A New Approach for TLP Installation in the Gulf of Mexico

Abstract

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.