Paper Session 12 Bru Ening

8/12/2019 Paper Session 12 Bru Ening

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PIANC World Congress San Francisco USA 2014

1

ASSESSING MOORING FORCES AT AN OFFSHORE WIND TERMINALIN BREMERHAVEN, GERMANY

by

Anja Br ni ng1, Dr. O. Stoschek

2, D. Spinnreker

2and U.Kraus

3

ABSTRACT

Breaking mooring lines of vessels during port and terminal operations are one of the most disastrous eventsthat affect safety and productivity. Therefore dynamic mooring analysis of vessel motions and mooring

forces are a requirement for port authorities.

Especially in narrow channels and terminals with limited navigation width for ship traffic, long-periodtransient waves, drawdown, caused by passing vessels are not negligible for adjacent terminals. Forces onmooring lines induced by passing vessels cannot be established from the present guidelines. Therefore ajoint modelling approach using DHIs MIKE 21 hydrodynamic model and WAMIT is used.

The present case focuses on determining the operational safety of the new Offshore Terminal Bremerhaven(OTB) for most severe ship traffic situations. These are derived from a matrix of navigation simulations thatwere established for the project and determined the main parameters for the simulation of the vesselpassage. The input parameters regarding moored ships and berth layout are defined with the client basedon the expected operating vessels at the terminal and on information from guidelines and previously usedharbour equipment.

The method outlined in this paper was used for the first time in Germany for a permit process. The dynamicanalyses generally support the proposed mooring and berthing arrangement for worst-case passing vesselscenarios.

1. INTRODUCTION

To manifest its leading position as one of the main ports for the offshore wind industry in North Germany,Bremerhaven started the development of the former Fischereihafen. To support this industrial developingarea with best infrastructure connections, a new offshore terminal located in the Blexer Bogen a bend ofthe Weser River right before the estuary mouths into the Wadden Sea is planned for offshore componentsshipment (Figure 1).

Figure 1: Location of p lanned Offshore Terminal Bremerhaven (Layout by br emenports GmbH &Co.KG) at t he Blexer Bogen (Image OpenStreetmap)

1DHI, Agern All 5, 2970 Hrsholm, Denmark [email protected] GmbH, Max-Planck-Strae 6, 28857 Syke, Germany3bremenports GmbH & Co.KG, Am Strom 2, 27568 Bremerhaven, Germany


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The water depth at the proposed terminal will be dredged to -14.1m MSL, while the channel has waterdepths up to -19.0mMSL in the vicinity of the new Terminal.

In order to ensure a safe operation and berthing of the designated vessels, mooring lines and fenders haveto resist the different external forces that they are exposed to. Endangerment to both terminal and vesselevokes, besides inexperienced handling of mooring, from external forces that lead to significant shipmovement and vice versa high restoring forces within the applied mooring equipment.

Influencing external forces are typically: Wind Currents Waves (wind-sea and swell)

Passing vessels (primary and secondary wake wave phenomena)

Figure 2: Definition of vessel motions

At the Blexer Bogen, large bulk carriers are passing this planned terminal at short distance offapproximately 270m (see Figure 3, green line). Therefore interplay of ship traffic and mooring forces willoccur.bremenports GmbH & Co. KG asked DHI to investigate the safety of moored ships in front of this terminal.The main objective was to document that drawdown generated by passing vessels does not endanger theproposed mooring and berting system.

Figure 3: Layout of the Offsho re Terminal Bremerhaven includi ng marked navigational channel(bremenports GmbH & Co.KG)


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Figure 7: left: Passing shi p hull inc luded in the mesh in fron t of the OTB; right: Bathymetry andextraction points of surface elevation at the Terminal

Furthermore, the hydrodynamic model includes: stationary boundary conditions: constant water level, no ambient tidal or net currents displaced water volume (in time and domain) included as moving pressure field inital conditions:

two-dimensional constant water level including the displaced water level at the starting point of themoving ship/pressure field

The water level was based on information derived from the tidal gauge at Bremerhaven Alter Leuchtturm.

Validation of numerical model

The approach outlined above was used for studies undertaken in navigation channels for harbours aroundthe world, but never before within the specific conditions found in northern german estuaries. In order tovalidate the approach for its applicability within the Weser River, available in-situ measurements at thelocation of Dedesdorf were used (BAW, 2006b). The measuring campaign documented mainly two types ofpassing vessels, which were also relevant in size and speed for the later simulation at the OTB.Observations of the produced drawdown for a large bulk carrier (Panmax size) and a General Cargo shipwere taken into account for the validation.

Four different scenarios including the effect of different tidal water levels, distance to the measurementdevice, vessel type and speeds, could be investigated to quote the effect on the resulting drawdown andtherefore the quality of the approach. Table 1 summarizes the parameters.

Date/TimeWater

level

Vessel data Passing distance

Type Draft SOGHeading

NKleinensiel

D1Dedesdorf

D223.10.2005/

15:19+0.91mMSL

Weserstahl(Bulk carrier)

10.4 m 10 kn 179 318 m 414m

24.10.2005/07:06

+1.00mMSL


7.1 m 12 kn 356 381 m 351 m

05.11.2005/20:07

+0.74mMSL

Star Ikebana(GeneralCargo)

8.7 m 14.1 kn 355 390 m 342 m

08.11.2005/03:30

+1.26mMSL


10.1 m 9.3 kn 184 278 m 454m

Table 1: Parameters of validation scenarios


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Figure 8 shows the simulation domain of the Weser River at Dedesdorf. Measuring devices were located atboth sides at the landing piers of a former ferry connection Kleinensiel (D1)-Dedesdorf (D2).

Figure 8: Bathymetry of Weser River at Dedesdorf inc luding locations of measurement devices

The maximum modelled and measured drawdown values are compared in Table 2. Figure 9 shows agraphic presentation. It is seen that the modelled drawdown is in excellent agreement with themeasurements in most cases though with a trend of overestimating the measured drawdown.

Date Type

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Modelled results Measurements

Drawdown zA[m]

Primary waveheight HP[m]

Drawdown zA[m]

Primary waveheight HP[m]

D1 D2 D1 D2 D1 D2 D1 D2

23.10.2005Weserstahl

(Bulk carrier)0.14 0.11 0.11 0.10 0.08 0.07 0.08 0.12

24.10.2005 Weserstahl(Bulk carrier) 0.13 0.17 0.11 0.12 0.15 0.12 0.10 0.10

05.11.2005Star Ikebana

(GeneralCargo)0.48 0.83 0.31 0.71 0.31 0.50 0.39 0.59

08.11.2005Weserstahl

(Bulk carrier)0.11 0.08 0.08 0.06 0.07 0.04 0.08 0.02

Table 2: Modelled results vs. in-situ measurements


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Figure 10 shows an example of a passing bulk carrier (STW = 13.0 kn) and the resulting water leveldepression at the OTB for an initial still water level of +1.76 m MSL. The significant two-dimensional patternindicates the transient wave (consisting of bow wave, drawdown and primary stern wave). The model doesnot resolve secondary wave effects.

Figure 10: Passage of a bulk c arrier. Shape of the transi ent wave (bow wave, drawdown and primarystern w ave)

A signal of the surface elevation was extracted at several points along the quay within an approximatedistance of 20m off the wall (Error! Reference source not fou nd.).

Figure 11: Extracted signal of sim ulated surface elevation for s ingle bulk carrierer sailingdownstream on an easterly pathway Worst Case(Tidal level : +1.76 mMSL)


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The model results for the scenarios studied showed that the worst case lead to the highest drawdowneffects of approximately 40cm at OTB. Therefore, this most conservative approach was chosen toinvestigate the resulting ship motions and mooring forces.

4. ASSESSMENT OF SHIP MOVEMENTS AND MORING FORCES

To estimate the influence of calculated first order ship waves (drawdown) and their corresponding fluxesreceived from the MIKE 21 model on moored ships in front of the terminal, the results were coupled to thetime domain simulation package, WAMSIM, in the vicinity of the terminal area. WAMSIM includes thepre-processed ship geometry of special installation vessels affected by the induced drawdown andcalculates the mooring forces due to the relative movements of the floating ship hull. Further advantage isthis model approach accounts for the non-linear interaction between external forces coupled with the

characteristics of the mooring arrangement (fender and lines) and not only provides static assessments ofmotions and forces.

Model set-up

To determine ship motions and mooring forces using the approach outlined above, best knowledge of theapplied mooring arrangement (harbour and deck layout, fender and mooring line characteristics) as well asthe physical parameters of the considered ship (size, draft, displacement and vertical centre of gravity) iscrucial for the liability of results. The simulations undertaken in the study included the set-ups for differentvessels listed in Table 4. Additionally, a friction coefficient of = 0.4 between ship hull and fender as well asa pre-tension within the mooring lines of 10t were assumed based on experience and best practice.

In consultation with bremenports GmbH & Co.KG for the terminal layout, following assumptions based onthe equipment used for the nearby Container Terminal CT4 were made:

Single block distance: 20m (0.25 LoA 0.25 * 90m = 22.5m, see (PIANC, 2002)) Doubble bollards: max. Force 200t, max. 4 mooring lines Fender : Diameter 2.00m; Length 3.50m

One bollard associated with a fender system in front was placed in each section center. The characteristics

of the fender are listed in Table 3.

Fabricate TypeDiameter

[m]

Length

[m]

Energy

[kNm]

Reaction

[kN]

Trelleborg Sea Guard 2.00 3.50 454 845

Table 3: Fender characteristic s

The ships of interest regarding their motion behaviour and mooring forces were chosen by bremenportsGmbH & Co.KG based on the envisaged terminal operations. Information regarding eg ship sizes, mooringarrangement on deck and used mooring lines was jointly acquired from ship owners. For the investigations,it was concerted to study predefined loading conditions (ballasted/loaded) resulting in specific draft anddisplacement. The principle dimensions of the ship for this study are given in Table 4, while characteristicsof mooring lines are given in Table 5. Digital hulls from the ship archive (representing the shape of the shipin question) were scaled accordingly and used in the numerical model. An example is shown in Figure 12.

Ship typeLoA

[m]

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[m]

Draft

[m]

Breadth

[m]Cargo ship P2-class (balasted) 168.68 155.79 9.50 25.20

Pontoon (ballasted) 90.00 90.00 2.00 32.00

Pontoon (loaded) 90.00 90.00 5.00 32.00

Jack-up ship 1 (ballasted) 100.00 99.20 4.44 40.00

Jack-up ship 2 (ballasted) 147.50 146.80 7.00 42.00


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Figure 13: Cargo ship P2-800: Initial mooring arrangement (top) and optimi sed set-up with sh oretensions (bottom)

In the following, Figure 14 and Table 6 compare the maximum motions and line forces occuring during theship passage for both mooring configurations. The recorded results of relative motions are based on theinitial position of the ships center of gravity. To relate the motions to the drawdown, the surface elevationmeasured at the center of gravity is shown. In general, focus was laid on the assessment of the occuringmaximum values.

Figure 14: Surface elevation (top) and ship motion s (middle: translation; bo ttom: rot ation)


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The initial movement without optimisation is shown as dotted lines. A large drift motion (sway) towards thenavigation channel was prevented by the activated pre-tension within the mooring lines that assured aconstant and therefore safe contact with the fenders. Heave motions are small and limited to the samemagnitude of the significant water level changes. Mainly a high surge motion (parallel to the quay) appliedfor the ship, while the rotation was small.

For the initial mooring arrangement, an unacceptable surge motion with up to +2.5m can be seen, whileother motions are relatively small. To optimise this, additional shore-based mooring systems were taken intoaccount to reduce the motions. The solid line shows that the surge movement significantly reduced to 1.2mfor the optimised mooring arrangement. The slight increase of the roll motion (approximately 0.5) wasneglectible for resulting forces.

Since spring lines are generally applied to compensate excessive surge motions, a resulting overloadoccurring for the initial mooring layout is unexpected.

OCIMF recommends using a safety factor to determine the maximal allowable force in the morring linesduring their life time. These reference values are based on the Minimum Breaking Load (MBL) defined foreach line and their material (OCIMF, 2008):

Wire: 55% MBL Synthetic ropes: 50% MBL Polyamide: 45% MBL

Results are marked in red when this value is exceeded.

No. of

linesPosition

Initial set-upOptimised set-up

incl. shore tension

Max. force [kN] Max. force [kN]

1 stern line 221 153

2 stern line 223 157

3 aft spring line 258 203

4 aft spring line 251 199

5 fore spring line 258 175

6 fore spring line 291 170

7 bow line 173 150

8 bow line 172 150

9 aft breast line (with ST*) - 157

10 aft spring line (with ST*) - 183

11 fore spring line (with ST*) - 183

12 fore breast line (with ST*) - 114

Table 6: Max. moorin g for ces per l ine compared t o maximal allowable force (0.50*MBL = 240 kN)

*ST=ShoreTension(www.shoretension.nl)

Summary

The highest mooring forces found are summarised in Table 7. Furthermore, it shows the rate of line usage.The results showed that for some cases additional mooring lines were needed to withstand the external loadof passing vessels. The Cargo Ship and the Jack-up ship were optimised to stay within the maximumallowed forces.


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Ship type MBL [kN]Reduced MBL

[kN]Max. force [kN]

Percentage of

line usage

Cargo ship P2-class(8 Lines)

480 240 291 121%

Cargo ship P2-class(8 + 4 lines with st)

480 240 203 85%

Pontoon(ballasted)

990 495 144 29%

Pontoon(loaded)

990 495 280 57%

Jack-up ship 1(initial set-up 6 lines)

512 230 263 114%

Jack-up ship 1(6 lines + 2x spring lines)

512 230 180 78%


850 425 277 65%

Table 7: Max. mooring forces per line occurr ing durin g dynamic load assessment

The maximum absolute ship motions occurring within the simulations are summarised for each investigatedset-up below. As a reference for safe operations, the PIANC Working Group no. 24 published recommendedvalues for maximum allowable ship motions during loading and unloading conditions for General CargoVessels (PIANC, 1995). These are based on experience and investigations and give a good guidance fordifferent vessel types. Table 8 lists the maximum motions derived from simulations. Red numbers indicatean exceedance of the values recommended by PIANC.

Ship type Surge [m] Sway [m] Heave [m] Roll [] Pitch [] Yaw []

Cargo ship P2-class(8 Lines)

4.49 0.06 0.40 1.62 0.19 0.16

Cargo ship P2-class(8 + 4 lines with st)

2.72 0.05 0.40 0.60 0.19 0.14

Pontoon(ballasted)

0.19 0.03 0.39 0.11 0.18 0.08

Pontoon(loaded)

0.94 0.17 0.41 0.34 0.20 0.51


2.40 0.12 0.40 0.21 0.21 0.40

Jack-up ship 1(6 lines + 2x spring lines)

1.58 0.12 0.40 0.13 0.19 0.32


0.94 0.17 0.39 0.22 0.20 0.14

Table 8: Max. motions absol ute values assessed from shi p motion s imulations

In general, the simulation results were significantly lower than recommended values. Still the surge motion

was critical for some cases, eg the Cargo ship P2-800 where even an optimized mooring layout with shoretension was not able to reduce the motion to an acceptable value. In this case, the recommended value wasexceeded by 36% when a maximum 2.0m surge was assumed. Thus it has to be stated that this extremeevent would only occure rarely, and operations could be stopped for the duration of such a vessel passage.

The exceedence of surge motions for both Jack-up vessels could be neglected since loading operations willmost probably take place at a jacked position.


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5. CONCLUSION

This study consisted of a central question challenging the permit process of the Offshore TerminalBremerhaven: Can large vessels pass the terminal at their required speed for manoeuvring withoutendangering the moored vessels at the berth?

The results of the study showed that with only limited improvement of the mooring arrangement byimplementing quay side operation system, all investigated vessels were capable to resist the forces inducedby the worst case scenario of a passing vessel. Dynamic loads induced into the fender system due to rapidship motions were analysed as well and found to be non crucial. Regarding operational safety, it wasconcluded that an interruption of the loading process should still be considered in certain cases due to highsurge motions.

Finally, it has to be mentioned that a detailed analysis of coincident wind forces was not pursued sinceloading operations will only occure during weather windows with low wind speed. Nevertheless, theresistance of mooring lines comprises auxiliary resistance that ensures safe mooring conditions as a resultof this study.

ACKNOWLEDGMENT

The author would like to thank bremenports GmbH & Co.KG for their kind permission to present this Casestory to a larger audience. Furthermore, the thanks go to all involved shipping and construction companyswho supported this project with relvant information, their help is highly appreciated.

REFERENCES

BAW (2006), Fahrrinnenanpassung der Unterweser, Gutachten zur ausbaubedingten nderungschiffserzeugter Belastungen, Bundesanstalt fr Wasserbau, Hamburg.

BAW (2006b), Naturmessungen zur schiffserzeugten Belastung der Unterweser, Bundesanstalt frWasserbau, Hamburg.

Briggs, M. J. (2006), Ship Squat Predictions for Ship/Tow Simulator, Coastal and Hydraulics EngineeringTechnical Note ERDC/CHL CHETN-I-72. Vicksburg, MS: U.S. Army Engineer Research and DevelopmentCenter.

Christensen, E.D., Mortensen, S.B., Jensen, B., Hansen, H.F., Kirkegaard, J. (2008), Numerical simulationof ship motion in offshore and harbour areas. Proceedings of the ASME 27 thInternational Conference onOffshore Mechanics and Arctic Engineering OMAE 2008, June 15-20, 2008, Estoril, Portugal.

DHI (2014), MIKE 21 & MIKE 3 Flow Model FM Hydrodynamic and Transport Module ScientificDocumentation, MIKE by DHI, Hrsholm.

Morgenstern, H. von (2011), Simulations-Studie Offshore Terminal Bremerhaven, Abschlussbericht,Bremen.

Mortensen, S.B., Alley, C., Kirkegaard, J., Hancock, R. (2009), Numerical modelling of moored vesselmotions caused by passing vessels, Proceedings of Coasts & Ports 2009, pp. 544, Wellington, NewZealand.

OCIMF (2008), Mooring Equipment Guidelines 3rdMEG3 Edition.

PIANC (1995), Criteria for Movements of Moored Ships in Harbours, A practical Guide, Supplement toBulletin No 88, PIANC.

PIANC (2002), Guideline for the Design of Fenders Systems.

WAMIT (2011), WAMIT User Manual 7.0, WAMIT Inc.

Wuebben, J.L. (1995), Winter Navigation on the Great Lakes, A Review of Environmental Studies, CRRLReport.

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