Mooring Load Analysis ASCE 1998

7/25/2019 Mooring Load Analysis ASCE 1998

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Presented at

ASCE Ports 98Long Beach, CA, March 8 - 11. 1998

Computer Mooring Load Analysis to

Improve Port Operations and Safety

John F. Flory, Stephen P. Banfield, Dr. Alan Ractliffe

Tension Technology International

Abstract

Mooring analysis of a vessel alongside a pier usually involves a number of non-linear

mooring lines, extending at different vectors in both the horizontal and vertical planes, with

elastic fenders, acted upon by wind, current, and sometimes other forces, which may vary in timeand direction. Computer programs are now available which can quickly and accurately analyze

such complex mooring arrangements.

In this paper, a number of example cases, calculated by a mooring analysis computer

program, are used to demonstrate that the characteristics, qualities, and arrangements of the lines

can greatly affect the mooring line tensions experienced at a pier.

Introduction

A vessel's mooring arrangement is seldom a concern, until a gust of wind, a passing ship, or

inattention to changes in tide or freeboard causes mooring line failure and sudden vessel

movement. Such an accident can result in costly damage to cargo handling equipment or other

nearby vessels and structures, oil or chemical pollution, and personnel injuries and fatalities.Proper analysis of the adequacy of a vessel's mooring gear, a pier's mooring points, and the

mooring arrangement can significantly reduce the likelihood of such mooring accidents. Some

ports which handle hazardous materials, such as LPG, require such an analysis for each vessel

mooring. Other ports simply prescribe the use of a minimum number of mooring lines, without

concern for the character, quality or arrangement of those lines.

In this paper, a typical large-tanker mooring is analyzed with variations of the number, type,

and arrangement of mooring lines. Cases are compared in which winch-mounted wire mooring

lines are used together auxiliary synthetic fiber lines. Cases involving several alternative

synthetic fiber ropes are examined. Finally, several variations of mooring bollard positions are

evaluated.

Mooring Analysis Simplified

Figure 1 shows a simplified mooring arrangement, comprised of bow and stern breast lines,

spring lines, and fenders. With only four mooring lines, the analysis involves six unknowns the

mooring line forces and the fender reactions. Only three force and moment summation equations

can be written. The vector and elasticity of each mooring line must also be considered. Thus a

complete, proper solution of even this simplified mooring arrangement is not an easy matter.

Real vessel mooring arrangements usually have many mooring lines. If fender deflection is

considered, the solution becomes more complicated. The solution becomes very complex if


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non-linear mooring line elasticity is included. Thus the typical mooring arrangement is too

complex to be properly analyzed by hand.

Computer programs are now available which can perform a complete mooring analysis of

even a complex system in a few minutes on a personal computer. One such mooring analysis

computer program is the Optimoor program, developed by Tension Technology International.

The Optimoor Mooring Analysis ProgramWithin Optimoor, the vessel is defined by its dimensions as well as by data on fairlead

positions and mooring line size and material. Based on the mooring line information, Optimoor

determines the appropriate break strength and non-linear force-extension characteristics. The

berth is defined by data on mooring point positions and fender characteristics. These data are

entered in spread-sheet-like screens and stored in files, which are then used in setting up

analyses cases.

The Optimoor user defines a mooring analysis case by calling up the vessel and the berth

files and describing which vessel lines are connected to which mooring points on the pier. Wind

and current velocities and directions are then entered. Optimoor contains appropriate wind and

current force and moment coefficients for typical vessels, and additional data files can be

prepared as necessary. Tide table data can be entered, and then used to determine tidal currentsas well as tide elevation with time.

As data is entered or updated, the program calculates the resulting mooring line loads due to

the wind and current conditions and state of tide. Wind and current velocities can be increased to

check limiting conditions. The wind vector can be swept through 360/to determine the most

severe direction. Mooring line pretensions can also be varied.

A mooring analysis can be set up based on the vessel's arrival and departure times and

corresponding drafts and trims. Tide elevation and current information can be input or called

from an existing data file. Optimoor can then perform a mooring analysis over time, predicting

the effects of changes of draft, trim, tide, and tidal current on mooring line tensions. This

technique can be used to determine when mooring lines may need to be tended and also to plan

the best way to tend the lines in order to minimize the need for further mooring line tending.

Example Mooring Arrangement Analyses

The following examples involve a 250,000 dwt tanker moored alongside a pier. The vessel

length is 330 m (1080 ft) between perpendiculars. Its beam is 52m (170 ft), and its molded depth

is 24 m (79 ft). In the example cases, the vessel has a draft of 6 m (20 ft) and a trim of 5 m (16.4

ft) by the stern. This particular tanker is used because it was used in several mooring analyses

examples in the Oil Companies International Marine Forum Guidelines and Recommendations

for the Safe Mooring of Large Ships at Piers and Sea Islands.(1)The mooring forces calculated

by Optimoor are compared with those OCIMF examples in a previous paper.(2)

The mooring arrangement which is used in these analyses is shown in Figure 2. The

bollards, other than those used for spring lines, are positioned 38 m (125 ft) back from the fenderface. Later, the case of 53 m (175 ft) bollard setback is examined. The bollards for the bow and

stern lines are positioned substantially ahead and abeam of the moored vessel. Later, a case in

which these bollards positioned essentially in line with the bow and stern is examined.

In these examples, the forcing environment is a 60 kt wind pushing the moored tanker off

the pier combined with a 3 kt current from ahead. This corresponds with the criteria given in the

OCIMF Guidelines. Note that these guidelines apply to mooring gear onboard the vessel and do

not necessarily apply to mooring analysis of a particular pier. The pier should normally be

designed and outfitted in accordance with the most severe environment which is expected at the

site.


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All-Wire Cases

Case 1, 14 Wire Mooring Lines Alone

In Case 1 the vessel is moored by 14 wires, arranged as 2 head lines, 3 forward breast lines,

2 forward springs, 2 aft springs, 3 aft breast lines, and 2 stern lines. The wires are 45 mm (1-

in.) diameter steel with fiber core, with a rated break strength of 52.9 GN (235 kip). Each line is

pretensioned to 2.2 GN (10 kip).In the case tables at the end of this paper, connection designates the number of the vessel

line and the letter of the shore mooring point to which it is attached. Thus for example, 5-C

indicates that line 5 is attached to mooring point C. The spring lines are not shown in the tables,

because in these examples they were only lightly tensioned.

Line 8-G is tensioned to 58% of the rated breaking strength of the wires. This exceeds the

55% maximum mooring line tension criteria permitted by OCIMF. Thus this is probably an

unacceptable situation.

The common solution is to require the use of auxiliary lines. Here the term auxiliary refers

to lines other than winch-mounted mooring lines which are deployed on bitts to provide

additional mooring capacity.

Case 2, 14 Wire Mooring Lines plus 4 Wire Auxiliary Lines

In Case 2, four additional wire ropes are deployed as auxiliary lines. These are mounted on

bitts near the edge of the deck and run essentially in parallel with the breast lines. Auxiliary lines

11-D and 12-D are forward breast lines, and auxiliary lines 19-G and 20-G are aft breast lines.

Because the wire auxiliary lines are relatively short, as compared to the winch-mounted

wires, they are more heavily loaded. In this arrangement the most highly loaded line, 11-D, is

tensioned to only 45% of its breaking strength.

Wire and Synthetic Mixed-Line Cases

Figure 3 shows typical load-extension curves for wire rope and several types of synthetic

fiber rope. These curves are for broken-in ropes which have been cycled a few times to a modestload. Steel wire rope extends about 1% when loaded to 50% of its new breaking strength. Ropes

made of high-modulus fibers extend about twice as much as steel wire rope. Broken-in

polypropylene and polyester ropes typically extend about 6% at 50% of new breaking strength.

At 50% strength, nylon rope typically extends between 12% to 15%, depending on other

variables.

Case 3, 14 Wire Mooring Lines with 4 Polypropylene Auxiliary Lines

Polypropylene rope is frequently used as auxiliary mooring lines. Polypropylene rope is

light-weight, is easy to handle, and it floats, but it is relatively low in strength.

In Case 3, the auxiliary lines are 75 mm (3 in.} diameter polypropylene rope, with a rated

break strength of 24 GN (107 kip). Four auxiliary polypropylene lines are used in the samearrangement as was used with the auxiliary wires in Case 2. These are lines 11-D, 12-D, 19-G

and 20-G.

In Case 1, wires without auxiliary lines, the most highly tensioned line was 8-H. Through

the use of these polypropylene auxiliary lines, the load in that line was reduced by 4.2 GN (19

kip), a relative reduction of 14%. Similar tension reductions are achieved in several other breast

lines.

Note that the polypropylene ropes are only lightly loaded. These polypropylene auxiliary

lines are generally tensioned to only about 25% of the tension in the adjacent wire breast lines.


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This is because the polypropylene ropes are much more elastic than the wires. Thus they are not

very effective in reducing mooring tensions.

Use of High-Modulus Fiber Ropes

High performance fiber ropes are now sometimes used for mooring lines. The fiber

materials used in these ropes are much stronger and also stiffer than the conventional

rope-making fibers nylon, polyester, and polypropylene. Examples of such materials are aramid(duPont "Kevlar" and Akzo Nobel "Twaron") and high-modulus polyethylene (HMPE) (Allied

"Spectra" and DSM "Dyneema"). Because they are much stiffer, the ropes made of this new

class of fibers are called high-modulus fiber ropes. (3)

These high-modulus fiber ropes are almost as strong as wire ropes of the same size, and they

are also almost as stiff. At 50% of new break strength, wire rope extends about 1%, and

broken-in high-modulus fiber rope extends about 2%.

Case 4, 14 Wire Mooring Lines with 4 High-Modulus Auxiliary Lines

Case 4 illustrates the use of high-modulus fiber rope in place of polypropylene in the above

example. The high-modulus fiber auxiliary lines are used in the same positions as the auxiliary

lines in the preceding examples. And as before, they are pretensioned to 2.2 GN (10 kip).The resulting maximum line tensions are much less than with polypropylene auxiliary lines.

The tension in line 8-H, which was overloaded in Case 1, is reduced by 10.4 GN (46 kip), a 35%

relative reduction in tension.

Note that tensions are relatively equally distributed among the various lines, including the

high-modulus fiber auxiliary lines. The maximum line tensions in this case are lower than those

of Case 2 where wire auxiliary lines were used. The high-modulus lines, which are about twice

as elastic as the wire auxiliary lines, are not as greatly affected by the relatively short leads from

the bitts to the shore mooring points.

Use of Synthetic Fiber Mooring Lines

Case 5, All High-Modulus Fiber Mooring Lines and Auxiliary Lines

Some vessels now use high-modulus fiber rope instead of wire rope as winch-mounted

mooring lines. The principal advantage is the lighter weight, which requires a smaller deck crew

to handle the lines and also reduces the chances of injuries. Other advantages are; the

high-modulus fiber ropes do not corrode, do not require greasing, and generally last longer than

wire ropes in typical service. But the high-modulus fiber ropes cost more than conventional fiber

ropes and much more than wire ropes of the same strength.

In Case 5, 14 high modulus fiber ropes are mounted on winches in place of the wires, and 4

additional high modulus fiber ropes are deployed as auxiliary mooring lines. The high modulus

fiber ropes have a rated breaking strength of 56.7 GN (252 kip), slightly greater than the wires

which they replace. This strength could represent either aramid or HMPE rope of approximately50 mm (2 in.) diameter.

Compared to Case 2, in which wire rope was used in the same arrangement, the line tensions

are very similar. Compared to Case 4, in which wire was used as mooring lines and

high-modulus fiber rope was used only as auxiliary lines, the maximum tensions are about the

same, but now the relatively short auxiliary lines are more highly tensioned and the tensions in

the winch-mounted mooring high- modulus lines are less.


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Case 6, 14 High-Modulus Fiber Mooring Lines Alone

What would happen if only the 14 winch-mounted high-modulus fiber ropes were used,

without the use of auxiliary lines? With only 14 high-modulus fiber mooring lines, the maximum

tensions are within the OCIMF 55% limitation criteria.

This is in contrast to Case 1, with wire mooring lines, in which line 12-H was tensioned to

58% of its rated breaking strength. This is because the tensions were more equally shared among

the more elastic high-modulus fiber lines.

Case 7, 18 Nylon Mooring Lines

Nylon rope is much more elastic than wire rope and high-modulus fiber rope. In some mooring

and towing applications, this is a desirable property. But it is generally undesirable where the

motions of the moored vessel must be restricted. Case 7 shows what happens when nylon is used

for all of the mooring and auxiliary lines.

The line loads in this all-nylon 18 line case are similar to those for the all-high-modulus 18

line Case 5 above. However, the vessel motions are much greater. The moored vessel shifts

forward 3.4 m (11 ft) and moves out almost 6.1 m (20 ft) from the berth. These vessel motions

would be unacceptable for a tanker connected by hoses or loading arms to a terminal and would

probably be unacceptable in other applications.

Case 8, 18 Polypropylene Mooring Lines

Some vessels are equipped only with polypropylene mooring lines. Polypropylene fiber

ropes are relatively inexpensive. However, they are also weak compared with other conventional

fiber (nylon and polyester) ropes of the same size and with wire rope and high-modulus fiber

rope of the sizes typically used as vessel mooring lines.

The following case illustrates the use of 18 polypropylene ropes in the example situation

discussed here. The vessel movement is much greater than with wire or with high-modulus fiber

rope, although it is not as great as with nylon.

The mooring line tensions are similar to those of the above all-high-modulus fiber and nylon

rope cases. But because the polypropylene rope is not as strong, several auxiliary lines aretensioned to over 90% of the rated break strength and others are tensioned to over 80%. Several

breast lines are tensioned to over 70% of the rated break strength. Tensions in a number of other

mooring lines exceed the 55% criteria. Clearly, this is an unacceptable situation, even though a

total of 18 mooring lines are used.

Changes in Mooring Line Arrangement

Thus far only the effects of changing the types and numbers of mooring lines have been

considered. What if the mooring line arrangement is changed?

Case 9, 14 Wire Mooring Lines, Bow and Stern Lines to Alternative Mooring Points

As shown in Figure 3, two additional mooring points are available on the pier. Thesebollards B and I are positioned such that bow and stern lines respectively will be essentially

perpendicular to the vessel. Case 9 shows the 14 wire mooring lines of Case 1with the bow and

stern lines run to these alternative mooring points.

In this case, none of the mooring lines exceed the OCIMF 55% line tension limitation

criteria. The bow lines and also the stern lines now act essentially as breast lines, more

effectively sharing the force of the broadside wind, and thus relieving tensions from the other

breast lines. The spring lines still serve to resist the longitudinal force due to the 3 kt current.


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Case 10, 14 Wire Mooring Lines, Change in Bollard Set-Back Distance

In all of the above examples, the bollards (except spring-line bollards) were positioned at

38.1 m (125 ft) back from the fender face. What happens if this bollard set-back distance is

altered? In Case 10 these bollards are moved back to positions which are 53.4 m (175 ft) from

the fender face. The 14 wire mooring lines are placed on the same bollards as in Case 1.

In this case, none of the mooring lines exceeds the 55% OCIMF criteria. In all of the above

examples, the maximum tensions would be less if the bollards were moved back. Conversely, ifthe mooring bollards were moved closer to the pier face, the maximum tensions would be

greater. This is because the longer mooring lines provide greater elasticity which better shares

mooring loads.

Discussion

Many ports establish criteria for a minimum number of mooring lines, without regard to the

character, quality, or arrangement of the mooring lines which might be employed by a particular

vessel. This practice does not necessarily guarantee an adequate mooring.

Case 1 illustrated a situation in which 14 wire mooring lines were not adequate, meaning

that the OCIMF 55% maximum line tension criteria was exceeded. In Case 2, an additional 4

wire auxiliary lines made the situation acceptable.Case 9 and also Case 10 demonstrated how repositioning the original 14 wire lines achieved

an acceptable mooring arrangement. Case 6 demonstrated how 14 high-modulus fiber mooring

lines in the original arrangement provided an acceptable mooring situation. Thus 14 lines are

sufficient in some circumstances.

Case 3 demonstrated how the use of 4 polypropylene auxiliary lines turned Case 1 into an

acceptable situation. The use of 4 high-modulus fiber auxiliary lines in Case 5 created an even

better situation.

In Case 7, 18 nylon lines would be judged acceptable by the maximum tension criteria.

However, the greater elasticity of these nylon lines permit excessive vessel movement. In Case

8, a total of 18 polypropylene lines was not sufficient, because the break strength of at least one

of these lines was exceeded. Thus specifying a minimum of 18 mooring lines does not alwaysensure an adequate mooring.

These example cases demonstrate that each particular mooring situation should be

individually analyzed to determine its acceptability, instead of simply specifying a general

criteria without considering the character, quality, or arrangement of the mooring lines. In some

cases, 14 lines alone was sufficient, and the additional auxiliary lines were not necessary. In

other cases, even 18 lines was not sufficient.

References:1. OCIMF, Guidelines and Recommendations for the Safe Mooring of Large Ships at Piers and Sea Islands,

Witherby & Co., London, 1978.

2. Flory, J.F. and A. Ractliffe, "Mooring Arrangement Management by Computer", 1994 Ship Operations,

Management, and Economics Symposium, SNAME, Jersey City, NY, 1994.

3. Flory, J.F., H.A. McKenna, and M.R. Parsey, "Fiber Ropes for Ocean Engineering in the 21st Century", pp

934-947,Proceedings of Civil Engineering In the Oceans V, ASCE, New York, Nov. 1992


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Figure 1 Solving a simple mooring system with two breast lines, two spring lines, and two

fenders, requires three equations for six unknowns.

Figure 2 Vessel Mooring Arrangement Used In These Examples

Figure 3. Typical Wire and Synthetic Fiber Rope Load-Extension Curves


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Bow Lines Fwd. Breast Lines Auxiliary Lines Aft Breast Lines Stern Lines

Connection 1-A 2-A 3-C 4-C 5-C 6-H 7-H 8-H 9-J 10-J

Mooring line wire wire wire wire wire wire wire wire wire wire

Tension,GN

21.8 18.2 27.9 27.2 29.5 24.1 23.6 30.4 23.8 23.2

% ofStrength

41% 34% 53% 51% 55% 46% 45% 58% 45% 44%

Vessel Shift: 0.24 m forward, 0.73 m out, 0.1 Nto port

Case 1 - Base Case, 14 Wire Mooring Lines (4 spring lines not shown)


Connection 1-A 2-A 3-C 4-C 5-C 11-D 12-D 19-G 20-G 6-H 7-H 8-H 9-J 10-J

Mooring line wire wire wire wire wire wire wire wire wire wire wire wire wire wire

Tension,GN

11.9 8.6 19.8 18.4 19.6 23.8 23.0 19.8 18.6 18.6 13.7 16.4 20.7 19.6

% ofStrength

23% 16% 37% 35% 45% 43% 43% 37% 35% 27% 26% 31% 39% 37%

Vessel Shift: 0.44 m forward, 0..40 m , 0.1 Nto port

Case 2 - 14 Wire Mooring Lines with 4 Additional Wire Auxiliary Lines


Connection 1-A 2-A 3-C 4-C 5-C 11-D 12-D 19.G 20.G 6-H 7-H 8-H 9-J 10-J

Mooringline

wire wire wire wire wire Poly. Poly. Poly. Poly wire wire wire wire wire

Tension,GN

19.1 15.8 25.4 24.8 26.6 6.8 6.8 6.3 6.3 21.2 20.7 11.0 22.5 21.6

% ofStrength 36% 30% 48% 47% 50% 28% 28% 26% 26% 40% 39% 49% 42% 41%


Case 3 - 14 Wire Mooring Lines with 4 Polypropylene Auxiliary Lines



Mooringline

wire wire wire wire wire HM HM HM HM wire wire wire wire wire

Tension,

GN

14.8 11.7 21.4 20.5 21.8 18.0 17.3 15.3 14.6 16.6 16.4 20.2 20.5 19.6

% ofStrength

28% 22% 41% 39% 41% 32% 31% 27% 26% 32% 31% 38% 39% 37%


Case 4 - 14 Wire Mooring Lines with 4 High-Modulus Fiber Auxiliary Lines


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Mooringline

HM HM HM HM HM HM HM HM HM HM HM HM HM HM

Tension,GN

11.7 8.6 19.4 18.2 19.4 24.8 23.6 20.5 19.4 14.0 13.5 16.6 19.6 18.4

% of

Strength

21% 15% 34% 32% 34% 44% 42% 36% 34% 25% 24% 29% 35% 33%


Case 5 - 14 High-Modulus Fiber Lines with 4 High-Modulus Fiber Auxiliary Lines



Mooringline

HM HM HM HM HM HM HM HM HM HM

Tension,GN

20.7 16.9 28.6 27.4 30.2 23.6 23.0 31.0 23.0 22.9

% ofStrength

37% 30% 50% 48% 53% 42% 41% 55% 42% 41%


Case 6 - 14 High-Modulus Fiber Mooring Lines without Auxiliary Lines



Mooringline

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Nylon

Tension,GN 12.4 10.6 16.0 15.8 17.1 23.0 22.3 20.9 20.2 14.2 14.0 17.6 14.8 14.4

% ofStrength

24% 20% 31% 30% 33% 44% 43% 40% 39% 27% 27% 34% 29% 27%


Case 7 - 14 Nylon Mooring Lines with 4 Nylon Auxiliary Lines



Mooring

line

Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly. Poly.

Tension,GN

12.6 10.6 16.4 16.0 17.3 22.7 22.0 20.7 20.0 14.4 14.2 17.6 15.5 14.8

% ofStrength

52% 44% 68% 66% 72% 94% 92% 86% 83% 60% 58% 73% 64% 62%


Case 8 - 14 Polypropylene Mooring Lines with 4 Polypropylene Auxiliary Lines


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Mooringline

wire wire wire wire wire wire wire wire wire wire

Tension,GN

24.8 25.2 20.0 19.6 20.9 20.9 20.7 25.9 25.0 23.4

% ofStrength

47% 47% 38% 37% 40% 40% 39% 49% 47% 44%


Case 9 - 14 Wire Mooring Lines with Bow and Stern Lines Repositioned



Mooringline

wire wire wire wire wire wire wire wire wire wire

Tension,GN

21.6 19.6 25.0 24.8 26.8 23.4 23.2 28.8 22.5 21.6

% ofStrength

41% 37% 47% 47% 50% 44% 44% 54% 43% 41%


Case 10 - 14 Wire Mooring Lines with Bollards Moved Further Back

Mooring Load Analysis ASCE 1998

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