Modelling of Seabed Interaction in Frequency Domain Analysis of Mooring Cables

Mooring cables under wave loading interact dynamically with the seabed; this interaction is nonlinear and can be modelled in full only by performing lengthy time integration of the equations of motion. However, time domain integration is far too computationally expensive to be carried out for all load cases. A new method of modelling the interaction between a cable and the seabed in the frequency domain, but without considering frictional effects and impact, is therefore proposed. The section of cable interacting with the seabed is truncated and replaced with a system of coupled linear springs, with stiffnesses linearised from static catenary equations. These springs would model the behaviour of the truncated cable and hence the time-varying boundary condition at the touchdown. The entire cable-spring system is then analysed in the frequency domain with a centred finite difference scheme. The proposed method has shown to increase the accuracy of frequency domain analysis in certain cases with affordable computational overhead.

Drag Force Reduction and VIV Suppression of Circular Cylinders Using Radial Water Jets

Deep-water oilfield developments demand accurate predictions of vortex induced vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as drag amplification and increased operational costs. Therefore the quest for effective suppression devices with a minimum of such drawbacks is still ongoing. The present paper presents a novel approach for VIV suppression based on radial water jets from prescribed patterns of circular openings in the cylinder wall. Jet flow will introduce a disturbance that will change VIV amplitudes. The alternation of the flow pattern must be understood to have both 2-dimensional (2-D) and 3-D effects. 2-D effects will influence the local pressure on the cylinder surface by altering the separation point as well as creating a general disturbance to the flow, while the 3-D effects involve changes in correlation of the vortex shedding process along the span of the cylinder. Results will be presented from experiments in a towing tank testing four 2m long spring supported cylinders with diameter 0.1m and three different patterns of radial water jets. One cylinder has three straight rows of holes at angular positions 0° and ±120° with respect to the ambient flow. The second has two straight rows of holes at positions ±120° with respect to the ambient flow, while the last has one straight line of holes blowing directly upstream. Since the first can be said to consist of the two latter, comparing them to each other gives valuable information in order to understand the physics of the first. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and drag force coefficients are presented and compared to a smooth cylinder.

Wave Action on Double-Cylinder Structure With Perforated Outer Wall

Based on an eigenfunction expansion of velocity potential and a linear model between the pressure difference between two sides of a perforated wall and the fluid velocity inside it, a semi-analytic linear solution has been acquired for wave interaction with a combined cylinder with an solid interior column surrounded by a coaxial exterior column with perforated wall at a section in azimuthal direction. Numerical experiments have been carried out to examine the influences on the wave force and wave run-up on the combined cylinders with perforated wall by the porous coefficient, the size of the perforated section, and the ratio between the radii of the interior and the exterior columns. This paper also presents the comparison between the numerical experiments results and the physical experiments results. It is acceptable of the comparison of these two results. The combined cylinder may reduce both the wave run-up and the wave loads on it through combination of certain parameters.

Generation of Wave-Field Time Histories by the Modulated Discrete Fourier Transformation

In the time domain simulation of the response of an offshore structure under random waves, the time histories of the wave field should be generated as the input to the dynamic equations. Herein the wave field is the wave surface elevation, the water particle velocities and accelerations at structural members. The generated time histories should be able to match the given wave-field spectral descriptions, to trace the structural member motions if it is a compliant offshore structure, and be numerically efficient. Most frequently used generation methods are the direct summation of a limited number of cosine functions, the Fast Fourier Transformation, and the digital filtering model. However, none of them can really satisfy all the above requirements. A novel technique, called the Modulated Discrete Fourier Transformation, has been developed. Under this method, the wave time histories at each time instant is a summation of a few time-varying complex functions. The simulated time histories have continuous spectral density functions, and the motions of the structural members are well included. This method seems to be superior to all the conventional methods in terms of the above mentioned three requirements.

An Independent-Flow-Field Model for a SDOF Nonlinear Structural System: Part II — Analysis of Complex Responses

Complex responses observed in an experimental, nonlinear, moored structural system subjected to nearly periodic wave excitations are examined and compared with the simulations of a newly proposed independent-flow-field (IFF) model in this paper. Variations in wave heights are approximated by additive random perturbations to the dominant periodic component. Simulations show good agreement with the experimental results in both time and frequency domains. Noise effects on the experimental results including bridging and transition phenomena are investigated and interpreted by comparing with the simulations of its deterministic counterpart. Possible causes of a chaotic-like experimental result as previously observed are also inferred.

Monitoring Axial Strain in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers

Synthetic fiber ropes constructed of polyester are providing an important new technology for mooring deep-water drilling and production platforms. Considerable effort is being directed toward advancing and qualifying this enabling and cost-effective technology. To date, synthetic fiber mooring ropes have been successfully deployed in Brazil and they have seen limited service in the Gulf of Mexico. Synthetic fiber mooring ropes have high strength-to-weight ratios and possess adequate stiffness, but they are much more susceptible to damage than their steel counterparts. Future safe deployment of synthetic fiber mooring ropes would be significantly enhanced if a reliable technique were available to monitor the performance of the ropes in service and thus provide an early warning of the loss of structural integrity. Test data in the open literature indicates that the strain in the rope at failure is essentially a constant independent of load path or history. Measurement of the accumulated strain in the rope should thus provide a reliable benchmark with which to estimate the remaining life and establish criteria for rope recertification or retirement. This paper discusses the results of research and development activities aimed at developing a reliable, robust method for monitoring strain in braided and twisted strand Synthetic Fiber Mooring Ropes [1]. The strain transducer is a polymeric optical fiber, integrated into the mooring rope and interrogated with Optical Time-Domain Reflectometry (OTDR) to measure changes in its length as the optical fiber and rope are stressed. The method provides a direct measurement of large axial strains. Strains measured in polymeric optical fibers exhibit good one-to-one correlation with applied strains within the test range studied (10% or less, typically). The integrated polymeric optical fiber has been shown to withstand large numbers of repeated cycles to high strains without failure and to accurately track the hysteresis exhibited by polyester rope. Results are reported for tests conducted with polymeric optical fibers integrated into typical mooring rope elements.

The Shadow Effect on the Dynamics of a Shuttle Tanker Connected in Tandem With a FPSO

The ship based Floating Production Storage and Offloading system (FPSO) has been largely used in the recent offshore oil exploration. In most of the cases the oil stored in FPSO is offloaded to a shuttle ship that is connected by a hawser in tandem configuration. The problem of dynamic instability that arises in several ship mooring systems, like SPM and SMS subjected to the environmental forces, may also be present in the tandem system. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, there are none concerned with the environmental forces interference caused by FPSO on shuttle ship, here called as shadow effect. It is well known that the dynamic behaviour of a moored ship, in particular SPM system, is hardly affected by the environmental forces. Therefore, it is expected that shadow effect on the environmental forces acting on the shuttle ship will cause great influence in its dynamic behaviour, and consequently in the dynamics of whole FPSO-shuttle system. These phenomena could be observed in experiments with single point moored shuttle ships with and without the FPSO in upstream position. Therefore, the shadow effect should be considered in analysis of dynamic behaviour of two ships connected in tandem. Among the commercial simulators that analyse tandem systems there are none that consider shadow effect, making their analysis different from the real world. This paper presents an empirical model of the current shadow effect. The model was implemented in a numerical simulator, named DYNASIM. The comparison between numerical results and experimental one showed that the proposed model is effective.

The Ocean Thermal Gradient Hydraulic Power Plant and Its Scope

Heretofore, the concept of developing power from the tropical oceans, (Ocean Thermal Energy Conversion, or OTEC) has assumed the mooring of large platforms holding the plants in deep water to secure the coldest possible condensing water. As the Ocean Thermal Gradient Hydraulic Power Plant (OTGHPP) does not depend, on the expansion of a working fluid, other than forming a foam of steam bubbles. It does not need extremely cold water as would be dictated by Carnot’s concept of efficiency and the 2nd Law of Thermodynamics. Plants may be based on or near-shore on selected tropical islands, where cool but not extremely cold water may be available at moderate depths. This paper discusses the above possibilities and two possible plant locations, as well as projected power outputs. The location and utilization of large of amounts of power on isolated islands, where cabling of power to major population centers would not be feasible are discussed. Two that come to mind are the reduction of bauxite to produce aluminum and the of current interest is the electrolyzing of water to produce gaseous hydrogen fuel to be used in fuel cells, with oxygen as a by-product.

Hydroelastic Modeling of a Compliant Offshore Tower: Formulation

Offshore oil and gas can be produced using a variety of platform types. One option, the compliant offshore tower, has proven to be an economic solution in moderately deep water (300–600m). In this paper, the wave-induced global dynamic responses of a compliant tower in wind, current and waves are studied in the context of fluid-structure interaction. A beam undergoing transverse and axial motion models the vertical member of the tower. The beam is supported by a linear-elastic torsional spring at the bottom end and a point mass and a buoyant chamber is located at the top free end. The fluid forces on the beam are modeled using the Morison equation while the hydrodynamic forces on the chamber are obtained based on the three-dimensional diffraction-radiation theory. By applying Hamilton’s variation principle, the equations of motion are derived for the coupled fluid-structure interaction system. The non-linear coupled system equations that emanate from this new approach can then be solved numerically in the time domain.

Numerical Simulation of Vortex Shedding From a Circular Cylinder in the Presence of a Free Surface

CFD simulations of crossflows around a 2-D circular cylinder and the resulting vortex shedding from the cylinder are conducted in the present study. The capability of the CFD solver for vortex shedding simulation from a circular cylinder is validated in terms of the induced drag and lifting forces and associated Strouhal numbers computations. The validations are done for uniform horizontal fluid flows at various Reynolds numbers in the range 103 to 5×105. Crossflows around the circular cylinder beneath a free surface are also simulated in order to investigate the characteristics of the interaction between vortex shedding and a free surface at Reynolds number 5×105. The influence of the presence of the free surface on the vortex shedding due to the cylinder is discussed.