weight, elongation, breaking strength and pretension angle on the behavior of multi-component mooring line, were investigated by using an implicit iterative solution of the catenary equations. On the other hand, using linearized frequency domain analysis, the contributions of platform payload, platform dimensions, number of columns, number of mooring lines, the wave environment mathematical model, the wave characteristics and the operating (intact or damage) conditions to the responses of moored semi submersibles were investigated. The experimental and published results verified the efficiency of the developed numerical model for prediction of the wave frequency and low frequency motions and mooring dynamic tension responses of the semi submersible. Moreover, experimental results indicated that in addition to the modeling of the mooring system stiffness, typical or hybrid modeling of the mooring system and attachments are necessary for the critical assessment of the mooring system damaged conditions

Design of paired column semisubmersible hull

There is a constant effort to reconfigure column stabilized semisubmersible unit to meet the challenging demands associated with deep water exploration. Paired column semisubmersible platform is one of the recent column stabilized semisubmersible hull configured to allow top-deck well head compatibility for oil reserves in deep waters. Its unique ability to maintain reduced vertical motion in extreme weather conditions despite its hull size and payload create a high payload to motion ratio, as compare to conventional semisubmersible hulls. This unique feature makes it recommendable for other hull applications in ocean engineering. A study has been carried out to harness this high payload to motion ratio offered by this new hull concept in the development of drilling and production platforms in deep waters, support and foundation systems. Numerical models were developed to understand the semisubmersible hull (dynamics of the reduced vertical motion and its ability to withstand bending and twisting behaviour from extreme wave conditions). Prior to this, a preliminary CFD model was developed in to understand the vortex shedding effect on the arrayed columns. An experimental setup was also put together to understand this motion behaviour, alongside a detailed review of the first model. The motion response of a scaled hull model was studied in a wave tank with a Digital Image Correlation (DIC) system known as Imetrum. To further investigate its application for other ocean depths and support systems, series of hydrodynamic models were developed in ANSYS AQWA with weather conditions as recommended by API, DNV, and ABS. The AQWA model was validated with results recorded by Imetrum system from the wave tank experimental test. The wave forces and moments were studied for different draft sizes and ocean conditions, and their response where checked in ORCAFLEX. A finite element model was finally developed in APDL to understand the nature and effect of stresses from wave, current and wind loads, alongside topside integration. The results obtained from the FE model was use to postulate reinforcement during scantling, for different hull applications. The results for motion response showed favourable heeling moment for smaller draft sizes as recommended by regulatory bodies, but a reconfiguration for heave displacement might be required for smaller draft size. In such case, an increase in pontoon area or an additional heave plate attachment has been recommended. Furthermore, the effect of wavecurrent interactions was observed to create unique motion behaviour for all draft sizes at resonance frequency range. A fluid-structure interaction model of multi-phase flow will be required to understand this behaviour. The stress concentration on the columns generated from hydrodynamic loads was observed to be higher on the inner columns, relative to the outer ones

A parametric study of variable deck load for drilling vessels

Master's thesis in Offshore technologyThe size of semi-submersible drilling rigs has tripled over the past 50 years, with corresponding increase in cost. In order to change the direction of this development, the size of the rigs has to be challenged. Utilizing new technologies is the key for succeeding. By reducing the required variable deck load (VDL), existing rigs could increase their capacity, and the size of the future rigs could be reduced without jeopardizing their operational capacities. This thesis presents a parametric study of the VDL where the objective is to identify technologies that can reduce the required VDL, and attempt to quantify reduction potentials for key contributors of the required VDL. Theoretical background for the semi-submersible drilling rigs and VDL is presented. The identified technologies are presented and their reduction potential is established and discussed, as well as the increased operational capacity due to the identified technologies. The focus has been on technologies that can reduce the key contributors of the VDL. The capacity of the drilling rig Maersk Deliverer, together with the characteristics of the drilling rigs on the market today was used as a basis to identify the largest contributors of the VDL and the potential increase in capacity. The results show that there is potential to reduce the required VDL by applying new technologies. For existing rigs this means increased operational capacity, e.g. a 4th generation drilling rig has the potential to operate within the same operational range as a 5th generation drilling rig. The reduction in required VDL also leads to more free storage space, which is an advantage when drilling in remote locations. For the development of future generations of drilling rigs the results indicates that the size can be reduced without decreasing the operational capacity

Coupled Dynamic Analysis of Multiple Unit Floating Offshore Wind Turbine

In the present study, a numerical simulation tool has been developed for the rotor-floater-tether coupled dynamic analysis of Multiple Unit Floating Offshore Wind Turbine (MUFOWT) in the time domain including aero-blade-tower dynamics and control, mooring dynamics and platform motion. In particular, the numerical tool developed in this study is based on the single turbine analysis tool FAST, which was developed by National Renewable Energy Laboratory (NREL). For linear or nonlinear hydrodynamics of floating platform and generalized-coordinate-based FEM mooring line dynamics, CHARM3D program, hull-riser-mooring coupled dynamics program developed by Prof. M.H. Kim’s research group during the past two decades, is incorporated. So, the entire dynamic behavior of floating offshore wind turbine can be obtained by coupled FAST-CHARM3D in the time domain. During the coupling procedure, FAST calculates all the dynamics and control of tower and wind turbine including the platform itself, and CHARM3D feeds all the relevant forces on the platform into FAST. Then FAST computes the whole dynamics of wind turbine using the forces from CHARM3D and return the updated displacements and velocities of the platform to CHARM3D. To analyze the dynamics of MUFOWT, the coupled FAST-CHARM3D is expanded more and re-designed. The global matrix that includes one floating platform and a number of turbines is built at each time step of the simulation, and solved to obtain the entire degrees of freedom of the system. The developed MUFOWT analysis tool is able to compute any type of floating platform with various kinds of horizontal axis wind turbines (HAWT). Individual control of each turbine is also available and the different structural properties of tower and blades can be applied. The coupled dynamic analysis for the three-turbine MUFOWT and five-turbine MUFOWT are carried out and the performances of each turbine and floating platform in normal operational condition are assessed. To investigate the coupling effect between platform and each turbine, one turbine failure event is simulated and checked. The analysis shows that some of the mal-function of one turbine in MUFOWT may induce significant changes in the performance of other turbines or floating platform. The present approach can directly be applied to the development of the remote structural health monitoring system of MUFOWT in detecting partial turbine failure by measuring tower or platform responses in the future