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Dr Zaman Sarker, Principal Engineer atRoyal HaskoningDHV outlines a practicaland cost-effective method for the evaluation of likely operational downtimeat a berth based on waves in the initialstages of a project.

ulk Terminals are of critical importanceto the seaborne trade and transportationof dry bulk commodities such as

crude oil, iron ore, coal, grain, bauxite, aluminaand rock phosphate. Bulk commodities whichare those loaded or discharged in loose orfluid form account for the biggest share ofthe world seaborne trade. Crude oil andpetroleum products are tanker cargoes whileiron ore, coal, grains, bauxite, alumina and rock phosphate constitute the main dry bulkcargoes and are referred to as the major dry

bulks.They are traded in large quantities andshipped almost exclusively in specialised bulkcarriers of different sizes ranging from handysized (10,000 – 55,000 dwt) to large specialised ships of >200,000 dwt. Bulk terminalshave become one of the well-established featuresin international seaborne trade. Such terminalsmay simply have an anchorage or a berth orinclude items such as dredging and breakwaters.As a minimum a bulk terminal comprises aberthing facility for loading or unloading shipsand marine works for the safe access andoperation of ships. Operational downtime dueto adverse wave conditions for loading andunloading of vessels at berth is an important technical and commercial aspect in the planningand development of a bulk terminal or a port.This article describes a practical techniquefor the preliminary assessment of operational

downtime at a proposed dry bulk terminal.As part of the assessment process, numericalmodelling of wave transformation was carriedout using the MIKE21 Spectral Wave (SW)model developed by DHI to derive inshorewave conditions at the bulk terminal.Operational downtime due to wind-wavesand swell-waves was calculated for the headand beam seas separately for a wide range ofvessel sizes.The limiting wave heights H5% aswell as the significant wave height Hs wereused in the downtime calculations.Wave climatevaries throughout the year resulting in differentoperational downtime at different months ofthe year.Annual downtime assists with thefinancial feasibility analysis of a new berthwhereas a month by month downtime assessment assists with port management suchas planning for storage capacity.Therefore,besides an annual downtime assessment, it wasalso assessed for each month of the year.Themethodology has been successfully applied toa real project in the Black Sea, Russia and theresults presented in this paper are from thisproject.The methodology and lessons learnt fromthe study would be useful for the developmentof any sea port or bulk terminal worldwide.

The study site is situated between the -12mand -14m seabed contour lines. However, adredged depth of -17.3m will be maintainedat the berths, in the turning circle and thenavigational approach channel.Three-hourlytime-series wind and wave data for 10.83years (1 January 2000 to 31 October 2010)was purchased from BMT ARGOSS (2011).The wind and wind-wave roses are shown inFigures 1 and 2. Winds blow predominantlyfrom the north-east and south-west sectorswhereas most of the small wind-waves comefrom the north-easterly direction.The astronomical tidal level variation at the studysite is negligible and does not exceed 50mm.Global sea level rise for various scenarioswas extracted from the IntergovernmentalPanel on Climate Change (IPCC, 2007).A sealevel rise of 0.5m has been adopted for thenext century.

☛ Model SetupThe MIKE21 Spectral Wave (SW) model ofDHI was used to transfer offshore waves intothe study site [DHI, 2011].The model area covers the study site and the surrounding areaso that the impact of headlands and nearshorebathymetry is included in the simulations.Overall the area covered by the model was

Practical Technique for Preliminary Assessment of Operational Downtime ofSea Ports and Berthing Facilities through Numerical Modelling

B

Meteorological conditions

Numerical modelling: waves

Table 2 Annual downtime due to wind-waves

Table 3 Downtime event in February

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approximately 45kmx50km.A flexible (triangular)mesh was used in the study. Bathymetric data(Figure 3) were obtained from CMap (2011).

☛ Model Input ConditionsIn addition to offshore waves (from BMTARGOSS), the model requires input waterlevels.A water level of +0.74m was adoptedfor all model runs. This is the mean of HHWLand MHWL plus an allowance of 0.5m for futuresea level rise.The model was used to transformthe offshore time-series wave to inshore location.

☛ 3.3 Model ResultsModel results (wave heights and directions)were extracted for the entire modelling area (seeFigure 4) as well as at selected locations withinthe navigational areas and along the jetty ofthe proposed dry bulk terminal (see Figure 5).

☛ MethodologyThe wave model results were used to assessoperational downtime at the berths. Wavesaffecting the head and beam of a vessel (seeFigure 6) were considered separately.A widerange of vessel sizes was considered with displacement tonnage (DT) ranging from5,000 to >200,000 tons. H5% is used as thelimiting wave height as shown in Table 1.Operational downtime was also calculatedusing the significant wave height,Hs as a criterionwith limits of Hs = 1.0m for beam seas andHs = 1.5m for head seas where H5% = 1.2 Hs.

☛ Downtime criteriaThe limiting wave heights for downtimeassessment were provided by the clientbased on Russian norms (SNiP 2.06.04-82)

and are given in Table 1.

☛ Downtime ResultsAn assessment of downtime due to head andbeam seas was carried out based on wind-waves,swell waves and the resultant of the windand swell waves.Typical results from theseanalyses are provided below. Downtime dueto wind-waves is illustrated in Figure 7.Table2 presents the annual downtime due to beamand head seas for vessels of various sizes. Datafrom Year 2000 was analysed to determine theduration of the worst downtime events due tobeam and head (resultant) seas in each month ofthat year. The downtime in February is presentedin Table 3.The downtime results have assistedthe client with making a commercial decision.

Numerical modelling was undertaken using theMIKE21 SW model to derive wave conditionsat the berths of a proposed bulk terminal.Synthetic time-series data covering bothwind-waves and swell-waves between 2000 and2010 was used in the study.The model resultswere used to calculate operational downtime dueto head and beam seas for a range of vessel sizes.The downtime results are presented in Tables2 to 3 and are illustrated in Figures 7 to 8.The findings may be summarised as follows:☛ The study site is primarily affected byswell-waves from the south-westerly sectorand by wind-waves from the south-eastthrough to north-west sector☛ The operational downtime due to wind-wavesfor a range of vessel sizes is summarised in Tables2 and 3. From the results it can be seen that:- Operational downtime due to wind-wavesis significantly higher than that of swell-waves- Operational downtime for beam seas ishigher than that for head seas.The above assessment of operational downtimeat berth is presented for vessel displacementtonnage as defined in SNiP 2.06.04-82. For bulkcarriers the vessel displacement tonnage is about1.2-1.3 times the vessel deadweight tonnage (dwt).Based on vessel displacement tonnage of 1.25times vessel dwt, Figure 8 shows the operationaldowntime at berth due to resultant-waves forvessels in the range 5-160,000dwt.The approachdescribed in this paper provides a practicaland cost-effective method for the evaluation oflikely operational downtime at a berth basedon waves in the initial stages of a project. In thelater stages of a project numerical (or physical)modelling techniques to look at operationaldowntime based on ship motion at the berth arelikely to be required.The wave data from the studywill provide inputs to these ship motion studies.

Downtime assessment

Summary