Vessel Interaction A Case Study eter O Brien, Dr Terry O Brien, Chris Hens OMC International ty Ltd, Melbourne, Australia Abstract The growth in vessel sizes is lacing orts under increasing ressure to safely and efficiently eet the deand. One ajor issue river orts in articular are facing is the unaccetable otions of a oored vessel generated by large assing vessels. This is causing both safety issues, where resulting breakage of lines can cause significant injury or death, and efficiency issues, where berths are required to sto loading while a vessel asses. Decisions ade by ort oerators on what easures to take for assing vessel scenarios are tyically ad hoc and based only on exerience. This generic aroach ay fail to address iortant safety and efficiency issues. OMC has develoed a scientifically-based odel for deterining otiu vessel assing seeds and distances given the revailing environental conditions, tidal levels and characteristics of both the oored and assing vessels. The odel has been validated against full scale easureents, with excellent correlation between easured data and odel oututs. The odel rovides ort oerators with a tool for develoing reeatable and auditable rocedures for deterining safe assing conditions with otiu efficiency. A case study is resented which involves validation and design siulations of the OMC vessel interaction odel at ort Hedland, Western Australia. 1 Introduction ort Hedland is located at the outh of an estuary in the ilbara region of North Western Australia. ort Hedland ort Authority (HA) is the statutory authority with the riary urose of facilitating trade through the ort. The ain trade coodity of the ort is iron ore with BH Billiton exorting over 100 illion tonnes annually in Cae size vessels aking it Australia s largest ort with resect to throughut. Due to significant resent and future escalating global deand for iron ore fro China, the growth in iron ore exorts fro ort Hedland is redicted to trile over the next 10 years. Develoent lans for the ort include construction of a nuber of additional iron ore berths throughout the estuary. The layout of ort Hedland is shown in Figure 1. The iron ore berths are located at BH Billiton s Berths C & D at Finucane Island near the western entrance of the estuary and berths A and B at Nelson oint, the ost inland berths on the eastern side of the estuary. Additional berths at ort Hedland include two anaax berths, Berths 3 and 1 located seaward of Berths A and B. Cae vessels dearting Berths A and B ass within roxiity of Berths 3 and 1 and can, at ties, cause ooring interaction issues, including the occasional instance of arting of lines for the anaax vessels. Siilar issues can also occur for Cae vessels berthed at Finucane Island. Figure 1: ort Hedland ort Layout (source: Cardo Lawson & Treloar) The ort Authority has rocedures in lace to reduce the likelihood and iacts of ooring interaction incidents, including iosing seed restrictions and, at ties, stoing loading at these berths during such assing events. A assing event of a laden Cae vessel ast a berthed Cae vessel is shown in Figure.
Figure 3: assing Vessel Coordinate Syste Figure : assing Event at ort Hedland The interaction roble will be exacerbated as the need to increase the nuber of dee draft vessels transiting the ort grows with the ort exansion. In 006, HA coissioned OMC to undertake a shi interaction study. The riary objective was for HA to use the findings of the study to refine the ort User guidelines and oerating rocedures to address articular identified risk situations, both for the resent and throughout the exansion hases. The ajor objectives of the interaction study were to deterine: safe transit seeds for a range of shi oveents ast secified berths; whether loading at articular berths should sto for articular oveents; ooring line loads resulting fro vessel interaction; whether existing ooring arrangeents are adequate to iniise line breakages. assing Vessel/Moored Vessel Interaction Theory The roble of deterining vessel interaction forces is a colex one. However, a reasonable aroxiation ay be found by introducing silifications. Solutions for vessels travelling on arallel aths were rovided by Wang (1975) and Tuck and Newan (1974). Both solutions ake use of otential flow techniques and hence ily an ideal fluid. The roble can be further silified by assuing the assing vessel travels at low seeds, in which case free surface effects ay be neglected. This allows the ethod of reflection to be used with the water surface as a lane of syetry. A scheatic overview of the adoted coordinate syste is shown in Figure 3. As er Wang (1975), the velocity otential Φ of the assing vessel in water of infinite deth ay aroxiated by odelling a distribution of doublets with strength µ in a unifor flow with a velocity of V. It is convenient to write the velocity otential relative to the shi-bound coordinate syste of the oored vessel. In doing so, Φ can be written as a function of the assing vessel seed (V), searation distance (d) and stagger distance (X). See Eq. 1. The doublet strength µ is a function of the sectional area distribution Φ V µ = ( x, y, z) S of the assing vessel. µ ( x) ( x x X ) L ( x x X ) + ( y d) + S ( x ) V ( z ) = π 3 / dx (Eq.1) The velocity otential generated by the assing vessel needs to be balanced by an interaction otential at the oored vessel to satisfy its boundary condition (Eq. ). Φ n = 0 (Eq. ) The flows as a result of the interaction otential are of oosite and equal strength to the velocities induced by the assing vessel on the body axis of the oored vessel (Eq. 3) u v ( x ) = ( x,0,0) ( x ) = ( x,0,0) Φ x Φ y (Eq. 3) The interaction otential ay now be found by lacing the oored vessel hull, which again ay be aroxiated by a distribution of doublets, in a unifor flow of u( ) x in longitudinal direction and
( ) v in cross direction. Once the interaction x otential has been deterined, the unsteady Bernoulli equation (Eq. 4) can used to deterine the ressure distribution on the oored vessel hull. Φ V + t + + gz = ρ C( t) (Eq. 4) The surge, sway and yaw forces ay be found by integrating the ressure distribution over the length of the oored vessel hull. Corrections for finite deth effects and current velocities have been alied to the derived results. Figure 4 shows tyical diensionless surge and sway forces and Figure 5 shows tyical yawing oents. The diensionless forces and oents are shown as a function of the stagger distance X (the relative osition of the assing to the oored vessel). ositive surge and sway forces corresond to oored vessel dislaceents in ositive X and Y direction resectively. Siilarly a ositive yaw oent corresonds to a rotation of the bow of the oored vessel to ortside. See also Figure 3 for an overview of the adoted coordinate syste. F / V^ [ton / kn^] 3.0.0 1.0 0.0 -.0-1.0 0.0 1.0.0 Sway (finite deth) Sway (infinite deth) -1.0 -.0 X / (0.5*L + 0.5*L) [-] Surge (finite deth) Surge (infinite deth) Figure 4 Tyical Surge and Sway Interaction Forces on Moored Vessel Mz / V^ [ton / kn^] 150 100 50 0 -.0-1.0 0.0-50 1.0.0 Calc (finite deth) -100-150 X / (0.5*L + 0.5*L) [-] Calc (infinite deth) Figure 5 Tyical Yaw Moents on Moored Vessel These induced forces and oents on the oored vessel in the horizontal odes can only be absorbed through the inial daing forces in these odes and the ooring line configuration. The induced hydrodynaic forces on the oored vessel can exceed the breaking strength of the lines, resulting in line breakage, occuational health and safety issues and otential daage to vessel and wharf structures. 3 OMC Interaction Model The OMC vessel interaction odel considers the dynaics of the vessel and ooring line and fender configuration to rovide tie doain redictions of the surge force, sway force and yaw oent and the induced vessel otions and ooring line loads. The riary influences affecting the intensity of the assing forces and oent between vessels are included in the interaction odel and are suarised as follows; dislaceents of the berthed and assing vessels seed of the assing vessel through the water searation distance between vessels UKC of the vessels ooring configuration environental conditions at the tie of transit (wave, wind, current) The ain coonent of the interaction odel is the nuerical odel Siulation ackage for the Motions of Shis or SMS (O Brien 00), which has been develoed by Dr. W.T. O'Brien of OMC for the analysis of various robles associated with the otions of oored and free oving vessels. The ethodology for the OMC interaction odel involves two stes. (i) Deterination of the tie series of exciting forces and oents induced by the assing vessel on the oored vessel, using the theory outlined in Section above. Assutions ade include that the vessel asses arallel to the berthed vessel at a constant seed and that the flow around the assing vessel is not affected by any reflections fro the surrounding harbour.
(ii) Inut of the exciting forces and oents, together with the design environental conditions, into the SMS odel to obtain tie doain siulations of the six degree-of-freedo otions, fender forces and ooring line loads induced on the oored vessel by the assing vessel. 4 Case Study: ort Hedland Interaction Study The ort Hedland interaction study was searated into two coonents: art A: validation of OMC s shi interaction odel through full scale DGS easureent of assing shi events. art B: utilisation of the validated OMC shi interaction odel for analysis of design interaction scenarios fro which HA can forulate the oerating guidelines. 4.1 art A: Full Scale Validation To validate the OMC interaction odel at ort Hedland, a site visit to ort Hedland was undertaken in Noveber 006. The objective was to record high accuracy DGS easureents on the anaax vessel Santa Isabella oored at HA No. 1 Berth. The easureents involved laceent of two roving Trible 5700 DGS instruents on the vessel and one Trible 5700 DGS base station on the shore near the HA ort Control Tower. An instruent was located on both the bow and stern centreline to enable the vessel horizontal dislaceents (surge, sway) and rotation (yaw) to be resolved. The instruents recorded vessel osition every second. Figure 6 is a hoto of the DGS receiver setu on the stern of the Santa Isabella. At the tie of each event, the loading condition of the Santa Isabella was recorded. The assing shi details and trajectory and seed were also recorded. The ooring configuration of the Santa Isabella, including nuber and location of lines, line tyes and retensions and fender tyes was also recorded for each event. The tidal level at the tie of each easureent was recorded and used in the analyses. The DGS data for each event was ost-rocessed and resolved into tie series of horizontal excursions (surge, sway and yaw) for validation against the OMC interaction odel. The following general observations were drawn fro analyses of the data: 1. The iortance of vessel seed and searation was clearly shown with the saller anaax vessel, which assed closer to the oored vessel at significantly greater seeds, inducing interaction forces of siilar agnitude to the uch larger Cae Vessels.. There was significant interaction fro all three events, with the easured surge in the order of at least 1 for all three events, and the stern dislaceent exceeding 1 for all three events. The ooring conditions at the tie of each event were recreated in the OMC interaction odel. Hydrodynaic odelling was conducted using the SMS for the Santa Isabella oored in each loaded state and water level to deterine added ass and daing coefficients for otions in six degrees of freedo. For each siulation, the exciting forces and oents were deterined based on the recorded assing vessel characteristics, the assing vessel seed and the searation distance between oored and assing vessel as detailed in Section 3. These exciting forces and oents, together with the frequency-indeendent added ass and retardation functions, were used in SMS to deterine the tie doain series of induced vessel dislaceents, ooring line tensions and fender reactions for that scenario. Figure 6: DGS instruent on stern of Santa Isabella The instruents recorded three assing vessel events, naely an inbound anaax vessel and two outbound laden Cae vessels. The OMC odel dislaceents were then coared to the easured dislaceents for each event. Figure 7 deicts the tie series of easured and redicted sway otion of the Santa Isabella during the assing of the Cae vessel Challenge lus. The validation easureents deonstrated that the OMC odel correlated very well with the results fro the easureents. In articular the agnitude and hasing of dislaceents are well within the exected
accuracy, given assutions ade on ooring line condition and retensions and assing vessel seed and trajectory. The validation study rovided full confidence in the validity of the outut fro the OMC odel for use in the design siulations in art B of the study. the odelled current streas through the ort is given in Figure 9 for a tyical sring tide. The largest current seed given by CLT for each berth off the fenders was used in the odelling as this rovided the greatest oored vessel resonse during the assing event. DGS Measured Sway "Challenge lus" 0.5 Sway Motion () 0.00 13:00:00 13:05:00 13:10:00-0.5-0.50 Measured Tie redicted Figure 8: ort Hedland Sring tides (source: Cardo Lawson & Treloar) Figure 7: Exale Tie History of Measured & redicted Sway Motions 4. art B: Design Scenario Analyses Four berths and rototye vessels were selected for the dynaic vessel interaction analyses utilising the validated odel, as follows: (i) BH Berth A - laden Cae vessel (ii) HA Berth 3 - laden anaax vessel (iii) HA Berth 1- laden anaax vessel (iv) BH Berth D - laden Cae vessel. The assing vessel for each berth was assued to be a Cae vessel; ballast for inbound, laden for outbound. In consultation with HA and the ilots, an enveloe of assing seeds and searation distances for both outbound and inbound vessels was selected. The enveloe covered the axiu and iniu realistic seeds and searation distances for assing each berth. The values have been deterined as realistic and reresentative for a laden Cae vessel leaving fro BH Berth B and ballast Cae vessel transiting to Berth B. The ooring and fender configuration at each berth were rovided by HA. The environental conditions at each berth were rovided by Cardno Lawson & Treloar (CLT). CLT undertook current, tidal and wave odelling for HA as art of this and other studies. The assued design current conditions iosed on each vessel during the interaction odelling were rovided for tyical sring and nea tides. A lot of The tyical wave cliate at each berth as rovided by CLT was insignificant in ters of induced ooring loads and otions for these vessels and as such was not odelled. The vessel hull shaes for each design vessel were odelled nuerically for the secified loading conditions. Hydrodynaic odelling was conducted using the SMS for each of the design shis oored in their secified loaded state and water level. A nuber of siulations were then undertaken for the enveloe of design assing seeds and searation distances for each berth. For each siulation, the exciting forces and oents were deterined for the design conditions and then inut into SMS tie doain siulations. The outut was a tie doain series of induced vessel dislaceents, ooring line tensions and fender reactions for that scenario. For each scenario, two outcoes were exained: 1. Whether the odelled assing scenario would involve risk of line breakage. In such cases it was recoended that such a assing event not occur, and that a reduced seed or increased searation distance be adoted.. Where there was no threat of line breakage, whether the resulting horizontal otions would be greater than recoended for loading. For these cases it was recoended that loading should sto during such a assing event.
5 Discussion / Conclusion The vessel interaction study has rovided data to enable HA to refine the ort guidelines and oerating rocedures with resect to safe assing vessel events. For all berths, assing scenarios have been identified whereby the design vessel ay ass the design berthed vessel without risk of line breakage and/or the requireent to sto loading during the assing. Secifically, laden vessels rovided the ost critical assing events; however, for cases where the assued inbound ballast vessel seed was significant coared to the outbound, then siilar significant interaction ay also occur. This inforation obtained fro this study can also feed into future design araeters both in ters of roxiity of new berths to assing vessels but also in ters of siulation odelling of future ort and channel caacity during the available tidal cycles. 6 Acknowledgeents The data collection and analyses described in this aer has been funded by ort Hedland ort Authority and the authors are grateful for erission to ublish this work. Cardno Lawson & Treloar ty Ltd has rovided the environental data described in this reort and the authors gratefully acknowledge this. References O Brien, W.T. (00) Exerience using dynaic underkeel clearance systes,: selected case studies and recent develoents, roceedings of the 30 th IANC Congress, Sydney, Australia, -6 Seteber 00. Wang, S. (1975) Dynaic Effects of Shi assage on Moored vessels, ASCE, Journal of Waterways, Harbors and Coastal Engineering Division, WW3, 47-58. Tuck, E.O. and Newan, J.N. (1974) Hydrodynaic interactions between shis, roceedings 10 th Syosiu on Naval Hydrodynaics, Cabridge.