This paper describes the procedures used to design bumper piles for the docking installation of a large North Sea platform jacket over a subsea drilling template. Model tests are described and their response data presented and interpreted for design use. Procedures are presented for evaluating ultimate load and energy absorbing capacities of the docking piles together with the jacket bottom horizontal framing. Conclusions are drawn and observations offered concerning analytical methods for determining docking pile design loads, and the feasibility of mating large platform assemblies offshore.
In late September, 1981, Amoco successfully docked and installed its Northwest Hutton Platform jacket over a subsea drilling template. The maneuver was performed in 473 feet of water in the UK Sector of the North Sea and was the first operation of its kind in that hostile environment. The drilling template had been installed two years earlier as part of an early production system which permitted several wells to be pre-drilled (with a semi-submersible) while the jacket was being designed and fabricated. Seven of the twenty available well slots had been drilled at the time the jacket was installed and precise mating of the two structures was required to facilitate tieback of these wells to the surface.
The docking maneuver utilized a mooring line system integrated with the jacket to provide a controlled horizontal approach to the template (see Figure 1). Following a conventional launch and upending operation, the jacket was ballasted to rest on the seafloor near the template. A winch and control package was installed atop the jacket and the mooring lines were deployed and pre-tensioned in preparation for the docking approach. Under favorable weather conditions the structure was deballasted about 9 feet off bottom and slowly winched toward the template with the aid of sophisticated position and motion monitoring systems, until it was finally positioned against two bumper piles which had been previously installed near the template. The jacket was securely latched onto the piles and then ballasted to the seafloor to conclude the docking maneuver. Detailed presentations of the docking installation operation and the position monitoring system are contained in References 1 and 2, respectively. The motion monitoring system, which evaluated the jacket dynamic response to ensure that operating limits would not be exceeded during the docking operation, is described in Reference 3.
The bumper piles functioned to protect the template and its wells from possible impact damage during docking, as well as to help orientate the two structures correctly. As the moored jacket approached the template, it responded to the surface wave action with rigid body motions. The surge and sway components of this motion at the bottom of the jacket resulted in random impact loads to the bumper piles. The piles were located adjacent, but not rigidly connected, to the template so as to avoid any direct load transfer (see Figure 1) and had to be designed to absorb the impact loads without damage to themselves or the jacket horizontal mudline bracing.
