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**Industry**

**Oilfield Places**

**Technology**

The analytical solutions by Wu (1987), Lee, Yates & Wu (1989) and Shen (1996) for predicting the amplitude and period of resonantly forced long waves by submerged moving disturbances are further investigated by comparing the analytical solutions with both numerical results based on the KdV model and with data from the towing tank experiments: The objective is to examine the validity of the analytical solutions for predicting waves generated by disturbances of different lengths. Our results show that the analytical solutions by Wu (1987) and Lee, Yates and Wu (1989) are valid for a broad range of disturbance length, while the recent solution by Shen (1996) gives excellent prediction for forced waves generated by relatively shorter disturbances such as those with length shorter than five times the water depth. INTRODUCTION It is known that, when being resonantly forced by a submerged moving object in shallow water, a series of solitary waves are generated periodically in front of the disturbance and move upstream with supercritical speed. A sketch of this phenomenon is shown in figure 1, where a. is the amplitude of the solitary waves, , the wave elevation, h the unperturbed water depth, d the depth of the depressed region behind the disturbance, and U and L the speed and length of the disturbance, respectively. The dominant forcing factor that affects the wave amplitude a. and wave period T. is the blockage ratio b which is defined as the ratio of the blockage area Ad of the disturbance over the unperturbed channel cross-sectional area A as shown in figure 1. The blockage ratio b generally varies in the longitudinal direction. The maximum value of b is called the blockage coefficient and its effect was discussed in Ertekin (1984), Ertekin, Webster and Wehausen (1984, 1986), Mei (1986), Ertekin and Qian (1989), and Teng and Wu (1990, 1997a,b,c).

ISOPE-I-98-227

The Eighth International Offshore and Polar Engineering Conference

A force allocation strategy used in connection with dynamic positioning of ships and semi-submersibels is presented. The strategy dynamically allocates force, and in case of azimuth thrusters - force directions, to a set of thrusters. The strategy handles n thrusters (n≥2) in an arbitrary configuration. The decision making of the strategy is based on minimum power consumption taking into account effects of reversing thrusters, thruster/hull interaction and partly the effects of interaction between thrusters. The functions of the full allocation strategy is illustrated through a number of examples. Finally the strategy is compared with a more classical allocation strategy to show that significant reductions in energy consumption can be achieved. INTRODUCTION Dynamic positioning interpreted as an automatic control of the horizontal position and the heading of a vessel may be applicable in a number of fields within the maritime industry. One such example is the docking of a ferry and more directly related to the present development is positioning of a drilling platform/vessel. In both cases the horizontal position and heading of the platform or vessel shall be controlled. The precision required to the positioning of an offshore structure yields quite a challenge. It is therefore necessary to be assured that the most efficient use of the thruster capacity is acquired. In general terms the problem has three constraints, the desired position and heading. With a ship or semi- submersible platform utilising more than two thrusters or combinations of propeller/rudder and thrusters more than three parameters are available to adjust the control of the position resulting in an mathematically underdetermined problem. Often this problem of having an under-determined system is solved by allocating individual units to perform a certain task i.e. azimuthing thruster in the bow of a FPSO are allocated for heading control and consequently provides only a side-force near the bow.

ISOPE-I-98-052

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines:

The use of Rectangular Hollow Sections (RHS) in different structures has been expanded dramatically, and longitudinally oriented plates have been used widely to connect braces and other attachments to RHS members. Since plate-to-RHS connections are very flexible, some factors have to be considered in the design of this connection type, such as large deformations on connecting faces and non-uniformly distributed stresses in plates. The existence of an axial load on an RHS member also affects the strength of a plate connected to the RHS member. Based on an investigation of different failure modes of plate to- RHS connections under different loading cases, a tentative design method is proposed for the connections. Two deformation limits are used to control the deformation of the connecting face in an RHS member. The deformation under the ultimate load is limited to 3% of the RHS member width and the deformation under the service load is limited to I % of the RHS member width. The Connections were tested to rupture and the load-displacement behaviour of the column faces and stress distributions m the plates were obtained. The test results generally confirm the proposed design method for plate effective width and connection to the column face.

INTRODUCTION

As a common means of connecting a brace member to a column member, for example in braced frames, a gusset plate is typically welded in the longitudinal direction to the column wall. When the column is an RHS, the connection is extremely flexible to a load or a load component perpendicular to the column axis because of the flexibility of the RHS connecting wall, unless so called through plates are used, the fabrication of which is expensive, or the plate is welded to the RHS corner, which requires the RHS to be orientated at a 45° angle.

ISOPE-I-98-299

The Eighth International Offshore and Polar Engineering Conference

A Volterra-model method is used to create the time series of second-order nonlinear wave force by employing the frequency response function, which forms the Fourier transform pair with the impulse response function. The linear and quadratic frequency response functions for the wave forces are hydrodynamically computed using HOBEM. The convolutions of the impulse response functions with the given random wave and random wave*wave yield the nonlinear wave forces in the time domain. As an example, this method is applied for the simulation of nonlinear responses of a TLP in the long-crested seas.

This paper presents a practical method for the time-domain simulation of nonlinear wave forces on a floating structure in random seas. Time histories of nonlinear wave forces are essential for the simulation of nonlinear responses of offshore structures. For instance, the slowly and rapidly varying dynamic responses of a Tension-Leg-Platform (TLP) are due to the above nonlinear hydrodynamic wave forces. Conventional methods to generate the time series of wave elevation employ the given wave spectrum and linear superposition theory (random phase). These may lose natural randomness and groupness as discussed by Burcharth (1981) and Tucker et al. (1984). Besides, it needs enormously dense frequency resolution to produce a long Gaussian wave time series. To avoid the foregoing difficulty, frequency-disturbance method was introduced by Shinozuka and Jan (1972), but it failed to produce Gaussian waves. These disadvantages can be avoided if we use a linear filter with Gaussian random signals, where the linear filter is derived from the specified wave spectrum. The wave time series derived in this manner will have practically infinite repeating period with randomness and groupness resembling the natural waves. Zhao (1996) developed an authentic Volterra method recently.

ISOPE-I-98-272

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines:

This paper describes a recent technical innovation in design, called ''''Design Through Analysis" led by the authors_ In the new approach, finite element methods are used to simulate global behavior and detailed structural strength. The global analysis and local analysis are integrated to determine the governing limit-states and optimize the design. The advantage of using such advanced engineering is a substantial reduction of project CAPEX and OPEX. The paper presents the following technical developments: (1) Design Through Analysis (DTA) concepts, (2) non-linear finite element simulations of global behavior and detailed local strength, (3)limit-state design criteria, (4) vortex-induced vibrations and fatigue (5) design for trawling loads, (6) wall-thickness design based on LCC (Life-Cycle-Cost) optimization, (7) reliability-based calibration of safety factors and inspection planning, (8) design of in-field flowlines. INTRODUCTION A recent technical revolution in the design process has taken place in the Offshore and Marine industries. Advanced methods and analysis tools allow a more sophisticated approach to design that takes advantage of modem materials and revised design codes supporting limit state concepts and reliability methods. At JP Kenny we call the new approach "Design Through Analysis" where the finite element method is used to simulate global behavior of pipelines as well as detail structural strength. The two-step process is used in a complementary way to determine the governing limit states and to optimize a particular design. The advantage of using advanced engineering is a substantial reduction of project CAPEX and OPEX by minimizing unnecessary conservatism in the design through a more accurate determination of the effects of local loading conditions on the structure. Rules and design codes have to cover the general design context where there are often many uncertainties in the input parameters and the application of analysis methods. Where the structure and loading conditions can be accurately modeled, realistic simulations reveal aspects of the design codes,

ISOPE-I-98-103

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines: Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Offshore pipelines (1.00)

A numerical wave tank is presented, in which the interactions of wave packets with a vertical cylinder are simulated in the time domain, using a 3D boundary element method. Nonlinear free surface boundary conditions and Neumann conditions on material boundaries are modeled through ad hoc Taylor series expansions including contributions up to second order. Unlike previously published second order approaches of the diffraction problem in the time domain, the problem is formulated and solved for the total velocity potential, without separation of incident and diffracted components, with the objective of simulating as closely as possible the conditions of real wave tank experiments. Accordingly, waves are generated by a paddle wave maker following the experimental wave maker motion signal, while waves are absorbed at the opposite side of the numerical wave tank by an absorbing zone with dimensions and efficiency similar to the physical wave tank beach. In both numerical and experimental tests, the wave maker motion is built according to linearized theory in order to obtain a prescribed short duration first order wave elevation history (called target signal) at a given distance from the wave maker. First and second order wave elevations as well as in line forces on a bottom-mounted vertical cylinder submitted to transient wave packets of different shapes are presented and compared with experimental records.

Due to a demand from the offshore industry, motivated by the discovery of phenomena such as ''springing'' and ''ringing'', there has been recently an increasing interest in the hydrodynamic research community on the development of experimental, theoretical and numerical models of nonlinear wave diffraction, aiming at a better understanding and more accurate predictions of the effects of extreme waves on offshore structures. One now common theoretical/numerical approach is the frequency domain second order wave radiation-diffraction theory (Molin, 1979, Kim & Yue, 1990).

ISOPE-I-98-246

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines:

In the paper the modifications of a wave field due to a non-linear interaction with an opposing jet-current are analysed. Hydrodynamic governing relations are represented by the mass conservation and 2DH momentum equations. Wave refraction is obtained by the irrotationality of wave number, the energy conservation and Doppler effect equations. A numerical solution based on a "two-level iteration" method, is proposed and some applicative examples are also reported. INTRODUCTION The influence of a current on wave motion is a very common physical phenomenon, characterised by the generation of geometric and kinematic modifications of the flow field (such as wave front deformation, wave height and length reduction or increase, tidal current development etc.). In coastal areas this subject assumes considerable interest in the examination of the effects of an opposing jet-current on waves. This situation can be, in fact, the basic scheme of some important technical applications (e.g. controlled sea discharges from plants or sewage outfalls, natural discharges, river mouths etc.). The hydrodynamic study of this problem has been treated by several researchers with mathematical models, where some appropriately simplified schemes are considered. A good synthesis of these researches is reported by Peregrine (1976) and Jonsson (1990). Ismail (1981) and Ismail and Wiegel (1983) treated the problem analytically, evaluating only the spreading of a surface jet due to an opposing wave and also reported some experimental results. This aspect is very interesting because the laboratory studies on the subject request large experimental facilities (wave basin). More recently, Yoon (1987) and then Yoon and Liu (1989) studied the problem considering only the long waves (e.g. tidal flows near an inlet or estuary entrance). By applying the parabolic approximation criterion, the Authors obtained a particular linearised expression of the Boussinesq equations.

ISOPE-I-98-218

The Eighth International Offshore and Polar Engineering Conference

A finite difference model based on potential flow theory is proposed for estimating the equilibrium scour hole underneath offshore pipelines. The model solves the Laplace equation for velocity potential in a curvilinear coordinate system to cope with the irregular and free boundaries involved in this problem. A local method is used to determine the free boundary formed by the eroded seabed via the equilibrium of all forces acting on a sediment particle on a slope bed. The major features of the present model are: 1) that it takes into account the nonlinear interactions between the flow, pipe and the changing bed topography in calculating the equilibrium scour hole, 2) that the shear stress on sediment particles is represented in terms of a characteristic near-bed velocity, creating a dynamic link between the flow and the sediment movement, and 3) that the model predicts the equilibrium scour hole without using any sediment transport formula, which usually contains many empirical parameters or constants. The maximum scour depth and the upstream part of the scour hole predicted by the model compare well with the experimental data published in the literature. INTRODUCTION Many offshore pipelines are directly laid on the erodible sea bed and are exposed to currents, waves, and storms. Even with moderately strong currents, local scour can occur, and this will cause the pipelines to be suspended in water. As the scour hole develops, the flow around the pipe will exert a downward force on the pipe instead of an uplifting force. This force together with the self-weight of the pipe will tend to sink the pipe section into the scour hole and cause additional stress on the pipe joints. Consequently, the excessive deflection of the pipeline may result in damage and failure. Therefore, the prediction of local scour around pipelines is of great importance to pipeline design.

ISOPE-I-98-112

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines:

An accelerated aging method was developed to evaluate losses in mechanical properties in carbon composite materials. Carbon fiber reinforced polymer (CFRP) composite coupons were tested under tension and bending to establish their strength and stiffness. After accelerated environmental conditioning reductions in mechanical properties in mechanical properties of CFRP were established, and preliminary calibration was carried out using natural degradation rate of CFRP materials. The maximum reduction in these values after accelerated aging was found to be 7%.

This paper explores the rate of degradation in mechanical properties of carbon fiber reinforced plastic (CFRP) plates under accelerated aging conditions. Characteristically, these CFRP plates are lightweight and have extremely high strength and stiffness, excellent fatigue properties, and outstanding corrosion resistance [Meier and Kaiser, 1991]. The literature reviews dealt with the effect of environment on fiber reinforced polymer (FRP) composites and adhesives. More specifically, the goals of this investigation are: to establish rate of degradation of strength and stiffness of the CFRP material (manufactured in cooperation with Creative Pultrusions Inc.) under an accelerated aging procedures, and to establish mechanical fatigue effects on the CFRP material. A total of seventy two coupons were prepared for coupon level testing. Thirty six 1" x 12" 3116" coupons were prepared for static tension tests and referred to as tension specimens. Four of the thirty six specimens were left unconditioned and used as base values for comparison with the conditioned tension specimens. Additional thirty six coupons were prepared for coupon level three point bending tests and are referred to as bending specimens. Following ASIM D 790 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical insulating Materials, the bending specimen dimensions were established to be 9 112" x 112" x 3/16".

ISOPE-I-98-331

The Eighth International Offshore and Polar Engineering Conference

SPE Disciplines:

DREA and ISER are evaluating issues facing the development of a Canadian Remote Minehunting System (CRMS) for the Canadian Navy. The CRMS is an autonomous, snorkelling drone towing a deployable, active towfish which houses a side scan sonar for route surveying and mine location on the sea floor. With route surveying, sonar images are obtained from an area where mine hunting is anticipated, in order to provide a reference against which future mine hunting images can be compared. A high degree of towfish stability is required to get good images, and the absolute location of the towfish must be known for differencing images with those from subsequent mine hunting surveys. The semi-submersible DOLPHIN (Deep Ocean Logging Platform for Hydrographic Instrumentation and Navigation) Mk1 vehicle developed by ISE Research Ltd. is a proven stable remote platform for hydrographic instrumentation. It can operate in up to sea state 5 and has successfully demonstrated its towing capability (Preston and Shupe, 1993). DOLPHIN Mk2 (Figure 1) is currently a candidate drone for the CRMS.

ISOPE-I-98-138

The Eighth International Offshore and Polar Engineering Conference

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