Installation Penetration of Gravity Installed Plate Anchors - Laboratory Study Results and Field History Data
Authors: Shelton John T. , Nie Chunyong , Shuler Dillon 
Published: October 2011

The installation penetration of gravity-installed anchors is a critical factor in determining ultimate anchor capacity. Previous studies on installation penetration of gravity-installed anchors were limited to torpedo anchors and mostly focused on the geotechnical aspect. Recently, a new type of gravity installed anchor, constructed mainly of flat plate, has emerged.

Compared to the torpedo anchor, flat plate type anchors typically have larger surface and projected frontal areas with more complex geometry and less mass. The hydrodynamic properties of gravity installed anchors directly affect the seafloor impact energy and free fall behavior. The geotechnical properties are still an important aspect and are of great interest. This paper describes a series of laboratory tests conducted at Texas A&M University's Haynes Coastal Engineering Laboratory and Oran W. Nicks (OWN) Low Speed Wind Tunnel, regarding both the hydrodynamic and geotechnical properties of gravity installed plate anchors. Based on parameters obtained in these laboratory tests a numerical tool was constructed which accurately predicts anchor penetration of gravity installed plate anchors. Nearly 160 gravity installed plate anchor installation records were presented in order to calibrate the numerical tool. Published torpedo anchor installation data was also used for validation purposes.

Introduction

The offshore oil and gas exploration and production industry continues to require increasingly higher safety standards, especially regarding floating facilities in extremely harsh environments. An anchoring system that provides reliable capacity in extreme conditions while maintaining a low cost has raised the interests of mooring system engineers. Delmar Systems' patented gravity-installed plate anchor, the OMNI-Max, is an available solution to satisfy these anchoring requirements.

OMNI-Max anchors, along with Petrobras' torpedo anchors, are the two available types of gravity-installed anchors currently used in various offshore mooring applications.

The OMNI-Max anchor is capable of being loaded at extreme tensions at any direction, 360-degrees, around the axis of the anchor. In addition to being able to be loaded omni-directionally, it can also be loaded at very high uplift angles. The high uplift capability offers the benefit of reducing potential risk to subsea infrastructure in the field. The free rotating load arm mounted between the lower and upper fins is a key feature of the OMNI-Max anchor (Figure 1). The OMNI-Max anchor is distinct from the torpedo anchor, whose loading padeye is also the installation padeye located at the tail of the anchor. When a mooring load is applied, this feature allows the OMNI-Max to rotate from the original vertical position and penetrate to increasingly stronger soil gaining additional anchor capacity. Load arm rotation allows the anchor to withstand out of plane loading during large offset drilling operations as well as extreme station-keeping damage and failure events. With the exception of the Mk1 version, the OMNI-Max anchor's load arm is equipped with a shear pin. A shear pin is used to increase anchor structural capacity by releasing bending moment on the anchor axial shaft at a specified load. In the event a mooring system becomes damaged causing multiple line failures, the OMNI-Max anchor may allow the system to survive longer since the anchor's load angle can change without adversely affecting the capacity of the foundation. With proven in-field installations, established performance during severe events, and successful recoveries, the gravity installed plate anchor is widely accepted by the offshore Gulf of Mexico community and has shown higher cost efficiency than most other gravity installed anchors.