This paper presents a direct method for determining natural frequencies of lateral modes of vibration for marine risers in deep water. This method applies to marine risers that are vertical, relatively straight, and attached at both ends. The method is particularly useful for determining natural frequencies of higher modes that are sometimes difficult to obtain analytically or numerically. Comparisons of numerical results with published data show that even though the method of solution is approximate, the calculation procedure gives useful engineering results. The algorithm is based on classical vibration theory and can easily be programmed on portable computers for direct use on offshore oil rigs.

Several mathematical models are currently available to predict the mechanical behavior of twisted wire cables and ACSR electrical conductors subjected to axisymmetric loads. The equations for each model are presented in a standardized form in the linear case and the differences illustrated are shown. Numerical results obtained with each model are presented in tabular form and a comparison made with experimental results reported in the literature.

An analysis of the variation of forces acting on the upper ball joint of a riser string due to the drill ship motion and riser tensioner dynamic has been conducted. The analysis includes the effect of breakaway torque on the tensioner sheaves while assuming vessel and upper ball joint motion to be independent. General equation for the tensioner cable forces and for the forces exerted on the riser upper ball joint by the ship joint-tensioner system derived in Part I are solved. The variation in the tensioner cable forces is compared to data generated in field operation.

General equation for the tensioner cable forces and for the forces exerted on the riser upper ball joint by the ship joint-tensioner system are derived. An analysis of the variation of forces acting on the upper ball joint of a riser string due to drill ship motion and riser tensioner dynamic has been conducted. The analysis includes the effect of breakaway torque on the tensioner sheaves while assuming vessel and upper ball joint motion to be independent.

A fatigue model which predicts cycles-to-failure for helically armored cables subjected to fluctuating axial tension is proposed. Electrical-optical communication cables, power cables, and bridge and track strands normally derive structural strength from two or more layers of round steel wires contrahelically laid around a cylindrical core. In cases where wires are laid in direct contact with wires in adjacent layers, Hertz contact stresses produce wire failures leading to ultimate cable failure at tensions well below the static breaking strength. The proposed model treats cross-wire Hertz contact stresses as equivalent geometric notches in conjunction with the numerical solution of the governing helical wire cable equations. Model and physical test results show good agreement.

Previous solutions to the vortex-induced vibration of structures have been primarily based on modal analysis, using a one or two-mode approximation. Such an analysis is generally meaningful only when the vortex shedding frequency is locked onto a natural frequency of the structure. In very large structures, typical of those found in some ocean engineering applications, modes are closely spaced, and it is not reasonable to assume total spanwise correlation in the fluid forces or response. The approach used herein avoids the limitations associated with the modal solution of such problems by implementing a solution based on the traveling wave nature of the response. Results presented indicate that the amplitude of response of a long cable is smaller than is predicted by a conventional modal analysis. The drag forces on such a structure may therefore be overestimated by current design approaches.

An anchoring system for an offshore structure must meet certain prescribed requirements controlled by factors such as the site environment, operational constraints and the vessel employed. Its adequacy, survival and ability to stay on site must, therefore, be checked out with proper methods of analysis. The inclusion of cable dynamics is an important consideration in the dynamic analysis of a moored vessel. In this paper, mooring line equations of motion are derived for a multi-component, n -segment model using Lagrange’s modified equation, permitting anchor motion, and then numerically solved to yield time histories of cable displacements and cable tensions for the various cable configurations that can occur. Initial conditions can be provided through the mooring line static catenary equations. The nonlinear restoring force terms in the vessel equations of motion are generated through the dynamic tension- displacement characteristics of individual lines. The equations of motion of the moored vessel subjected to an open ocean environment are then numerically solved to yield time histories of vessel motions and cable tensions. An example involving a moored production barge is examined and results are compared with those of previous work in which a quasi-static cable configuration is employed.

Cable is normally recovered by hauling in from ahead, with the cable guiding the ship. In an early paper, Zajac presented a method for recovery, proposed by Shea, in which the cable is overrun, and hauled in tending aft. The potential advantage is lower tension in recovery. Cable was recovered with overrun in two recent exercises. The tensions and cable angles observed at the ship in these exercises are used to work out cable configurations and bottom tensions. The calculations show that the bottom tension was considerably less than it is for normal recovery in both exercises. As the overrun is increased, the tension decreases, and tends to a limiting value defined by the ship speed and the slack rate at which the cable was originally laid. Recovery with overrun is practical and advantageous.

The vortex-induced vibration response of long cables subjected to vertically sheared flow was investigated in two field experiments. In a typical experiment, a weight was hung over the side of the research vessel by a cable that was instrumented with accelerometers. A typical experiment measured the acceleration response of the cable, the current profile, the tension, and angle of inclination at the top of the cable. Total drag force was computed from the tension and angle measurements. Two braided Kevlar cables were tested at various lengths from 100 to 9,050 ft. As a result of these experiments, several important conclusions can be drawn: ( i ) the wave propagation along the length of the cable was damped, and therefore, under most conditions the cable behaved like an infinite string; ( ii ) response spectra were quite broad-band, with center frequencies determined by the flow speed in the region of the accelerometer; ( iii ) single mode lock-in was not observed for long cables in the sheared current profile; ( iv ) the average drag coefficient of long cables subjected to sheared flow was considerably lower than observed on short cables in uniform flows; ( v ) the r.m.s. response was higher in regions of higher current speed. A new dimensionless parameter is proposed that incorporates the properties of the cable as well as the sheared flow. This parameter is useful in establishing the likelihood that lock-in may occur, as well as in estimating the number of modes likely to respond.

During the 1982–1983 drilling campaign in the Mediterranean, two wells were drilled by the Discoverer Seven Seas in record water depths of 1714 m and 1252 m. The riser was equipped with instruments to measure tensions, moments and angles at points close to the extremities. Simultaneously ship motions and environmental conditions including current measurements at various depths were recorded by ELF AQUITAINE and I.F.P. A large quantity of data relating to riser behavior was gathered and recorded. The results available for publication include raw data relating to the evolution of current at different depths, long period lateral ship motion acting at the riser top end, induced by the dynamic positioning system. Simultaneous measurements of angles at extremities were recorded to confirm the validity of two-dimensional riser analysis. Measurements of riser parameters made with deliberately large ship excursions from the vertical, enabled the riser practical profile to be compared with the theoretical one. Measurements, made with the riser hung off in storm conditions, confirmed the existence of large dynamic tensions and enabled them to be compared with the theoretical values. Correlation of the fluctuation of cable tension with ship heave has led to practical values of tensioner stiffness.

Spiral ropes of high load-bearing capacity are usually made of cold-drawn steel wires of high tensile strength. This material has been investigated by many researchers, not only to determine the mechanical properties of cold-drawn wires, but also to find out more about their stress-strain behavior under tensile loading and local three-dimensional stresses. It is now possible to determine load ranges for fatigue as a function of: surface conditions of the wire; the length of the wire; the diameter of the wire; local stresses resulting from damage to the wire surface; local stresses resulting from transverse pressure at points of contact where wires cross; friction between the wires produced by changes in stress; friction between the wires produced by changes in cable curvature; friction between wires and fittings or anchorage structures. Cables can be protected from corrosion due to environmental influences. If this is done the fatigue behavior of a twisted cable primarily depends on the addition of friction energy given to the material where strong contact forces cause three-dimensional stresses. A method of determining the fatigue strength of tension members made of cold-drawn wires is described using statistical methods, results of fatigue tests on short specimens, and precision measurements of the cold-drawn wire. It is possible to reach the material dependent fatigue limit of about 300 N/mm 2 with a spiral rope by a welldone structural design and corrosion protection.

Exxon’s Mississippi Canyon 280-A (Lena) platform represents the first commercial application of the guyed tower concept for offshore drilling and production platforms. Unlike a conventional offshore platform, the guyed tower is held upright by an array of guylines attached near the upper end of the structure and radiating outward to anchor piles driven into the seabed. This paper describes the functional requirements of the various components of this unique guying system and shows how each component was designed to meet those requirements. Among the design parameters discussed are guying system stiffness and strength, fatigue and wear life, corrosion protection, and assembly. The major length of the guyline consists of spiral-wound bridge strand, most of which is sheathed in polyethylene. The cable constructions and terminations are described. Near the tower, the sheathed cable is connected to a length of bare cable which passes through a special fairlead arrangement to direct the cable to the upper attachment above the water line. The fairlead arrangement and the upper attachment are described as well as the means provided to protect the bare cable from corrosion. Unique pinned connections in the system are designed to ease assembly while still providing the required load capacity and service life. A special anchor pile attachment eye design allows for large tolerances in anchor pile orientation. A description of the clump weight shows how this component is designed to achieve the desired system stiffness while also providing stability against overturning or excessive settlement into the seabed.

Twenty full-scale test runs were conducted during the cable strumming experiments reported in this paper. These consisted of ten pairs of equivalent tests conducted in air and in water with a cable fitted with arrays of attached masses. The measured in-air natural frequencies are in good agreement with computed code predictions for the second and higher (up to n = 5) cable modes. The first mode frequency apparently was influenced by the sag of the cable. The measured mode shapes of the cable vibrations in water also are in agreement with the computed mode shapes. For the experiments in water the computed and measured natural frequencies are in good agreement. Drag coefficients in the range C D = 2.4 to 3.2 were commonly observed when the cable-attached mass system was strumming due to the water flow past it.

On designing optical fiber cables, it is necessary to deal with the weakness of optical fibers, such as a small breaking elongation compared with metals and excess optical loss under both lateral and hydraulic pressure. This paper presents a structural design method for submarine optical fiber cables, based on the study of both lateral and hydraulic pressure characteristics. This paper also clarifies that a composite tension member increases the lateral and hydraulic pressure strength of the cable and can protect optical fibers from extremely large force during laying and recovery.

The object of this paper is to discuss the use of empirical factors in the prediction of armored cable axial stiffness. The stiffness of the armor wires and core that compose an armored cable are obtained from empirical data. To predict the net axial stiffness of a cable from the stiffnesses of its components a model of how the pitch diameter of an armor wire decreases as the cable is tensioned is required. The pitch diameter contraction is modeled in this paper by an experimentally determined Poisson’s ratio for the cable. The accuracy of cable axial stiffness predictions using these empirical factors is illustrated by comparing the results of mathematical models of armored cable to tension-elongation test data for ten different cables.

For burying submarine cables under a seabed by a water jet cable burier, it is important to improve digging depth of water jet. This paper discusses digging properties with air-bubble water jet, and clarifies the relation between digging depth and water jet parameters such as traverse speed of nozzle V n (m/s), air content ratio C , and jet momentum per unit time M (N). It is found from experiment results that the digging depth H* (m) is represented by H * = ( 1 + 0.26 C 0.65 ) ( 10.31 − 4.54 V n ) M 0.44 × 10 − 2 .

During the past decade, there has been a proliferation of operational systems which include electromechanical (or electro-optical-mechanical) cables as vital system components. Examples of such systems include: 1) unmanned remote operated vehicles (ROVs) used for subsea exploration, inspection, recovery, or repair; 2) towed arrays or towed electronic packages used for subsea maping, exploration, or surveillance; 3) moored subsea systems used for surveillance or acquisition of environmental data; 4) tethered aerostats used for communication and surveillance; 5) data acquisition systems used in oil, gas, and geothermal wells. For each of these systems, the success of the mission is contingent upon the reliable operation of the electromechanical (EM) cable which provides the strength, power, and communications link. As the applications for EM cables have become more varied and as the operational conditions for these cables have become more demanding, there has been a corresponding increase in the sophistication of cable design and manufacturing methods. Most contemporary EM cables are highly complex machines which are designed specifically for the intended application. Unfortunately, the complexities and the operational idiosyncrasies of these cables are often misunderstood by the cable user with the result being less than optimum cable performance. As cable technology continues to advance, it becomes increasingly important that cable users devote ample engineering and monetary resources to the development of cables and cable handling systems which are required for critical applications. This paper discusses many of the operational characteristics of typical EM cables and highlights the important points which must be considered during cable design. Examples are given of how cable designs may be tailored to achieve specific performance goals in terms of cable strength, elasticity, torque and rotation characteristics, bending fatigue life, and twist tolerance. The paper also compares and contrasts the operational characteristics which are achievable with cables having metallic strength members versus cables having nonmetallic (typically Kevlar) strength members. The advantages and short-comings of these two basic classes of cables are described for various strength member configurations. A list of references is provided to assist the reader in further investigations into cable response to tension, bending, and twisting.

The problem of the bending of a spiral strand or armored cable is addressed, with particular reference to the “free bending” situation where pulleys or other restraints are absent. The analysis, based on a study of the properties of the layers of wires which form the strand (or armor), treats the “free field” bending, remote from the termination. A parallel treatment of the situation close to a termination is summarized. In the former case, limiting values for the effective bending stiffness of the strand are presented; in the latter case an examination of the behavior of an outer layer of wires sliding (with known frictional characteristics) over an inner core has led to predictions of the strains and movements between the individual wires and the inner core (as a function of wire position in the strand) which are reported in the paper. The results offer an explanation of some experimental observations from fatigue tests on a large (39mm) strand under combined steady axial load and lateral movements causing bending adjacent to the restrained termination. In particular the observed wire failures close to the socket, not at the extreme fiber positions but at the neutral axis, can be explained in terms of the much larger slip on the interlayer contact points there.

A simple model for a complex system is presented that contains the essential properties of the guyed tower. The dynamic response to wave load of a guyed tower is represented by a single degree of freedom model. Tower response, with different wave heights and hydrodynamic coefficients, using statically and dynamically determined mooring properties were calculated for a specific tower in 1100 ft (335 m) of water. The importance of this study lies in a good modeling of the cable forces, including the hysteretic effects of line damping, and in identifying cases where the quasi-static model may fail.

To transport highly viscous crude oil efficiently through pipelines, some electric heating systems have been conventionally used. But they include heating cables or tubes, which are troublesome in submarine pipeline construction. A new electric heating system without a cable has been developed and is applicable to long-distance submarine pipelines. This system has a coaxial steel pipe structure, using the pipes as a heater by applying AC current to this coaxial circuit. In this paper, electromagnetic analysis on this system, experimental study using a 30-m length test pipeline, and mechanical tests on the electric insulating structure of the system are described.