Installation of Vertical Loaded Anchors Using a Subsea Tensioning Device in Ultra- Deepwaters in the Gulf of Mexico

Abstract

This paper presents a method for the installation of anchors for deep water moorings, based on the use of a subsea tensioning device to generate the high installation loads required. Subsea tensioning devices have been used in waterdepths of up to approximately 200 m for the installation of conventional drag embedment anchors and pre-tensioning of piles, but have not yet been used for a deep water application. The proposed method of installation will be presented, and is based on data gained from full scale tests with vertically loaded anchors (VLAs) in approximately 1000 m of water offshore Brazil and also on simulations performed on the computer. The results of the computer simulations and the field tests show that using a subsea tensioning device in deep water can significantly reduce the required time to install the mooring system as the installation and pre-tensioning of the anchors can be performed with two anchors simultaneously, instead of installing them one by one. The use of the subsea tensioning device also requires less powerful vessels for the installation as only vertical pulling capacity is required and not bollard pull. The simulations and field tests show that by using a subsea tensioning device a vertical pulling capacity of approximately 40% to 50% of the required anchor installation tension is required, meaning that the winches of current anchor handling vessels are quite sufficient for deepwater installation of anchors. Introduction The use of catenary moorings systems in deep and ultra deep water requires long and heavy mooring lines. As a consequence the floater has to support a large weight and the mooring footprint becomes large and may interfere with neighbouring infrastructure. Installation of these mooring systems becomes very expensive due to the vessels that are required. For deep and ultra deep water mooring systems the more appropriate moorings system is the taut leg mooring, using shorter and lighter mooring lines. The mooring loads arrive at an angle between 30 and 45 degrees with the bottom, requiring the anchor points to resist not only horizontal, but also considerable vertical loads. As conventional drag embedment anchors are not capable of withstanding large vertical loads, a new type of anchor had to be developed with the capability of withstanding the large vertical loads associated with a taut leg mooring system. Research in finding a solution for a Vertical Loaded Anchor (VLA) capable of resisting large uplift loads started with laboratory testing of models in sand, laponite, bentonite and mud. This resulted in the fine tuning of an anchor shape that is stable in the soil during penetration, able to resist large normal (vertical) loads and easy to handle. These model tests were followed by large-scale tests in the Gulf of Mexico and offshore Brazil. Petrobras initiated a test program on several deep-water sites in Campos Basin where the handling, installation and pullout capacities of VLA anchors have been verified.