Trench Dredging and Backfilling for
the 'Mumbai High Fields - Uran Trunkline Project'

The 'Mumbai High Fields - Uran Trunkline Project' in India, that is carried out on behalf of the government owned Oil and Natural Gas Corporation (ONGC), is currently the biggest pipeline project in the Asia-Pacific region.


      Onshore excavation

The project involves laying two subsea pipelines of 204 km in length each, one a 30 inch for oil and the other a 28 inch gas line, to replace the ageing trunk lines at the Mumbai High offshore field in the Arabian Sea.
The existing pipeline system was laid in 1978 and after more than 25 years of successful operation has surpassed its design life.
The system is used to transport crude oil and natural gas that is produced from various offshore platforms at the Mumbai High fields to the Uran Onshore Terminal.

In 2004, Hyundai Heavy Industries Co. Ltd. (HHI) has been awarded the contract to carry out this $600 million project.
Subsequently HHI has subcontracted the pre-trenching, backfilling and landfall works that are part of this project to Boskalis Westminster Middle East Ltd.

The scope of work for Boskalis Westminster Middle East Ltd. consists of:
-Dredging the trench (pre-trenching) from the landfall point (LFP) to the Port Limit which is 19.5 km offshore.
-Dry excavation of the onshore / tidal section of the trench.
-Dredging of a Lay-Barge Access Area.
-Maintenance of the trench and Lay-Barge Access Area during the period in between the completion of the pre-trenching and the start of the laying of the first pipeline.
-Backfilling of the trench from the landfall point to the Port Limit.
-Backfilling of the trench in the onshore / tidal section with dry equipment.
-Backfilling of the Lay-Barge Access Area.


The backhoe dredger 'Colbart'

Soil conditions in this area vary between very soft silty clay to weathered basalt.
In the case of hard material a cover on top of the pipeline is required of 1 m and in the case of soft material a cover is required of 3 m.
A trench has to be realized with a width of 8 m in the section near the shore and 10 m further offshore.
In order to allow the Lay-Barge from HHI to approach the shore close enough to realize the beach-pull, an access channel had to be dredged from 1.6 km offshore to 3.0 km offshore with a width of 60 m and in which a water depth of 4 m below chart datum was guaranteed.
For the dredging operations Boskalis has mobilized the backhoe dredger 'Colbart' from Abu Dhabi and the trailing suction hopper dredgers 'Flevo' and 'Seaway' respectively from Portugal and Singapore.
The 'Colbart' was deployed to dredge the trench in the section close to the shore, where the water depth limited the activities of the hopper dredgers, and sections where hard material was encountered.
To pre-handle sections with hard material, the 'Colbart' was fitted out with a S35 hydrohammer.
The 2,130 m3 hopper dredger 'Flevo' was used to dredge in the shallow parts of the trench, while the larger 13,255 m3 'Seaway' operated in the offshore section.
The period between November and May, the dry season, was selected to carry out these operations. From May till September the Monsoon dominates the weather and sea conditions in this region make maritime operations practically impossible.


      The hopper dredger 'Flevo'

Operations started early November 2004 with two land-based excavators excavating the trench from the landfall point to approximately 200 m offshore.
By the end of November the 'Colbart' started dredging and after having removed all the dredgeable material the bucket was replaced by the hydrohammer and for 2.5 weeks the 'Colbart' hammered the remaining material consisting of layers of hard basalt with a thickness of up to 1 m.
Using hammer patterns based on previous experiences all the material was successfully broken after which all the remaining rocks could be dredged with the bucket.

The 'Flevo' started dredging early January, in water as shallow as 4 m. In less than 3 weeks, approximately 300,000 m3 was dredged and deposited in a Temporary Storage Area at 1 km distance from the trench. Choice for the location was based on optimal cycle times and guaranteeing that no material would flow back into the dredged trench.
Halfway January the 'Seaway' also started dredging and in 3.5 weeks removed approximately 1.5 million m3 from the offshore part of the trench. The material dredged by the 'Seaway' was also temporarily stored in an area at 1 km distance from the trench.


The 'Colbart' seen from the 'Seaway'

With the trench completed, HHI started the beach-pull with the Lay-Barge 'HD289' on 4 February 2005 and three weeks later the land-based equipment commenced the backfilling of the trench.
Subsequently the 'Colbart' started backfilling the section of the trench near the shore.
Currently (March 2005) the 'HD289' is still laying the pipeline and is expected to finish its operations within the Port Limits early April 2005.
The 'Flevo' is expected to start backfilling operations shortly, while the 'Seaway' has to remain standby until most of the pipeline has been laid.
The entire project is expected to be completed before the start of the monsoon season in May 2005.

- Source: Royal Boskalis Westminster, 1 March 2005 -