I.- DREDGING OBJECTIVES

The dredging tool was designed and manufacturedin order to perform the KITINA Template and Jacket mudmats area cleaning.

This Advanced Tool is based on 3 principles, oneof them being a principle of venturi tube similar to the one of the GEOCEANdredging equipment operated during the Scarabeo 4 dredging campaign whichefficiency was not proven in hard soil nor in very dense and viscuous mud.
 
 

Scope of the work:

- Dredging a trench corresponding to the KITINAPtf 14" Riser located under level 133. - Trench dimensions: 3m wide x 40m long x up to 1.5 m deep -

- Levelling (if requested) Kitina area sea bedcovered with a 1 to 3m layer of mixed cuttings, drilling mud and cementproduced while drilling the KITINA template wells.
 

II- DREDGING TOOL PRINCIPLE

The dredging tool is a venturi tube 14" wide and7 m long, including 3 systems located at 3 separate levels of the pipe

- Retrojetting system:

Having a Venturi tube shape, this system is awater lift based system dimensionned for providing a succion effect ontothe seabed/mud layer sufficient for rejecting the

sediment through the exhaust located at the upperend of the tool.

Retrojetting pipe end is located 80cm above thelower end of the tube

- Jetting system:

The jetting system is built of a ring located60 cm above the dredging tool lower end, heaving 6 jetting screwed capswith holes.

The purpose of the jetting ring is to breack /cut the hard layer or crust and to reinject

a big quantity of water in the high density andhigh viscosity muds.

- Air lift system:

This system is located 1.5 m above the retrojettingand venturi tube. it is considered

as a contigency system allowing to boost the retrojetting efficiency when necessary.

Note: This system induces a total lack of visibility(Visual monitoring not possible) and constitute a perfect screen for acousticwaves - not possible to use an acoustic positioning (HPR/LBL system) norlocate the dredging tool using a sonar.- no possibility of using a ROVin the vicinity).

Water pressure is provided by a drilling rig HPpump Pressure max 5000 psi.

Compressed air is provided by a SCHRAMM BP/HPair compressor Pressure max 350 psi.
 

III- DREDGING TOOL SPREAD

The spread used for the14"riser track route cleaningoperation is based on the Castoro 8 Assistance & Survey vessel spread:

· M/V TAURUS DP vessel and her full spreadincluding 20T aft crane, second 5 T crane.

· HP Pumps and Air compressor

· 5000 PSI HP water twin pump with 200m HP hoses.

· HP/BP Schram compressor with 200 m HP/BPhoses

· additionnal sea water pumps and watertank.

· Positioning spread

· DGPS 1&2, Gyrocompas, GNS navigationsystem,

· USBL acoustic positioning system

· LBL acoustic positioning & navigationsystem·

· ROV SCORPIO 59 equipped with:

· Gyrocompass,

· High definition sonar,

· Bathy system,

· Dual head Scanning Sonar

· 5F/7F manipulators

· Processing and Mapping full spread
 

IV- DREDGING TOOL DEPLOIEMENT

The HP pump and the air compressor are connectedto the dredging tool by means of two 200 m HP hoses built up in an umbilical.

The tool , weighing 2 tons, is lowered from aDP vessel using a 1" crane cable. Umbilical is fitted onto the cable every10 m.

On KITINA Jacket location WD (135 m)

Lowering operation duration is 45 mn.

Recovery operation duration is 20 mn
 

V- DREDGING TOOL OPERATION

This tool is designed in order to work deeplyburied into the mud & sediments layers (0.5m up to 1.5 m).

The three dredging systems are selectable duringoperation:

Air pressure directly from the surface (separatehose)

Jetting /retro-jetting by means of two 2" valvesoperable by a ROV.

Water pressure and flow rates are adjusted versusmud properties.

Control of the start of line and end of line positionis performed using LBL acoustic positioning; control of the trajectorybetween these two points is performed by the DP System itself.

The monitoring of the position is performed usingUSBL or LBL acoustic positioning system -both USBL and LBL transpondersare positioned directly above the wire/pipe connection -

The monitoring of the depth is performed by twoways:

- ROV direct visual control of the level of penetration(when possible)

- ROV monitoring of the attitude of the upperpart of the tool

Optimisation of the efficiency of the dredgingis performed after visual or sonar monitoring of the rejection at the exhaustlevel:

Tool depth small variation

water pressure and flow rate modification

Water pressure was increased from 1500 psi/100bar up to 2400 psi / 170 bar while flow rate increasing from 57 m3/h upto 75 m3/h.
 

VI- CLEANING OPERATION RESULTS

Dimensions of the trench 40 m long, 6 m wideand 1 to 1.2 m deep.

Estimation of the quantity of mud removed :1000 m3

Volume dredged : 250 m3

Dredging Operation duration : 20 hours