Glory Hole Excavation; Experience with a Trailing Suction Hopper Dredger
Authors: Grundlehner G.J. , Smits A.P. 
Published: October 2008

A full account is given of the excavation of five 10 meters deep glory holes in the Terra Nova offshore oil field, at an initial water depth of 96 meters, using Boskalis' jumbo size trailing suction hopper dredger Queen of the Netherlands. Theexcavation method is discussed in the light of the project requirements, seabed properties, environmental conditions and alternative methods. The project preparation is addressed, including dredging procedure, suction pipe configuration, engineering analysis and monitoring program. The paper describes the final project execution, assisted by operation support models, the dredger's tracking system and the 3-D seabed surveys. The predicted suction pipe response is compared with full-scale measurement data, acquired during the execution of the project. The achieved excavation performance is evaluated against the available geo-technical data, environmental conditions and working procedure. The paper concludes with an evaluation of the capabilities of the trailing suction hopper dredger method for these types of excavation projects.

Introduction

The Terra Nova offshore oil field, as under development by the Terra Nova Alliance, is located 200 nautical miles South East of St John's, Newfoundland, in an average water depth of 96 meters. The field is part of the Grand Banks, an area that is scoured by icebergs. The development scheme includes a number of subsea wellheads. In order to protect the wellheads against potential iceberg damage, they are accommodated in artificial depressions in the seabed, the so-called 'glory holes'.

Early 1999 the Terra Nova Alliance member Coflexip Stena Offshore Newfoundland Ltd., commissioned Boskalis Offshore bv to excavate four glory holes in the summer period of 1999 with the jumbo size trailing suction hopper dredger (TSHD) Queen of the Netherlands. The excavation work was executed under the responsibility of Boskalis Offshore bv, assisted by the various Boskalis group units, among which Paragon International, for engineering and operational support on the suction pipe.

This paper addresses the various technical challenges encountered during the project and the solutions found. It demonstrates the applicability of state-of-the-art dredging technology to meet offshore engineering standards under difficult environmental and geo-technical conditions.

Glory hole specification

The original project scope comprised the excavation of 4 glory holes. During the course of the project, the scope was extended by a 5th glory hole (Figs. 1 and 2, Table 1). The total theoretical excavation volume for the five glory holes is just over 200,000 m3. In the template area the required vertical tolerance is +00 / -50 cm.

Geo-technical situation

Various sampling data were available to assess the soil conditions. As the seabed of the Grand Banks is (highly inhomogeneous) glacial till, it was difficult to make consistent predictions of the excavation performance. In particular, the hardness and thickness of the cemented deposits (hard pan) and the amount and size of boulders was a question mark. Three strata were recognized in the available soil data:a top layer of loose to medium dense sanda second layer of dense to very dense sand and hard pan