This paper presents experience of subsea and marine systems gained from the first four years of production from the Ivanhoe/Rob Roy fields focusing particularly on those areas which are different from a fixed platform development. As an introduction the overall field layout is described and the success of the development is demonstrated by summarising availability of the facilities. The paper then describes the field development work since first oil, together with the inspection, repair and re-design work that have been necessary to ensure this success. The description includes work on all facilities on the sea bed, work on marine systems, and subsea well servicing. If thus includes the hull, stability and damage stability, bilge and ballast systems, subsea Christmas trees, moorings, flexible risers, flowlines and jumpers, valves, chokes, electro-hydraulic control system, umbilicals, downhole gauges, etc. The information can be used to lead towards more cost effective developments in the future. Introduction Ivanhoe/Rob Roy Fields The Ivanhoe/Rob Roy fields came on stream in July 1989 and have now been producing successfully for nearly four years. Typical production rates are in the region of 70,000 bpd. The fields are located in the UK sector Block 15/21a approximately 110 miles north east of Aberdeen in 140m of water. The layout of the field facilities is shown in Figure 1. From the centrally located Floating Production Facility, designated AH001, a flexible riser system (Fig. 2) connected to a Riser Base Manifold (RBM) conveys fluids up and down to the sea bed. From the RBM oil and gas export lines go to the Claymore and Tartan platforms respectively and infield flexible flowlines travel approximately 1.6 kms to the Ivanhoe and Rob Roy manifolds. There are a total of 16 subsea wells clustered around the manifold; Rob Roy manifold has 6 producers and 3 injectors (Fig. 3) and the Ivanhoe manifold has 4 producers and 3 injectors (Fig. 4). The electro-hydraulic control system consists of a dynamic umbilical at the aft end of the AH001, splitting three ways to distribution units and control modules on each manifold (Fig. 5). The control system also recovers data from a range of pressure and temperature gauges. The vessel is moored on station by a twelve point multi-component system (Fig 6), with the position being monitored by an acoustic system and also by reference to line lengths and tensions. The vessel draught, trim and stability are altered and maintained by a bilge and ballast system. P. 131^
This paper describes the Ivanhoe/Rob Roy dynamic flexible riser system. The design requirements and considerations are summarised and a general description of the system is given. The paper then concentrates on giving details of the main components involved, thus presenting information on various aspects which are needed to turn theoretical knowledge and techniques into a viable system.
TfdS paper was selected for presemtatlonby the OTC Program Committee followingreview of informationcontainedin an absIract submittedby the auihor(s). Contenis of the P*er, w prosonted,have not bee" reviewed by the Offshore TechnologyConference and am subject to mrrectlon by the author(s).The material, = presented, does not necessarilyref!eã ny positionof the OffshoreTechnologyConference or Its officers.Permissionto copy1srestrictedto an abstractof not more than 3W words. mustrations maY WJlbe u@ed. me abstract should contain$msplcuous acknow!edgmantof where and by whom the papar Is presented. ABSTRACTFive years of operating experience with a floating production system mooring are described. Irr-service chain diameter measurements and cathodic protection levels are described.Detaiied examination of removed components shows that wire ropes in a permanent mooring system degrade differently from those on a mobile unit and that careful consideration should be given to minimisirrg installation damage. Break testing of 84mm ground wires after five years service reveals a loss of strength of only 0.1% to 0.2% per year. Strength reductions of 1.5% to 2% per year are noted for the winch wires. In this case, the fairleader sections of the winch wires have suffered less damage than the winch sections. Chain breaking strength data is presented showing that used f02mm Grade 3 chain from the mooring consistently sustained loads some 13% higher than the catalogue breaking strength of new chain. References,nomenclature, Iables and figures al end of paperBecause of the large number of lines, the 100 year return environmental design basis and the high factors of safety, the Af-fOOl mooring system is arguably one of the most robust currently in operation in the North Sea. This paper brings together the original design assumptions for the mooring components, the subsea inspection data, and the results from the testing of removed components. Operational HistoryMooring system performance has generally been good. In particular, the vessel excursion and line tensions are lower than predicted. However, although no line failures have resulted from overstressing, fatigue damage, or corrosion, the AHOOfhas lost a mooring line on two separate occasions.On both occasions, approval was gained from the Certifying Authority to resume and continue production operations with revised operational criteria appropriate to an eleven line system. On neither occasion was the revised operational criteria exceeded.The first incident was due to failure of the bolted connection at the bottom bearing mounting on the line 1 fairleader. The fairleader then detached from the column foundation structure and parted the winch wire.Extensive investigations were conducted to determine the cause of failure, and to assure the integrity of all other components involved. It was concluded that the failure was due to the original bolts being of inadequate section and ductility. The integrity of the mooring system was restored to design levels by correcting these defects.The...
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