Dredging and Backfill Operations in a Hostile Environment

Category:

Production Monitoring

Description:

The Boston Rivers Project involved the construction of 55 diffusers on the ocean bottom, 10

miles offshore in Massachusetts Bay in 110 ft of water. The $93-million project for the MWRA

was undertaken as a joint venture of J.M. Cashman, a marine contractor from Massachusetts, and

Interbeton, a subsidiary of the Dutch HBG Group.

The project consisted of dredging, boring of vertical shafts, installation of risers and precast

diffusers, and armor rock placement. What is applicable to Corps dredging projects offshore is

beach restoration projects involving offshore borrow sources.

The work platform for the majority of the project construction was the Dredge "Lyons." The

Lyons is 120 ft in length, with a 55-ft beam, and a draft of 10 ft with 5 ft of freeboard. Stability

analysis of the dredge indicated that the rig could withstand a 3- to 4-degree roll from either

starboard or port, which was tested many times throughout the project duration. Stability from

bow to stern was adequate, provided the dredge was pointed into the direction of the wave attack.

The dredge was moored to the seafloor with a five-point anchor configuration.

A Project Installation Manual was used to provide strict quality control procedures for the

following:

1. ROV (remote observation vehicle) inspections.

2. Bathymetric surveys.

3. Dredging.

4. Protective dome movements.

5. Backfilling and landscaping.

6. Grout skirt placement.

7. Grouting of casting and skirts.

8. Underwater assembly of HPDEX domes.

9. Repairs to diffuser coating.

The project team monitored weather forecasts through faxes (commercial and public) to

continually modify schedules to maximize production.

Dredging was limited to sea conditions of less than 4 ft. The dredge used a computerized

positioning system, developed by SAIC, based on the Del Norte Range Range Microwave

system.

At the heart of the project was the heave compensation system. The TSS 320b heave

compensator was used. Bathymetric survey activities were central to dredging and backfill

material quality control. Bathymetric surveys were used to determine the existing seabed

elevation.

Reviewer:

J. Lally

Projects Completed:

Boston Risers Project

Location:

Massachusetts Harbor, Massachusetts

Project Contact:

Carlos Pena

Estimated Cost:

$93 million

Information Sources:

Literature:

Title:

"Dredging & Backfill Operations in a Hostile Environment"

Author:

Carlos Pena

Month:

Year:

1995

LitType:

Report/Proceedings

Publisher:

WEDA 16

Chapter 5 Other Technologies Reviewed