stat moor

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Introducing

Statmoor

A Mooring Statics Calculator

from

SeaSoftSystems

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SeaSoft Systems Statmoor Description

Table of Contents

I. Introduction .................................................. ........................................................ ..........1

Capabilities of Statmoor........................................................................................1

Special Applications .............................................. ...............................................2

Program Access.......................................................................................................2

II. Discussion Of Elasticity In Mooring Systems ..... ..... ..... ..... ..... ..... ..... ..... ..... ...... ..... ...... .....2

III. Technical Evaluation of Statmoor.................................................................................3

Comparative Analysis.......................... ................................. ................................3

Validation of Statmoor Predictions ..... ..... ..... ...... ..... ...... ..... ...... ..... ...... ..... ...... ..... ..4

IV. Treatment of Bottom Friction ..... ..... ..... ..... ..... ..... ...... ..... ...... ..... ...... ..... ...... ..... ...... ..... ...5

V. Description Of User Interface.........................................................................................5

VI. Input Description And Discussion ..... ..... ...... ..... ...... ..... ...... ..... ...... ..... ...... ..... ...... ..... ......6

General..................................................................................................................6

VII. Output Options ............................................... ........................................................ ....7

References..........................................................................................................................8

Sample Output...................................................................................................................8

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Executive Summary

Statmoor is a state of the art static mooring analysis program developed by SeaSoft Systems

and Richard J. Hartman, Ph.D. It is the outcome of the initial phase of a comprehensivemooring system analysis project undertaken in January 1982. Highlights of selected programfeatures follow:

Elasticity

An analytically exact treatment of mooring line elastic deformation allows accurate analysis ofthe steadily growing family of highly compliant synthetic materials in mooring systems.Commonly employed approximations to elastic deformation in mooring problems can in some

cases lead to errors of 100% in restoring force predictions. The exact solution used in Statmooravoids altogether the problem of approximating the effects of either gravity or elasticity inmooring materials.

Bottom Friction

The effect of bottom friction on mooring line tensions in a weakly dynamic environment is shownto be most realistically represented by averaging a double valued tension function.

Capabilities

Statmoor can analyze up to 24 mooring lines (12 on microcomputer versions). Each line can consistof 3 sublines including concentrated weights and/or buoys. The ocean bottom may be uniformlysloping or anchor depths can be independently specified. Zero-offset fairlead pretension,distance to anchor or line declination angle can be used to define pretension conditions. Englishor Metric units may be selected.

Versatility

Statmoors flexibility makes it useful in non-conventional applications including analysis ofrubber hoses used in subsea production, composite towlines, TLPs, guyed towers, tethered buoysand SALMS.

Usability

A powerful user interface provides a simple question-and-answer environment for preparationand editing of input files. No knowledge of computer programming is required. Special built-in

Help facilities can supply default values for most mooring line material properties orassistance in computing values from user-supplied data.

Access

Statmoor is available on an unlimited-use basis through a license agreement with SeaSoft.Statmoor is presently configured to run on many popular mini- and microcomputers In addition

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I. Introduction

Statmoor is a state of the art static mooring analysis program developed by Richard J.

Hartman, Ph.D. It is the outcome of the initial phase of a comprehensive mooring systemanalysis project undertaken in January 1982.

In developing Statmoor, special attention was given to a number of areas in which at least someof the more well known codes are deficient. The most notable of these areas is the treatment ofelasticity of mooring elements. Many modern mooring materials, in particular the synthetics(Kevlar, nylon, polypropylene, etc.), are notable in that their gravitational response toimposed loads is small compared to their elastic response. In the analysis of such materials,special care must be taken to account properly for this response. In particular, methods

employed to approximately account for the small elastic distortion of chain or wire rope understatic mooring conditions can be inadequate in many conditions of physical interest, inparticular when elasticity plays a dominant role. Statmoor incorporates an exact analyticalexpression for the combined elasto-gravitational static response of mooring line elements andthereby avoids altogether the problem of approximating the effects of either gravity orelasticity.* Furthermore, the exact equations used in the code apply to arbitrary degrees of non-linearity in the elasticity of the elements, thereby rendering the analysis of highly non-linearsynthetic elements completely straightforward.

Capabilities of Statmoor

1. Up to twenty-four individual mooring lines can be accommodated (12 mooring lines onsome microcomputer versions).

2. Each mooring line may consist of up to three independent sublines. Each subline can havespecifiable mass characteristics and non-linear elastic characteristics (tension-strainrelationship may include up to cubic terms).

3. A concentrated weight and/or line support buoy can be specified between appropriatesublines.

4. The ocean bottom may be uniformly sloping, or anchor depths may be independentlyspecified.

5. Either fairlead pretensions or pretension line angles can be specified.

6. System and individual line characteristics (net restoring force, line tensions, suspendedlengths, etc.) are output for horizontal offsets of any magnitude in any direction and for yaw

offsets both clockwise and counterclockwise.

7. English or metric units may be specified.

It should be noted that all offsets are by the Hand of God meaning that the vessel is notallowed to respond in any degree of freedom to the development of mooring forces as the offsetis made. The totality of force and moments acting on the vessel during the offset are, however,

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Special Applications

Statmoors power and flexibility make it useful for the analysis of virtually any problem inwhich elastic lines or cables play a central role. Some examples of recent non-conventionalapplications:

1. Tension-offset characteristics for composite rubber hoses of various types used in offshoreproduction facilities.

2. Tension-offset characteristics for composite towlines.

3. Surge and yaw restoring characteristics and tendon tensions for tension leg platforms andguyed towers.

4. Restoring force characteristics for submerged tethered buoys and SALMs.

Program Access

Statmoor is available on an unlimited-use basis on a users own equipment through a standardlicense agreement with SeaSoft. The license agreement permits the user to utilize Statmoor for

any purpose other than resale or transfer to a third party. Statmoor is presently available torun on many mini- and microcomputers. In addition, individual timesharing accounts can beestablished. Special arrangements to configure Statmoor to run on any computer system with asuitable Fortran 77 compiler can be made.

II. Discussion Of Elasticity In Mooring Systems

The large and growing family of synthetic materials is gradually setting the scene for a

revolution in the design of offshore mooring systems. Ropes of nylon, Kevlar, Dacron andPolypropylene have already proven themselves superior to steel ropes for many offshoreapplications and their future utilization in mooring systems has been anticipated by a steadilygrowing number of studies of synthetic-steel composite mooring lines, especially for deep watermoorings. [Ref. 3-5]. The virtues of synthetics include unsurpassed corrosion and fatigueresistance, light weight, ease of handling, and high elastic compliance which greatly reducesthe tension peaks arising from wave-frequency fairlead motions. The inevitability of increasedutilization of synthetics in mooring systems requires that state of the art mooring analysesaccurately incorporate the (generally non-linear) elastic response characteristics of mooring

materials in addition to the classical gravitational response associated with mooring linecatenary adjustments. Most mooring analysis codes in wide use today treat line elastic responsein an approximate manner. This approximation becomes increasingly unsatisfactory as therelative effects of elastic response increases and that of gravitational response decreases. Inextreme cases these approximations can lead to line tension estimation errors of more than 100percent (see Section III). Even for conventional mooring systems comprised only of wire rope thecommonly utilized approximations to the elastic response can result in line tension estimate

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III. Technical Evaluation of Statmoor

During the development and check-out ofStatmoor it was found that at least some mooring

programs currently in wide use are notably deficient in the analysis of mooring systems forwhich elastic deformation of the mooring line plays a role comparable in importance to theadjustment of the line shape under the action of gravity (catenary adjustment). This section isintended to identify some of those mooring configurations for which significant errors arisewhen using commonly applied approximation methods. The following conditions, in particular,are subject to error in elastic modeling by many conventional mooring programs:

1. Systems in which light, elastic synthetic materials (Kevlar, nylon, polypropelene) areprominent elements in the mooring system. These include the family of composite lines which

have in recent years received attention for deep water mooring systems. Computational errorsfor these systems can be substantial, in particular, under conditions of large pretension.

2. Systems in which very large clump weights and high pretensions are used to create a stiffmooring system for operational conditions which soften during large-excursion survivalconditions. This would include, for example, guyed tower applications.

Comparative Analysis

To illustrate the magnitude of the errors which can develop, the output of Statmoor,Ultramarines OSCAR and an early (1972) version of the Exxon CALMS program were compared(during fall 1982) in four systems:

System Description WaterDepth

SublineComposition

Line AE

(lbs)*Weight

inWater(lbs/ft)

LumpedWeight(kips)

Case 1:

Kevlar/LumpedWeight/Wire/AnchorPretension 300 kips

1500 2000'x4" Kevlar

l000'x3.5" Wire

1.20x107

8.65x107

1.01

17.10

280

Case 2:

Kevlar/Pile AnchorPretension 60 kips

4500 6000'x4" Kevlar 4.00x107 1.01 N.A.

Case 3:

Wire/LumpedWeight/Wire/AnchorPretension 300 kips

1500 2500'x3.5" Wire

l000'x3.5" Wire

8.65x107

8.65x107

17.10

17.10

280

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Note that the systems described here are not intended to be optimized designs in any way. Theyhave been selected only to illustrate the various general observations made above. All casesutilize a simple two-line moor (lines at 0 and 180) with offsets towards one anchor. Case 1

represents a highly pretensioned, highly elastic system which might be a candidate conceptfor a future guyed tower design utilizing the excellent fatigue and corrosion properties of Kevlarin a circumstance where the possibility of chafing of the Kevlar elements would be minimal.The offset producing clump liftoff is a particularly sensitive function of the treatment ofelasticity; for this simple system, Statmoors results can be demonstrated to be correct. (Figure l)

Case 2 represents a mooring which might be considered for a deep water production platform.The restoring force characteristics are again very strongly dependent on the elastic properties;Statmoors results can be shown to be correct; see the following subsection. (Figure 2)

Case 3 represents a more-or-less conventional guyed-tower type moor with heavy wire ropemembers and a large clump weight. Because of the high pretensions used the elasticity of thewire rope is quite important, as indicated by differences in excess of 30% between the exact(Statmoor) treatment and the approximate treatments. (Figure 3)

Case 4 represents a typical Kevlar-chain composite system which has been proposed for deepwater mooring systems. A particularly interesting feature of this example is the largeoverestimate of anchor uplift resulting from the errors in conventional modelling of the elastic

behavior of the Kevlar. (Figure 4)

In summary, note should be made of the following:

1. In highly tensioned deep water elastic systems, errors of 100% are possible (Figures 1, 2).

2. Although the largest errors are associated with light, highly elastic syntheticmaterials, the performance of wire rope can under special circumstances be significantlymisjudged - see Figure 3, where the maximum errors amount to nearly 40%.

Validation of Statmoor Predictions

Statmoors predictions have been checked by hand in a wide variety of limiting cases includingthe totally inelastic limit (catenary) and the totally elastic limit (neutrally buoyant lines).An example of one of these checks is outlined below for Case 2 (Figure 2) of this section.

For the large tensions associated with Case 2, Kevlar can be considered neutrally buoyant andthe restoring force from a single line can be computed with high accuracy from purelygeometrical considerations applied to the assumed linear elastic properties of the Kevlar rope:

1) Line length at zero tension 6000

2) Line length at 60 kips pretension 6009.00

3) Line length for 100 offset 6075 73

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IV. Treatment of Bottom Friction

Traditionally bottom friction has been accommodated for in static mooring programs by the

specification of a friction coefficient and incorporation of a simple tension correction calculationbased on the assumption that the tension in each line under all conditions of offset has beenreached by a uniform monotonic increase in tension from low values. This assumption is clearlyfallacious since many of the mooring lines, for any offset condition other than zero, have alower tension than they had at zero offset. Furthermore, under weakly dynamic conditions, thetension in each line will oscillate slightly about the mean. It is a consequence of theirreversibility, or hysteresis, inherent in all dissipative processes such as friction that therecent tension history for each individual line must be considered in the determination of itsinstantaneous state. A careful analysis of the consequences of bottom friction for the calculation

of static offset restoring characteristics shows that the complete neglect of bottom friction willmost realistically reproduce the average static offset characteristics one would measure in aweakly dynamic environment. This approach has been adopted in Statmoor. The physicalbasis for this procedure is best understood by considering the friction-associated hysteresiscurve for a single line, as depicted in a schematically exaggerated way in Figure 5. As afairlead undergoes even infinitesimal horizontal oscillations about a particular point, forinstance its zero offset point, the line tension jumps discontinuously between the two hysteresisbranches due to the action of elastic elongation in the presence of friction. Since everyenvironment is at least weakly dynamic, it is clear that the most reasonable single value of

tension for a given fairlead-anchor distance is given by the average of the double - valuedtension function, which average is adequately represented by the curve representing the case ofzero friction. It must be stressed that these comments apply to static offsets only. Whenconsidering quasi-static or fully dynamic conditions, the energy loss due to friction. effects canplay an important role and must be carefully accounted for.

V. Description Of User Interface

The user interface, a flexible and easy-to-use input file editor, was created to provide a user-

friendly question-and-answer environment for the preparation and editing of the input filerequired by Statmoor. The interface has been designed to permit the first-time user to executeStatmoor without studying the input file structure and format requirements or learning aspecialized and easily forgotten nomenclature. A knowledge of computer programming is notrequired for utilization ofStatmoor. The following special features of the user interface shouldbe noted:

1. Help facility for effective weight/unit length and breaking strength of mooring lines: Thisprovides approximate default values for breaking strengths and for effective weights of a wide

variety of mooring materials in seawater, fresh water and air. Although variations in lineproperties between different manufacturers are generally small, it is recommended thatmanufacturers data, when available, be used in lieu of default values. Reference 1 containsspecific illustrations of the Help facility.

2. Help facility for elastic coefficients of mooring lines: This provides two types of help:(i) default values for a wide variety of mooring materials; and (ii) a curve fitting routine to

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VI. Input Description And Discussion

General

Before Statmoor can be exercised, all relevant physical data for the mooring system to beanalyzed must be gathered. When available, manufacturers data for the specific materials tobe used should be obtained.

The list of input parameters which should be prepared before attempting to execute Statmoorfollow:

A. General Layout

1. Number of mooring lines.

2. Plan view layout of the lines with the fairlead-anchor direction for each. Every mooringline is assigned a unique number; the number serves to associate each line with its physicalcharacteristics. Thus, the first fairlead-anchor direction to be specified is associated withLine Number 1, etc. Numbering should normally begin with the most forward line on thestarboard side and progress in a counter-clockwise direction. See Figure 6 for a samplelayout.

3. Spatial coordinates (x,y,z) of each fairlead. x (forward) and y (starboard) coordinatesare required only if yaw restoring characteristics and pitch/roll moments are desired.

4. Depth of water beneath vessel.

5. Slope of ocean bottom, if uniform, or depth of each anchor.

6. Direction of maximum downwards slope (the slope direction).

B. Details of Mooring Lines

1. Complete description of each type of mooring line to be used, including length,weight/unit length in water, nominal diameter and breaking strength of each sublinecomponent of a particular composite line. Each line can consist of at most three independentsublines. There are no restrictions on the composition of the sublines. A lumped weight orsinker can be specified between the two anchor-side sublines, and a mooring buoy can bespecified between the fairlead-side sublines.

Note: Default weight/unit length values and breaking strengths for a wide variety ofmooring materials including chain, IWRC wire rope, Kevlar and nylon are availablethrough the Help facility of the user interface sub-program described in Section V.

2. Complete description of the elastic characteristics of each subline component of aparticular composite line. For elements with approximately linear elasticity, such as chainand wire rope, the EA value for the specified line should be obtained. For elements withnonlinear elastic properties manufacturers data for the elastic component of elongation

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C. Zero-Offset (Pretension) Conditions

1. Zero-offset line tensions at fairleads (total tension or horizontal component), declination

angles at fairleads (measured from the horizontal), or fairlead-anchor horizontalseparation.

D. Offset Specifications

1. Direction of each desired lateral offset sequence.

2. Actual lateral offset magnitudes from the zero-offset position. (Default offset increments

are 1 percent of water depth).3. Direction (parity) of any desired yaw offset sequence. The possibilities are only two,clockwise and counter-clockwise.

4. Actual yaw offsets magnitudes from the zero-offset condition. (Default offset incrementsare 2.5 degrees).

Miscel laneous Notes

1. Dimensions: Either English or Metric units can be used in Statmoor. All lengths, depths,etc., should be input in feet (meters) except mooring line diameters, which should be inputin inches (millimeters). All weights, forces or line tensions should be input in kips (metrictons) except line weights which should be input in pounds/feet (kilogram weight/meter).

VII. Output Options

The output stream from Statmoor consists of two distinct output types: mandatory and elective.

The mandatory output includes:

Tabular output of selected input parameters of importance.

Tabular output of the zero-offset condition including distances to anchors, slope of bottomtowards anchors, fairlead and anchor line tension components, and suspended line lengths.

Tabular output of the physical characteristics of the most exposed (most highly loaded)line during each requested offset sequence.

Tabular output of mooring system restoring characteristics for each requested offsetsequence.

The elective output can be included or excluded as desired. The elective output includes, forevery line and every specified offset condition:

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References

1. Program Statmoor Static Mooring Analysis Code: Users Manual, 1989. Available from

SeaSoft Systems, 250 Ribera Drive, Suite B, Santa Barbara, CA 93111.

2. Fylling, I.J., Anchor Line Forces in the second WEGEMT (West European Graduate EducationMarine Technology) Graduate School on Advanced Aspects of Offshore Engineering, TheNorwegian Institute of Technology, January 1979, Vol. II.

3. A Study of the Comparative Mooring Performance of Kevlar and Steel Ropes, E. I. duPont deNemours & Co. Report. Available through: Textile Fibers Department, Kevlar Marketing,Centre Road Building, Wilmington, Delaware 19898.

4. Pollack, J. and Hwang, Y., A Permanent Mooring for a Deepwater Production System, 1982Offshore Technology Proceedings, Paper # OTC 4347.

5. Niedzwecki, J. M., A Comparison of Non-Metallic Ropes with Wire Rope and Chain MooringLines for Deep Water Applications, 1978 Offshore Technology Conference Proceedings, Paper#OTC 3207.

6. Finn, L. D., A New Deepwater Offshore Platform - The Guyed Tower, 1976 Offshore

Technology Conference Proceedings, Paper #OTC 2688.

Sample Output

The following pages comprise a portion of the output from a Statmoor execution .

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SeaSoft Systems Statmoor Manual

H.1

SeaSoft Systems Software Library

Volume 7

Catenary Mooring Statics Calculator

------------------------------

Statmoor Version 4.60

Copyright (C) 1993By Richard J. Hartman, Ph.D.

------------------------------

Statmoor Trial RunDeep Water Semi

Executed at 10:42 on 9/8/93

***** INTERPOLATION TABLES FOR LINE NUMBER 1 *****

INDEX THTOP TOPT TZANC THANC XANC SL

1 .078 4.801 .000 .078 2591.719 2001.8452 7.483 8.890 .000 7.483 3765.668 2194.2713 15.590 16.312 .000 15.590 3933.556 2374.3124 24.464 24.930 .000 24.464 4020.668 2521.7205 34.178 34.513 .000 34.178 4073.284 2605.0836 44.812 45.068 .000 44.812 4109.604 2693.6247 56.453 56.657 .000 56.453 4137.911 2785.7808 69.196 69.363 .000 69.196 4161.581 2881.0689 83.147 83.285 .000 83.147 4182.236 2979.44910 98.418 98.535 .000 98.418 4200.779 3081.05711 115.135 115.235 .000 115.135 4217.754 3186.11012 133.436 133.522 .000 133.436 4233.521 3294.86013 153.469 153.797 .000 153.469 4262.283 3368.84614 175.399 176.141 .000 175.399 4286.140 3446.882

15 199.406 200.702 .000 199.406 4305.658 3531.60716 225.686 227.661 .000 225.686 4322.051 3623.05017 254.455 257.219 .000 254.455 4336.191 3721.29418 285.948 289.598 .000 285.948 4348.714 3826.46219 320.424 325.044 .000 320.424 4360.087 3938.71220 358.164 363.829 .000 358.164 4370.659 4058.23321 399.478 406.253 .000 399.478 4380.694 4185.24022 444.703 452.646 .000 444.703 4390.398 4319.97023 494.212 503.370 .000 494.212 4399.933 4462.68024 548.409 558.847 9.009 548.409 4409.231 4500.00025 607.738 619.581 22.064 607.738 4417.992 4500.00026 672.685 686.069 36.357 672.685 4426.426 4500.00027 743.782 758.854 51.996 743.782 4434.729 4500.00028 821.611 838.531 69.098 821.611 4443.069 4500.00029 906.811 925.748 87.789 906.811 4451.591 4500.00030 1000.078 1021.217 108.208 1000.078 4460.429 4500.000

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H.4

*************** ZERO-OFFSET CONDITION SUMMARY ***************

ANCHOR- MEAN FAIRLEAD- UNSTRETCHED ZERO-OFFSETTO-FRLD SLOPE TO ANCHOR SUSPENDED --- FAIRLEAD TENSIONS --- LINE

LINE HEIGHT ANCHOR DISTANCE LENGTH TOTAL HORIZ. VERT. DEC.NO. (FT) (FT/FT) (FT) (FT) (K.LBS) (K.LBS) (K.LBS) (DEG)

1 1000.0 .000 4177.7 2957.8 80.00 79.85 -4.81 3.5

2 1000.0 .000 3837.9 2000.0 80.00 69.95 -38.82 29.0

3 1000.0 .000 3837.9 2000.0 80.00 69.95 -38.82 29.0

4 1000.0 .000 4177.7 2957.8 80.00 79.85 -4.81 3.5

5 1000.0 .000 4177.7 2957.8 80.00 79.85 -4.81 3.5

6 1000.0 .000 3837.9 2000.0 80.00 69.95 -38.82 29.0

7 1000.0 .000 3837.9 2000.0 80.00 69.95 -38.82 29.0

8 1000.0 .000 4177.7 2957.8 80.00 79.85 -4.81 3.5

******************** OCEAN BOTTOM SUMMARY *******************

BOTTOM: DEPTH AT VESSEL 1000.00 FTBOTTOM SLOPE .000 FT/FTSLOPE DIRECTION .0 DEGREESFRICTION COEFFICIENT .000

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H.17

stat moor

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