Subsea gas transition hubs

Abstract

Gassco is the operator of two platforms with gas transport functions. These platforms are getting old and require large modifications to extend the lifetime and maintain their functions. Rapid development in subsea technology the recent years enables functions that earlier where performed by offshore platforms to be converted into subsea systems. Subsea developments offer the potential of reduced CAPEX, OPEX and risk reduction in terms of HSE. This makes subsea developments an attractive alternative to conventional topside developments. As a case study, this thesis evaluates the challenges and opportunities related to moving the functions of Heimdal Riser Platform into a subsea system. The requirements of a mid/downstream operator such as Gassco have not been widely evaluated by the subsea industry. A subsea gas transition hub is fundamentally different from other subsea developments due to the fact that there is no production involved. Subsea production systems have an increasing demand for power due to their complexity. A subsea gas transition hub is simpler and the power demand is low compared. With no nearby topside host facilities, there are challenges with respect to the subsea power supply and communication. Traditionally these requirements are provided by an umbilical which are tied-back to a host platform. Considering the low power demand and the long offsets to nearby host facilities, a long and costly umbilical may be hard to justify. Hence has the focus of this thesis been to eliminate the requirement of an umbilical. Many R&D projects, with the objective of reducing costs and risks related to umbilicals, have commenced the later years. Although many of the projects have been successful, the umbilical maintains as the only option to meet subsea production systems requirements. In this thesis, based on earlier studies, alternative solutions for power supply and communication have been evaluated. All the equipment that are required to maintain the gas transport functions will be incorporated within a 230 tons subsea manifold(excluding protection structure). This includes a remotely operated subsea flow control valve, a subsea High Integrity Pressure Protection System (HIPPS) and a subsea pig launcher guiding base. The structure also incorporate isolation valves which facilitate the opportunity to retrieve the HIPPS and flow control modules if maintenance is required. The subsea control system is based on the All-Electric technology which eliminates the requirement of hydraulics for valve actuation. The control system is powered by a rechargeable Lithium-Ion battery package which requires periodic recharging of energy from an intervention vessel. Communication between the subsea system and the master control station is provided by a connection to the integrated subsea fibre network in the North Sea. This subsea concept implements technologies that have not been widely used by the industry, hence a qualification program must be initiated before a fully functional subsea gas transition hub is ready for installation.