oceans2009finalpaper-mooringlinetechnology

7/25/2019 Oceans2009FinalPaper-MooringLineTechnology

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Improved Mooring Line Technologyfor Tankers and Gas Carriers

at Exposed BerthsStephen J. Banfield

Tension Technology International Ltd.36 Huggetts Lane, Willingdon,

Eastbourne, East Sussex BN20 0LU, UK [email protected]

John F. FloryTension Technology International LLC

4 Tower LaneMorristown, NJ 07960

[email protected]

Abstract

Fixed-pier berths for tankers and gas carriers are now sometimes installed and operated in locations exposed towaves and swell. In these conditions, the moored vessel can experience large motions which overload and fatiguemooring lines.

Tails are short lengths of synthetic fiber rope which are placed in series with the vessels winch-mounted wires to

decrease mooring line stiffness and thus to reduce peak line loads and fatigue due to wave-induced vessel motions.Past guidelines for conventional tankers and berths recommended the use of 11m long nylon tails on mooring lines.Those guidelines were developed many years ago when tankers were relatively small and when fixed-pier berths werelocated in protected harbors. But when such short nylon tails are used on large vessels at exposed berths, the mooringloads are high and the nylon tails tend to fatigue quickly and fail.

Tension Technology International (TTI) recently conduced a study to investigate these problems and makerecommendation for mooring lines and tails for use on modern large tankers and gas carriers, especially at exposedlocations. The study assessed how waves influence vessel motions and line tensions and how tail material and lengthinfluence mooring line loads and fatigue.

New recommendations were prepared for tail material and length for use at berths where vessel motions aresignificant. Large vessels can accommodate longer tails to decrease mooring line stiffness and reduce peak loads.Polyester rope is stiffer than nylon, but longer polyester tails can achieve loads similar to short nylon tails. In some

situations, polyester tails should be preferred as that material has much better fatigue performance than nylon,especially in wet condition.Many vessels now use HMPE fiber rope mooring lines instead of wires. These HMPE mooring lines are

essentially the same size and as strong as the wires they replace. They are much lighter and easier to handle and thushelp crew safety. They are not as stiff as wire rope.

The recommendations of this study are now published in mooring guidelines and used by tanker and gas-carrier operators.

Introduction

Until recently, most oil tanker and gas carrier terminals were built in protected harbors or behind breakwaterswhich protected them from waves. Terminals are now being installed and operated in locations which are exposed torelatively high wave heights. (In this paper waves also includes swell) The resulting wave-induced vessel motions

cause high tensions and reduce the service lives of the mooring lines.Fiber rope tails are customarily fitted on winch-mounted mooring lines to reduce peak tensions produced by wave-induced vessel motions. Past industry recommendations suggested the use of 11 m (36 ft) long tails. Suchrecommendations were frequently interpreted as requirements which prohibited the use of longer tails.

Several incidents were recently reported in which 11 m nylon tails failed due to tension fatigue in a short time ongas carriers moored at new terminals exposed to waves. With the support of a number of leading oil tanker and gascarrier operating companies, Tension Technology International (TTI) conducted a study to evaluate the benefits of using longer nylon tails and alternate tail materials on winch-mounted wires, and also of using HMPE ropes in placeof wire mooring lines. This paper summarizes the results of that study.[1]


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Analysis MethodsMooring systems for an intermediate and a large LNG carrier were analyzed in this study. These vessels are

described in Table 1. The mooring arrangements are shown in Figures 1 and 2. These are variations of the mooringarrangement depicted in the OCIMF Mooring Equipment Guidelines.[2]

The Optimoor Seakeeping mooring analysis computer program was used to analyze these mooring systems. Peak mooring line tensions were calculated for many different situations, encompassing various wind, wave, and currentenvironments.

The various ropes investigated for use as mooring line and tail are given in Table 2. Traditional 11 m double-braidnylon tails were analyzed as the benchmark. Longer nylon tails were analyzed. As an alternative, 11 and 22 m polyester/polypropylene tails were analyzed. HMPE winch-mounted mooring lines were also analyzed with andwithout tails.

The fatigue life of the wires and the nylon tails were also estimated using Optimoor with actual wave statisticswhich had been recorded at an exposed LNG pier.

How Wave-Induced Motion Effect Mooring Line TensionThe principal concerns at moorings which are protected against waves are wind and current forces on the vessel.

These static applied forces are distributed among the mooring lines.But when the moored vessel is exposed to waves, it moves - surge, sway, yaw, roll, pitch, and heave. These wave-

induced motions cause the vessels fairleads to move. The fairlead motions can significantly increase the distance

between the fairlead and the bollard on the pier. And as a result, the mooring line must become longer.Figure 3 shows how wave-induced vessel motions can increase mooring forces. The mooring line is mounted on awinch on the vessel, passes through the fairlead on the vessel side, and is connected to a mooring point on the pier.The length of that line between the fairlead and the mooring point when stretched by applied wind- and current-induced forces is designated by Line Length 1. When wave-induced motions cause the vessel fairlead to move further from the mooring point, the line stretches to Line Length 2, as depicted in the figure. Delta Length designates theresulting line length increase.

Figure 4 shows how this wave-induced line length increase changes the tension in the line. The non-linear stretchcharacteristic of a typical nylon rope is illustrated. Tension due to the applied wind and current forces initiallystretches the rope to Line Length 1. When the rope stretches to Line Length 2, due to wave-induced vessel motions,the tension increases. Because of the non-linear stretch characteristic, the increased caused by wave-induced vesselmotions can be relatively high.

The Optimoor program, which was used in these analyses, carries out calculations to depict the effects of wave-induced vessel motions as described above.

Comparisons of Rope Stiffness CharacteristicsThe load-strain characteristics of typical steel wire and nylon, polyester, polypropylene, and HMPE (high modulus

polyethylene) fiber ropes are shown in Figure 5. (See End Note 1)These are base on tests of broken-in ropes. New fiber ropes are not as stiff, but the broken-in rope properties used

here are reached after only a few loadings. Steel wire rope stretches essentially linearly and breaks at about 2%stretch.

HMPE is a synthetic fiber material which can be made into large ropes which are essentially as strong as but aboutone-eight the weight of steel wire of the same size . It is known by the trade names Dyneema and Spectra. HMPErope belongs to the high-modulus category of rope. This category also includes aramid fiber , know by the trade

names Kevlar and Twaron, and LCP fiber (liquid crystal polyester or polymer), known by the trade name Vectran.These other fibers are heavier than HMPE. All of these ropes are also high-strength. All of these ropes stretchessentially linearly and break at about 3% stretch in broken-in condition.

Nylon double braid rope breaks at about 24% strain in broken-in condition. Note the non-linear load vs. straincharacteristic of nylon.

Polyester and polypropylene have similar, non-linear stretch characteristics. Ropes are also commonly made of combinations of polypropylene and polyester. This category of rope is called PP/PE in this paper. It typically breaksat about 10% strain in broken-in condition.


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Polyester and wet nylon ropes have similar break strengths. Their break strengths are about one-fourth that of fiber-core wire rope.. Dry nylon rope has a slightly higher break strength, but that is not applicable to marine use.The break strength of polypropylene rope is less, about one-sixth that of fiber-core wire rope. Some PP/PEcombination ropes have break strengths essentially as high as polyester alone, and such strengths are assumed here.

When a synthetic tail is used in series with a winch-mounted wire, the stiffness characteristic of the combinationdepends on the relative lengths of the wire and the tail as well as their individual stiffness characteristics.

The length of the wire is a function of the mooring layout. The onboard length of the wire, from the fairlead to the

winch, should be included. In the case of a short outboard mooring line and a short onboard length, the tail may be aslong or even longer than the wire, and in this case, the stiffness characteristic of the tail predominates. When a verylong wire used with in series with an 11 m tail, the wire stiffness is also a factor. But note that as the total length of line increases, its stiffness decreases.

Table 3 shows the OCIMF Mooring Equipment Guideline recommendations for minimum wire and synthetic fiber rope safety factors in various sercices.

Effect of Nylon Tail Length on Wire LinesA nylon tail attached to the end of a steel-wire winch-mounted mooring line can substantially increase the amount

of stretch which can be tolerated before the line breaks.In the first phase of the study, the following environment was applied:

35 knot wind at 135 E (offshore),

5 knot current at 170E

(10E

off stern), and2 m significant wave height, 10 sec mean-period wave at 225 E

Figure 6 shows the peak line loads predicted in this wave environment on the 138 k LNG and 267 k LNG carriersfor various nylon tail lengths, 11 to 22 m, The static mooring line loads due to wind and current forces alone (withoutwaves) were only about 10% of break strength. The peak line loads caused by wave-induced motions are muchhigher.

On the 138 k LNG vessel, the maximum line tension is 77% of break strength with an 11 m tail, a very high value.This load could cause failure in a damaged wire or tail. As discussed later, this load can also cause fatigue failure in awet nylon rope. With a 22 m tail, the maximum tension is only 48%.

The larger, 267 k vessel responded less to the waves and the resulting fairlead motions were less, producing lower peak loads. The maximum line tension is 59% of break strength with an 11 m tail. With a 22 m tail, the maximumtension is 42%.

Seastate Limits, Short and Long Nylon Tails on HMPE LinesIn the second phase of the study, HMPE winch-mounted lines were analyzed with 11 and 22 m long nylon and

PP/PE tails. The effects of wave height and period were investigated. The OCIMF line and tail strength criteria of 50% for other synthetic lines, discussed above, was used.

The seastate limits (wave height and period) for the 138k gas carrier with nylon tails are shown in Figure 7. Thetolerable wave at any period is about twice as high with the longer 22 m nylon tails.

The seastate limits for the 267k gas carrier with nylon tails, are shown in Figure 8. Again the tolerable waveheight at any period is about double with the longer 22 m nylon tails.

The results for these cases with wire instead of HMPE lines are almost identical. The HMPE lines are not as stiff as the wire lines. The 55% strength criteria for wire lines is less stringent than the 50% criteria for HMPE lines.

Seastate Limits, Long Nylon and PE/PP Tails on HMPE LinesThe seastate limits for the 267k gas carrier with 22 m nylon and PP/PE tails are shown in Figure 9. The tolerablewave height at any period is about half with the stiffer PP/PE tails than with the nylon tails of the same length. Butthe tolerable wave height with the 22 m PP/PE tails is about the same as that with the shorter 11 m nylon tails.

Service life limitations may make the PP/PE tails a better choice, as discussed below.

Fatigue Life Considerations


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[1]. Banfield, S. J and K. Black, Mooring Analyses & Safe Mooring Practices for Exposed Terminals,Tension Technology International, Eastbourne, UK, 2006

[2]. Oil Companies International Marine Forum, OCIMF, Mooring Equipment Guidelines , 3 rd ed., WitherbySeamanship International, London, 2008

Tension cycling of rope causes relative movement between the strands. This is true in wire rope and in fiber rope.This fatigue effect is especially severe in wet double-brain nylon rope.[3] [4] (See end Note 2.) It occurs in wire rope.It is essentially negligible in polyester and HMPE ropes.

In the third phase of this study, the fatigue-life calculation tool within Optimoor Seakeeping was used to estimatethe relative cyclic tension fatigue service life of several combinations of wire and HMPE winch-mounted lines witheither 11 or 22 m nylon tails. This program feature uses fatigue data for a number of fiber ropes based on data fromseveral past TTI studies. It used API wire rope fatigue data.[5]

The environment which was analyzed was 35 knot wind from either offshore (135E

) or onshore (225E

) and 5 knotcurrent at 170 E (10 E off stern). Wave statistics for a typical 1.5 m significant wave height were used to estimatevessel motions and consequential rope fatigue life.

Table 4 summarizes the results of the fatigue study.In the case of the 138k vessel with wire winch-mounted lines and 11 m nylon tails, the estimated service life for

the tails was only 0.3 years. This corresponded with the service life of 11 m nylon tails experienced by a gas carrier of this class in this environment.

With 22 m nylon tails, the predicted tail service life increases to 2.2 years. Note that in this case, the service life of the wire lines is also severely effected, being only about 1.8 years with the 11 m nylon tails.

In general, the service life of the nylon tail increased by a factor of about 10 when the length of the tail wasincreased from 11 m to 22 m.

In many cases the analysis was stopped when the wire service life exceeded 999 years. For those cases in which a

comparison can be made, the use of 22 m nylon tails increased the wire service life by a factor of about 20.

CONCLUSIONS AND RECOMMENDATIONSThe use of longer synthetic-fiber rope tails on winch-mounted mooring lines significantly reduces peak mooring

line loads and increases the service life of both the tails and the winch-mounted lines. 22 m long tails are preferredover 11 m long tails, and this longer length can be accommodated on most large vessel moorings. In some cases withhigh wave height, even longer tails may be needed.

Wet double-braid nylon ropes suffer severe strength loss when cycled only a few thousand times to high tension,while polyester and polyester/polypropylene ropes do not have this problem. Tails of polyester or polyester/

polypropylene combination ropes are stiffer than nylon tails, but the stretch characteristics of 22 m long PP/PE tailsare similar to those of 11 m long tails and produce almost the same peak tensions in the same environment. Polyester or polyester/polypropylene combination rope tails should be preferred unless nylon must be used to limit peak

mooring loads. When nylon tails are used, they may need to be replaced frequently to avoid failure due to fatigue.(See End Note 2)Winch-mounted HMPE mooring lines with either nylon or polyester/polypropylene tails are a good choice for

large vessels. The HMPE lines are lighter and easier to handle and are less hazardous to crew members. They cansignificantly reduce fatigue loading tail.

End Notes:(1) Strain is the ratio between length change caused by applied tension and the original length before application of load. It is a

basic property of the rope. Stretch is the length change of a particular rope of a particular length and is calculated bymultiplying strain by the rope length.

(2) Recent tests on of parallel-strand nylon rope with good marine finish now demonstrate that this type of rope has much better tension cyclic fatigue properties than double-braid nylon rope tested in the past. This type of rope is a good candidate for useas tails.

References


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Vessel length, LBP Beam Draft, ballased

138k LNG Carrier 266 m 43.4 m 8.1 m

267k QMax LNG Carrier 333 m 55 m 9.8 m

Table 1 Vessels Investigated

Winch Line Tail

wire 6 strand, IWRC nylon double braid

HMPE fiber 12 strand nylon, double braid

HMPE fiber 12 strand PP/PE (Polypropylene/Polyester) 8-strand

PP/PET represents the load/strain characteristics of polyester, polypropylene, and polyester/polypropylene blend ropes, whichare essentially identical.

Table 2 Ropes Used in Investigation

Safety Factor % Break Strength

steel wire mooring lines 1.82 55%

nylon (polyamide) mooring lines 2.22 45%

other synthetic mooring lines 2.0 50%

nylon tails on wires 2.5 40%

other synthetic tails on wires 2.0 50%

nylon tails on synthetic lines 2.75 36%

other synthetic tails on synthetic lines 2.5 40%

Table 3 OCIMF MEG 3 Safety Factor Recommendations

Winch Line Life Tail Service Life

Vessel winch line wind 11 m 22 m increase 11 m 22 m increase

138 k wire offshore 1.8 36 20 0.3 2.2 7.33

onshore 2.5 45 18 0.2 2.7 13.5

HMPE offshore 137 > 999 7.29 0.3 2.9 9.67

onshore 163 >999 6.13 0.3 3.4 11.33

267 k wire offshore 6 117 19.5 0.5 6.3 12.6

onshore 8 143 17.88 .6 7.7 12.83

HMPE offshore >999 >999 0.8 8.1 10.13

onshore >999 >999 1 10 10

Table 4 Estimates of Winch Line and Tail Service Lives with 11 and 22 m Nylon Tails


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Figure 1. Arrangement for 138k LNG Carrier at Typical Berth

Figure 2. Arrangement for 267k LNG Carrier at Typical Berth

Figure 3 Wire and Fiber Rope Stiffness Characteristics


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Figure 4 Effect of Vessel Motion on Mooring Line Length

Figure 5 Effect of Vessel Motion on Mooring Line Tension


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Figure 6 Effect of Nylon Tail Length On Maximum Line Tensions


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Figure 7 Seastate Limit Curves, 50% Criteria, for 138k LNG Carrierwith 11m and 22m Nylon Tails on HMPE Lines

Figure 8 Seastate Limit Curves, 50% Criteria, for 267k LNG Carrier

with 11m and 22m Nylon Tails on HMPE Lines

Figure 9 Seastate Limit Curves, 50% Criteria, for 267k LNG Carrierwith 22m Nylon and PE/PP Tails on HMPE lines.

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