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**Industry**

**Oilfield Places**

**Technology**

The project "Stress Concentration Factors for Simple Tubular Joints" was a UK Department of Energy (DEn) sponsored project, completed in 1990. The primary objectives of this project were to assess current methods for deriving SCFs in simple tubular joints, to discuss the applicability of the more commonly used parametric equations, and to derive a new set of parametric equations that would reduce some of the anomalies in the existing formulae. In this paper the principal conclusions of this project are summarised and the new Lloyd''s Register parametric equations estimating SCFs for simple tubular joints are presented.

In the 1970''s with the increasing development of the hot-spot stress S-N approach to nodal joint fatigue life estimation, it became clear that the determination of reliable SCFs for tubular joints was fundamental to this concept. The first parametric SCF equations covering simple tubular joints were derived by Toprac and Beale (1967) using a limited steel joint database. The prohibitive cost of testing scaled steel models led Reber (1972), Visser (1974) and Kuang et al. (1975) to use finite element (FE) analyses based on analytical models of cylindrical shells. Subsequent equations by Wordsworth and Smedley (1978) using acrylic model specimens and by Efthymiou and Durkin (1985) employing 3-D shell FE analyses, have made considerable advances both in the accuracy of parametric equations and in the range of joints covered. Over this period, differences arose between the experimental procedures used to derive stress concentration factors for simple tubular joints. These differences led to inconsistencies both in the measured SCFs themselves, and also in the SCF parametric formulae based on these measured SCF values. These inconsistencies in SCF derivation are reflected in the hot-spot S-N curves used to estimate fatigue lives for simple tubular joints. In the UKOSRP II project (DEn, 1987), a limited programme of work investigated anomalies between the existing simple joint parametric SCF equations. A test programme on unstiffened tubular joints performed by Lloyd''s Register, (Smedley and Fisher, 1990), has further emphasised inconsistencies between test results and the more commonly used parametric equations. While current guidance merely states that ''the appropriate SCF'' should be used, it is the intention of future guidance (Reynolds and Sharp, 1990) to give more specific directions on which parametric SCFs may be employed in fatigue design, and the procedures that should be considered prior to performing either experimental tests, FE analysis, or in deriving parametric equations.

ISOPE-I-91-282

The First International Offshore and Polar Engineering Conference

analysis, axial, brace, chord, concentration, configuration, database, difference, equation, extrapolation, factor, ofthe, saddle, SCF, steel, stress, value, weld, weld fillet, Wordsworth

It is shown that the equivalent non-linear equation method (ENLE) can be used to obtain the approximate solutions of the non-linear rolling equation of ship motion using the results of the average method and that the proposed scheme is to be preferred over the average method when the damping effects become dominant.

The present study investigates a method of predicting nonlinear rolling motion of a ship in irregular waves. Since the nonlinear nature of the rolling motion of a ship in waves is very complicated, a complete analytic solution of the problem has not been proposed so far. In the past, an equivalent linearization approach has been widely adopted in random vibration problems for nonlinear systems. But when the nonlinearity is strong, the higher order moment statistics may not be reliable. A better approximate method known as the equivalent nonlinear equation method has been introduced. Most of the proposed equivalent non-linear equation methods have treated non-linear damping, but only in the special case of linear restoring. In this study, the equivalent nonlinear equation method largely developed in the field of random vibration was adopted to solve the problem of nonlinear rolling motion of a ship with non-linear restoring in irregular waves.

The difference between the ENLE method and the average method becomes large. The differences between the ENLE and the average method, in the case of large damping, are thought to be due to the fact that the average method assumes light damping. Similar features are also shown in the figures 8 and 9.

ISOPE-I-91-203

The First International Offshore and Polar Engineering Conference

The civil engineering construction of Britain''s first prototype wave power device was completed in November 1988. The device is located on the Isle of Islay, one of the southern islands in the Inner Hebrides. The unit has been built in a natural rock gully in a headland north of the village of Portnahaven. The station has a biplane Wells rotor direct coupled to a 75kW would rotor induction generator. During the past twenty months the pneumatic power output has been measured in a wide range of conditions from comparative calm to severe storm. During the past year the turbine generator plant has been designed and manufactured and was installed during the winter of 1990/91. The unit

This paper provides a synopsis of the shoreline wave energy research being undertaken by Queen''s University centered around a prototype installation on the island of Islay. The criteria for selecting sites for coastal wave power stations are discussed with particular application to the coasts of the United Kingdom. This is followed by a detailed description of the Islay prototype station. The hydro-pneumatic performance of the system is discussed with particular emphasis on the correlation between the results obtained from the prototype and fiftieth scale model tests. The paper concludes with a general discussion of the construction techniques used and how the technology can be applied to larger systems.

ISOPE-I-91-053

The First International Offshore and Polar Engineering Conference

In most experiments, and in real applications, it has been observed that fatigue crack growth is much faster at the surface along the weld line when compared to the through thickness direction. Surface cracks may extend up to and over 1,000 mm in some cases before a 20 mm through thickness crack develops. The paper is dealing with this problem using analytical methods. As a 3 dimensional FEM approach is too demanding a line-spring element method has been developed to calculate the distribution of stress intensity factors along the growing crackbody interface. Based on the relation between displacements { 8 } and general stress { a } the stiffness of the line-spring element can be derived by using the virtual work principle: [k] =

The study of fatigue behaviour of tubular joints, is very important for the purposes of design, manufacture and service as it relates to the offshore structures. In most experiments and in real applications, it has been observed that the progress of through thickness cracks is much slower than the surface crack growth along the weld line and the ratio of surface length of cracks to the through thickness crack length is high, of the order of 20-50 (Wyled and McDonald, 1981).

ISOPE-I-91-275

The First International Offshore and Polar Engineering Conference

Drilling ships positioned with a mooring system perform various motions in waves. These motions consist of first-order motions of wave frequency and of low-frequency motions (second order) from wave-drift forces. The amplitudes of low-frequency motions are influenced by the mooring positioning system. In this paper, the influence of some parameters, such as pretension, unit weight, number of chains in the system and water depth on ship motions has been presented. First and second-order ship motions as well as natural frequency of low motions within a horizontal plane have been shown. Low frequency amplitudes of motion have been calculated for wave groups.

A moored ship is influenced by forces from wind, current and waves. The forces from the ocean environment push the ship off the datum position. At the same time dynamic forces from waves cause motions. Dynamic forces of wave frequency(first-order forces) and low-frequency wave-drift forces(second-order forces) occur in the wave. As a result there are Ship motions of a low-frequency and of wave frequency. Low-frequency motions of the ship are caused by additional restoring forces such as spring forces from the mooring positioning system. Low-frequency motions occur in three degrees of freedom :surge, sway and yaw directions where hydrostatic restoring forces do not exist but only spring forces from the mooring system. The displacements of low-frequency motion depends, among others, on restoring spring forces, that is to say, on parameters of the mooring positioning system.

ISOPE-I-91-080

The First International Offshore and Polar Engineering Conference

SPE Disciplines: Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Mooring systems (0.92)

Recent studies were performed in the development of a lifted jacket for water depth of greater than 100 metres. The 9500 tonne jacket was to be barge. transported horizontally, lifted off the transportation barge with a tandem crane arrangement over the stern of a semi-submersible crane vessel, and upended in the water prior to docking on the seabed with a pre-installed subsea drilling template. Three methods of lifting and upending the jacket were considered:

ISOPE-I-91-007

The First International Offshore and Polar Engineering Conference

arrangement, buoyancy, capacity, configuration, crane, DERU, hook, installation, jacket, LIFT, Load, method, pile, release, requirement, trapeze, trunnion, upend, water, weight

Oilfield Places:

- Europe > United Kingdom > North Sea > Central North Sea > Block 16/7a > Brae Field (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Block 16/3b > East Brae Field (0.99)
- Europe > United Kingdom > North Sea > Central North Sea > Block 16/3b > Brae Field (0.99)
- (2 more...)

Presently a different generation of pipes in the form of clad pipes are under consideration of oil industry as cost effective alternative over conventional solid pipes made of corrosion resistant materials. Mater1als such as Austenitic t Duplex Stainless Steels and Nickel based alloys are used as clad metals and the same are examined under accelerated test conditions as per ASTM and MTI standards to evaluate their corrosion resistance. Incoloy cladded carbon steel line pipe material is tested to assess galvanic coupling effect, intergranular corrosion resistance. Tafel pots are plotted and polarisaton resistance values are calculated separately for both carbon steel and incoloy clad material. Coupled potentials and corrosion rate of steel are also evaluated. The pitting and crevice corrosion resistance of Stainless Steels such as 304 t

Production of sour oil and gas has posed great challenge to engineers to select suitable material with proven fabrication specifications and performance. Conventional carbon steel line pipes are prone to severe hydrogen induced cracking or sulfide stress corrosion cracking. Several corrosion resistant alloys have been developed to enable them to withstand the hostile sour operating conditions. Most of these alloys have also been used as internal cladding material for carbon steel line pipes. Internal cladding on carbon steel enables conventional external protection of coating and cathodic protection. Austenitic Stainless Steels t Duplex Stainless Steels and Nickel alloys are used as cladding materials. The selection of the material normally depends on operating conditions economic factors and ease of fabrication. Operating parameters and fluid environmental factors in particular PH, temperature, oxygen, carbon dioxide, hydrogen sulphide, water content and chlorides play considerable role in case of material corrosion failures besides metallurgy.

ISOPE-I-91-249

The First International Offshore and Polar Engineering Conference

SPE Disciplines:

- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)

The present paper attempts to provide a critical review of various parameters that should be considered while designing a pipeline system for transporting waxy crude (high paraffin content). In designing such a system both steady state and transient state have to be considered as the former is relevant during normal operation of the pipeline while the later is relevant for the start-up up operation after shut-down. Considerations regarding heat transfer coefficient and thermal conductivity variations of the wax deposition over a period of time are essentially required for simulating near pour point conditions in the pipeline. These considerations also play an important role in determining optimum shut-down periods after which the pour point zone is likely to be reached resulting in wax deposition in a large scale. The simulated pipeline temperature profile along with the rate of cooling down to the pour point zone are also presented which in turn determine the optimum shut-down period of the pipeline. A method for quick calculation of restart pressure has been put forward. The results of the case study are presented with critical analysis.

Waxy crude oil transportation, particularly through long distance pipelines requires critical attention by pipeline designers and operating personnel as the transportation process can create potential operational problems. The prime mover (i.e. pumps) availability in this regard plays an important role as sudden reduction 1n flow may cause gelling of wax inside the pipeline requiring higher start-up pressures. Evaluation of paraffin inhibition requirement in pipelines also requires to be dealt with care as often these inhibitors are very costly. Thus before recommending the optimum operational modes, a designer has to consider several aspects and in most cases has to recommend a number of options in order or their priorities.

ISOPE-I-91-108

The First International Offshore and Polar Engineering Conference

case, crude, flow assurance, formation evaluation, heat, offshore pipeline, oilfield chemistry, operation, point, pour point, pour point temperature, pressure, production control, production logging, production monitoring, pump, restart, study, temperature, transportation, wax deposition, waxy crude

SPE Disciplines:

In the design of cylinders sitting on a permeable seabed, the wave induced uplifting forces are important factors. This paper studies the nonlinear wave action on the bottom of a circular cylinder by using cnoidal wave theory. The results show that when

Circular cylinders are often used in coastal engineering. In the design of such structures, in addition to the horizontal wave forces, the wave induced uplifting forces acting on the bottom of the cylinder should be taken into account. Among the past researches of this problem, linear wave theory has been used. But in coastal regions, due to the relatively large wave height to water depth ratio, big error often occurs if linear wave theory is used to describe the real wave profile. Therefore it is needed to study the nonlinear effect of waves on the uplifting forces. In deep water regions, that is, in the regions where the values of wave length L to water depth d ratio are relatively large, for nonlinear waves with large wave height and wave steepness, good results can be achieved by using higher order Stokes wave theory to describe the wave profile. But in shallow water regions, when

ISOPE-I-91-185

The First International Offshore and Polar Engineering Conference

Mavrakos, S.A. (Dept. of Naval Arch. & Marine Engrg., National Tech. University) | Papazoglou, V.J. (Dept. of Naval Arch. & Marine Engrg., National Tech. University) | Triantafyllou, M.S. (Dept. of Ocean Engrg., Massachusetts Inst. of Technology) | Brando, P. (Tecnomare S.p.a.)

The dynamics of mooring lines for deep water applications with submerged buoys attached to them are studied both experimentally and numerically. The experimental setup, as well as the data acquisition procedures are first detailed. This is followed by a presentation of the obtained results and their comparison with numerical predictions using both time and frequency domain computer codes. Very good correlation is obtained. Finally, the beneficial effects of buoys in reducing the mooring line dynamic tension is shown, provided that proper selection of their size, number and location is performed.

The need for incorporating dynamic considerations in a rational design procedure for deep water mooring applications has been addressed in several recent publications (Tein et aI, 1987; Kwok and Huang, 1988; Bergdahl and Rask, 1987; Taylor et aI, 1987; Fylling et aI, 1987; Kwan, 1990). This has led to a modification of the design rules and guidelines of various organizations. The quasi-static design approach, which has been proven to be a proper design tool for mooring systems in shallow water, is generally considered inadequate for the case of deep water applications. In such cases the maximum attainable value of the dynamic tension amplification, which is approximately equal to the elastic stiffness of the line, ,is shifted within the wave frequency range, thus causing large dynamic tension values due to the vessel''s firstorder motions (Triantafyllou et aI, 1985). It is therefore evident that the elastic stiffness of the cable represents the principal parameter affecting the mooring line''s dynamic response and, hence, that its reduction would improve the dynamic performance of the line. It has been recently shown both numerically and experimentally (Mavrakos et aI, 1989a and 1989b; Papazoglou et aI, 1990b), that this reduction can be achieved by inserting submerged buoys along the mooring line.

ISOPE-I-91-098

The First International Offshore and Polar Engineering Conference

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