tiarch 1990
DELFT UNIVERSITY OF TECHNOLOGY WORKGROUP OFFSHORE TECHNOLOGY P.O. BOX 5048 2600 GA DELFT THE NETHERLANDS Telephone: 31 (0)15 784614 Telefax: 31 (0)15 786993 E-mail: HASSIE@TUDCOT.TUDELFT.NL Sche?pshydromechWica Archief Mekeiweg 2,2628 CD D&ft TO1578883FaxO157818213
PREDICTION 0F CABLE SERVICE LIFE in the MARINE ENVIRONMENT A PROJECT DEFINITION by W.W. Hassle, MSc, P.E. Offshore Coordinator
TU Delft
Deift University of Technology Offshore Technology
J U S TIF IC A T I O N
The use of more modern (and expensive) materials for all types of offshore cables makes their unit replacement cost higher. Use of poor
(underdesigned or worn out) cables impairs safety; overdesign or too frequent replacement impairs the budget; neither of these is
desirable. Work in deeper ;ater, for longer times, and with more severe (loading) conditions makes cable performance more critical. Often the relation between cable failure and its cause is not well defined; many cable failures seem to have mysterious causes.
Apparently, there is a weakness in the present methods used to predict cable system performance and dependability (lifetime). One way of
increasing system dependability is via more frequent cable
replacement; this shortens the depreciation period, too however. An alternative is the use of other (more expensive) materials and/or more conservative design. These alternatives often increases cable cost. Both investment cost and replacement frequency contribute, therefore, to the total cost represented by cable systems in offshore operation; cable use optimization is called for.
The emperical basìs by which most offshore cable systems are now
selected for a given application, makes economic optimization of their original installation or replacement impossible. Cable performance is
actually determined by a complex synthesis of dynamic loads occurring in the cable with the cable's ability to resist those loads. Those users and suppliers able to carry out an optimization which includes both of these factors iill have a definite competitive advantage.
PROPOSAL OBJECTIVE
The ob.iective of the proposed study is to develop the knowledge and systems to carry out this optimization by predicting both the dynamic
loads occurring in a cable (system) as well as its ability to
withstand these loads. Studies which concentrate on only one of these factors cannot result in a responsible optimization.
This proposal involves the development of an optimization procedure so that cables and cable systems can be optimized for a given
application. This will involve the accumulation of new knowledge as iell as the development of tools to work with that knowledge.
PROBLEM ANALYSIS
As can be seen in the table below, there are a great variety of offshore cable applications and application characteristics. Only
the most relevant and spectacular characteristics needed for this discussion are included here.
The table includes a wide spectrum of marine cable applications: This can involve umbilical communications cables for under-water equipment such a cameras to photograph the 'l'itanic and/or mooring lines in any
system configuration. These cables can be fabricated from any of a variety of materials including synthetics even chain can be
considered for certain applications. As can be seen from the table, all or these applications have many common characteristics. The optimization procedure to be developed is independent of the
application - at least initially. Restriction of the applicability of the procedure to be developed can be introduced later, if needed for financial or technical reasons. On the other hand, it is relatively simple to extend this project to include such additional items as flexible hoses, riser pipes or exposed submarine pipeline segments.
Hydrodyn. Degradation Application Flexure Tension Torsion Interact. Sag Intensity
Moor ing
(tension)
little high & none varying
Salvage conc. & extreme high severe & vary.
Towing cont. & high & light, intense yes high
small varying cont. axial (wear, chem.) Umbilical cont. & constant yes intense no moderate
small (towing) (wear, chem.)
Note: Each of these applications is explained a bit in an appendix. PROBLEM SCHEMATIZATION
The development of a cable optimization procedure involves a complex interaction of many factors. A first schematization of these factors -more or less in the form of a number of questions - are listed below. They are worked out further in separate sections under the heading
'Solution Approach
I. Cable Segment Properties:
(These properties result from the properties of the chosen material combined with the way it is used in the cable.
. What are the properties of given cable under a given loading?
What are the limitations of a given cable with a given type of conditions?
How can general hydrodynamic interaction best be described? Hoisting severe constant little axial & no high (wear,
(running) transv. UV, etc.)
Hoisting conc. & constant some mixed no little except
lrlggíng) severe storage damage
varying light intense yes locally high
cont. transv. (wear, chem.)
transv. no low
little no locally high: (short life). transv. yes high wear
when exposed
Moor ing cont. &
(cat enary) small
Sea bed some low & light,
II. Conf Iqurat ion Behavior:
(This portion concentrates on the prediction of cable system [made up of seymentsl behavior.
Establish a dynamic model to simulate cable behavior. (uses results of steps A and C.)
What loading situations lead to especially unfavorable conditions for the cable?
(uses D with results from B.
How can the cables be improved so that unfavorable conditions are avoided?
III. Supply:
G. Fabricate a cable with optimum properties as defined in step F.
SOLUTION APPROACH
What are the properties of given cable segment under a given loading?
The properties of the individual rau materials used in cables seem to be well (if not all too publicly) known. Data on this can - and
hopefully iill be provided by various (synthetic) raw material manufacturers. The properties of cables - often fabricated from a
composite of materials in a variety of configurations is usually quite unknown. The mechanical characteristics of the cable will be needed
for the computer simulation of its behavior in the sea. These include the cables torsion, axial and bending properties (elastic, plastic and damping). The mechanical characteristics will probably be related to each other as iell as to the external (environment and loading) conditions. Wear resistance under a variety of conditions, internal heat generation and dissipation, and cable crushing resistance are also of importance. All of this information will have to be gathered via mechanical testing in a facility which can provide water
submergence as íell as ultraviolet light exposure. This can be
arranged in the Mechanical Engineering Cable Testing Laboratory of the Delft University of Technology.
Ihat are the limitations of a given cable with a given type of conditions?
The limits (such as fatigue life) of a cable can also be determined using - in principle - the same facility as for step A. Now, however, the testing will have a much longer duration.
How can general hydrodynamic interaction best be described? The hydrodynamic coefficients for a (moving) cable can best be
determined by towing and oscillating a (cable-like) cylinder in the Marine Engineering Towing Tank of the University. For a vertical cylinger, comparison of results with two different cylinder
penetrations will eliminate the water surface and lower end effects. Special attention will have to be paid to the proper definitions of
lift, drag, inertia, and axial hydrodynamic force components. The coefficients associated with each of these force components are
expected to be dependent upon the caule orientation in the flow, the cable motion itself (amplitude, frequency, [relative] direction) and the cable form (streamlined or round) and surface roughness.
Establish a dynamic model to simulate cable behavior.
Significant non-linearities are expected in the results of the above analyses. Consequently, a dynamic, time-domain, discrete element, computer simulation model is chosen as a 'working tool'. In generai. it can handle nearly any type of non-linearity. The NOSDA software set, already in use by the Workgroup Offshore Technology for the simulation of the behavior of other slender, flexible structures In
íaves and currents can be extended easily using the results of steps A and C, from above. This work can best be carried out within the
above Workgroup, itself.
What loading situations lead to especially unfavorable conditions for the cable?
The results from step B, above, will indicate which cable loading will lead to unfavorable (short lifetime, for example) behavior of a cable. Here, one will attempt to determine the external conditions which
causes that unfavorable behavior. This work can initially be done by those iho develop the computer models; later work can take place In industry using the same models.
How can the cables be improved so that unfavorable conditions are avoided?
By 'playing' further iith the cable properties in the computer model established in step D, It is possible - as one alternative - to
identify the external excitations which cause unfavorable behavior. A second alternative is to determine a set of internal cable properties which reduces or eliminates the effects of the unfavorable external conditions.
Fabricate a cable with optimum properties as defined in step F. This step is the responsibility of the cable manufacturers. Their
task becomes one of creating a cable which provides the mechanical and hydrodynamic properties found to be necessary In step F, above.
Economics can impose additional restrictions - and thus coupling with step F. Indeed, several types of cable can potentially provide the same or equivalently behaving sets of cable properties. The choice between them can then be influenced by economics. etc.
RELATION TO OTHER RESEARCH
This project will make use of work already completed on the
development of NOSDA for non-linear time domain dynamic analysis of structures schematized via Discrete Element Modeling. Further
development of NOSDA within this project can be restricted to refining its description of general hydrodynamic interaction and the tailoring of its user interface.
The proper description of the hydrodynamic Interaction between the sea and a slender (non-circular?) cylindrical segment is general to many offshore problems. Combination of the work necessary for step C, above, with that dictated by other offshore projects can be
iork on much more limited scale is now being carried out via a British Joint Industry Pro.ject (Tethers 2000) on properties of tension leg cables. It should be obvious from the above discussion that the work proposed there cannot be sufficient to meet the oblectives formulated
in this proposal.
The objective proposed here will not be reached overnight. Indeed, this will involve a longer-term (in the order of 5 years) research and testing effort; the University is an excellent setting for such
research.
TU DELFT EXPERTISE and FACILITIES
Steps A and B. above will rely heavily on the expertise of the Transportation Engineering Group in the Faculty of Mechanical and Marine Engineering; Wiek will be a key figure in this. Static and dynamic tests would be carried out with various cables, excitations, and exposures Lo determine their mechanical properties and endurance life. During this testing, using cable samples from industry, it will be possible to determine realistic limits for the properties of given types of cables; these limits can serve as guidelines when 'playing with properties in steps F, below.
Step C can be examined using the towing tank from the Ship Hydromechanics Group of the Faculty of Mechanical and Marine
Engineering using, possibly, an instrumented cylinder section from the Hydraulic Engineering Group in Civil Engineering. Pinkster, Journee and i4assie will be involved in this. By towing and simultaneously oscillating a section of a rigid cylinder, it is possible to determine hydrodynamic interaction coefficients. The tests can also include
studies of the effects of cable shape, cable roughness and waves. Steps D, E and F will utilize NOSDA, a software package for the simulation of the nonlinear dynamic behavior of a (moving) structure
(here a cable) in the sea. This simulation is very flexible in its dccommodation of nonhinearities; its development has been funded in part by the Dutch Technology Foundation (STW). Liu, from the Workgroup Offshore Technology, is a key figure for this.
Step G will have to be carried out via an intense cooperation between industry and the University. (It is assumed here, that it is still impossible Lo precisely predict all the properties of a cable having a
neu construction form or using new materials or combinations of materials.) This cooperation can involve any or all of the above groups.
ACKNOWLEDGEMENT
The assistance of Ir. J. Hogervorst from MATech, Zoetermeer with the formulation of this pro,iect definition is gratefully acknowledged.
APPENDIX I DESCRIPTIONS OF CABLE APPLICATIONS Hoisting (runningi
This application involves the cables reeved through the blocks of a crane. La any given hoisting operation, tensions will remain more or
less constant and bending will take place in the sheaves. In general the cable will be running axially through the water. Weight
considerations make the use of synthetic cables attractive in deep water; studies carried out in Delft result in cautious optimism about
this.
Hoisting (rigging)
Rigging cables are typically those between the hook and the ob3ect being hoisted. Host any materials can be used for this. Synthetics are attractive for heavy lifting because of their relative handling ease.
Mooring (catenary)
Catenary mooring cables can include all materials. Wear and corrosion restistance. weÌght. and handling and storage ease can all be
selection criteria in addition to performance. Heavy lines can be desirable in some cases while light ones are needed in others.
Mult-segmented lines are being considered for deep water applications. The catenary sag can amplify local cable motions thus intensifying the hydrodynamic interaction.
Mooring (tension)
The performance of tension moorings is dependent to a great extent on the elastic properties and 4for very deep water) the weight of the line. Cable strumming can be caused by hydrodynamic interaction with deeper water applications.
Sal vage
Working cables used in salvage are usually roughly handled and are written off quickly. Overloading can be expected to occur.
Sea Bed Cables
Communications or power cables laying on or in the sea bed can become exposed due to morphological changes in the sea bed. They are then subjected to abrasive action and hydrodynamic loads caused by currents and (in some locations) waves. This same discussion can hold for
Towing
Towing cables have a very prominent hydrodynamic interaction with the
sea. This interaction can lead to shock loads and premature cable
failure. This can be reduced by adjusting both cable weight and cable elasticity.
limb il ical
Umbilicals are very special in terms of their material composition. They always include electrical conductors and may even include hoses
for a diving bell, for example. Cable motion resulting from hydrodynamic interaction in deeper water can lead to transmitted signal distortion and to rapid conductor failure.
