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to

GoA general numerical approach is presented which is capable of predicting both the steady and unsteady heave response of a tethered subsea unit during deployment from a floating support vessel. A transformation is proposed to cope with the moving boundary. The examples given here focus on the subsea unit motion and the tension in the umbilical cable which have direct applications to subsea intervention activities.

1 INTRODUCTION

The deployment of a tethered subsea unit consists of four rather distinct stages:

1. Passage through the air gap. During this stage, the pendulum motion of the subsea unit in air may cause difficulties in control. Generally, the gap between the handling system on deck, such as cranes or A-frames, and the water surface is very small. Nevertheless, care must be taken to prevent the possible collision with other structures nearby Of the vessel itself.

2. Transition through the splash zone. This is often a crucial phase of the process involving very complicated hydrodynamic phenomena which may cause impact loading of the subsea unit and/or snap loading in the cable (Dutta, 1986; Oritsland and Lehn, 1989).

However, due to the exponential decay of the wave forces on the subsea unit as it is being lowered, the depth of this zone is much less than the total length of the cable. 3. Lowering to the working depth, or lifting from the working depth to the splash zone. This stage covers most of the time of the deployment phase, especially in deep waters. 4. Operation at a certain depth to fulfill the designated tasks. If a subsea unit is deployed in a weak current, or is constrained by taut vertical guide lines, the whole system can then be approximated as an one dimensional problem (see Figure 1).

ISOPE-I-92-302

The Second International Offshore and Polar Engineering Conference

SPE Disciplines: Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Subsea processing (1.00)

Characteristic groups of random sea waves on deep water and finite depth are introduced by adopting and extending the well-known concept of characteristic waves. The characteristic groups are defined starting from random wave groups formed by zero upcrossing waves obtained by numerical simulations carried out in the time domain with a random phase method. The behaviour of the heights and periods of the nominal waves which form the characteristic groups and the energy conveyed by these groups are examined. Tests of some results with suitable field data are carried out and the effect of the spectral shape is then highlighted.

The phenomenon of wave grouping in the random wave field, i.e. the formation of wave sequences, has been examined in the time domain by several authors with reference to two different approaches, a discrete one concerning the random wave heights and a continuous one concerning the envelope function of the random wave process. As regards the discrete approach, Goda (1970, 1976) introduced the concept of wave grouping as a threshold level-crossing problem associated with the heights of successive random waves, both for the ordinary groups and the conditional ones, i.e. those which include the maximum wave in each sea state. The quasi-deterministic groups considered in the Boccotti theory (1989) can be referred to this approach, although those groups do not need any threshold. As regards the continuous approach, LonguetHiggins (1962) referring to Rice (1945) introduced the concepts of wave grouping as a threshold level-crossing problem associated with the envelope of the random wave process. This approach may be used to define the mean value of the time duration of the continuous group, bounded by two successive crossings, and the mean number of waves which belong to the group.

ISOPE-I-92-231

The Second International Offshore and Polar Engineering Conference

account, behaviour, central wave, characteristic group, characteristic index, deep water, dimensionless energy, energy, field, frequency, frequency spectra, group, increase, nominal wave, numerical simulation, reservoir description and dynamics, reservoir simulation, sea, spectra, spectrum, State, Wave

The crack propagation characteristics of steel A537. under constant load, were studied at 238K.273K and 303K. using fracture mechanics. The fatigue crack propagation in steel A537 at the above testing temperatures was in accordance with Paris law. As the temperature decreased, the fatigue crack growth rate decreased for most region of the crack propagion and the Paris exponent n greatly increased. This caused a much higher acceleration of growth rates at lower temperatures. The curves of the propagation diagram at different temperatures tend to converge with the increase of stress intensity factor range ∆K and intersect at a certain high ∆K level. Thus. after the intersection, the crack growth rate at lower temperatures was inversely higher than that at room temperature. The crack growth rates are represented by the Yokobori formula. The fracture surfaces were observed under a scanning electron microscope. showing that the fracture of steel A537 at all the testing temperatures took the form of microvoid coalescence. which characterizes ductile fracture.

In cold climates, structures such as bridges, towers and buildings may be subjected to temperatures that fall as low as 220K. The materials used in constructing these structures are low-carbon structural steels, rather than steels designed for low temperatures. Because of reduced fracture toughness of such low-carbon structural steels. fatigue cracks are quickly propagated and become unstable at low temperatures

ISOPE-I-92-330

The Second International Offshore and Polar Engineering Conference

An experimental study is currently being carried out to investigate the hydrodynamic force characteristics produced on vertical multi-cylinder structures when subjected to wave motions. Of particular interest to this study is the modification to the force characteristics attributed to the relative spacing and pattern of a cylinder group on individual members when this loading is in the Morison force regime. Testing is being conducted in the Michell laboratory of the University of Melboume on cylinders with a fixed diameter of 0.02m located in a 1m later depth. A variety of wave types have been used in this program including a range of irregular Pierson Moskowitz waves and sets of both Swept Sine Waves (SSWs) and regular waves of different amplitudes and frequencies. The cylinders in the group include ones that contain are-locatable 0.10m length active segment from which both the in-line and transverse force components are able to be measured simultaneously under test. Results from this study for a two-cylinder and four-cylinder group are provided in this paper that indicate the tendency observed in force coefficients from this interference effect with orientation and distance of separation between the cylinders.

Oil production riser systems used in offshore platforms normally consist of a group of long slender vertical cylindrical pipes that form a pattern which varies with the number and relative spacing of the constituent1members. Because of the proliferation of geometric configurations that have been adopted in the design of riser groups, and the current lack of a good understanding of the wave interaction effects on individual members, it is not possible at this point in time to adequately model the complex realization of the hydrodynamic loading on such members on a theoretical basis.

ISOPE-I-92-265

The Second International Offshore and Polar Engineering Conference

SPE Disciplines: Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Risers (0.88)

This paper investigates temperature rise distribution near a crack tip of CT specimens of a mild steel due to plastic work under intermediate loading rate for the purpose of considering the criterion for crack instability under arbitrary loading rate. The dynamic thermal elastoplastic FEM analysis shows the good agreement with experimental temperature rise distributions by using thermo viewer. The calculation results of yield stress in the process zone in the vicinity of a crack tip fairly keeps constant because of opposed effects of strain rate and temperature on yield stress. Moreover yield stress in the process zone increases with increasing loading rates up to a certain rate due to the increase in strain rate, however, the increase in loading rates over the certain value leads to decrease in yield stress which is caused by local heating near a crack tip due to plastic work because there is not enough time to dissipate heat. From above information, the criterion of fracture initiation under arbitrary loading rate is conducted. Experimental Kc values of HT-50 TMCP steel with various loading rates support quantitatively the criterion.

1 Introduction

It is well known that crack instability is strongly affected by loading rate and temperature especially in body-centered cubic metallic material as steel. In other words, fracture toughness decreases with increasing loading rate in the region of intermediate loading rates including quasi-static condition and then reaches a minimum and increases again if the loading rate exceed a certain limit (KalthotJ,1986). In the previous study (Toyosada et.al.,1989),we found out the possibility that fracture toughness depends only on strain rate-temperature parameter R defined by Bennet(196S) if the thickness of material is fixed.

1. A quantitative estimation of strain rate dependency on fracture toughness is plausible in the low toughness range since the plastic work is quite small.

ISOPE-I-92-327

The Second International Offshore and Polar Engineering Conference

This paper deals with an advanced stochastic analysis for deep water offshore structures under wave-current and structure-fluid interactions. Having described the random sea and transfer functions of water velocities and accelerations in a wave-current field, wave forces, hydrodynamic damping ratios and added masses are calculated on the base of using the Morison''s equation. The drag force term of the Morison''s equation is linearized by using an equivalent second-moment criterion and the relative water velocity concept. Then, attention is paid on the structural response analysis. It is emphasized that a proper and correct analysis can be fulfilled by considering the wave-current and fluid-structure interactions.

Wave-current and fluid-structure interactions are important subjects to be considered in the analysis of offshore structures. Since the process of wave loads in an ocean environment is stochastic, spectral methods are usually applied to calculate response statistical characteristics of structures. In this calculation, the effect of current is reflected on response results in two ways as a) the existence of current alters the sea spectrum, b) wave loadings and damping ratios are dependent on the current velocity so that loading and structural transfer functions are also affected by the current. In'' general, wave-current interaction is a complicated and difficult subject. Under certain conditions which will be mentioned later, spectral description of sea states in a wave-current field can be obtained in terms of sea spectra in a quiescent area. Since the non-linear drag force term of the Morison''s equation is dependent on the total water velocity, the linearized form of this term contains two components as being timeinvariant (assuming that current is not a function of time) and stochastic. The time invariant component results in mean value responses.

ISOPE-I-92-307

The Second International Offshore and Polar Engineering Conference

To increase the safety and reliability of pipelines and to assess their service life and to predict their durability it is necessary to analyse the serviceability of pipeline members taking into account their actual operating loads at various time periods. The Paper offers an analysis of the actual operating loads acting upon oil and gas pipeline members and their failures in the process of operation. It shows the variations in the workload of the pipelines transporting oil and gas, as well as pump station pipelines and the dependence of the physical causes and mechanisms of failures on the nature of acting loads. the The Paper also analyses the criteria and design diagrams for the evaluation of the durability of pipeline members in the conditions of actual random loading taking into account the low and high-frequency components of the stress variables. The Paper shows the need to consider the nature of loading while specifying the defect acceptance levels for various-purpose pipelines.

Concord of Independent States is currently operating a large-scale network of oil and gas pipelines totaling over 250,000 km. The pipelines were designed basically taking into account the need to ensure the structural strength of the pipeline under the internal pressure static loading of transported product. the The analysis of failures indicates, however, that in some cases the initiation and propagation of pipeline member failures in the process of operation cannot be explained on the basis of their insufficient strength under static loading. There are some cases of fatigue failures while running oil-product pipelines and pump station pipelines. Therefore, it is of interest to analyse the actual operating loading and serviceability of pipeline members used for various purposes, as well as the criteria and design diagrams used to evaluate their strength and durability under various workloads.

ISOPE-I-92-119

The Second International Offshore and Polar Engineering Conference

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

It can be recognized recently that the gap has been steadily decreasing between practice and theory of automatic controls. For instance, considerable applications are available in the case of LQG, i.e. Linear Quadratic Gaussian Control, one of the principal modern control theories. This paper is to discuss or criticize its applicability in the ocean engineering fields. The LQG Control involves an assumption of constant disturbance characteristics, usually those of normally-distributed white noise. In marine environments, however, it can hardly apply, especially when the sea winds, waves and currents are considered as the main disturbances, from the viewpoint of their spectral density functions. The author discussed, therefore, about applicability of some control methods other than LQG control for deep sea mining operations. Considering the disturbance of drift force type, discussions were made on lateral vibrations of the hauling pipe induced by “vortex shedding”. The necessity of a newly revived frequency method was then suggested for compensation of disturbances with different frequency ranges.

Control theories have been grouped into the two categories; that is, the classical or scalar and the modern or vector ones, established mainly in 1930''s and in 1960''s respectively. Those theories can be also distinguished by the different domains in which they are developed; the former in the frequency domain and the latter in the time domain. In recent times since the year of 1981, however, there became available a new theory developing in the frequency domain. Someone call it a new frequency response method'', while the other says ''Here comes a Renaissance of control theory''. On these backgrounds the author considers it significant to investigate here into some application problems of control theories in ocean engineering fields.

ISOPE-I-92-073

The Second International Offshore and Polar Engineering Conference

Industry:

- Information Technology (0.49)
- Materials > Metals & Mining (0.35)

The liquefaction of seabed constitutes an important consideration in the planning and design of various offshore facilities. The stresses and the pore water pressure in the sediments, induced by the action of waves during a storm, may cause them to liquefy leading to seabed instability. In this paper the possible mechanism of such failures are discussed and the conditions necessary for them are formulated through simplified analyses. These provide the combinations of wave, site, and sediment parameters likely to cause liquefaction which may be used to identify potentially unstable sediments in a given offshore region during a storm. The numerical results are presented in convenient graphical forms. Some illustrative field studies are also presented.

The stability of seabed is important in connection with the planning and design of various offshore facilities such as pipelines, anchors, and gravity or platform structures. The occurrence of wave induced liquefaction in seabed have been reported by various authors (Christian et al, 1974; Nataraja and Gill. 1980; Zen et alI991). Water waves propagating on the ocean may create significant dynamic pressure on. the seafloor. This pressure field induces a stress field and an associated pore water pressure in the seafloor sediments. In most cases the pore pressure will be oscillatory in nature, however in case of cohesionless sediments of loose to medium density, pore water pressure may build up progressively. In case of unsaturated sediments or in case where the pore pressure may build up progressively, the sediments may experience liquefaction - leading to seabed instability. The possibility of wave-induced liquefaction in saturated ocean floor sediments was first recognized and analyzed by Bjerrum (1973) in connection with the deign of foundations for deep water North Sea structures.

ISOPE-I-92-056

The Second International Offshore and Polar Engineering Conference

As part of a research programme dealing with the flexibility behaviour of multiplanar joints in circular hollow sections, the flexibility of various types of multiplanar joints are investigated. This paper deals mainly with the flexibility behaviour of multiplanar XX-joints, with comparisons on the influence of the finite element (FE) modelling and computer costs (disc space and CPU-time). For the whole range of the joint parameters β, γ and τ, the joint flexibility has been investigated with and without inclusion of the weld shape for various load cases, such as axial loading, in-plane bending and out-of-plane bending. The results obtained can be used in determining the local joint flexibility of tubular joints, and calculating the deflection and load distribution of structures containing multiplanar joints. ** Introduction.** Multiplanar joints are frequently used in tubular structures e.g. in offshore jacket structures, triangular or rectangular girders, towers, etc. Local joint flexibility affects the load distribution and deflection of such structures and this work shows some quantitative results. There is little information [2-6] on the flexibility behaviour of tubular multiplanar joints and, to the authors'' knowledge, insufficient information on FE modelling for joint flexibility is available. The results of the numerical investigation (in the linear-elastic range) are presented using different types of FE models for comparison purposes. Several conclusions are established from the results, which should be considered when tubular joints are analysed numerically.

ISOPE-I-92-371

The Second International Offshore and Polar Engineering Conference

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