Remote power generation for deployment of subsea technologies in deepwater marginal fields
Christian A. Cermelli, Dominique G. Roddier, Alexia Aubault - Marine Innovation & Technology
DOT International presents the technology that manages industry risks, resources, and rewards
The Deep Offshore International Conference & Exhibition (DOT), to be held Feb. 12-14 in Houston at the George R. Brown Convention Center, will feature a program of more than 90 presentations on deepwater E&P technology.
ExxonMobil hosts this event, the 20th DOT, and provides the keynote speaker and a number of technical papers and presentations. The conference theme, “Risks, Resources & Rewards,” captures three key challenges facing the deepwater industry – safely and successfully managing risks, completing projects on time and under budget with limited resources, and assuring profitability in the face of increasing costs and difficult operating environments.
More than ever, the industry relies on innovation in technology to unlock deepwater assets. New and improved technology, much of which will be presented at DOT in Houston, must provide the capabilities to succeed in the face of challenging environments and operating conditions.
This DOT Preview presents selected papers from each of the DOT tracks and serves as a sampling from the more than 90 presentations that make up the conference program.
This paper investigates a number of novel subsea technologies that the industry recently implemented or is expected to implement in the near future which allow production from remote marginal fields in deep and ultra deepwater. Of particular interest is remote power generation for such operations.
Novel subsea technologies have evolved significantly in recent years with improved functionality, reliability, and water depth capability. These include subsea separation, multiphase pumping, electrically heated flow lines, artificial lift with subsea electric pumps, and subsea gas boosters, among others.
Minifloat-III 4MW and 30,000 b/d water-injection facilities in 7,000 ft water depth
Power requirements for such subsea applications are typically large, ranging from hundreds of kilowatts to tens of megawatts. Conventional power supply comes from a host platform or from shore. For marginal deepwater assets, however, the cost of bringing power by subsea cable from afar may make it uneconomical. Therefore, a workable alternative would be the availability of a minimal floating facility with sufficient payload capacity to generate power for subsea equipment below.
Fundamental characteristics of such a minimal floater are:
- It must not be normally manned, so that exposure to offshore personnel is reduced
- It must be accessible by boat and by helicopter for inspection or maintenance
- It does not require active ballast
- Its availability must be sufficient to achieve the required production targets
- Its motion response must be as good as that of existing floaters, so that topsides equipment does not require special provisions or additional testing
- It must be able to remain operable in a 10-year winter storm, and to support steel catenary risers (SCRs) or umbilicals
- It can be relocated easily, since marginal fields will typically not produce for many years
- Its cost must be sufficiently low to meet economic targets.
Such a minimal floater – dubbed the Minifloat – has been developed to satisfy these requirements and also enables application of key subsea technologies by offering the ability to produce power locally and to provide additional payload for functionalities such as chemical injection, telecommunication, fuel storage, controls, etc.
A feasibility study by the authors involves a minimal floating platform with a 4-MW power generation requirement and the ability to treat and injection into the field some 30,000 b/d of water via three SCRs. Alternatively, an electrical cable could be provided to supply power to subsea facilities. In this case, the equipment size would be reduced by 200 tons (181 metric tons).
The Minifloat concept is a three-column semisubmersible floater with a horizontal water-entrapment plate extending from the base of each column. The design was conceived to provide a stable support for small offshore facilities, and the water plates assure excellent motion characteristics.
Several feasibility studies were conducted to assess the platform’s potential to carry various payloads. The smallest has 50 tons (45 metric tons) payload and provides power to subsea instruments in the Southern Ocean ultra deepwater. The platform, namedMinifloat-I, is less than 200 tons (181 metric tons) in displacement. On the other side of the payload spectrum, the concept was shown to be attractive to support a 40,000-b/d full oil process facility with 2,000 tons (1,814 metric tons) of equipment for the Gulf of Mexico. Displacement of the platform, labeled Minifloat-IV, is 14,000 tons (12,701 metric tons). For larger payloads, the conventional or deep-draft semisubmersible concepts are viable and more economical.
One of the key features of the Minifloat is that it can be fully integrated and pre-commissioned at quayside. Both the deck and the hull may be fabricated on the US Gulf Coast since the tow from a Far East shipyard would eat up the savings presented by cheaper fabrication there.
