Font size

iv

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

=Â»ERIOO COVERED

THE 1980 CEL MOORING DYNAMICS SEMINAR

Final; )an 1980 - Oct 1980

â€¢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 , 'Â°r73 1473 ED,

MOV 65 IS OBSOLETE

Unclassified

MBL/WHOI

D301 004Q3b7 1

Unclassified

SECURITY CLASSIFICATION OF THIS PAGEfWien Dmim EnlÂ»rÂ«<<;

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

â€¢ Mooring Dynamics Models Dr. B. J. Muga

â€¢ Mooring Cable Dynamics Dr. R. L. Webster

â€¢ Diffraction Theory, Including

Mean Drift Forces Dr. C. J. Garrison

â€¢ Vessel Equations of Motion Dr. R. Bhattacharyya

Mooring System Excitation and Analysis

â€¢ Second-Order Drift Forces Dr. W. McCreight

â€¢ Random Wave Characteristics Dr. M. K. Ochi

â€¢ Spectral Analysis Dr. J. S. Bendat

â€¢ Mooring System Analysis Dr. J. R. Paulling

â€¢ 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 â€” 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:

â€¢ 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.

â€¢ 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

â€¢ Increasing Accuracy of Models â–

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

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

=Â»ERIOO COVERED

THE 1980 CEL MOORING DYNAMICS SEMINAR

Final; )an 1980 - Oct 1980

â€¢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 , 'Â°r73 1473 ED,

MOV 65 IS OBSOLETE

Unclassified

MBL/WHOI

D301 004Q3b7 1

Unclassified

SECURITY CLASSIFICATION OF THIS PAGEfWien Dmim EnlÂ»rÂ«<<;

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

â€¢ Mooring Dynamics Models Dr. B. J. Muga

â€¢ Mooring Cable Dynamics Dr. R. L. Webster

â€¢ Diffraction Theory, Including

Mean Drift Forces Dr. C. J. Garrison

â€¢ Vessel Equations of Motion Dr. R. Bhattacharyya

Mooring System Excitation and Analysis

â€¢ Second-Order Drift Forces Dr. W. McCreight

â€¢ Random Wave Characteristics Dr. M. K. Ochi

â€¢ Spectral Analysis Dr. J. S. Bendat

â€¢ Mooring System Analysis Dr. J. R. Paulling

â€¢ 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 â€” 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:

â€¢ 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.

â€¢ 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

â€¢ Increasing Accuracy of Models â–

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