Industry Trends for Design of Anchoring Systems for Deepwater Offshore Structures

This paper summarizes the current industry practice for the geotechnical design of anchoring systems of deepwater production units (TLPs, Spars, FPSOs, Semi-submersibles, etc.), with emphasis on in-place design conditions. The discussion includes anchoring system strengths and limitations, design criteria used, issues encountered, future studies needed, etc. Some deepwater anchoring systems, such as VLAs and suction piles, still do not have uniform or universally accepted design approaches. This paper discusses the importance of site-specific geological and geotechnical characterization, in order to identify the foundation strength and deformation parameters, as well as geohazards that may affect a project’s risk and reliability evaluations. Other issues discussed in this paper include short and long-term reverse end bearing for suction piles and holding capacity of VLAs; effects of cyclic and long-term environmental loading (e.g., Loop currents), such as soil cyclic degradation and creep; and soil set-up development with time. Introduction Heading more and more towards exploration and exploitation of hydrocarbons from ever-deeper waters, the industry has faced the need for more robust and specialized anchoring systems for drilling and production units (TLPs, Spars, FPSOs, Semi-submersibles, etc.). With the greater depths come greater mooring loads, new load sources such as different kinds of surface and deep currents, as well as the advent of taut and semi-taut leg moorings that have incurred greater uplift components. In particular, moving from shallow water to deepwater has called for qualitatively different tools to safely transfer predominantly tensile rather than compressive loads. For the most part, three distinct technologies are currently used in these new, uncharted waters, gaining more experience and confidence in their versatility and capacity; namely slender anchor piles, suction piles and Vertically Loaded Anchors (VLAs). This paper gives a snapshot of the current situation and state of the technology for these different types of anchoring technologies, citing their strong points and limitations, and pointing to areas that are in need of more development for better understanding of their behavior and more efficient designs. This paper gives the state of the industry, rather than the state of the science, which may involve further developments that are yet to percolate through to the application arena. The authors, however, would like to point out the proprietary nature of much of the developments in this new field, which may, inadvertently, result in not presenting some efforts and developments in the area. This paper presents information based mainly on projects in which the American Bureau of Shipping (ABS) has participated. These projects cover major parts of the world, including the Gulf of Mexico, Offshore Brazil and South East Asia. Through the classification and certification of deepwater offshore projects, ABS has been involved in the assessment of unique aspects of anchoring system designs that are normally not the design concerns for shallow water foundations. Facing these new design challenges and lack of existing industry standards, ABS has been working closely with the offshore industry to seek solutions and achieve the proper implementation of new designs. For new designs, information (analyses, test data, etc.) is generally required, so as to establish that the strength and reliability of the design is comparable to those of established designs. Normally, anchors need to be designed geotechnically for installation conditions and holding capacity as well as structurally for strength and fatigue under handling, installation and in-place conditions. Considerations for anchoring production units are generally different from those of Mobile Offshore Drilling Units (MODUs). This paper, however, addresses only the geotechnical design of the three main types of anchoring systems used for deepwater production units, under in-place conditions. Site Investigations The need for reliable and economic design of anchors for deepwater production units calls for a more detailed geotechnical investigation and advanced testing as compared to those done for less sizeable shallow water projects. An economic design of slender and suction piles requires less variability in soil parameters. More advanced testing is required to derive design parameters relevant to issues such as soil set-up and long-term, cyclic and anisotropic strengths. VLAs are known to call for the least of geotechnical information and analyses amongst different types of anchors. This may be attributed to several factors, such as their commonly simplified design apOTC 15265 Industry Trends for Design of Anchoring Systems for Deepwater Offshore Structures By: Ayman Eltaher, Yatendra Rajapaksa, and Kuan-Tao Chang, American Bureau of Shipping