Subsea hot tapping of pipelines is performed for a variety of reasons, including tie-ins, pipeline repair, insertion of instrumentation, facilitating chemical injection or providing access for temporary isolation tools.
The full hot tap process — that is, installing the hot tap assembly, performing the tap and recovering the hot tap machine — is normally conducted with diver assistance. After bolting the assembly of the machine, isolation valve and fitting to the pipeline (or machine and isolation valve to a pre-installed flanged membrane on the pipeline), the divers then operate the machine to perform the tap, under instructions from — and supervision — by hot tap technicians located on deck of the diving support vessel (DSV). Subsequent unbolting and removal of the hot tap machine is also carried out by the divers.
The demands of deep water have necessitated development of a totally diver-less, remote-controlled system. Diver operations are limited to a maximum of 300 meters of water depth, whereas a significant portion of existing subsea field infrastructure, as well as projected future developments, are in deeper waters in depths up to 3,000 meters. In addition, diver safety concerns in shallow water, as well as impaired diver efficiency in difficult environmental conditions such as wave breaking zones, prompts the call for a reduction of diver exposure or complete elimination of diver assistance.
The recent completion of a remote-controlled hot tap machine (Subsea 1200RC) is an important step toward developing a totally diver-less system. The installation of the hot tap assembly and subsequent removal of the machine still require diver assistance, but the performance of the tap itself is remotely controlled by a hot tap technician from the deck of the DSV. The concept is a topside-driven hot tap machine with “passive Remotely Operated Vehicle (ROV) interface”, which means a stationary ROV with its hydraulics and control system is attached to the hot tap machine and operated from an onboard laptop. This results in a light weight hot tap frame and total direct control of the cutting process. The machine has been designed, built, tested and successfully deployed on a recent subsea tap for a pipeline operator in Asia.
This technology promotes the “separation of man and machine” proposition. It reduces risk by reducing diver exposure, enhances safety, provides direct control and visibility from a laptop and facilitates fast and accurate execution. Ultimately, the concept may be extended toward onshore hot tap applications in risky environments calling for remotely operated systems. Diverless tapping is now also qualified and offered by others.