Five years ago when Port of Melbourne Corporation (PoMC), Australia was planning to deepen its main shipping channel, serious questions emerged: How to effectively remove hard rock from the entrance to the channel of Port Phillip Bay; how to ensure that more than 22 million cubic metres of sand and silt could be removed without significant environmental impacts on the nearby marine park and the Bay’s flora, fauna and Ramsar wetlands; and how to effectively communicate these works to sceptical stakeholders.
To add to the challenges, the entrance to Port Phillip Bay is one of the most turbulent and potentially dangerous stretches of water in the Bass Strait area, with an unusually hard seabed. Until 1986, the entrance was deepened using explosives. Current environmental regulations prohibit this approach because of the risk of rock damaging deep reef habitats. Normally, a large stationary cutter dredger would be used, but with the sea’s extreme turbulence and the frequency of shipping movements, a cutter was not the solution.
In May 2004, PoMC and Boskalis signed an Alliance Contract, which is especially useful on high-risk projects. In this type of contract, contractor and client go through the development phase together working with a ‘no blame-no claim attitude,’ striving for ‘win-win’ solutions, and thus avoiding many potential conflicts. At Melbourne, specialised equipment, dredging strategy and channel design details were thoroughly discussed by the Alliance Team, and cost-savings were realised that would have been very difficult in a traditional contract.
The ripper draghead
The Boskalis Research & Development, Central Technical and Dredging Departments, working as an interdisciplinary thinktank, found an innovative solution to deepen the entrance to Port Phillip Bay: A new ripper draghead, strong enough to cut through hard rock, with excellent suction characteristics, yet able to be mounted on a trailing suction hopper dredger (TSHD). Trailers are more flexible than cutters and can work in extremely severe weather conditions (Figure 1).
After extensive research and field tests, a ripper draghead mounted on the TSHD was designed (See Figure 2). It worked extremely well. All material was removed, even the strongly cemented edges of the rock formations, and the structural strength of the ripper draghead was not compromised.