TOGI Service Lines and Cable Installation: Concept Selection and Field Experience

ABSTRACT This paper presents the concept for the TOGI service lines and cable bundle used to provide hydraulic and electric power and chemical injection fluid for the TOGI Subsea Station. The paper also presents the fabrication and installation methods used and some of the technical challenges experienced during the fabrication and installation phases. The paper includes descriptions of MSV "Regalia" used as the laying vessel and the water jetting system CAPJET 500 used for trenching. INTRODUCTION The TOGI service lines and cable bundle consists of three 34 mm ID duplex steel service, and one 75 mm OD electrical cable. The length of the service lines and cable bundle is 48km equal to the distance between the Subsea Station at Troll and the Oseberg Field Center. The waterdepth at the end points is 303m and llOm, respectively. See Fig. 1. The four bundle elements were all prefabricated onshore in their full lengths and stored on three service line installation reels, and one carousel type basket for the cable. During the summer of 1990 the TOGI service lines and cable bundle were successfully installed between the TOGI Subsea Station and the Oseberg Field Centre. The laying and later trenching of the three duplex service lines and the electrical cable marked the completion of four years period of engineering, fabrication and installation efforts, starting with the conceptual engineering in mid 1986 and ending up with the fabrication and installation contract which in spring 1989 were awarded to Ugland Coflexip A/S (UCAS), a joint venture between Ugland Offshore and Coflexip Norge. The work included new installation methods and equipment not earlier used in the offshore pipeline industry. This paper is one in a series of six, which are presented at the OTC 1991. TOGI CONTROL BUNDLE CONCEPT The TOGI subsea wells are controled and monitored via a multiplexed electro-hydraulic system operated from the Oseberg A platform some 48 km away. The hydraulic supply and communication lines are gathered into a service line and cable bundle, consisting of three separate 34 mm ID duplex steel service lines and one 75 mm OD electrical cable, as shown in Fiq. 2. One of the service lines is used for chemical injection, and the other two for hydraulic pressure supply. The electrical power and signal cable consists of 12 pairs of conductor elements, where the control signals and power supply are transferred through common elements. The bundle concept was established in the basic engineering phase of the project, and the following principles were adopted for the specification of service lines:One dedicated high pressure line (345 bar) for the surface controlled subsurface safety valves (SCSSV's) and one separate medium pressure line (207 bar) for the X-mas tree valve functions.One chemical injection feeder line with distribution chokes to each tree.No hydraulic fluid return lines.No spare lines, neither for the chemical injection nor the hydraulic system.