Off take of oil from the Heidrun field is achieved through a Direct Shuttle Loading (DSL) system. This approach eliminates the need for an intermediate storage facility, allowing continuous production and transfer of oil directly from the Heidrun TLP to shuttle tankers. Purpose-built or appropriately converted tankers with an integral bow turret locate and connect to a Submerged Turret Loading (STL) buoy which functions both as a tanker mooring point and a termination for the flexible offloading line. The system is designed to permit the tankers to remain connected during loading and to disengage km the STL buy on completion of loading in all weather conditions up to and including the 100 year storm. This paper describe a implementation of the Heidrun DSL system from conception to first oil. It gives the background for choosing the DSL system and information on the data generated to support the selection process. Design, fabrication and installation of various components are explained to give an sight into the challenges that had to be overcome for realization of this 'first-of-its-kind' system in a record time of about one year Installation of the complete DSL system in the summer of 1994, approximately one year ahead of the original plans, mabled full scale in situ testing of the system with a purpose modified shuttle tanker. The two-month program provided the equivalent of one year of operational experience with the system before first oil. The paper addresses data obtained during the full scale testing, and comparison with analytical results. The operation of the Heidrun DSL system is also described, These data together with the experience gained during realization of this bold concept will give key information on how such a concept can be effectively applied to any major or marginal field development scenario either as an off take system or in conjunction with an FPSO/FSO. Introduction The DSL represented a method of offloading oil which required a bold decision for its implementation for a major development like Heidrun. When in summer of 1992 Conoco decided to use this concept for Heidrun, the STL technology was still in it a infancy and had not advanced beyond the preliminary design stage. Nevertheless, the idea was promising and offered potentially substantial cost savings when compared with other alternatives. However, the 'first-of-its-kind' and uninvolved nature of the STL technology, combined with the extremely tight schedule for its implementation, presented the Conoco Project Team with a formidable challenge. An STL system was used for the first time in October 1993, albeit in a permanent connection mode, for the UK Fulmar field. Some Fulmar experience was useful, including the marine operation aspects, in service data, etc. which led Conoca to adopt a modified approach to marine operations and very rigorous testing and trial programs for implementation of the Heidrun DSL system.
In recent years, in response to persistent low oil prices, the demand for affordab1e technology has led to a considerable effort intended to reduce the overall oil field development costs. One of the aspects of this search for innovation resulted in studies on the use of concrete for Tension Leg Platforms (TLP) hulls. These studies have evidenced a promising potential for an overall cost reduction for the installed platform. This paper presents a cost comparison of a series of TLP configurations, with steel and concrete hulls for potential application in the Gulf of Mexico. The paper includes a description of the sizing strategy, detail s of the configurations developed, global cost estimates and sensitivity of cost estimates to water depth and payload variations. 1.0 INTRODUCTION The unpredictable and dramatic reduction of oil prices in the mid eighties has forced operators and designers to explore innovative ways of developing oil fields at affordable cost. The need for cost reductions while moving into deeper waters or harsher environments presents an unusual set of challenges, which can only be met by developing alternative, unconventional strategies. The Tension Leg Platform (TLP) technology as a means of developing oil fields was first proven with the Hutton Field installation, in the UK sector of the North Sea. However, the search for cheaper alternatives has been leading designers towards variations to the basic early concepts. One notable result is the first Tension Leg Wellhead Platform (TLWP), just recently installed in the Gulf of Mexico, in a water depth of 1,760 (FT). Another promising direction forth is type of structura1 concept is the use of concrete rather than steel as structural material for the hull. A great deal of work in this conceptual development has been carried out, and the recent announcement by Conoco Norway of the forthcoming use of a concrete TLP for the Heidrun Field development, in the Norwegian sector of the North Sea, has considerably heightened the interest in this evolutionary application,[l], [2], [3]. Concrete has already been used Quite extensively for offshore applications, particularly in the North Sea. Some of the heaviest and largest offshore platforms have concrete Substructures. The long term performance of the material has proven Quite satisfactory, with very 1ittle need for expensive underwater maintenance. A number of other advantages have been claimed for this material, which is, in fact, experiencing a newwave of popularity in the UK sector of the North Sea even for relatively shallow water applications. A number of potentia1 benefits have been suggested from the use of concrete for TLP hull s. Among others:–The shorter fabrication schedule, compared to a steel hull, with possible economic benefits;–The possibility of increasing the platform operating draft, without substantial structural penalties. This in turn may lead to higher displacement and payload carrying capacity than a comparable steel TLP, and eventual lower tendon dynamic loads;–A high resistance to damage from impact loads;–The inherent resistance to corrosion, with consequent decreased requirements for inspection and maintenance;
During the past few years, BP Exploration has conducted nonsite specific engineering studies on deep water producing systems for the Gulf of Mexico (GOM). One of these studies [1] investigated the use of Tension Leg Platform (TLP) concepts in 3,000 (FT) water depth. Primary objectives of this stUdy were to assess various technical alternatives cost and schedule associated with the application of TLPs for this water depth. Various TLP configurations with different number of wells and payloads were considered. The components of these TLPs and their selected fabrication and Installation scenarios were the outcome of a series of tradeoff studies. Trade-offs encompassed safety, technical, operational, and economical issues to determine the installed cost and schedule of these TLPs, use of existing technology was assumed to the maximum extent possible. This paper discusses the TLP installation technique based on an improved way to achieve the installed system that was developed. The technique did not require access to a protected area remote from the final TLP installation site for the hull-deck mating operation, Also the technique minimized installation risks that could occur with the narrow weather window in the GOM. The method of installation was based on a modularized deck concept. In this method, the TLP hull was towed to the final installation site and all tendons were installed and locked down prior to setting the deck. A heavy lift vessel would then install the deck modules on the Module Support Frame (MSF) of the hull. INTRODUCTION TLPs with capabilities for simultaneous drilling and production operations tend to have very deep float out drafts. This is particularly true for concrete hull TLPs, [2], [3], [4], [5]. The conventional installation methods proposed to date for these TLPs rely on using an offshore deck and hull mating operation. The mating operation is done in shallow water protected areas, and the mated hull and deck are subsequently towed out to the final TLP installation site [6], [7], [8]. This approach has its merits, but it is best suited for the areas such as the North Sea where fjords and islands provide relatively deep water and sheltered sites adequate for doing the hull-deck mating operation [9], [10], [11]. However, it presents several drawbacks in the GOM which require careful consideration. The major shortcomings of this method are due to the narrow weather window and the lack of a suitable sheltered water mating site. Also, the method requires using a costly temporary mooring system for carrying out the mating operation. This temporary mooring system cannot be used readily for other purposes. Further, the mated deck and hull form a top heavy floating structure that requires adequate stability reserve for the tow-out to the final installation site. In some instances, this may control the TLP configuration rather than the long-term, in-place conditions. From a schedule standpoint, any delay in the deck fabrication will postpone the TLP hull deployment timing, resulting in an overall schedule delay.
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