**Source**

**SPE Disciplines**

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**Concept Tag**

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**Industry**

**Oilfield Places**

**Technology**

The shoreline of the mainland and barrier islands of the U.S. Beaufort Sea from Bullen Point to Brownlow Point were studied to determine the long-term (1908-1982) and short-term (July-September 1982) rates of change from historical records, charts and air photos, and from 67 survey stations at nearly evenly spaced intervals along the area. The short-term rates were determined from profile line measurements taken at the beginning of the open water season (mid-July) and again in early September. Several of the measurement site were revisited in 1983 and 1988 and these data were added to the data set. Island migration rates of 24m per year have been documented over the ''long term (1908-1950), and much higher rates have been observed over shorter intervals. The historical records also show the bluff erosion rate on Flaxman Island to he about 5m per year (long term) and is sporadic from season to season. The data collected during the Summer 1982 surveys indicate erosion rates as high as 7.5m on Alaska Island between July and September. This was due primarily to a minor storm which occurred in late July. As expected, greatest changes were recorded near the ends of the barrier islands, and least changes were observed on the mainland shore that lies in the lee of the barrier islands. Survey markers were specially designed and placed along the shoreline at the 67 locations. Several markers have been over-washed by storm wave processes, however, we believe that few have been lost, thereby allowing this survey data base to be extended into the future.

Studies of coastal changes in the Pt. Thomson region of Northern Alaska on the southern shore of the U.S. Beaufort Sea (Figure 1) have been conducted to establish an environmental baseline for the Point Thomson area.

ISOPE-I-92-183

The Second International Offshore and Polar Engineering Conference

Industry:

- Energy > Oil & Gas > Upstream (0.69)
- Government > Regional Government > North America Government > US Government (0.47)

The load level on a jack-up platform is typically governed by the drag term of the Morison equation. Thus, the variabilities related to the extreme wave induced particle speed are very much amplified by the non-linearity of the loading mechanism. The total variability of the wave induced speed consists of both aleatory uncertainties and epistemic uncertainties. The first category is caused by the inherent randomness of e.g. the annual largest wave, while the latter are introduced by the imperfect modelling of this quantity and the calculation of the corresponding particle speed. Subsequently, the various sources of aleatory and epistemic uncertainties related to the calculation of the base shear of a jackup are identified. Uncertainties related to the wave climate description are established from data for two North Sea locations, while uncertainties related to the calculation of kinematics are quantified based on available literature. The relative importance of the epistemic uncertainties of the kinematics are assessed using a first order reliability method. Finally, the adequacy of the commonly adopted load factor of 1.3 for environmental load is discussed in view of the properties of and the loading on an idealized jack-up platform.

The overall uncertainty related to the design of marine structures is usually dominated by the uncertainties related to the load calculation. Uncertainties may be divided into two main categories; aleatory (inherent) - and epistemic uncertainties. The aleatory variability of the extreme forces is mainly produced by the inherent randomness of the environmental conditions e.g. the largest wave and the simultaneously occurring wave period, current, water level and wind. The aleatory uncertainty is accounted for by introducing the primary environmental parameters as random variables described by a joint probability density function.

ISOPE-I-92-276

The Second International Offshore and Polar Engineering Conference

SPE Disciplines:

Generally, we increase the cross sectional area of circular transverse frames or decrease the overall buckling length for increasing the critical buckling load of the ring-stiffened cylindrical shells. However, it is of no use to a large submarine . The curve r-p _{eo} and formula are presented here, which determines whether the result of ordinary calculated methods is correct or not, estimates the maximum buckling load, determines the thickness of the shell, decides the reinforcements of structure. There is only one curve . Therefore it is convenient to use. The curve has been checked by both model experiment and exact solution. **INTRODUCTION** Because of the high-duty alloy being used in the large submarine, the ratio of the thickness to the radius of the ring-stiffened cylinder shell reduces and the lateral rigidity of the frames increases . For this kind of structure, the critical buckling load obtained by the ordinary method is much larger than the exact solution, the ordinary reinforcement is useless . For the structure above, in this paper, the methods are given to calculate the critical buckling load, to decide the reinforcement, to determine the maximum buckling load and to design the dimension of the structure. 1. Introduction to the problem by two practical submarines The formula for calculating the critical value of overall buckling of ring-stiffened cylindrical shell under combined axial lateral compressure is as follows [6] We find Eq.2 depending only on parameters A, B and r when the materials have been determined. Giving two examples of overall buckling and comparing the results of various reinforcements of the ring-stiffened cylindrical shell, we can find that there is a abnormous buckling property.

ISOPE-I-92-377

The Second International Offshore and Polar Engineering Conference

In Part I of this paper, a general model is derived that governs the geometrically nonlinear, three-dimensional response of suspended cables that support an array of offset bodies. An asymptotic form of this model is derived herein which describes the linear response of the suspension in its equilibrium plane. The resulting model is amenable to closed-form analysis and is rich enough to capture dominant sagged cable effects including small equilibrium cable curvature and dynamic cable tension. Free response characteristics are computed using a closed-form solution strategy based on transfer matrices. Example solutions highlight how the natural frequency spectrum, vibration mode shapes and the dynamic cable tension depend on suspension symmetry.

A general model is developed in Part I that describes the geometrically nonlinear, three-dimensional response of a cable/body suspension about a sagged equilibrium. Upon linearization, the equations governing out-of-plane response decouple from those governing in-plane response. Moreover, for sagged cable/body suspensions with small equilibrium curvature, the out-of-plane model reduces to that of a taut string supporting an array of pendula. The ''present investigation of in-plane response requires a richer model for the cable/body suspension that captures the dominant sagged cable effects reviewed in (Triantafyllou, 1984a). These effects, which derive from cable elasticity and sag, are first noted in the small-sag model of Shea (1955) and later in (Simpson, 1966; Soler, 1970). A comprehensive investigation of smallsag cable theory is provided by Irvine and Caughey (1974) who show that in-plane vibration depends on a single cable parameter, .A2 , that describes the cable equilibrium tension and geometric and material properties. For horizontal suspensions, the in-plane modes are either symmetric or anti-symmetric with respect to the cable mid-span.

ISOPE-I-92-146

The Second International Offshore and Polar Engineering Conference

In this paper, the directional analysis is applied to study the behaviour of a floating platform connected to a deep water riser. Initially, numerical simulations of the sea surface elevations are carried out from wave spectra with directional spreading. The wave elevations are obtained for fixed positions in space which represent wave sensors, and they are analysed by the Maximum Likelihood Method (MLM) producing estimates of the directional wave spectra. The coupled system (platform/riser) is modelled and solved for a range of frequencies and directions of wave incidence, producing directional Response Amplitude Operators (RAO) for the six degrees of motions of the platform and for the riser bending stresses. With the platform motion RAO and the estimated wave spectra, directional response spectra are obtained and short-term prediction, in frequency domain, is performed to evaluate the first order motions of the platform in directional waves. The influences of the spreading parameter for head sea conditions and of the principal direction of wave incidence are investigated. Finally, short-term analysis is performed to evaluate the effects of wave spreading and of the wave principal direction on the total bending stresses of the riser.

The directionality of ocean waves has been recognized for a long time in the past. However, the effect of the wave spreading over a principal direction of propagation, or even of two or more simultaneous principal directions (crossed sea), in the analysis of the behaviour of offshore structures has not had a wide application. Many reasons could be listed to explain this fact, such as complexity and not enough knowledge of the directional properties of waves in a given location of the sea, and large computational effort needed when directional properties of waves are included in the analysis of the behaviour of ocean systems.

ISOPE-I-92-299

The Second International Offshore and Polar Engineering Conference

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

An analysis of interwire slippage in an axially loaded cable with finite friction is presented. It is shown that during the elongation the slippage starts in the middle of the cable and propagates towards the terminals. Explicit expression for the losses during one cycle of deformation are derived. It is shown that these losses depend on the parameters of the cable as well as on the magnitude of the load. For a cable modeled as an equivalent linear visco-elastic element, the equivalent elastic and viscous parameters are given as functions of cable parameters.

Cables are commonly used tension members in modern light and flexible structures, and in many applications it is important to know their dynamic properties. A conventional way to account the energy losses in cables is to assume a constant coefficient for viscous damping. However, many experimental studies have shown that losses in cables are mainly amplitude-dependent (Yu,1952; Claren and Diana, 1969; Vinogradov and Pivovarov, 1986). An attempt to explain structural losses in a bent cable was made by Vinogradov and Atatekin (1986). They proposed a theoretical model of the interwire Slip caused by the twisting of the wire and found the corresponding cyclic losses. The problem of hysteretic losses in a tension cable was considered by Hobbs and Raoof (1982), who represented a cable as an equivalent mutilayer continuum and predicted the losses resulting from the slip inside each of these cylindrical layers. The possibility of microslip in cable was also considered by Utting and Jones (1989) who used in their study of tension cables. Recently, Vinogradov and Huang (1991) developed a new model of an axially deformed cable, which shows that twisting and bending deformation of wires is the cause of microslip and the corresponding friction losses.

ISOPE-I-92-153

The Second International Offshore and Polar Engineering Conference

This paper describes the philosophy of analysis for the design of lifting eyes (padeyes). A systematic ordered approach to the design using simple beam theory is given. Typical i modes of failure for this connection are bearing, pullout arid tension tearing, as well as shear, axial and bending stresses. The validity of the use of simple beam theory for this type of design was checked by developing several finite element models to access the stress fields under various loadings and the limitations on the use of the theory in the design of padeyes is given.

In the past when crane ·lifting capacities were relatively small and fabrication costs low, it was possible to grossly overdesign the lifting eye without any significant economic impact. This is no longer the case, however especially in erecting offshore facilities. Offshore lifting has now reached the 5,000 ton (4535 metric ton) capacity, and the cost of materials and fabrication of lifting eyes is rapidly escalating. This paper describes a philosophy of analysis for the design of lifting eyes utilizing a systemaic ordered approach: The design procedure given here IS based on Gelpi (1976),Gelpi, Cayll, Coyne and Russo (1990) and Russond Hall (1977). The approach uses simple beam theory. Typical modes of failure for this connection are bearing, pullout and tension tearing, as well as, shear, axial and bending stresses .. The validity of the use of simple beam theory for this type of design was checked by developing numerous finite element analysis models to assess the stress fields under various loadings. It is not possible to analyze all configurations and load cases with finite element techniques; however, It IS felt that from the finite element analysis that simple beam theory does present a reasonable approach to lifting eye design.

ISOPE-I-92-045

The Second International Offshore and Polar Engineering Conference

analysis, area, Beam Theory, cheek plate, cross section, design, element, inch, lifting eye, Load, meters, padeye, pin hole, plate, section, stress, Tension

SPE Disciplines:

The present paper discusses structural damping in nonbonded flexible pipes based on experiments and numerical simulations. The main conclusions from the study are: • Structural damping in flexible pipes is high and influence dynamic response significantly. • Refined cross section models allowing for hysteretic behaviour improve the accuracy of the results from dynamic simulation of flexible risers. The experiments are performed on a free hanging fullscale 4" internal diameter, 8 m long nonbonded pipe pressurized to different levels. Additionally, the influence on bending stiffness from excitation frequency and curvature amplitude are studied by cross-sectional tests. A hysteretic bending model is implemented in a computer program. Structural damping and bending stiffness description in flexible pipe simulations are discussed, and a consistent procedure for simulations is presented.

Several damping mechanisms are present in flexible pipes during dynamic excitation. The motivation for the present study was the reported high structural damping levels, especially in nonbonded pipes (e.g. Otteren and Hanson, 1990). The damping level is high compared to other offshore structures, and will thereby influence dynamic response significantly. It is therefore important to apply an adequate damping model in numerical simulations. There are several problems linked to the modelling of structural damping in flexible riser analysis: • Damping properties for flexible pipes are generally not specified by the producers, and may be difficult to obtain for specific pipe structures. • Damping may be characterized in several ways, and it is not obvious which model will be best suited for a particular method of analysis. • Characterization of damping properties should be compatible with the particular damping model and method of analysis. There are no standard methods of measuring damping properties, and choice of experimental method may significantly influence the results.

ISOPE-I-92-125

The Second International Offshore and Polar Engineering Conference

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

The effect of hydrogen contents on crack nucleation time was conducted by using compact tension specimens of 473K tempered AISI 4340 steel in three kinds of environmental solutions under various electrode potentials. The crack nucleation at the notch root is determined by the electrical potential method. When the crack initiates, the voltage differences between the notch mouth starts to increase. The crack initiation times depend on hydrogen contents and the apparent stress intensity factors. These dependencies are successfully explained by a dislocation pile-up model with the hydrogen atom interaction.

1. INTRODUCTION

Considerable works have been done to clarify the crack nucleation mechanisms of hydrogen assisted cracking (Troiano, 1760, Beachem, 1972, Leeuwen, 1975, Oriani, 1974, Page, 1982 and Hirose, 1977). In the present paper, the effect of hydrogen contents on crack nucleation time was investigated for 473K tempered AISI 4340 steel in three kinds of environmental solutions; distilled water, 3.5% NaCl solution and 0.1N H2S04 solution. A new dislocation model is proposed to explain the crack nucleation phenomenon. Experiments are performed to compare with the present theory.

2.EXPERIMENTAL PROCEDURE

The material used was AISI 4340 steel (wt.% ; 0.39C, 0.74Mn, 1.38Ni, O.78Cr, O.23Mo)'' Compact tension specimens (see Fig. 1) with notch radius, ρ = 0.5mm and thickness, B = 5.5mm were machined from cutting slice at the right angle to the axis from hot rolled round bar with 100mm diameter. The specimens were normalized at 1153K for 1 hour and austenized at 1123K for 1 hour and then oil-quenched and tempered at 473K for 2 hours. After the surface layer was removed by grinding, the notch was made carefully by an electrical discharge machine to minimize the worked layer. The specimens were finally finished by electro-polishing. The mechanical properties of the material after heat treatment are given in Table 1.

ISOPE-I-92-332

The Second International Offshore and Polar Engineering Conference

SPE Disciplines:

- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (0.51)
- Well Drilling (0.48)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (0.41)
- Reservoir Description and Dynamics > Reservoir Characterization (0.34)

Hydrodynamic interactions between a threedimensional body of revolution and an infinitely-long cylinder moving relatively in an in viscid fluid at rest at infinity are studied by the boundary-integral method. A set of four integral equations of the second kind are solved numerically, and a numerical technique is developed to evaluate integrations over steep peaks accurately and effectively. As a practical example. the moving trajectories of a sphere, conveyed by a uniform flow, around a fixed circular cylinder are computed and presented.

For design of offshore structures in arctic and subarctic waters, it is important to determine the interaction forces acting on the structures due to the unsteady motion of a floating ice floe in the near field. In general, the total interaction force consists the drag component and inertia component, and may be approximated by the Morison equation if the Reynolds number based on the size of an offshore structure is less than 2X10

ISOPE-I-92-292

The Second International Offshore and Polar Engineering Conference