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AN OUTFALL DESIGNED AND INSTALLED IN HARSH ENVIRONMENTAL CONDITIONS

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AN OUTFALL DESIGNED AND INSTALLED IN HARSH ENVIRONMENTAL CONDITIONS Powered By Docstoc
					          MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                   and Marine Environment
         IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                           for Coastal WWTP, Outfalls and Sea lines


    An outfall designed and installed in harsh environmental
    conditions. The case of AGADIR south outfall-MORROCO
         Bernard Lhuillier1, Bruno Maerten2, Roger Sahyoun3, Abdelahad Fettahi4

                                               Keywords
Steel outfall, bottom pull,Morocco,

SUMMARY
To improve the quality of the inshore seawater in this tourist area, the collection of the
50000cmper day waste water has been directed to an existing waste water treatment plant
located 10 km south of AGADIR city.
The required outfall has been design and constructed on an area with hard environmental
conditions: natural reserve, west coast exposed to Atlantic Ocean waves, permanent NS current,
mobile sandy sea bed, existing beach outfall.
These conditions and local survey has led to design an outfall with heavy displacement steel
pipes and gravity circulation.
The outfall was 1000mm ID 500 m long onshore, 800m long offshore.
To safe guard stability against waves and currents forces and soil stability, the outfall as been
weighted with heavy continuous concrete coating and installed in a predredged trench.
After ID and OD coating, the steel pipes were welded into 50 m prefabricated string.
A temporary 250m jetty and cofferdam has been erected to maintain the trench wall in the surf
zone.The outfall has been installed by pulling on the trench bottom using floats and linear winch.
Construction works started March 2005 and commissioning test was performed September
2005.




                                              Costal dunes
Introduction

The Agadir sewerage development scheme has selected the routing of waste water of Agadir
and the south neighbouring cities toward south near the site of M’zar, in order to reduce
pollution around the Agadir harbour zone.
1
  Technical manager, GEOCEAN AUBAGNE,France;e-mail:b-lhuillier@geocean.fr
2
  General manager,GEOCEAN AUBAGNE,France;e-mail:b-maerten@geocean.fr
3
  Managing Director,SOMAGEC,Casablanca, Maroc;;e-mail:somagec@somagec.ma
4
  South Area Manager,SOMAGEC,Casablanca, Maroc; e-mail:somagec@somagec.ma




                       MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
     B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 1 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines



The M’zar site is located 10km south of Agadir, and has been selected taking into account the
general circulation of marine current, indicating a main direction towards the south.

Waste waters are treated with a first anaerobic phase and a second phase with sand screening.
The treatment program includes in this first phase the replacement of the existing shore outlet
with a short sea outfall allowing to improve the dilution of effluents, and to reduce the negative
impacts of outlet at a rate of 50 000 m3 per day on the marine environment.
This sea outfall must be designed to accommodate a future extension corresponding to the full
use of the treatment plant in 2010.

The order for the design and construction of the sea outfall was given in March 2005, and the
construction was completed on September 7th 2005.
The final test was completed on November 14th, 2005.

The project has been conducted by RAMSA (Regie Autonome Multiservices d’Agadir) for the
government of the Kingdom of Morocco, under a loan of EIB (European Investment Bank)




                                   Workzone and existing outfall
Environment

 The location of the outfall was dictated by the location of the present treatment plant.
Environmental conditions were thus quite severe:
- Landfall in a dune area subject to erosion and scouring
- No access road
- Work zone in a national park, with a special fauna and flora protection programme
- North South coast line extremely exposed to Atlantic Ocean swell
- Seabed made of very unstable loose sands subject to sedimentary transport
- Permanent current on top of tidal currents
- 10 years return maximum wave height 15m
- Necessity to restore and maintain in service the existing beach outlet by installing a by pass

Design and Engineering

The complete sea outfall system is made of
- An onshore pipeline 1000mm diameter running from the treatment plant to a connection
chamber




                       MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
     B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 2 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines


- A connection chamber receiving the effluent and dispatching it through a valve system either
in the main sea outfall or to the by pass
- A 1000mm diameter by pass with the outlet at marine chart zero level
- The outfall itself made of an onshore part and an offshore part with a diffuser.
- A diffuser made of a vertical tube, allowing the effluent to exit above the seabed level through
four horizontal outlets. This can guarantee a satisfactory diffusion whatever the current
direction is.
- The offshore end part of the outfall has been designed to take into account
        - Future connection of an extension when effluent flow rate will have increased in 2010
        - expected sedimentary transport or scouring

       Hydraulic design

The effluent is to be evacuated by gravity at the rate of 80 000 m3 per day from a connection
point situated at the exit of the treatment plant.
The outfall is designed for a daily flow rate of 50 000 m3 per day by gravity, thus a design
speed of 2, 5 m3/s.
Higher flow rates have been checked to evaluate the possibility to accommodate the future
gravity disposal of effluents (80 000 m3/d in 2010)

       Marine environment Oceano Meteo datas

The site is directly exposed to North Atlantic Ocean swell from west and North West.
The selected project wave is the 50years return wave Hs= 8,8m, period 11s, direction 300°
Maximum tidal range 4m
Taking the above conditions into account, the route selected for the outfall is in the direction of
the prevailing swell.
The currents used in the design are
   - permanent current parallel to the coast line, directed towards the south, speed 0,2 m/s
   - tidal rotating currents not exceeding 0,1 m/s

       Soil conditions

An additional soil investigation was carried out. An underwater soil jetting operation has been
implemented along three different profiles, as well as a video survey of the seabed along the
offshore part of the outfall.
CPT soundings have been carried out at low tide in the tidal zone and on the onshore part of
the route.
The above investigation concludes that there is coarse sand all along the route, with a thickness
in excess of 9m. Consequently, sheet pile driving feasibility could be confirmed for the
temporary construction of a double wall at the landfall.
Bathymetric studies also revealed sand movements as a result of winds and currents.
Sediment transport studies concluded that the burial of the outfall was necessary on the whole
length, with a cover of 2m TOP, with backfilling at the end of the installation.

       Stability

Although buried, the outfall submerged weight has been designed to ensure stability of the
outfall under project waves at the breaking limit.
The static vertical stability of the outfall during operation had to be checked with water fill at
75% in case of a significant wave height, Hs of 9,2m, direction 330° and period 11 to 12 s.




                       MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
     B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 3 of 9
                   MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                            and Marine Environment
                  IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                                    for Coastal WWTP, Outfalls and Sea lines




                                                              LONGITUDINAL PROFILE
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       D i f f u se u r                                                -6                                                         f ond t r anchée
                                                                       -7                                                         haut du t ube
                                                                       -8                                                         Limit e palplanche
                                                                       -9                                                         Air e de pr éf ab
                                                                      -10                                                         Mar ée haut e
                                                                      -11
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              Cathodic protection

The material constituting the outfall must allow a 50 years life time for the outfall within normal
operating conditions.

In order to cope with installation stresses and stability criteria, the selected material has been
carbon steel with anti corrosion protection coating and weight concrete coating:
-external diameter 1016 mm, thickness 12 mm
-external anti corrosion coating polyethylene 4mm, internal anti corrosion coating epoxy 300µ,
concrete weight coating 130mm thickness
Additional anticorrosion protection is ensured with sacrificial anodes welded every 40m on the
outfall.
On the overall outfall length, 140 anodes have been installed and allow a 50 years protection
against corrosion.

Installation method and site preparation

        The outfall being buried on its whole length, specific structures had to be installed near
the coastline, both along the offshore and the onshore part, to be sure that the trench will
remain open during preparation and installation phases.
        A protection dike, about 260m long, has been built to the North of the outfall route. This
dike is used as a protection and construction support for a double wall sheet pile 268m long.
This sheet pile construction is used to stabilize the installation trench.
Concrete weighting of pipes for the offshore part has been done on site on a specifically
prepared weighting area close to the prefabrication area. The outfall has been installed by
pulling through an offshore submarine return sheave and a linear winch onshore. The pulling
cable was connected to the pipe through the submerged return sheave which was held in
position using a high holding capacity anchor.
In order to minimize the outfall installation time, 48m meters sections, made of four pipes, has
been pre assembled on a prefab area about 200m inshore from the beach. The launching of the
actual outfall until the pre opened trench has been made use a roller support ramp.

A secondary pipeline, used as a bypass for the outfall during the maintenance or inspection of
the outfall, has to be brought back in service to allow the installation of the main outfall. The
outlet has been derived and is now situated on the beach, north of the outfall.

A connection chamber has been built to allow the connection of the outfall to the existing
onshore concrete pipeline. This chamber is used to direct the effluent either through the outfall
or through the secondary pipeline.




                         MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
       B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 4 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines



In order to protect the bypass structure, a small dike made out of piled rocks has been built,
ensuring continuity between the dunes north and south of the outfall.




                                       Prefabrication area
Installation work
       -Pipes concrete weighting
The steel pipes arrived in Agadir harbour on May 3rd, 2005. Concrete weighting has been first
preliminary tested to define the exact final submerged weight.

The test included actual weighting and allowed to validate the concrete thickness, the necessary
shattering, the amount of reinforcement steel, the characteristics of the poured concrete as well
as the curing time before removal of shattering, and the final curing time.




               Concrete coating                              Check of the submerged weight



       -By pass construction

The successive damages to the previously installed pipelines have conducted the need for a
secondary outlet able to withstand the site wave conditions.




                      MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
    B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 5 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines


The protection of the by pass has been designed to avoid any scouring of the sand bed under
breaking wave actions and to resist to the direct impact of waves.

This protection is made of a piled rock anti scouring bed 1 to 3 tonnes, buried 2m below the
beach surface plus a concrete protection above the top of the pipe.

       -Dike and sheet pile walls

        A 268m long temporary protection dike has been erected to install the sheet pile wall.
Dike was composed of rocks ranging 200kg/5 tons. 30000 cubic metres of rocks have been
necessary and installed in less than 30 days. The sheet pile wall is a temporary structure
installed to maintain open the trench within the surf zone. This double sheet pile wall has been
installed using a vibrohammer from the protection dike. The distance between the two walls
was 4,5m and the total length was 268m.
1 500 tonnes of sheet piles have been driven in 45 days.




       -Pipe sections prefabrication and launching ramp installation

In order to reduce offshore pulling time, the outfall has been prefabricated in sections. The
overall pull time has been thus reduced to 4 days.

Prefabrication and concrete weighting areas have been installed on site.

Six independent shattering, made in two parts each, have been used for concrete coating.

The prefabrication area was wide enough to allow welding and storage of 4 joints sections. Such
sections were 80 tonnes in weight and were moved using wagons on rails and hydraulic jacks.

The launching ramp, made of 27 roller supports, has been built for an easy pull of the outfall in
the final trench.

       -Dredging

         The selected trenching methodology was to open trenches prior to the installation of the
outfall.
Dredging inside the double sheet pile wall cofferdam has been performed simultaneously using
an excavator and a submerged dredge pump.




                      MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
    B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 6 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines


The offshore dredging, from the offshore end of the sheet pile wall, until the diffuser, has been
performed using a suction hopper dredger.

60 000m3 sediments have been moved in 25 days.




Dredging depths were selected to allow a burial of the top of the outfall pipe under 2 meters of
sediments.
As the suction hopper dredger was unable to access the very shallow water areas near the
cofferdam end, a smaller cutter suction dredger was used from -2m LAT contour to the
cofferdam end.

       -Installation by pulling

One of the many difficulties of pulling a large pipe into the sea is the proper choice of the
submerged weight. On the one hand, it has to be minimum to reduce the necessary pull force;
on the other hand it has to be maximum to withstand the wave and current forces acting on the
pipe during the pull in case weather deteriorates rapidly.

It has been selected to pull a heavy pipe, equipped with additional buoyancy, thus allowing to
have quickly additional weight in case of need, by removing the additional buoyancy.

Pipe assembly was made by welding using a pre approved procedure, and welding control using
also a pre approved procedure.

Each weld was then covered with heat shrink sleeve corrosion coating and concrete weight
coating to reconstitute the pipeline continuity.

To optimise operation time, the diffuser, at the offshore end of the outfall, and the end blind
flange, were pre installed before pulling. This solution allows to reduce the diver’s operation
time.

                                                 The offshore pipeline was pulled using a 120
                                                 tonnes linear winch installed onshore. The
                                                 1800m pulling cable was 52mm diameter and
                                                 was running through an offshore submarine
                                                 sheave installed about 60m away from the
                                                 final diffuser’s position.

                                                 The sheave was hold in place with a Stevpris
                                                 MK6 HHP anchor of 400 tonnes capacity. The
                                                 pulling cable was connected to the pull head
                                                 of the outfall pipeline




                      MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
    B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 7 of 9
         MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                  and Marine Environment
        IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                          for Coastal WWTP, Outfalls and Sea lines




During the pulling, the pipeline was air filled and the pull force was further reduced using
buoyancy tanks attached on the pipeline. In case of deteriorating weather conditions, the
buoyancy tanks could be removed in order to safely stabilize pipeline on the bottom of the
trench.
A diver’s inspection was made immediately after the end of the pull to check the position and
the integrity of the pipeline.




                    Launching ramp                            Outfall pulling head



      -Connection chamber and connection of the pipe

The connection chamber is a reinforced concrete structure ensuring interconnection between
the incoming pipe from the treatment plant with the offshore sea outfall and the secondary
outlet. This reinforced concrete chamber has been built after the completion of the outfall
installation while the flow in the secondary outlet was maintained.

      -Environmental issues

The environment preservation measures have been the subject of a particular plan established
in close cooperation with the concerned Moroccan administrations.
This plan was aiming at having particular attention to the local fauna and flora, and to the
stabilisation of the sand dunes.
The work area has been completely enclosed with a 2m high fence, protruding 50cm into the
ground, to avoid intrusion of the wild animals living in the national reserve (boars, ostriches,
gazelles….).
The earth works in the dunes have been subject to replant of vegetation to avoid wind erosion,
and the dune tops have been stabilized using vegetal barriers.
The construction site has been reinstated to its original state upon completion of the works.




                      MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
    B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 8 of 9
          MWWD 2006 - 4th International Conference on Marine Waste Water Disposal
                                   and Marine Environment
         IEMES 2004 – 2nd International Exhibition on Materials Equipment and Services
                           for Coastal WWTP, Outfalls and Sea lines




Conclusion

Thanks to extremely detailed engineering and working procedures, as used in the offshore oil
industry, and despite extremely difficult local conditions, the project has been completed ahead
of the anticipated schedule. The construction teams have all been extremely professional and
reactive when confronted to new situations.

Thanks

The authors thank the following actors of this success for their efficiency and professionalism.

Régie Autonome d’Agadir (RAMSA)
Entreprise Générale : consortium SOMAGEC (Morocco) and GEOCEAN (France)
Drapor (dredging)
Ulis(Underwater works)

_________________________________________________________________________________________________

           GEOCEAN- 520 avenue de Jouques 13685 AUBAGNE FRANCE Tel 0033442180218
           Fax 0033442180220
           info@geocean.fr web site: www.geocean.fr
           2
            Bruno MAERTEN (Mr.), General manager,
           b-maerten@geocean.fr , GEOCEAN AUBAGNE FRANCE

           1
            Bernard LHUILLIER (Mr.), Technical manager,
           b-lhuillier@geocean.fr , GEOCEAN AUBAGNE FRANCE.




                 Angle rue Mohamed El Mesfioui & Corbi – Oukacha – CP – 21600 CASABLANCA –
               MAROC Tel 00 212 2235 4945 Fax 00 212 2235 4424
               somagec@somagec.ma
               3
                   Roger SAHYOUN (Mr.), Managing Director,
               4
                   Abdelahad FETTAHI (Mr.), South Area Manager




                       MWWD-IEMES 2006-Antalya, Nov. 6-10, 2006
     B.LHUILLIER-An outfall installed in harsh environment. The case of Agadir- page 9 of 9

				
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