Abstract
Over the years, offshore decommissioning and abandonment has matured and become a considerable part of the portfolio of many offshore contractors. Nevertheless, there is a continuous effort to find innovative methods to create more economical and sustainable ways of working. One of the areas of development is the optimization of use of available equipment and crane capacity.
About a decade ago, Heerema Marine Contractors introduced an unconventional method to remove and transport large jackets. The jackets are lifted as one single piece and transported to the recycling yard whilst being suspended from both cranes of one of Heerema's Semi-Submersible Crane Vessels. Two purpose built gripper structures at the stern restrain the jackets from horizontal motions during transport. Benefits of this robust method are a shorter offshore project duration due to a reduced subsea cutting scope, and substantial cost savings due to reduction of subsea lifting tool and marine spread requirements. Besides this, safety of the operation is significantly improved due to fewer offshore activities and by preventing back loading operations to a barge at open sea.
A downside is that each jacket is different, which implies that adjustments have to be made to the gripper structures. Removal of topsides introduces additional complications, because structural strong points to attach the grippers are not always easily accessible due to protruding platforms and walkways. In an effort to find solutions to these challenges, Heerema investigated the feasibility of omitting the restraints when transporting the platform component to the recycle yard. This idea sounds simple and has been done before. However, it has a lot of implications. Opposed to a load which is horizontally restrained, a load freely suspended in the cranes significantly influences the vessel's motion behaviour, which makes the transportation more weather sensitive. To ensure the loads stay within the capacity of the cranes, numerous hydrodynamic calculations and simulations were performed to investigate the effect of expected environmental conditions on the behaviour of vessel and load.
The result is an effective removal method which is predictable, safe and time efficient. The method has been successfully applied for the removal of a number of structures (weighing up to 7000 mT) in the North Sea. Moreover, it has proved to offer substantial cost savings for installation of new-built structures by lifting straight from the quay of the fabrication yard and transportation to site suspended in the crane, as such eliminating the need for a barge transport.
