Pile-Driven Plate Anchors For Fleet Moorings

Abstract

ABSTRACT A means of providing high efficiency driven plate anchors to satisfy Naval requirements for hurricane moorings in congested, confined or complex seafloor areas is under development. Driven plate anchors resist loading in any direction, including uplift, and offer extremely high holding capacity to size ratios. Where space is limited and drag or clump anchors or stake piles are unsuitable, small, inexpensive plate anchor configurations can provide very large holding capacities and be installed with readily available marine equipment. Driven plate anchors can be fabricated from common structural steel configurations and materials. Anchor installation is achieved by attaching the plate anchor to a structural member, called a follower, and driving it vertically into the seafloor using standard pile driving techniques. The follower is retrieved and the anchor is then loaded to properly position it in the seafloor and verify it's capacity. This paper describes plate anchor theory, installation techniques and the results of proof tests in a number of different seafloor soils. These include soft organic silts, over-consolidated clays, dense sands, glaciated soils and corals. Also included is a brief overview of completed installations and a design guide currently in development by the Navy. INTRODUCTION Downsizing of the Fleet is taxing the Navy's ability to provide secure moorings for the Inactive Fleet. These moorings must be designed for hurricane conditions, and often occur in congested areas and at sites with complex seafloor conditions. Conventional drag and pile anchors are either unsuitable for the site conditions or are extremely expensive. In light of today's shrinking economy and the urgent need to provide new moorings, technology for designing and installing simple plate anchors using common pile driving equipment and marine gear is being developed by the Navy. The technology is proven through many mooring installations, resulting in substantial savings to the Navy. Plate anchors installed using standard pile driving techniques (Pile-Driven Plate Anchors) provide an alternative to conventional anchors that can be used today by the offshore industry. This report is an effort to introduce this new technology to the Offshore Industry. PLATE ANCHOR THEORY Plate anchors are driven into the seafloor with the plate parallel to the direction of driving and then rotated or "keyed" by pulling on the anchor line (Figure 1). The anchor is connected to a structural member called a follower and driven vertically into the seafloor using a pile-driving hammer. The follower is retrieved and the anchor is proof loaded. This can be accomplished either vertically from the installation platform immediately after installation or later by pulling horizontally against another anchor leg. The proofing process keys or rotates the anchor plate to a more resistive position, near-normal to the loading direction. Fig. 1- Driven plate anchor - concept of operation.(Available in full paper) Current techniques for determining the holding capacity of plate anchors have been adapted from conventional foundation design practice. For cohesionless soils (c = O), static anchor holding capacity, F, can be determined by the simplified relationship. (Available in full paper)