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MMM




Ta



TN no. N-1604



m^




title; THE 1980 CEL MOORING DYNAMICS SEMINAR



author: paui a. paio

date I March 1981
sponsor! Naval Facilities Engineering Command

program nos: yf59. 556. 091. 01.400



CIVIL ENGINEERING LABORATORY

NAVAL CONSTRUCTION BATTALION CENTER

Port Hueneme, California 93043

This publication is required for official use or for administrative or operational

purposes only. Distribution is limited to U.S. Government Agencies. Other

requests must be referred to the Civil Engineering Laboratory, Naval

Construction Battalion Center, Port Hueneme, CA 93043




no



J \la(A



Unclassified



REPORT DOCUMENTATION PAGE



TN-1604



2, GOVT ACCESSION NO

DN787011



5 TYPE OF REPOR



=»ERIOO COVERED



THE 1980 CEL MOORING DYNAMICS SEMINAR



Final; )an 1980 - Oct 1980



•JG ORG, REPOR



7. AUTHORfs;

Paul A. Palo



9. PERFORM



sID ADDRESS



CIVIL ENGINEERING LABORATORY
Naval Construction Battalion Center
Port Hueneme, California 93043



62759N;
YF59.556.091. 01.400



CONTROLLING OF



Naval Facilities Engineering Command
Alexandria, Virginia 22332



12. REPORT DAT

March 1981



173



Unclassified



DECLASSIFICATION DOWNGRADING
SCHEDULE



I DISTRIBUTION STATEMENT (of fhrs Reporl)

This publication is required for official use or for administrative or operational purposes
only. Distribution is limited to U.S. Government Agencies. Other requests must be referred
to the Civil Engineering Laboratory, Naval Construction Battalion Center, Port Hueneme,
CA 93043



Mooring, numerical mooring models, mooring simulation.



This report describes the CEL Mooring Dynamics Seminar, which was held on 10-11

January 1980. Nine experts, selected to represent the major disciplines relevant to mooring

analysis, were invited to attend and informally discuss the field of mooring simulation.

These discussions resulted in identification of the present state-of-the-art and promising

research topics in mooring simulation. Suggestions were also made towards advancing the

state-of-the-art in nonlinear systems identification techniques. This report summarizes

(continued)



DD , '°r73 1473 ED,



MOV 65 IS OBSOLETE



Unclassified



MBL/WHOI



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Unclassified



SECURITY CLASSIFICATION OF THIS PAGEfWien Dmim Enl»r«<<;



20. Continued

the discussions and presents overview papers and seminar conclusions contributed by each
attendee.



Library Card

Civil Engineering Laboratory

THE 1980 CEL MOORING DYNAMICS SEMINAR (Final),

by Paul A. Palo

TN-1604 173ppillus March 1981 Unclassified

1. Mooring simulation 2. Computer analysis, moorings I. YF59.556.091.01.400
This report describes the CEL Mooring Dynamics Seminar, which was held on 10-11
January 1980. Nine experts, selected to represent the major disciplines relevant to mooring
analysis, were invited to attend and informally discuss the field of mooring simulation.
These discussions resulted in identification of the present state-of-the-art and promising
research topics in mooring simulation. Suggestions were also made towards advancing the
state-of-the-art in nonlinear systems identification techniques. This report summarizes the
discussions and presents overview papers and seminar conclusions contributed by each
attendee.



Unclassified

SECURITY CLASSIFICATION OF THIS PAGECl



CONTENTS

Page

INTRODUCTION 1

SEMINAR 1

Background 1

Participants 1

Format 2

Session I 2

Session II 3

Session III 3

CONCLUSIONS FROM SEMINAR DISCUSSIONS 4

Present Mooring Analysis Capabilities 4

Problem Areas and Uncertainties 4

Guidelines for Navy Mooring Research 6

State-of-the-Art Advances 7

SUMMARY 7

ACKNOWLEDGMENT 7

PRESENTATIONS 9

Dr. B. J. Muga 11

Dr. R. L. Webster 25

Dr. C. J. Garrison /r

Dr. R. Bhattacharyya 55

Dr. W. McCreight 87

Dr. M. K. Ochi 53

Dr. J. S. Bendat 109

Dr. J. R. Paulling 125

Dr. S. Calisal ^47

SEMINAR CONCLUSIONS BY PARTICIPANTS 153



INTRODUCTION

The Civil Engineering Laboratory (CEL) started its research in
mooring simulation in FY 78 under the sponsorship of the Naval Facilities
Engineering Command (NAVFAC) . The goal of this development effort,
called the Mooring Systems Prediction Project, is to develop and demon-
strate a validated mooring analysis capability; the effort is being
supported under the Ocean Facilities Engineering Exploratory Development
Program (YF59.556).

NAVFAC provided CEL with two mooring analysis computer models for
continued development. The first model, DESMOOR (for DESign MOORings),
is an inexpensive, simplified model which gives approximate answers.
The second model, DSSM (Deep Sea Ship Moor), is an advanced finite
element mooring model for use in the final design stage. Neither model
was verified by experimental or field measurements.

After the first year's effort into the problem of mooring simula-
tions, it was realized that the behavior of moored ships involved complex
mechanisms that were deeply interrelated. It was clear that an under-
standing of each of the fields related to the mooring phenomenon was
necessary before rational decisions could be made regarding the develop-
ment and use of a general mooring analysis capability.



SEMINAR

Background

CEL sponsored a workshop seminar at the beginning of 1980 for the
purpose of reviewing the NAVFAC/CEL mooring analysis development effort.
Specifically, the objectives of the seminar were as follows:

1. To define the present state of mooring analysis and simulation.
Included here would be an evaluation of the framework of
NAVFAC s mooring analysis capability, namely, DESMOOR and DSSM.

2. To identify the problem areas and uncertainties in the present
state-of-the-art .

3. To develop specific guidelines for the further development of
the Navy's mooring analysis capability.

4. To identify promising research topics for advancing the
state-of-the-art of mooring analysis.

Participants

CEL invited nine prominent experts to attend. These participants
were not necessarily experienced in the mooring dynamics area but were
recognized for their expertise in subjects integral to the mooring
phenomenon. A list of the attendees and their affiliations follows (in
alphabetical order) :

1



Dr.


B.


J.


Mug a


Dr.


M.


K.


Ochi


Dr.


J.


R.


Paulling


Dr.


R.


L.


Webster



Dr. J. S. Bendat - J. S. Bendat Company

Dr. R. Bhattacharyya - U.S. Naval Academy

Dr. S. Calisal - U. S. Naval Academy

Dr. C. J. Garrison - C. J. Garrison and Associates

Dr. W. McCreight - David Taylor Naval Ship Research

and Development Center

Duke University

University of Florida

University of California, Berkeley

Thiokol Corporation

The participants displayed a spirit of cooperation that led to the
success of the meeting.

Format

The two-day seminar was organized into three sessions. Session I
covered the first day and consisted of short presentations by each
attendee summarizing the problems associated with his particular area of
expertise. Guidelines as to subject matter for these presentations were
outlined in advance by CEL to insure complete coverage of the disciplines
associated with mooring analyses. A short discussion/ question period
followed each presentation.

Session II was held on the second morning and was the most important
part of the seminar. Session II was an informal discussion which allowed
the participants to build on the presentations of the first day. The
intent of Session I was to brief each participant on the problems and
limitations within each discipline (vessel motion, cable dynamics,
etc.), while Session II was an opportunity for a free exchange of questions
and ideas aimed at evaluating and extending the state-of-the-art in
mooring analysis.

Session III was held at the end of the second day and started with
a description of the Navy's mooring needs as seen by CEL. The intent of
Session III was to get specific recommendations from the attendees
regarding the development of a mooring analysis capability. This dis-
cussion was purposely placed last to avoid any bias in the Session II
discussions on mooring models in general.

Session I

The seminar attendees were carefully chosen to represent all the
"building blocks" necessary to evaluate and assemble mooring models.
The presentations on the first day were divided into two groups: mooring
system behavior and mooring system excitation and analysis. The following
presentations were made during Session I:

Mooring System Behavior

• Mooring Dynamics Models Dr. B. J. Muga

• Mooring Cable Dynamics Dr. R. L. Webster

• Diffraction Theory, Including

Mean Drift Forces Dr. C. J. Garrison

• Vessel Equations of Motion Dr. R. Bhattacharyya



Mooring System Excitation and Analysis

• Second-Order Drift Forces Dr. W. McCreight

• Random Wave Characteristics Dr. M. K. Ochi

• Spectral Analysis Dr. J. S. Bendat

• Mooring System Analysis Dr. J. R. Paulling

• Mooring Dynamics Model

Development Example Dr. S. Calisal

Each attendee delivered a short paper concerning his particular
subject; these are included in a separate section in this report. Each
of these papers gives a concise overview of the fields important to
mooring simulation by emphasizing assumptions, limitations, and applica-
tions. These papers are subjective in nature, and as such they are
easily read and understood.

Session II

The discussions on the second day were intended to give CEL better
insight into existing state-of-the-art mooring models and to allow the
participants to delve into new ideas and approaches to mooring analysis
problems. Both of these goals were achieved.

Much of the discussion centered on the calculation and importance
of the slowly varying drift force. Although this force is small, it can
become very important because it exists at frequencies close to the
natural frequency of many moored systems. When this dynamic force is
negligible, a linearized frequency-domain dynamic analysis can be used
at a very small computation cost. However, if the force has a signifi-
cant effect on the mooring, a nonlinear time-domain model is required.
This model requires a great deal of computer time because the statistics
of the system behavior must be calculated indirectly from several brute-
force simulations. Thus, the magnitude of the slowly varying drift
force determines whether an inexpensive frequency-domain or an expensive
time-domain dynamic computer model is required. Other topics of discussion
are included in the CONCLUSIONS FROM SEMINAR DISCUSSIONS section.

The discussions throughout the second day did result in progress
toward extending the state-of-the-art in mooring analysis. As Dr. Muga
points out in his paper, a nonlinear stochastic model would be the ideal
analysis tool for moorings. This imaginary model would give statistical
information directly and would eliminate the expensive intermediate
results necessary with current time-domain nonlinear dynamic models.
Dr. Bendat stated that he felt the time was right to extend linear time
series analysis techniques and modify existing nonlinear techniques to
obtain the necessary mathematics to describe nonlinear system behavior.



Session III

The final session began with an explanation by Dr. Webster on the
DSSM computer model and a short summary by CEL on Navy mooring appli-
cations. This was followed by discussions on the strong and weak points
of the DSSM model and suggestions on how to improve it.



A collection of conclusions from each participant, which addresses
both general and specific conclusions from the seminar, is included in
this report. Many of the items listed below are developed in these
summary reports.



CONCLUSIONS FROM SEMINAR DISCUSSIONS

Th CEL Mooring Dynamics Seminar fully satisfied its objectives
(i.e., to define the present state-of-the-art in mooring analysis and
simulation, to identify the problem areas and uncertainties associated
with available mooring models, and to recommend guidelines for the
development of mooring models to suit Navy needs). The Seminar discus-
sions also initiated a development effort that may lead to significant
advances in the analyses of nonlinear dynamic systems. Some of the
major contributions within each objective are outlined below.

Present Mooring Analysis Capabilities

As illustrated in Figure 1, there are several models available for
mooring analysis. Each analysis technique is useful because of trade-offs
in accuracy versus computational costs, which allow the mooring analyzer
to choose the model that best suits his particular needs. For example,
the fully nonlinear time-domain model, although the most accurate, is
certainly not necessary for all applications. Alternatively, applying a
large factor of safety and omitting the dynamic analysis, although it is
very inexpensive, is likewise not appropriate in all cases.

The majority of available mooring analysis models known to CEL
assume a ship-dominated system, with the mooring lines treated as massless
springs. System response is determined in either the time or frequency
domain. Mooring line tensions are determined in a subsequent quasi-static
analysis with the ship displacement imposed on the cable. The most
accurate mooring models have no major restrictions or assumptions, and
are based on a time-domain representation of vessel and cable response.

It was generally agreed that DSSM is a very cost-effective mooring
model. As demonstrated in Figure 1, DSSM uses a fully nonlinear static
analysis model (finite element) and a fully coupled, but linearized,
frequency-domain dynamic model. The only major improvement possible
would be to add a nonlinear time-domain model, that would add at least
an order of magnitude to the computation costs. Since the degree of
nonlinearity (i.e., effect of the slowly varying drift force) for moorings
involving Navy (intermediate-sized) ships is unknown, the need for a
nonlinear dynamic solution is unknown. It was recognized that the
accuracy of the linearized dynamic solution in DSSM might be adequate
for Navy applications, and that major model improvements might not be
necessary. Further details regarding the evaluation of DSSM are included
after the next section.

Problem Areas and Uncertainties

Identification of the problems associated with state-of-the-art
mooring simulation will be discussed without reference to any particular
applications or computational cost limitations. Evaluation of these
items is left to the reader. Some of the most significant problems are
discussed below:

4



1. The most accurate and complete time-domain models have only one
restriction — that the buoyancy of the vessel be linear with immersion.
This does not result in any significant errors for moderate vessel
motions. However, this restriction does introduce errors for severe
vessel motion when bow/stern submergence occurs. This restriction is a
consequence of the mathematics required to transform frequency-domain
vessel motions to the time domain. Since no available vessel motion
models can handle extreme vessel motions, this restriction is unimportant.
However, recent efforts in the OTEC project towards the development of

an extreme vessel motion model may spur research aimed at the development
of a corresponding mooring model.

2. At the present time, there is no accepted technique for pre-
dicting and simulating the slowly varying drift forces on a floating
vessel. These forces can be significant in comparison to the other
environmental loads. Approximate techniques of unknown accuracy are
available for estimating this load.

3. Another limitation in reducing the errors associated with
mooring simulation comes from the uncertainty in defining the environ-
mental loads, particularly the wind and wind-driven surface waves.
Errors associated with the use of a wind wave model (Bretschneider,
Pierson-Moskowitz, etc.) have been shown to approach 100% for spectral
components as compared to actual measurements. These errors can seriously
affect the simulation due to the frequency sensitivities of the vessel
response and the use of the wave spectrum in determining the mean and
slowly varying drift forces.

The development of spectral families for wind wave models by Dr.
Ochi is an important development reported at the seminar. By identifying
the error bounds (admittedly a statistically averaged value) in these
wind wave spectral models, much of the uncertainty in the final results
can be reduced. For many mooring models, the error introduced by using
a single spectral model was significant compared to the error due to
approximations in the mooring model itself.

4. It was also pointed out that developing and using a very accurate
mooring model may not be cost effective if the criteria by which the
results are evaluated are not well-defined. This is illustrated in
Figure 2, which shows the uncertainty (also probability) in the simulated
results, p(s), and the uncertainty in the criteria, p(c); the bandwidth

of either curve is analogous to the standard deviation of the error.
The area of overlap gives an indication of the probability of system
failure. For example, in long-term applications, p(c) for failure loads
may be large due to uncertainty in the corrosion, wear, etc. of system
components. This has important implications because the mooring designer
could simulate such a system with an inexpensive, simplified model and
save computation costs from a more refined model. A more detailed
illustration of model errors versus evaluation criteria is shown in
Figure 3, using CEL's mooring models as an example. Definition of p(c)
is dependent on each application, so generalizations would be difficult.
Recognizing that the evaluation criteria play a role in the choice of
analysis models is the first step.



Guidelines for Navy Mooring Research

The conclusions listed in the previous section are universal and
are somewhat independent of specific needs. The conclusions in this
section are applicable to the development of a Navy mooring analysis
capability. An overview of this development within the CEL Mooring
Systems Prediction Project can be found in CEL Technical Memorandum
M-44-80-9."

Some of the specific recommendations made during and after the
seminar are listed below:

• A few minor additions could be made to DSSM to improve its
generality. Examples are:

(1) Build in additional wind wave spectral models and allow
for shoaling.

(2) Allow for wave orbital velocities in the dynamic analysis.

In shallow water, these velocities would approach the magnitude
of the dynamic motions and should be accounted for.

(3) Allow for unsteadiness in the wind loading by introducing
a wind spectrum.

(4) Allow for cylindrical surface buoys to complement the spherical
buoy dynamic characteristics already in DSSM. Results from an
extensive investigation into the dynamic characteristics of
floating cylinders will be reported soon from the U.S. Naval
Academy.

• The relative effect of many of the idealizations used in the
DSSM model can be determined through parametric studies.
Examples of this include:

(5) Errors caused by the use of spheres to represent all surface
buoys. The dynamics of buoys were considered of secondary
importance compared to the ship when this section of the model
was formulated.

(6) Errors associated with the linearization of the mooring cable
response for the frequency-domain analysis. The moored ship
response is first calculated in the frequency domain. A
second time-domain analysis with fully nonlinear cable response
is then performed separately, using the linear ship response

as excitation to the top of the cables. Relative comparisons
of the cable responses would help determine the significance
of this linearization.

(7) Importance of the inclusion or exclusion of the dynamics of
surface buoys in the dynamic analysis. The wave-induced
motions of the buoys certainly contribute to the loads in



*Civil Engineering Laboratory. Technical Memorandum M-44-80-9: A
review of the CEL mooring systems prediction product area, FY 79 and
FY 80, by P. A. Palo. Port Hueneme, Calif., Sep 1980.



the hawsers and mooring lines, but the relative size of this
contribution relative to the ship-induced dynamic loads has
never been determined. This is important, since surface buoys
add additional degrees-of-freedom to the solution and increase
computation costs.

(8) Errors associated with the use of current and wind loads
versus relative heading in the static analysis. Determining
the sensitivity of typical mooring systems to changes in the
static load coefficients would be extremely valuable, since
the available data exhibit a large scatter.

(9) Determining the sensitivity of the model to errors in any
input variable would be valuable and practical, particularly
for actual studies where many values can only be estimated.

State-of-the-Art Advances

Discussions which evolved from the use of bispectra for ship resis-
tance measurements indicated that an advance in the state-of-the-art may
be possible in the analysis of nonlinear dynamic systems. Development
of nonlinear systems identification techniques, as discussed under
Session II, would be a major breakthrough not only for mooring analysis,
but for all nonlinear dynamic systems. Efforts in this area have been
initiated.



SUMMARY

The two-day Mooring Dynamics Seminar satisfied all of its objectives.
Recommendations for development of a mooring analysis capability were
made, and a potential contribution towards advancing the state-of-the-art
in nonlinear dynamic analysis was initiated. The presentations and
summary reports included in this report form a unique primer on the
mooring analysis problem and state-of-the-art analysis techniques.



ACKNOWLEDGMENT

CEL gratefully acknowledges the cooperation and enthusiasm shown by the
participants, and hopes that they, too, benefited from the discussions.



Static Analysis



Weightless
Cables




Simple
Catenaries




Dynamic Analysis




Nonlinear



Frequency
Domain



Time
Domain



Uncoupled
Ship Cables



Coupled
Ship Cables



System Response



= DSSM
Mooring
Model



• Increasing Accuracy of Models ■



Figure 1. Hierarchy of deterministic mooring models.




1. X = parameter under
investigation

s = calculated value from
the simulation

c= criteria for evaluation
of system reliability/
performance

2. p(x)= uncertainty:
where

p(x)dx = 1



S C

b. Well-defined evaluation criteria



X



Figure 2. Role of uncertainty in system evaluation.



Uncertainty in Static
Environmental Loads



Uncertainty in Hardware
Specifications, Description



N.X"



Errors from
Approximations
(Catenaries, etc.)



DESMOOR



PSSM




No Significant
Model Errors



Uncertainty in
Dynamic Environ-
mental Loads



Cumulative Uncertainty
in Static Model Results



Uncertainty in
Slowly Varying
Drift Forces



Uncertainty in Vessel

Hydrodynamic

Characteristics




Uncertainty from
Linearizations



DESMOOR r



Total Cumulative
Uncertainty in
Simulation Results



Uncertainty from use
of Factor of Safety
Instead of Dynamic
Analysis




Uncertainty in
Evaluation Criteria



Risk/Payoff



Relative System
Reliability /Performance



Figure 3. Mooring system evaluation using CEL computer models.



SEMINAR PRESENTATIONS



1980 CEL Mooring
Dynamics Seminar



MOORING DYNAMIC MODELS
By
Dr. B. J. Muga



INTRODUCTION

It is particularly appropriate that this seminar is being
sponsored by the Civil Engineering Laboratory. As far as is
known, this Laboratory was the first (at least in modern times)
to begin the systematic study of moored ship behavior. The
earliest group of studies consisted of (1) an aircraft carrier
moored alongside a conventional pier at Bremmerton, Washington,

(2) another aircraft carrier anchored off San Nicholas Island
in what has come to be known as a single point mooring, and

(3) an LST moored off a drilling platform in the Gulf of Mexico
in what is known as a multipoint mooring or spread mooring or
what in the industry is referred to as a sea berth mooring.

In addition, a study of the motions of the CUSS I vessel as a
part of Phase I of Project Mohole was carried out here at this
Laboratory .

All of these studies were, for the most part, data collec-
tion efforts having rather specific objectives which were satis-
fied by what would be regarded as crude data analysis. In brief,
they were field studies of prototypes. It should be remembered


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Online LibraryPaul A PaloThe 1980 CEL mooring dynamics seminar → online text (page 1 of 11)