Offshore site investigation byrotary drilling from a diving bellby H. C. van de GRAAF and A. P. SMITS'

DIVING BELL for carrying out sea-bottom site investigations has been in al-most continuous use since 1976 whenMISSION I (Manned Installation for Sub-sea Soil Investigations'.') was developedand built as a joint venture of Vriens Div-ing Company and Delft Soil MechanicsLaboratory for use on the Eastern Scheldtstorm surge barrier. MISSION I was usedinitially for the preconstruction site inves-tigation and is now doing the control testsfor the soil improvement work. It is fittedwith a 60 tonne hydraulic jack and doescone penetration tests, 66mm continuoussoil sampling, in-situ density measw'e-ments, etc.

The success of MISSION I led to theconstruction of a larger diving bell, suit-able for offshore site investigations andincluding the capability for doing rotaryand percussive boring. Although equippedand used for cone penetration testing, etc.,the award of a contract in April 1981 fora project in Newfoundland provided theopportunity to install rotary drilling equip-ment.

The Canadian I ower Churchill Develop-ment Corporation (LCDC) intends to con-struct a sea-bed high voltage electricitypower cable from Labrador to Newfound-land across the floor of the 18km wideStrait of Belle Isle. The electricity cablemust be laid in a trench on the sea bed sothat it is protected from the bottom scourof the numerous icebergs that passthrough the Strait. The sea bottom strataconsisted of overburden (sand, gravel,cobbles and boulders) above bedrock(limestone, dolomite and sandstone).

Water depths extended to about 100m.There are strong tidal currents up to about4 knots; even in summer, the seas are

*Delft Soil Mechanics Laboratory, Stieltiesweg 2,PO Box 69, 2600 AB Delft, Holland

frequently rough and on occasions thework must be interrupted in order to getout of the way of icebergs.

Boreholes are required in order to inves-tigate the route and design of the trenchfor the sea-bed electricity cable. In 1976,a borehole investigation, carried out from aconventional drill ship, failed to give satis-factory results.

In these difficult conditions, it was con-sidered that only a submersible drillingsystem was capable of giving useful bore-hole results. In April 1981, LCDC award-ed a contract to Beaver Dredging for seis-mic and other surveys, together with mar-ine boreholes in the overburden and rock.A Dutch consortium, comprising Osiris-Cesco (project management), ACZ Mar-ine Engineering 8t Construction (work ves-sel), Smit-Vriens Offshore Diving andDelft Soil Mechanics Laboratory was form-ed in order to do the marine borings.

MISSION II, consisting of a diving belland heavy base plate, was fitted with drill-ing equipment. The diving system wassupported by the multi-purpose stonedumping vessel MV Rocky Giant ownedby ACZ and especially modified for thisjob. The Rocky Giant was kept on locationby means of a dynamic positioning systemwith an optional back-up system of con-ventional anchors.

The drilling equipment comprised adrilling mast for operating wire-line equip-ment with 1m long rods and tools, hydrau-lically-driven power swivels and a winchfor running the wire-line tools. One of twoalternative power swivels could be mounted on the mast —either a Wirth B1A forrotary drilling or a Klemm drifter 0-4025 forrotary percussive drilling, HQ, NQ and BQsize triple tube core barrels and rods wereprovided, together with PW, HW, NW andBW size casings.

For drilling through overburden, the Nas-

sovia NSK wire-line rotary barrel was pro-vided. This is a triple tube core barrel withalternative inner barrels —a plain barrelfor coring in rock and a barrel with corecatcher for sampling in overburden. Fordrilling by rotary methods through obstruc-tions, a wire-line full hole bit was available.The dimensions of this equipment aregiven in Table I.

TABLE I.

Casing (mm) Core barrel (mm)Size OD ID Hole size Core size

NSK 150.0 105.0PW 139.7 127.0HW 114.3 101.6HQ 96.0 61.1NW 88.9 76.2NQ

BW 73.0 60.3BQ

75.5 45.0

The drilling equipment was operated fromwithin the diving bell by a crew of twodivers, who had been trained and laterwere directed and instructed, by an exper-ienced rotary drilling supervisor. The div-ing bell was operated through the moonpool, set near the centre of the vessel,Rocky Giant.

First, the base plate was lowered to thesea-bed at the borehole location. Next, thediving bell, together with the divers, waslowered to the base plate guided by thehoisting wires of the base plate, whichwere kept under tension by the heavecompensators. After making adjustmentsto set the drill rig and diving bell trulyvertical —sea-bottom slopes up to 20'anbe coped with —the bell was clampedonto the base plate and drilling couldcommence.

a>el

xx '"

The Rocky Giant vessel from which the diving bell was operated, through a 7m x 7m moon-pool

18 Ground Engineering

i

ve; tZ

l

Il~Drill mast—

Power swwel

Diving bell

~ %~ hRa~

I

'

1~Base plate

"@+7%.Ih"-'.

(Above and right) MISSION II diving bell employed on the Straitof Belle Is/e sea-bed investigation. (Below) View within thediving bell, showing drill head and rods inside the 1.85m dia.(internal f operating chamber

The two divers, who operated the drillequipment, were instructed and directedby geotechnical engineers and the drillsupervisor over a telephone and closed-circuit television link.

The divers lived and worked under sat-uration diving conditions using an atmos-phere of helium. There was pressurisedliving accommodation on deck which couldbe locked onto the diving bell. After ap-proximately eight operational hours, thebell returned to the surface and the crewwas changed. Using this procedure, sixdivers under saturation could achieve around-the-clock operation.

Water depths were mostly between 55mand 105m, with one location only 20m. At17 borehole locations, a total of almost90m was drilled, out of which 46m wasin overburden and 42m in rock. The aver-age drilled in rock was 2.5m, while over-burden thickness varied from 0.4m to morethan 6.5m. At each location, at least onestandard penetration test was performed.The borings were carried out in less than25 operational days and there was hardlyany down-time due to weather. On twooccasions a borehole was abandoned whenthe Rocky Giant had to be moved in orderto get out of the way of icebergs. Verygood samples of rock and overburdenwere recovered using mostly the HQ, NQand NSK equipment.

In addition to the borehole proceduresdescribed in this article and the cone pene-tration testing and continuous samplingalready undertaken on other jobs, thediving bell is also equipped for vane test-ing, permeability testing and pressure met-er testing.

The MISSION II system, with its pro-vision of borehole procedures, as well as60 tonnes of thrust for static penetrationprocedures, can perform almost any typeof sampling or testing in either soil or rock.It has now become the deep water siteinvestigation system that can succeedwhen more conventional methods havefailed.

Referencesf. Wakeling, T. R. M. (Kroenena M.) (19BO): "The

Mission system; site investigation from a div-ing bell" Offshore Services 7, July.

2. Vermeiden, J. (1977): "The submersible work-ing chamber". Delft Soil Mechanics Laboratory.

January, 1983 19