The Heidrun Field - Marine Operations

ABSTRACT The Heidrun Field development concept required an exceptional number of inshore and offshore marine operations to assemble the final product, Contained within the marine operations program were a number of innovations and advancements, such as subsea installation of heavy structures without the use of cranes, pre-installation of tethers, simultaneous leek-off of a TLP, and execution of pipeline bundle installation with a record tow length and water depth. In addition, the size of the structures, the remote field location, and a diverless philosophy in relatively deep water (345m) also contributed to making theconduct of successful operations a challenge. This paper describe the major offshore marine operations, including the tether foundation, tether, and TLP installations, as well as those for the export pipeline and direct shuttle loading systems, Highlights of the TLP template, water injection system, and TLP inshore operations are also briefly mentioned. INTRODUCTION Development of the Heidrun Field required an extensive effort in the area of marine operations. The operations took place over three years, commencing with TLP pre-drilling template installation in August of 1992 and finishing with TLP installation in July 1995. The most intense period of operations was the summer of 1994. The total marine operations program comprised more than 130 main events (Table 1). Supporting the operations were design, planning, contracting, and verification/testing activities, all coordinated by a Conoco Norway, Inc. (CNI) Project marine group which. at its peak, consisted of no more than 15 people. The operations took place inshore, near-shore, and offshore. A total of approximately 60 vessels of all types contributed, through seven main contractors and numerous subcontractors. During the busy 1994 season, up to 14 vessels performing under several contracts were present simultaneously at Heidrun. A clear system of priority and field control was set up by CNI to ensure that vessel clashes and contractor downtime were avoided. Throughout the operations, safety of personnel and equipment was given prime focus, with the result that only two Lost Workday Cases, both of a minor nature, were experienced in three years of offshore marine work. This paper presents a brief technical overview of the majoroffshore operations, including a description of success factors and difficulties encountered. A layout of the Heidrun Field is contained in Figure 1. TETHER FOUNDATIONS The four tether foundations were installed in July 1994. Their main particulars are given in Table 2, and their general arrangement is described in Reference 1 and Figure 2. The installation of the tether foundations represented a significant breakthrough for installing large structures without the use of expensive heavy lift vessels. Each foundation unit, weighing over 20,000 tonnes in air. was wet-towed from the construction site at Hanoytangen near Bergen to Heidrun Field where the main installation spread awaited. After initial surface hookup and ballasting through the waterline, each foundation was lowered to near seabed by controlled sluicing of water.