In offshore construction work related, to platforms, one of the difficult operations has been the connection of add-on sections to pin piles as they are driven into the sea floor. The reason for this difficulty has been the relative motion of the derrick barge and the fixed platform. This paper describes the design of and the application of a pile aligning system that makes this operation much easier to accomplish. It emphasizes the design of the system and its field operation conducted in the Gulf of Mexico. It describes the design concepts, the actual hardware used to implement these concepts, and the successful operation of this hardware on the actual job site.
Steel platforms are conventionally pinned to the ocean floor with large diameter piling, either driven through the legs of the platform and/or through guides around the base of the platform. When platforms are set in appreciable water depth it is not feasible to handle the pipe in one section and it becomes necessary to add-on additional sections as the pile is advanced to the ocean floor and then driven into the ocean floor. The platform itself is grounded to the ocean floor, whereas the derrick associated with a derrick barge for handling the pile add-on sections is floating on the ocean surface. Consequently, there is relative motion between the section of piling to be added-on and the pile that has already been installed in the leg or guides around the platform itself. The amount of motion is, of course, a function of the wave action or sea state on the derrick barge. When the weather becomes a factor, it becomes a common practice to wait for calm seas even though the operation may be costing upwards of $50,000 per day to stand by.
Piling commonly used today is either 48" diameter or 54" diameter, with some piling over 90" in diameter. The accepted method of joining the add-on sections to the pile is by welding. Because this welding must be performed in the field, the connection time can approach three to twelve hours per connection, or longer depending on wall thickness. A time saving solution has been to use mechanical type connectors which theoretically eliminate the welding time and pay for themselves even though they may never be recovered. Experience to date, on the other hand, has proven that the motion described above makes it virtually impossible to align the connectors and successfully connect them. Sometimes the operation can take longer than welding would have in the first place; consequently, there has been a reluctance to use these connectors.
Finally, in the case of pile that is driven' through guides around the base of the platform (skirt piles), it is sometimes desirable to cut off the piles at the base of the platform and to remove the upper section of the pile since it is not needed to improve the stiffness of the jacket and thus considerable savings can result.