Dispersion of Contaminants in the Ocean

Transcription

1 Dispersin f Cntaminants in the Ocean André Luiz da Fnseca Fadel 1, Jã Trnvsky 2 and Flávi Martins de Queirz Guimarães 2 1 Petrbras/CENPES, Cidade Universitaria, Ri de Janeir / RJ - Brasil 2 Chemtech Engenharia, Av. Nil Peçanha 50 G. 1509, Ri de Janeir / RJ Brasil CEP , Phne: ; Fax: ja.trnvsky@chemtech.cm.br Abstract The present wrk describes the simulatin f cntaminants in pen cean waters. An aqueus mixture f cntaminants is released frm ff-shre il platfrms thrugh submerged headers several meters belw the surface. The discharge results in the develpment f a submerged plume due t the presence f mderate currents. Plume behavir is influenced by its initial mmentum, cncentratins, temperature and, specially, the gradients f temperature and cncentratin in the cean layer, t which the plume trajectry and leveling depth shwed t be extremely sensitive. Three classes f cmpnents were cnsidered: disslved salts, flating particles and sinking particles. A simplified apprach fr phase slipping was intrduced in GROUND thus aviding the use f less flexible internal mdels such as the algebraic slip mdel (ASLP) r mre cmplex nes such as the inter phase slip algrithm (IPSA). Cntaminant turbulent dispersin, velcity and temperature prfiles f the current, sea water density and bundary cnditins fr the exit f the dmain were prgrammed in GROUND with the aid f PLANT. Preliminary tuning f the mdels was achieved in cmparisn t data reprted in literature with gd agreement between described characteristic behavir and simulatin results. 1. INTRODUCTION The increasing envirnmental awareness has mtivated remarkable advances in industrial practices during the last years. Althugh pprtunities fr advances culd be easily sptted in many situatins, in thers the pssibilities f imprvement were still limited by the lack f infrmatin n the effective extents f the eventual impacts caused by the activities. In that matter, the benefits frm actins were als difficult t evaluate. Cmputatinal fluid dynamics (CFD) has been applied successfully in many situatins related t envirnmental studies. In fact, any wrk riented t ptimizatin f resurces (energy, raw materials, etc.) can be acknwledged as envirnmentally friendly, n matter the much this factr has effectively riented the slutin. Specifically in the area f risk management and cnsequence analysis, CFD has made pssible the systematic apprach t situatins which culd never be experimented, e.g. txic spills and majr fires r explsins. Cmparing t

2 ther types f mathematical mdels based mainly in statistical regressins r empirical crrelatins, CFD prved t be a much mre realistic apprach. Operatins in the petrleum explitatin and refining industry are undubtedly amng the greatest cncerns f envirnment prtectin entities. This is justified by the huge amunts f ptentially hazardus materials being handled in the mst diverse areas n land and sea. Investments in safety and mitigating actins receive therefre a majr attentin frm cmpanies in this industry as a way f minimizing impacts and assuring sustainable peratin. This wrk was dedicated t the develpment f a mdel fr assessing the ptential impact f peratins in ff-shre il platfrms. The fcus f these simulatins was the release f an aqueus slutin resulting frm the pre-treatment f petrleum. The expected results will allw a systematic analysis f the prblem and the evaluatin f the need and ptins fr slutins. 2. DESCRIPTION OF THE SYSTEM AND MODELLING 2.1. Gemetry and Grid The regin f interest was the cean layer dwnstream the release pint. It was cnsidered fr the develpment f the mdel that the cean flr was flat and at cnstant depth. The submerged fractin f the platfrm was disregarded fr the sake f simplicity, being the header the nly structure reprduced in the simulatin because f its great influence in the near-field f the release. Figure 1 illustrates the simulatin gemetry. Figure 1 Gemetry f the release cean flr Simulatin was assumed t be symmetric in the plane defined by the header centerline and stream directin, allwing the dmain size t be cut by half. The dmain was cmpsed by very distinct regins. In the near-field (i.e. clser t the surce) the grid shuld be well refined in rder t cpe with the dimensins f the header and the aggressive shifts in plume behavir and turbulence effects. In the far-field the grid culd be carser t save cmputatinal effrt. Increasing cell sizes frm the release pint t the limits f the dmain was set therein.

3 2.2. Prperties Density The liquid released cnsists f a mixture f salt water, lighter and heavier than seawater ils dispersed in small cncentratins. The salinity f the effluent is much higher than that f the sea and s is its temperature. The influences f these prperties n the density f the mixture were mdeled and implemented in GROUND. Because f its lw cncentratins, it was cnsidered that the il fractins wuld nt interfere in this calculatin, resulting in an expressin fr the density depending nly n the temperature and cncentratin f salt. Data frm the literature [3] shwed that the islated effect f the cncentratin n the density culd be linearly fitted, while the same apprach fr the temperature wuld lead t a third rder plynmial. A reasnable accurate mdel culd be btained by cmbining thse cntributins ver a reference state, set frm seawater at 15 C, 35 psu (practical salinity units g/kg). ( C3, T) ( C3, T ) ( C3, T) Eq ( C3, T ) C Eq ( C3, T) T T T Eq. 2.3 Diffusin Cnsidering the nature f the flw and the dimensins f the dmain, laminar diffusive effects culd be neglected. Nevertheless, the diffusin variable was manipulated in rder t achieve the expected spread f the cntaminants due t turbulence and mixing in the edges f the plume. A mdel fr the diffusin cefficients (PIL cmmand PRNDTL(phi) = - ###) was set in GROUND as a functin f the micr-mixing rate (EPKE). The parameters fr this mdel were tuned in accrdance with validatin experiments as described belw 2.3. Surces and Envirnmental cnditins Sinking and Flating The il drplets were nt simulated as dispersed phases. Instead, they were intrduced by means f the cncentratin variables C1 and C5, frm which the diffusin effects were eliminated. The main reasn fr this decisin was the need fr a custmized mdel fr the density, which wuld nt be pssible if an internal mdule such as the Algebraic Slip Mdel was used, unless mdificatins t the riginal files were made. Sinking and flating were therefre represented by a surce equatin resulting frm the mass balance in each cell, as illustrated by the fllwing figure and equatin. This surce was prgrammed in GROUND.

4 C1 H Z C1 V1 ( C1H C1L ) t Z Eq. 2.4 C1 L Figure 2 Mass balance fr the finite vlume due t sinking. The terminal settling velcity, V1, was estimated using traditinal mdels frm the literature [2]. Turbulence The standard k- mdel was used. Stream Prfile The stream prfile is an extremely imprtant factr. It is knw frm ceangraphic research that the temperature, salinity and velcity alng a vertical line can vary substantially. This phenmena may lead t well knw effects such as thermal inversin r thermcline. Fr the develpment f the mdel, a cnstant salinity was used, while temperature and velcity were assumed t have linear prfiles frm the bttm t the surface. This crrespnds t a mderately stratified envirnment, i.e. reduced vertical streams and cnsequently reduced mixing f the plume (which is a cnservative apprach t the prblem). The equatins fr these prfiles were set in GROUND and ught t be mdified shuld different envirnmental cnditins be desired. Relaxatin Due t the wide range f cell sizes, relaxatin fr the velcity variables culd nt be kept as a single cnstant value. The false-time-step relaxatin was therefre calculated in GROUND using the apprximate in-cell characteristic time [4]: 2 X TFAL Eq X v 10

5 Dilutin FD Rati 3. RESULTS Due t the difficulties in btaining gd experimental data with extensive cncentratin prfiles alng the dispersin regin, the validatin f the mdels, specifically fr the diffusin, was achieved in reference t the dilutin curve f the system. This curve represents the decaying rati f the maximum cncentratin f the plume as a functin f the distance frm the release pint. A pst prcessing sftware was used t interpret the PHI file and retrieve the maximum cncentratin pints at each crss sectin f the plume. By dividing each f these values by the cncentratin f the effluent, the dilutin curve fr simulatin is btained. Data frm literature [1] was presented in tw distinct envirnmental cnditins: unstable density layers (greatest dilutin) and stable-stratified layers (less dilutin). Gemetry frm the related experiment is very similar t that described in Item 2.1. Figure 3 presents the validatin simulatin in cmparisn t experimental data. 600 Validatin Validaçã Meta 100 Unstable mist. Meta Stable estrat Distance Distância (m) Simulaçã Simulatin Figure 3 Validatin f the mdel with experimental data frm literature It can be seen that the simulatin curve lays between the tw experimental curves, what is ttally reasnable cnsidering the intermediate envirnmental cnditin used. Cncentratin prfiles were similar fr the three types f cntaminants. This represents the reduced influence f sinking r flating f particulate matter, fr the cncentratins and density differences used. Belw is the cncentratin prfile fr each cntaminant, where the generic aspect f the plume can be nticed.

6 C1 C3 C5 Figure 4 Cncentratin prfiles fr each cntaminant The initial mmentum ceases very sn after the release. The fllwing sinking f the entire plume ccurs because f its higher density cmparing t the surrundings. As turbulent mixing vanishes, the dilutin rati tends t stabilize. At this mment, the plume will start seeking fr the layer that can suit its wn density better. This prcess may cause the plume t stabilize in different depths, depending n the density prfile f the water bdy. Sme mre dilutin may ccur in this stage, after which cncentratin changes will tend t be diffusin driven nly. 4. CONCLUSIONS The simulatin f a submerged cntaminant plume in the cean was presented. The develpment f this mdel invlved several particular mdels, which were intrduced in PHOENICS thrugh GROUND. The results shwed reasnable quantitative agreement with experimental data and gd qualitative behaviur. The simulatin served as an imprtant tl fr understanding the sensibility f the plume t peratinal and envirnmental cnditins. One f the mst imprtant cnclusins f this study is that the maximum cncentratin falls t acceptable values very shrtly after the surce pint.

7 5. BIBLIOGRAPHY 1. Mrgan, P. SEAWATER Library Versin 1.2e, CSIRO (1993) 2. Perry, R., Green, D.W. Perry s Chemical Engineer s Handbk, Mc Graw Hill, 7a Ed. (1997). 3. Reprt N. 2.62/204, Nrth Sea Prduced Water: Fate and Effects in the Marine Envirnment, E&P Frum Publicatins (1994) 4. Zhurbin, S., persnal cmmunicatins 6. NOMENCLATURE C1 C3 C5 T T TFAL V1 v X Z - nrmalized cncentratin f heavy il drplets - nrmalized salinity - nrmalized cncentratin f light il drplets - temperature, C - reference temperature, C - false time step, s - terminal settling velcity fr the heavy il drplets, m/s - resulting velcity f fluid in cell, m/s - minimum dimensin f the cell, m - thickness f the cell in vertical directin, m,,,,, - cnstants - density, kg/m 3 - reference density, kg/m 3 - density variatin, kg/m 3