**Source**

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**Industry**

**Oilfield Places**

**Technology**

The "hopping" phenomenon in the phase space of a ship rolling model with two coexisting attractors, representing the heave and heave/roll responses, is investigated. The noise-induced erosion of basins of attraction is studied using the simple cell-to-cell mapping technique. The power spectra of a function, which monitors the hopping phenomenon and determines the phase durations of roll motions, are shown to exhibit 1/f noise, or power-law behaviour. The behaviour is interpreted using a recently proposed general principle based on the extremal dynamics; the related implication is also discussed.

In recent years, attempts have been made to interpret the qualitative behaviour of ship instability in terms of the safe basin of attraction in the phase space (Thompson, Rainey and Soliman, 1990; Nayfeh and Sanchez, 1990.) . For the well-known instability phenomenon of large rolling motions of ships in longitudinal waves, the problem has been studied using coupled heave-roll models of parametric excitations (Sanchez and Nayfeh, 1990; Liaw, Bishop and Thompson, 1993). In the case that the natural frequency of a ship in heave is about twice its roll frequency and is close to the wave frequency, large-amplitude roll motions, attributable to "principal parametric resonance", and capsizing can occur even if there is no direct roll excitation. Heave excitations are necessarily simplified in order to keep the numerical efforts of solving the nonlinear dynamic problem to a manageable level. The heave excitations due to regular longitudinal waves are very often represented by harmonic forces. If only the instability conditions for parametric resonance are of interest, the roll angle responses are assumed to be small and the governing equations can then be simplified to either a Mathieu or Hill equation (Liaw, 1994) . Analytical solutions for the instability conditions can be readily derived from the Mathieu or Hill equation.

ISOPE-I-95-244

The Fifth International Offshore and Polar Engineering Conference

The paper describes the level of investigations of Sakhalin shelf for 17 years of prospecting and exploration. 8 oil and gas fields have been discovered with the total recoverable reserves ca. 272 million tons of crude oil and 819 billion m3 of gas. In the nearest 5-7 years the intensive exploration of these sectors by the companies-winners of tenders is expected. The volumes of drilling can reach 45,000-50,000 m/year. The deterrents are severe environmental conditions on the Sakhalin shelf, which are characterized by a short summer season and complicated ice conditions in winter. Offshore fields of Sakhalin can become one of the most promising sources of import of crude oil and LNG to the countries of Asia Pacific region. The Northern Sakhalin is one of the oldest oil and gas producing areas in Russia. Commercial production here began in 1928. In the history of oil and gas exploration the two basic stages are distinguished by being remarkably different in efficiency and final results. . Before the middle of 1970s the oil and gas prospecting was carried out only onshore of Sakhalin. The works were carried out practically in all promising areas of the island, but the major part of the fields (over 50) and commercial hydrocarbon reserves were discovered in the north-eastern part of Sakhalin. Oil and gas, gas and gas condensate fields with small reserves were discovered in the central and north-eastern parts of Northern Sakhalin. Three small gas fields were discovered in Aniva District on the Southern Sakhalin. An oil field was discovered in Pogranichny graben (Eastern Sakhalin). Totally on the island there were discovered 62 fields: 20 oil fields (with oil reserves being over 90%), 13 gas fields, 17 gas and condensate fields and 5 oil and gas condensate fields.

ISOPE-I-95-002

The Fifth International Offshore and Polar Engineering Conference

block, CNG, compressed natural gas, condensate reservoir, development, exploration, field, gas monetization, liquified natural gas, LNG, Offshore, oil, pipeline, production, project, recovery factor, region, reserve, resourse, Sakhalin, shelf, strategic planning and management, structure, sub-sea system, subsea system, ton, well, year

Oilfield Places:

- Asia > Russia > Sakhalin Island > Piltun-Astokhskoye Oil and Gas Field (0.99)
- Asia > Russia > Sakhalin Island > Odoptu Field (0.99)
- Asia > Russia > Sakhalin Island > Lunskoye Oil and Gas Field (0.99)
- (2 more...)

SPE Disciplines:

Miyajima, S. (Institute of Industrial Science, University of Tokyo) | Maeda, H. (Institute of Industrial Science, University of Tokyo) | Jo, Hyo Jae (Dept. of Naval Architecture, Pusan National University)

It is very important to estimate exactly low-frequency motions in designing mooring systems of floating offshore structures. However the sufficient number of directions of multi-directional waves and the sufficient length of duration of simulation to estimate the statistical values of low-frequency motions in directional waves have not been clarified yet. In this paper, we simulate motions of a floating offshore structure with low-frequency motions in multi-directional waves, varying the number of directions and the length of duration of simulation. We observe the statistical values of time histories of simulated motions. From these results, we propose the sufficient number of directions and the sufficient length of duration of simulation to estimate the statistical values of low-frequency motions of a floating structure, such as expected maximum amplitude, significant amplitude and mean amplitude, in multi-directional waves.

INTRODUCTION

The estimation of statistical characteristics, such as extreme values, expected maximum amplitudes, mean and significant amplitudes, of motions of a moored floating offshore structure are very important to design the positioning system and to estimate the workability of the floating structure on site. Stansberg et al( 1988) carried out model tests on non-linear slow-drift oscillations using moored floating box-shaped bodies in irregular beam seas and they analyzed the probability of extremes from experimental data and simulated results. They concluded that the probability of extremes of system with very small damping such as slow-drift sway motion was only slightly larger than for a Gaussian random model. On the other hand, Kinoshita et al (1989) and Kato et al (1990) derived new formulas of probability density function (p.d.f.) of total second order responses (combined first and second order low frequency response) that can be represented by the generalized Laguerre polynomials of which the first term is a Gamma p.d.f. with three parameters.

ISOPE-I-95-249

The Fifth International Offshore and Polar Engineering Conference

SPE Disciplines:

- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Mooring systems (1.00)
- Facilities Design, Construction and Operation > Facilities and Construction Project Management > Offshore projects planning and execution (1.00)

This paper presents the development and application of reliability based inspection planning techniques for floaters. Based on previous ,experience from jacket structure applications optimised inspection planning (OIP) techniques for floaters are developed. The differences between floaters and jacket structures in relation to fatigue damage, redundancy levels and inspection practice are examined and reflected in the proposed methodology. The application and benefits of these techniques is demonstrated through representative analyses and important trends are highlighted through the results of a parametric sensitivity study.

INTRODUCTION

Inspections and inspection planning form an integral part of the integrity monitoring of offshore structures. Until recent years the various inspection criteria were combined in a qualitative manner based on engineering judgement to produce the inspection plans. Such criteria include fatigue lives, member criticality, stress levels, past inspection data, previous experience and cost considerations. In more recent years reliability based inspection planning techniques were employed mainly on jacket structures (Onoufriou, Fowler and Smith, 1994) which encompass all these criteria in a quantitative manner through a procedure which enhances the decision making process. OIP techniques provide a more rational approach to inspection planning which is geared towards maintaining consistent levels of safety and reliability in the structure arid achieving higher efficiency from the inspection operations. This is achieved through the use of probabilistic limit state functions for fatigue which take into account the uncertainties associated with the parameters that determine fatigue life. The criterion used for inspection is maintaining a level of target reliability throughout the life of the platform. The knowledge obtained from inspections is incorporated in the analysis models to update the reliability of the joints and determined when the next inspection needs to be performed. The uncertainty associated with the inspection methods used is also taken into account.

ISOPE-I-95-334

The Fifth International Offshore and Polar Engineering Conference

SPE Disciplines:

Endo, Ryuji (The Polytechnic University) | Hamamoto, Takuji (Musasi Institute of Technology) | Kato, Takehiko (The Polytechnic University) | Hayashi, Takayuki (Musasi Institute of Technology) | Tosaka, Nobuyoshi (Nihon University)

This paper is concerned with experimental modal analysis to estimate the dynamic properties of unit-linked large floating structure models. The fluid-coupled vibration test were carried out by using an impact hammer. The hammer impact test to be used in our experiments must be performed by hitting force transducers arranged with each unit by an impact hammer made of very soft rubber. The vertical acceleration responses are measured on upper surfaces of the models by arranging acceleration sensor on each unit, respectively. The transfer functions defined by normalized input and output data are analyzed by means of a multi-channel modal analysis software. The modal properties such as natural frequencies. damping ratios and mode shapes were estimated by using the curve fitting of the transfer functions which were obtained in the vibration test. These experimental results were compared with theoretical values obtained by using a lumped-mass model with the aid of BEM to take into account fluid-structure interaction.

ISOPE-I-95-024

The Fifth International Offshore and Polar Engineering Conference

Industry:

- Transportation > Infrastructure & Services > Airport (0.54)
- Transportation > Air (0.54)

SPE Disciplines: Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Floating production systems (0.62)

Solutions for stress distributions in hollow spheres and cylinders of uniform thickness undergoing uniform radial and temporal changes in temperature are presented. Typical applications include a pressure vessel or pipeline in a polar environment or other low temperature service that is subjected to a sudden hot process flow. Nine types of thermal boundary conditions, as given by Ozisik (1980), are considered for each geometry including: insulated interior and/or exterior surfaces; convection on the inside and/or outside boundaries; sudden cooling, or heating of a surface, ''etc. These transient stresses may be momentary and rapidly decay as the temperature diffuses into the wall and a contraction or expansion of the outer layer of the vessel or pipeline is permitted. However, they co.uld be sufficient to produce a permanent set in the material thus causing residual stresses to be present when the temperature of the vessel or pipeline has stabilized and thus should be considered. The worst case scenario,'' that of thermal shock, is also presented. Examples are presented giving the temperature and stress distributions through the wall of a pipeline due to a sudden change at its surface.

This paper presents solutions for stress distributions in hollow spheres and cylinders in which the temperature is a function of the radius and time. Numerous variations are presented depending upon the thermal boundary conditions, including combinations of insulated surfaces; convection on the interior and/or exterior surface; sudden cooling or heating of a surface, etc. These transient stresses may be momentary and rapidly decay as the temperature diffuses into the wall and a col)traction or expansion of the outer layer of the sphere cylinder is permitted. Ozisik (1980) presents, nine different combinations of boundary conditions for each geometry. These conditions are applied in the work presented here.

ISOPE-I-95-110

The Fifth International Offshore and Polar Engineering Conference

The assessment of onshore wave energy resource should in general be based on computational shallow water wave models. The paper summarises the variety of such models and indicates their merit for different coastal situations and type of study. Case studies for Ireland and Azores islands, Portugal, are presented.

Evaluation of the wave energy resource is not easy: sea states are difficult to describe and highly variable on both short and long time scales. Because of this variability, a description of wave climate (i.e, the long-term distribution of sea states at a site) should be based on tong data sets. In the open ocean the data situation is good. The wave climate varies only slowly, and high quality data sets are available for European Atlantic sites, both from measurements and as hindcasts produced by numerical windwave models. For the Mediterranean the situation is not so good: there are no measured data sets for the West Mediterranean and accurate results from numerical wind-wave models are available only for a shorter period of time, At the shoreline, and in shallower or sheltered waters, the resource can vary significantly over distances of a kilometer, or much less if the coastline is very indented. Consequently, this requires much more detailed study. Again. good computer models exist for ,calculating the losses, focusing and defocusing which affect wave, energy in these situations, though there are certain situations in which their accuracy may be limited. This approach has major advantages of flexibility and long life, in that both components can be updated individually as longer data sets and better models are available. The level of detail required for these inshore calculations is such that it is not, at least at present, practical to carry them out for all of the European coastline of wave power interest.

ISOPE-I-95-089

The Fifth International Offshore and Polar Engineering Conference

A Boundary Element Model is used in this ,paper to calculate solitary, wave motion in front of a vertical ''wall. Its fundamental formulation is based on Cauchy''S integral theorem, similar to that of Vinje and Brevig (1981). ,The mathematical and numerical formulation of the model is described. A method of dealing with the reflected waves is proposed. ''The comparison between numerical and experimental results shows good agreement.

Vertical walls are widely used in coastal and port engineering. ,Wave forces are the main active load on the structures. In coastal areas, due to the large value of ratio between wave height and water depth, waves are nonlinear. Using linear, wave theory to calculate waves in front of a vertical wall often leads to under estimated values. It is needed to use nonlinear wave theory. Linear or weakly nonlinear irrotational surface waves evolving slowly ''can be considered as time-harmonic which enables the time dependency. to be separated from the wave equation. A simple relationship between the surface displacement and the velocity potential can be established. For strongly nonlinear waves, such as solitary waves, there is no such simple relationship as the waves are no longer time-periodic. More sophisticated techniques are therefore required. Among these techniques the one based on Boundary Element Method (BEM) can be considered as one of the most efficient methods. The problem of determining solitary waves in front of a vertical wall usually arises in the study of the coastal effects of tsunamis. Tsunamis are long water waves of small steepness generated by impulsive geophysical events on the ''ocean floor or at the coastline. Solitary waves are believed to model some important aspects of tsunamis well Tsunamis frequently ,happen and lead to huge loses of property and life.

ISOPE-I-95-274

The Fifth International Offshore and Polar Engineering Conference

It is beneficial to perform a dynamic lifting analyses for safe and cost effective installation of subsea structures, for example, production templates, foundation bases and guide frames. A study combining experimental and theoretical approaches in simulating realistic conditions encountered offshore has been carried out to establish design and operational criteria for the main installation stages. Computer simulations using non-linear time domain software and laboratory model tests have been performed to investigate the influence of the following factors: sea state conditions, parameters of the structure, type of installation vessel and crane characteristics. Derived results and conclusions are intended for use during assessment of feasibility, planning and final engineering of installation operations.

Dynamic lifting analysis forms part of the engineering process required for efficient installation of subsea structures, e.g. templates, riser bases and guide frames. However, if a preliminary dynamic analysis is conducted earlier, during the conceptual or design phase, a preliminary assessment of the main factors of dynamic loading, may allow integration of structural design criteria and operational requirements. Lifting analysis can help to quantify dynamic forces and thus identify the critical stages of the operation. In cases where the hydrodynamic forces will determine the sea state limitation, an analysis can potentially enable an expansion of the operational weather window and optimisation of the project schedule.

During installation, the lifted structure is exposed to dynamic loading due to the motion of the installation vessel and the direct action of waves.

3.1 GENERAL

ISOPE-I-95-031

The Fifth International Offshore and Polar Engineering Conference

SPE Disciplines:

Offshore structures should have a high degree of structural safety even under extreme environmental loadings. The authors have studied a composite steel - concrete structure of sandwich system for the strength members of huge offshore structures, where concrete is placed between steel plates. In the previous paper, [1] [2] the authors carried out both experimental and theoretical investigations into the strength of the composite structure. It was clarified that the composite structure has various excellent properties : the ultimate load - bearing capacity of the composite structure is very high and it can absorb a great deal of energy until failure under any type of loading conditions. In the present paper, the authors carried out both experimental and theoretical investigations into the dynamic toughness of the sandwich composite structures. Experiments were carried out using the two dimensional models of composite structure under high speed loading. A nonlinear analysis was developed to predict the toughness of sandwich beam under dynamic load. In the analysis, the material non linearity of both concrete and steel with strain rate. The nonlinear analysis accurately represented the behavior and toughness of the sandwich beam structure.

In designing, a composite steel/concrete structure, there is a tendency to design the structure with sufficient ductility, especially in consideration of collision loading. Ductility implies the ability to sustain significant inelastic deformation after the ultimate load without a significant variation in the resisting capacity prior to collapse. This ability is, generally quantitatively described by a parameter called the ductility ratio and the toughness which paraphrase is deformation energy. The ductility is defined as the ratio of ultimate deformation to yield deformation. The research program is composed of experiments and theoretical studies. Experiments include a series of dynamic test under shear and combined shear and bending.

ISOPE-I-95-318

The Fifth International Offshore and Polar Engineering Conference

Industry:

- Energy > Oil & Gas > Upstream (0.69)
- Materials > Construction Materials (0.68)