Seabed Intervention with Motion Compensated Grab
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
H.R. Maliepaard, J.C.L. Frumau
Abstract: On the White Rose Oilfield, off the coast of Newfoundland, Canada artificial recesses called "glory holes" had to be dredged. Boskalis was awarded the dredging contract in 2001. The location of the work site, combined with adverse environmental conditions, the narrow tolerances by which the glory holes were to be realized and the nature of the soil provided a serious technical challenge.
In 1999 Glory holes were dredged on the Terra Nova field with Trailing Suction Hopper Dredger (TSHD) "Queen of the Netherlands". In 2001 all TSHD had a high occupation rate. This led to development of the GES as a revisit of the 1996 grab dredger idea. Production rates of the GES had to equal the productions of a TSHD in similar conditions.
The GES concept consists of a lifting system with 2 main hoisting wires and a ROV controlled clamshell. The control system of the GES is an innovative composition of winch control, touch down control, clamshell control and motion control. The complete system can be operated in three modes: individual, combined winch and combined winch and thruster mode. The combined winch and thruster mode is used for normal dredging activities. In this mode the complete GES is controlled via 1 joystick and the positioning of the grab on the bottom is fully automated.
Especially when digging the Northern Glory hole the system showed it production capabilities. In 18 days the glory hole was dredged and conditioned. The realized levelness was within 30 cm Conclusion: The GES is an accurate, efficient offshore dredging tool with production rates in line with those of a TSHD. The system's automated positioning capability with full motion compensation allows for accurate dredging at depths over 100m, in a maximum seastate of 3m Hs.
Keywords: ROV, Grab, Heave compensation, Motion compensation, Excavation, Offshore dredging