The Heidrun Field - Heidrun TLP Tether System

ABSTRACT The Heidrun TLP was installed in July, 1995. The tether system was developed to meet the following requirements preinstall the tether strings before platform installation; commission the tether tension monitoring system before platform tow to field; lock-off on all sixteen tethers simultaneously 50-year platform design life inspection, maintenance, and repair capability over the operating life. The system and component designs, operations trials, and experience to date are summarized. INTRODUCTION This paper presents a brief description of the Heidrun tether system, which anchors the platform to the seabed foundations. The overall system is shown in Figure 1. A description of thefunctional requirements for both present and installation phases provides the background for the system configuration. The global platform response gives the performance requirements. As the Heidrun platform differs considerably from previous tension leg platforms, a number of challenges had to be faced. The main ones related to loads, installation and schedule. The tether system was divided into component assemblies or packages, bawd on logical interfaces and contractor capabilities. An outline of the assembled unit is given in Figure 2. The features of the component assemblies are described next. The main packages are the bottom connector assembly, the flex element, the tether string, the top connector assembly, the installation equipment, the tether tension monitoring system and the tether inspection system. As part of the system design, the main components were subject to prototype testing. In addition, trials of the inspection maintenance, and repair equipment were performed onboard the platform before the tow to field. Further, the installation equipment was tested and the full installation crew was trained on board before low. The paper is concluded by the main experiences from implementing the chosen solutions. SYSTEM CONFIGURATION From experience gained during earlier tether system work, it was apparent that development of the Heidrun tether system should simultaneously consider both temporary and permanent phases when defining system requirements. Thus, a systematic approach was applied which brought together consideration of in-place operation, installation, fabrication, and integration inter facing with the platform's substructure and foundations. During the course of preliminary engineering in 1991, the Heidrun tether system's overall configuration was therefore development in parallel with the development of the marine operations required for installation of the tethers and platform. From the beginning of this work it had been assumed that thetethers would be of single-piece, neutrally-buoyant design. Other options such as the use of connectors and welding in-situ were investigated but the single-piece welded system was the most economic concept for this environment.