Downtime Analysis Methods for Offshore Dredging Operations

G.J. Grundlehner1, R.J. van der Wal2, and G.J. de Boer3

Abstract: The operation of dredgers at sea may be seriously affected by the marine environment. This can lead todowntime during operations. The question whether a system is able to operate in specific seastates is defined as'workability'.

In recent decades improvements have been made in the hydrodynamic modelling of marine structures. However, thecoupling with the actual workability for a certain project and location is less developed. This paper presentsDredsim2000 as the result of a Joint Industry Project to develop an integrated tool to determine the workability.

The paper further focuses on two different methods to determine the workability (or downtime) in an accuratemanner, using the results of (hydro-)dynamic analysis as input. The first method is widely used in the industry:prediction of the downtime on the basis of wave scatter diagrams. The second method is less common but resultsin a more reliable downtime prediction: determination of the 'job duration' on basis of scenario simulations.

The analysis based on wave scatter diagrams simply checks which entries of the diagram satisfy the operationallimits. The combined probability of all acceptable entries results in the workability. The workability is used to correctthe required nett productive hours to determine the gross project duration. This approach can be used for eachoperational mode individually, however applying this approach for a combination of modes is principally impossible

In the determination of the gross project duration on the basis of scenario simulations, long term seastate time recordsare analysed by checking for each subsequent time step which operational mode is applicable. Past events andweather forecast are taken into account. The gross project duration is defined by the consumed time between startof the operation and the moment when the nett dredging hours are achieved.

In a case study the two methods are compared and discussed for a realistic dredging project. The clear differencesbetween the methods will be presented and suggestions for further applications in offshore dredging operations aregiven.

Keywords: downtime analysis, workability, scenario, criteria, operational limits

1 Baggermaatschappij Boskalis bv, Rosmolenweg 20, P.O. Box 43, 3350 AA Papendrecht, the Netherlands, Telephone +31(0)78 6969 359, Fax +31 (0)78 6969 300, g.j.grundlehner@boskalis.nl 2 Maritime Research Institute Netherlands MARIN, Haagsteeg 2, P.O.Box 28, 6700 AA Wageningen, the Netherlands, Telephone+31 (0)317 493 495, fax +31 (0)317 493 245, R.J.v.d.Wal@marin.nl, www.marin.nl3 Maritime Research Institute Netherlands MARIN, Haagsteeg 2, P.O.Box 28, 6700 AA Wageningen, the Netherlands, Telephone+31 (0)317 493 266, fax +31 (0)317 493 245, G.de.Boer@marin.nl, www.marin.nl

1. Introduction

By their nature, dredgers are exposed to the marine environment. This means wind, waves and current may hamperdredging operations or even may cause them to be suspended. The prediction of this 'Wait-on-Weather Downtime',or its more optimistic conjugate 'Workability', is one of the elements to be addressed in the cost estimation ofdredging projects. In practice the nett required operational hours are corrected with a 'workability percentage' to findthe gross project duration, which plays a primary role in the pricing of a project. Other delay factors may alsoincrease the project duration, but these are no subject of this paper and left out of consideration.

Which environmental factors have the most impact on the workability depends on their respective magnitudes, theresponse of the dredging spread and the ability of the spread to deal with this response. In offshore / nearshoreconditions, waves are normally the most important factor. Section 2 will mention some typical effects that wavesmay have on the most common dredger types Cutter Suction Dredger and Trailer Suction Hopper Dredger.

In order to quantify the workability for a certain project execution period, the workability prediction should addressthe following issues:

i) Assessment of the environmental conditions at the project location.This is still a challenging issue and subject to considerable research effort initiated by some dredgingcompanies. It will however not be addressed in this paper.

ii) Determination of the operational limits of the dredger and its auxiliary equipment, in view of projectrequirements and operating methods.In last decades major improvements have been made in the (hydro-)dynamic analysis of marine structures,among them dredgers. This paper addresses a recent improvement for this purpose: 'Dredsim2000'.

iii) Combination of the environmental conditions and operational limits into a workability, with due accountof the limitations and opportunities within the project.Dredsim2000 features two essentially different methods to predict workability:- Directly based on scatter diagrams, further referred to as 'Scatter approach'.- Project scenario analysis based on long term seastate time records, referred to as 'Scenario approach'.

A fictive, though realistic, case study will be used to illustrate the differences between the above two predictionmethods and their practical consequences. Under full appreciation of the effects that wind and current may have insome cases, the case study will focus on workability determined by waves only. Both prediction methods canhowever also be used for workability limited by wind or current or combination thereof.

2. Factors limiting the operability in waves

Each type / design of dredging equipment has its own particularities with regard to workability due to waves. Thissection suffices to mention a few aspects for two common types of dredgers.

2.1 Trailer Suction Hopper Dredger (TSHD)

- Due to its hinging suction tube, a TSHD is relatively tolerable to wave induced motions, provided that thedraghead lifting wire is heave compensated. Still a number of aspects deserve attention, like: deployment andrecovery of the suction tube from a rolling vessel may involve the risk of the draghead damaging the hull.

- Heave of the draghead gantry may exceed the heave compensator stroke, causing slack lifting wire andsubsequent snap loads.

- Vessel motions may impose backward motions of the draghead, which may get stuck in the seabed and thusinduce large buckling forces in the suction tube ('stand on the pipe').

- Connecting the floating discharge hose to the dredger's bow coupling.- Keel clearance margins in shallow waters are further reduced by the vessel's motions, allowing less spoil to be

carried.

It is evident that some of the limiting factors can be made less harmful by changing working method or improvementof sub-systems. Furthermore not all factors illustrated above can be clearly measured and given clear operationalcriteria for the crew to decide upon. Therefore the specific skills and experience of the key crewmembers stillremains a major factor in the final workability that can be achieved.

2.2 Cutter Suction Dredger (CSD)

The CSD's rigid spud system is capable to keep the cutter accurately in the breach, even in hard soils. A disadvantageof the spud is that it makes the CSD a mass spring system in the horizontal plane, with natural periods within thewave period regime. Large dynamic spud reactions are the consequence, in particular when the cutter is lifted. Whenthe cutter is in the breach, pitch motions cause the cutter to pound into it, leading to problems like slack ladder hoistwire, stalling cutter drive and even damage of teeth or blades.

On top of that, handling of the swing wire anchors, auxiliary boat operations and floating discharge line will put otherrestrictions on the operability of the CSD.

Again, improvements in work method and subsystems combined with an experienced crew can improve theworkability, but still the limitations are evident.

3. Description of Dredsim2000

A great number of the workability phenomena addressed in the previous section, can be analysed by accepted (hydro)dynamic ship motion analysis programs. Dredsim2000 was a joint industry effort to integrate and customisenumerical programs into a complete workability assessment tool for use by the dredging industry.

This section briefly addresses the program and concludes with the differences between the Scatter approach andScenario approach.

3.1 Joint Industry Project

In 1998 the following dredging companies together with Maritime Research Institute Netherlands (Marin), launcheda Joint Industry Project to investigate the feasibility of developing a computer program to predict the workabilityin a structured and integral manner, based on state-of-the-art analysis techniques. The participating companies were:- Ballast Nedam Baggeren BV*- Baggermaatschappij Boskalis BV- Hollandse Aanneming Maatschappij BV*- IHC Holland NV- Tideway Marine and Offshore BV- Van Oord ACZ BV*) Now merged to Ballast-HAM Dredging

During the feasibility study and subsequent project execution phase the 'Vereniging van waterbouwers in Bagger-,Kust- en Oeverwerken' (VBKO) acted as Marin’s contract partner, on behalf of herself and above participants. Theseparties together jointly funded the project. Governmental support was obtained through Senter, the Dutchgovernmental agency responsible for the execution of grant schemes for developments in the field of technology,energy, environment, exports and international partnerships.

3.2 Global set-up

Dredsim2000 is basically an integrated suite of computer programs in the field of wave loading, dynamic responseand operability prediction. The hydrodynamic programs were already existent within Marin, the workabilityprograms were defined in close consultation with the participating dredging companies.

Dredsim2000 is capable of calculating the workability of a dredger in wind, waves and current. Fig. 1 gives theglobal set-up of the program.

Dredger Environmental conditions

Motion Characteristics Waves, wind, currentMotions/loads

Downtime/workability

Criteria and typical projectinformation

Fig. 1: Global set-up Dredsim2000

The dredger types which are incorporated in the program are:- Cutter Suction Dredger (CSD), including spud, Xmas tree, swing wires and a model for cutter–seabed

interaction (Wichers 1987).- Trailing Suction Hopper Dredger (TSHD), including the effects of the suction tubes and dragheads.- Plain Suction Dredger (PSD), including the effects of suction pipe and spread mooring.- Stone Dumping Vessel (SDV), including DP capability check and effect of dumppipe.

Dredsim2000 distinguishes the various operational modes specifically for each type of dredger: dredging, stand-by,sailing, (dis-)connecting, discharging and survival, etc. For each mode the operational limits can be specified orcalculated and their effect in the total workability can be quantified.

3.3 Response analysis

The response of the dredger is calculated for a number of relevant single seastates. The single seastates are describedby:- Wave direction relative to the vessel.- Seastate characteristics which are generally described by a wave spectrum.- Wind and current speeds and directions.

Further the response analysis requires:- The (hydro-)dynamic properties of the vessel.- Loads or other effects resulting from the dredging process (like draghead force, cutter RPM, swing speed,

etc.).

Dredsim2000 implements two different response analysis methods:- The free floating TSHD and SDV respond practically linear with the wave height. In that case the

numerically efficient frequency domain analysis can be used.- For strongly non-linear dynamic systems, like a CSD with its cutter pounding in the breach, Dredsim2000

resorts to time domain simulations.

For both types of analysis the results are mainly expressed as statistically extreme responses for the subject irregularseastate.

It is evident that these analyses are already a challenge on their own. They will however not be further detailed inthis paper. Instead reference is made to previous publications (Wichers and Claessens 2000).

3.4 Operational limits

In general the dredger / mode specific operational limits are expressed by 'downtime lines': the maximum allowablewave height, as function of primarily wave direction and period. Seastates higher than the downtime lines areconsidered not-workable for the subject operational mode, as they result in responses that exceed maximumallowable response values (the 'criteria'). Downtime lines are either defined by the user or computed from thedredger's extreme responses for a suitable grid of single seastate analyses. In the latter case the user has to specifythe limiting criteria. In principle there can be several downtime lines, each of them governed by its own criteria, forinstance a downtime line for maximum allowable dynamic spud reaction and one for cutter torque variations.

3.5 Workability prediction based on the Scatter approach

Wave climates are normally given as wave scatter diagrams: the joint probability of occurrence of specific Hs – Tzcombinations. Often these diagrams are given for different wave directions and seasons. Fig. 2 illustrates how onlythe area enclosed by all the downtime lines can be marked as workable.

2 3 4 5 6 7 8 9 10 11 120.25 0 0 0.019 0.004 0 0 0 0 0 0 00.75 0 0 0.043 0.091 0.021 0.004 0.001 0 0 0 01.25 0 0 0.003 0.083 0.123 0.009 0 0 0 0 01.75 0 0 0 0.010 0.161 0.082 0.002 0 0 0 02.25 0 0 0 0 0.023 0.152 0.023 0 0 0 02.75 0 0 0 0 0 0.039 0.059 0.002 0 0 03.25 0 0 0 0 0 0.001 0.027 0.009 0 0 03.75 0 0 0 0 0 0 0.002 0.008 0 0 04.25 0 0 0 0 0 0 0 0 0.001 0 04.75 0 0 0 0 0 0 0 0 0 0 05.25 0 0 0 0 0 0 0 0 0 0 05.75 0 0 0 0 0 0 0 0 0 0 06.25 0 0 0 0 0 0 0 0 0 0 06.75 0 0 0 0 0 0 0 0 0 0 07.25 0 0 0 0 0 0 0 0 0 0 0

Sign

ifian

t wav

e he

ight

Hs

Zero upcrossing period Tz[ ]

workable

DTL3

DTL2

DTL1

Fig. 2: Determination of workability by scatter approach.

The sum of all workable entries (shaded area) is finally the workability for the subject operational mode. Thisworkability may still depend on wave direction and operating season.

The Scatter approach is simple and gives a quick overview of the dredger’s operational limits relative to theprevailing weather conditions.

It should be noted however that each operational mode will have its own workability. The question is how therespective operational modes of a dredging operation contribute to the gross project duration. Here we encountera major limitation of the Scatter approach, as will be illustrated by the following example.

Assume a dredging spread whose workability is governed only by two operational modes:- A dredging mode that can be sustained up to Hs = 2 m, in this example corresponding with workability of 70

%. For Hs > 2 m a floating discharge hose must be disconnected and the spread will have to wait on weatherat the dredge location.

- Re-connecting the discharge hose is the other mode. This is a weather sensitive mode that can only beaccomplished for Hs <1 m, now corresponding with 25 % workability. Connecting takes 2 hours.

The workability of the total project will be somewhere in the range 25 - 70 %, but based on the Scatter approach itis principally not possible to be more precise:- The number of periods with Hs > 2 m determines how often must be re-connected.- The time after a bad weather period before Hs < 1 m determines how long must be waited before re-connecting

can commence.- The probability of a period with Hs < 1 m being longer than 2 hours determines how often re-connecting can

actually be completed.

Particularly in situations where there is a large difference in the operational limits of subsequent modes, the Scatterapproach is in principle unable to quantify the gross duration and one has to resort to a more advanced method: theScenario approach.

3.6 Workability prediction based on the Scenario approach

A job scenario analysis simply follows a long term seastate time record and determines for each time step whichoperational mode is applicable. Fig. 3 illustrates how the operational modes will follow a certain sequence, the so-called job scenario, depending on the status of the dredging spread.

Dredging Dredging Dredging

Time

Hs [m]

Stop

D

Hs,max for connecting floating hose:Hs,max for disconnecting floating hose:Hs,max for dredging:

C,StartC,Start

D

Stop R,Start

Fig. 3: Example of job scenario.

Fig. 3 shows that the status of the spread, in a certain time step, is determined by the operational limits and the pastweather conditions. When the CSD had to disconnect the floating hose, because of the past severe wave conditions(points ‘D’), a re-connection has to be accomplished before dredging can be resumed (points 'C'). This requiressmaller wave heights and longer waiting time than when disconnection was not necessary (point 'R').

Depending on the weather forecast, the dredge master could even decide to go to the harbour when he expects thatthe next workable period is so short that staying stand-by on location is not effective. In such case the dredger canprobably be of more use in the harbour. These kind of considerations are common in dredging projects, but oftendecisions resulting from them are still made by arbitrary judgment. By repeating a job scenario a sufficient numberof times, decisions can be made more rationally.

The input required for a scenario analysis is:- The sequence of operational modes to facilitate the dredging process. For the types of dredgers covered

in Dredsim2000 these have already been implemented as logic decision trees.- The downtime lines for each operational mode. As addressed in section 3.4 they can be directly specified

by the user or can be calculated by Dredsim2000 based on specified criteria.- Seastate time records that:

- apply for the project location and the appropriate seasons- are sufficiently long for the gross duration of the dredging project- are available for a sufficient number of years to produce sufficient job scenario analysis, thus a statistically

stable prediction of the gross project duration

3.7 Discussion on the use of Dredsim2000

As mentioned before, the response prediction of dredging equipment is largely based on existing programs.Dredsim2000 integrates these programs in one tool. It should be noted however, that an appropriate responseassessment still requires expertise to select realistic input values and interpret the results in a correct manner.

In essence the determination of downtime lines is a simple process and it is tempting to see it as a black box. Inpractice the results can, for instance, be dominated by a single phenomenon, which, when judged in a slightlydifferent way, may lead to a considerably different evaluation. The user should always have a critical attitude to theresults and / or the criteria used as input.

The traditional prediction of workability based on wave scatter diagrams gives a quick overview of the operabilityof a dredging spread in certain project conditions. In many cases they will provide a sufficiently accurate predictionof the downtime. Besides, these assessments take only little time.

However, in projects where downtime can be significant and a single, though short, operation may causeconsiderable waiting time, a more advanced scenario analysis will also give a more realistic prediction of the grossproject duration. CSD projects in relatively harsh offshore / nearshore areas are a typical example where a moreadvanced, but also more time consuming, method really pays-off.

For a long time, seastate time records over a sufficient number of years, were scarce. These time records are a pre-requisite for job scenario analysis. Developments in wave hindcasting based on meteorological models have howevercontinued, enabling the generation of these time records (Wichers and Claessens 2000). Also the emergence ofsatellite based wave measurements has given access to historical wave information. Together with improvementsin the models that transform offshore waves to nearshore waves it is now becoming more feasible to acquire realisticlong term historical seastate information for any offshore / nearshore project location.

4. Project example case

The following fictive project is meant as an example case to show in particular the effect of the used workabilityprediction method. It is evident that an actual project may be more complicated, however, principles will be fullycomparable.

4.1 Project description

The project is located in a coastal region. The basic scope is deepening of a harbour access channel, which is fullyexposed to the waves, and involves 500 nett dredging hours.

The entire project requires a CSD. The excavated material will be pumped ashore through a floating discharge line.The base case project will start June 1 and will continue until the work is finished. As a variation also start dateSeptember 1 will be considered.

4.2 Environmental conditions

The basic data apply for the access channel and reflects a realistic 20 year time history trace, consisting of significantwave height Hs and zero-up crossing period Tz. For simplicity the effect of wave direction is neglected.

The weather forecast is assumed 100 % reliable over each next 48 hrs.The following figures give an impression of the wave conditions.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1 2 3 4 5 6 7 8 9 10 11 12Month

Hs

[m]

Hs meanHs,mean - 2*StDev.Hs,mean + 2*StDev.

Fig. 4: Seasonal variation (monthly average Hs and 95 % confidence interval).

all years/all seasons/all directions:

Hs Av. Zero Cros Per (sec) Total (m) 3.00 4.50 6.00 7.50 9.00 10.50 12.00 13.50 15.00 16.50 (%) 0.25 4.45 12.27 3.30 0.45 0.08 0.01 0.00 0.00 0.00 0.00 20.56 0.75 1.72 18.91 11.37 2.61 0.20 0.02 0.00 0.00 0.00 0.00 34.84 1.25 0.00 4.36 13.94 2.47 0.21 0.01 0.00 0.00 0.00 0.00 20.99 1.75 0.00 0.07 7.85 3.52 0.16 0.00 0.00 0.00 0.00 0.00 11.59 2.25 0.00 0.00 1.22 4.65 0.24 0.01 0.00 0.00 0.00 0.00 6.12 2.75 0.00 0.00 0.01 2.35 0.75 0.02 0.00 0.00 0.00 0.00 3.13 3.25 0.00 0.00 0.00 0.40 1.07 0.02 0.00 0.00 0.00 0.00 1.49 3.75 0.00 0.00 0.00 0.01 0.66 0.07 0.00 0.00 0.00 0.00 0.73 4.25 0.00 0.00 0.00 0.00 0.13 0.19 0.00 0.00 0.00 0.00 0.32 4.75 0.00 0.00 0.00 0.00 0.01 0.11 0.01 0.00 0.00 0.00 0.13 5.25 0.00 0.00 0.00 0.00 0.00 0.05 0.03 0.00 0.00 0.00 0.08 5.75 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 6.25 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.75 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.25 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Tot % 6.18 35.61 37.69 16.46 3.50 0.51 0.05 0.00 0.00 0.00

Fig. 5: Year round Hs – Tz scatter diagram based on the average of the 20 years(the downtime predictions are based on monthly scatter diagrams, which are not presented).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Year [-]

Hs

[m]

Mean Hs June, per year

Average Hs in June (over 20 yr)

Mean Hs September, per year

Average Hs in September (over 20 yr)

Fig. 6: Variation of monthly average Hs over the subsequent years for months June and September.

4.3 Operational limitations

Fig. 7 and Table 1 summarise the operational modes and limits of the CSD. The presented downtime lines(operational limits) are just assumed here, but normally they result from a preceding analysis.

mode 1mode 2

modes 3,4,5,6harbour

waves

mode 7

Fig. 7: Overview of operational modes and limits (Base case)

Mode Name Downtime line Duration

1 Sail from harbour to dredge location unlimited 2 hours2 Sail from dredge location to harbour unlimited 2 hours

3Connect, before the CSD is able to dredge,the floating hose and swing wires have to

be connectedHs = 0.75 m 2 hours

4 DredgingHs = 1.5 m for Tz< 6s.

Hs = 0.75 m for Tz> 9s.Linear relation for 6s≥Tz≥9s

500 hours

5Disconnect before the dredger can sail

back to the harbour, the floating hose andswing wares will be disconnected

Hs = 2.0 m 0.5 hours

6 Stand-by CSD is stand-by, i.e. spud poleand ladder are raised Hs = 2.0 m n.a.

7 Wait in harbour CSD is waiting for betterweather conditions unlimited n.a.

Table 1: Overview of operational modes and limits (Base case)

4.4 Operational considerations

A dredging contractor is typically confronted with questions like:- What is the expected gross project duration and risk of exceedance?- What is the effect of delayed arrival of the dredger?- Which operational improvements can reduce gross project duration and to what extent?

The following section will discuss how these questions can be answered by respectively the scatter and the scenarioapproach.

5. Results and discussion

The Base case and a few variations will hereafter be discussed in more detail. An overview of the various key resultsfor the work in the exposed access channel is given in Table 2.

Case 1: Base caseCase 2: Identical to Base case only job starts in September 1Case 3/4 : Identical to Base case only connect limits are changed respectively – 0.25 m and + 0.25 mCase 5/6: Identical to Base case only dredging limits are changed respectively – 0.25 m and + 0.25 m

Overview consumed durations ScenarioCase number 1’Base case’ 2 3 4 5 6

Nett duration [hr] 506.5Mean duration [hr] 649 891 777 621 715 618

minimum duration [hr] 534 655 541 543 564 513maximum duration [hr] 916 1643 1196 801 942 872

Scenario

St.dev. of durations [hr] 98 242 169 77 108 94Overview consumed durations Scatter method

Nett duration [hr] 506.5Duration Month June [hr] 592 592 596 591 677 562

Duration Month September [hr] 690Scatter

Duration Month October [hr] - 780 - - - -

Table 2: Overview of results from calculations using Scenario and Scatter approach

5.1 Comparison of scatter and scenario approaches

Table 3 gives the expected gross durations of the various modes for the envisaged execution periods based on theScatter approach. Note that the scatter diagrams were derived from the same time records as used for the Scenarioapproach. Thus the subject wave climate is exactly the same.

Mode Dredging Connecting Disconnecting Sailing Total

Nett duration 500 2 0.50 2+2 506.5June 583.0 4.36 0.52 2+2 591.9July 563.2 4.05 0.52 2+2 571.8August 614.9 4.26 0.54 2+2 623.7September 680.0 5.24 0.56 2+2 689.8October 768.7 6.54 0.60 2+2 779.8Table 3: Consumed (gross) duration of Base Case using Scatter approach

As clarified in Section 3.5 the Scatter approach can only provide the workability for each mode separately. In orderto compare the scatter approach with the Scenario approach, we have to assume the number of times the CSD hasto sail to / from location and (dis-)connect. In the best case this is only once. Above results fit in the trends of Fig.4.

The 20 year of wave data allowed to perform 20 job scenario analyses. Fig. 8 illustrates a sample of the scenarioanalysis. The waiting time before a re-connection can be made is clearly presented, as are other steps in theoperational sequence. The scenario analysis continues until the total dredged hours equals the required 500 hr.

The following typical events can be distinguished (see also Fig. 8):1: After sailing from harbour and connecting the dredging mode starts2: Wave height exceeds the maximum allowable wave height for dredging: Stop dredging; Dredger is stand-by3: Dredging resumed (actual wave height is smaller than maximum allowable wave height for dredging)4: Wave height exceeds the maximum allowable wave height for dredging: Stop dredging. dredger is stand-by5: Dredging resumed (actual wave height is smaller than maximum allowable wave height for dredging)6: Wave height exceeds the maximum allowable wave height for dredging: Stop dredging; dredger is stand-by7: Wave height exceeds the maximum allowable wave height for stand-by: disconnection and Sail to harbour8: Sail from harbour to dredging location, connect and start dredging9: Wave height exceeds the maximum allowable wave height for dredging: Stop dredging; dredger is stand-by

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time [days]

Hs

[m]

dredging (Tz>9 )/ t

dredging (Tz<6s)

disconnect / stand-by

12

34

56

78

9

Fig 8: Sample Scenario analysis

Fig 9 summarises the scenario results for the Base Case:- Nett duration line (no downtime) added for reference.- Stochastic variation over the years is considerable. 7 out of the 20 analyses result in durations that are actually

less than the gross duration based on the Scatter approach (591.9 hr, see Table 3). These are the 'good luckseasons' where the wave conditions tend to be more favourable than in average.

- Unfortunately there are also 'bad luck seasons'. In one case the gross duration is even 1.55 x mean duration.- In average however the scenario approach yields a more conservative project duration than the scatter approach:

for the base case 649 hr vs. 592 hr, so 10% more.

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10 12 14 16 18 20Job no.[-]

Job

dura

tion

[hr]

Job durationMean of job durations

Nett duration (no downtime)

BASE CASEtotal: 20 jobs

Nett duration: 506.5 hr

Fig. 9: Results of 20 job scenario analyses (Base Case).

5.2 Effect of delay of start date

When the CSD is delayed on another project, the start date may enter a less favourable season. For this exampleSeptember 1 was chosen. Fig. 10 compares the results for start date September 1 with the results of the Base Case(start June 1):- The average gross project duration is now 891 hr for the Scenario approach against 690 hr for the Scatter

approach, so the difference between the two approaches is now increased to 29 %.- As now more re-connections are required there are now only 3 jobs where the scenario analysis gives a shorter

duration than the scatter approach.- The stochastic variation, presented by the standard deviation of the job durations, has increased considerably.

The worst case duration becomes even worse: 1.85 x mean duration.

0

200

400

600

800

1000

1200

1400

1600

1800

0 2 4 6 8 10 12 14 16 18 20Job no.[-]

Dur

atio

n of

job

[hr]

Start September 1stMean start September 1stStart June 1stMean start June 1st

Effect of delay of start datetotal: 20 jobs

Nett duration: 506.5 hr

Fig. 10: Effect of delayed start date using the scenario approach (basic scope only).

5.3 Effect of system improvements

Just to demonstrate the value of the Scenario approach, the connecting limit is varied by +/- 0.25 m. The effect isplotted in Fig. 11, which shows a histogram of the gross durations of the twenty simulations, and summarised inTable 2 (cases 3, 4). For the base case, a reduction of the connecting limit from 0.75 to 0.50 m increases the meanjob duration by 20 % and the variability even more. Note that with the Scatter approach these effects are not found.

Reduction of the dredging limit by 0.25 m (see Fig 12 and cases 5, 6 in Table 2) has a comparable effect in this case.

Improvement of connecting and dredging limits with 0.25 m leads to roughly 5 % reduction in mean job duration.For start date September 1 this would be more. In that case improvement of the connecting procedure / equipment(for instance quick connect / release hooks) may yield a considerable saving in downtime. Increase of the dredginglimit is probably more costly but likely yields no larger saving. The Scenario approach could be helpful to supportsuch decisions.

0

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5

6

7

8

9

10

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500Duration of job [hr]

Freq

uenc

y [-]

Hs_connect=1.0 m

Base Case

Hs_connect=0.5 m

Effect of Connect limitstotal: 20 jobs, class width: 100 hr

Nett duration: 506.5 hr

Fig. 11: Effect of variation connecting limit (basic scope only)

0

2

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6

8

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12

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500Duration of job [hr]

Freq

uenc

y [-] Hs = 1.25m (Tz<6s) - 0.5m (Tz>9s)

Base Case

Hs = 1.75m (Tz<6s) - 1.0 m (Tz>9s)

Effect of Dredging limitstotal: 20 jobs, class width: 100 hr

Nett duration: 506.5 hr

Fig. 12: Effect of variation dredging limit (basic scope only)

5.4 Comments on the practical application of the Scenario approach

The examples of the previous sections, and as summarised in Table 2, are just illustrations of the value of theScenario approach. Its value is most apparent in cases where the downtime is relatively large and / or someoperational modes can hold up the entire dredging process.

For an accurate prediction of the workability, realistic operational limits and reliable local wave data are crucial. Thisapplies however also for the Scatter approach. The major additional requirement that the Scenario approach imposesis the need for wave history time traces.

One may argue whether the observed differences between Scatter and Scenario approach are worth the additionaleffort. For a 1 month project, 10 % difference equals 3 days, which readily justifies the effort when considering theday rates of large CSD's. For longer and more sensitive projects the answer is even more pronounced.

Still, even in the case of perfectly executed scenario analyses with sufficient number of job scenario analyses, theactual realisation can be far off the prediction. This is an unavoidable 'fact of life', but at least we now have animpression of this variation and can use it in settling the contract. The scatter approach gives no insight in thesevariations at all.

This brings us to the convergence of the gross duration prediction based on the scatter approach.In the cases addressed above as much as 20 years were available, this means 20 realisations of the stochastic seastateprocess. In practice this will often be less. The question is how many realisations are required for a reliableprediction based on the scenario approach. A theoretical treatment of this matter is beyond the scope of this paper,but some feeling can be obtained by plotting the average duration for an increasing number of job realisations. SeeFig. 13. The order of the realisations will have some influence, but is maintained as per original data set.

0

100

200

300

400

500

600

700

800

900

1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Number of jobs used for average duration [-]

Aver

age

dura

tion

[hr]

Base CaseStarting date Sept. 1stMean duration Base caseMean duration Starting date Sept.1st

Fig 13: Convergence of mean project duration.

Because of the lower workability the case for start date September 1 converges slower than for start date June 1. Inany case 7 – 10 job realisations seem to be sufficient, but at least 5 seem necessary for this example.

6. Summary and conclusions

Dredsim2000 is the result of a joint industry development to make the workability (or downtime) prediction ofcommon dredging projects more structured and integral. It implements existing and new methods for thedetermination of operational limits and the prediction of downtime, using site specific environmental data as input.

The workability prediction based on scatter diagrams is simple and gives a quick overview of the operational limitsrelative to the prevailing environmental conditions. However, many projects involve operational modes that,although they may last short, have clearly lower operational limits than other modes. These critical modes may hold-

up the entire process, causing a considerable increase in downtime. Such effects cannot be quantified by the Scatterapproach, so that job scenario analyses need to be performed.

Pre-requisites for the Scenario approach is the definition of the sequential relation between the various modes andsufficiently long seastate time records so that a project can be analysed for at minimum 5 job realisations. The firstrequirement can easily be fulfilled and is implemented in Dredsim2000. The availability of representative long termseastate time records becomes increasingly accessible for the industry, due to developments in metocean engineering.

A simple CSD project example case is evaluated by the Scatter and Scenario approaches. The latter gives a moreconservative prediction, though more realistic because of its due inclusion of all modes and waiting times betweensubsequent modes. Furthermore the scenario approach gives much more insight in operational issues like:- stochastic variation of project durations- 'bottlenecks' in the dredging operation- effects of operational strategies and system improvements

The more accurate prediction of downtime and the increased insight in above kind of effects may in many cases bewell worth the effort that the Scenario approach requires.

References

1. Feikema, G.J. (1995) Assessment of the Downtime of Dredges in Waves, WODCON, Amsterdam2.Van den Bos, L. (1998) Bepaling van de werkbaarheid van materieel offshore, Master Thesis, Delft Universityof Technology, Delft (in Dutch).3 Wichers, J.E.W. (1980) On the Forces on a Cutter Suction Dredger in Waves, WODCON IX, Vancouver4. Wichers, J.E.W. (1981) Hydrodynamic Behavior Of Cutter Suction Dredges in Waves, World Dredging andMarine Construction, November 19815. Wichers, J.E.W. (1987) Handbook of Coastal and Ocean Engineering, Volume 3, Chapter 7. “The behavior ofdredging equipment operating in waves”, ISBN 0-87201-452-5.6. Wichers, J.E.W. and E.J.Claessens (2000) Prediction Downtime of Dredges Operating in the open sea.WEDA, Rhode Island

Acknowledgements

The authors wish to thank the organisations that participated in Dredsim2000, for their permission to present inthis paper the program and results obtained with it.