General Introduction

Torpedo-Anchor-desingOffshore industry is going deeper and deeper and so are the challenges and costs. Finding the solution for deep water anchoring method, which is more efficient and economical is a big challenge.

Torpedo anchors are used as foundations for mooring deep-water offshore facilities, including risers and floating structures. They are cone-tipped cylindrical steel pipes ballasted with concrete and scrap metal and penetrate the seabed by the kinetic energy they acquire during free fall through the water. A mooring line is usually connected at the op of the anchor. The design of such anchors involves estimation of the embedment depth as well as short-term and long-term pullout capacities.

The first commercial application of torpedo anchors was conducted in the Campos Basin, offshore Brazil . To prepare it for installation, it should be first placed in a vertical position; after it is launched, the momentum caused by its own weight will drive it down into the soft seabed. The installation of the torpedo anchor does not require any external source of energy and it is a quick process requiring only one or two anchor-handling vessels and a limited use of ROVs (Remotely Operated Vehicles).

Due to the compact size of this kind of anchor as compared to other anchors, transportation to the site is easier and fabrication is more cost-effective. Further, no proof loading is required because of the very large kinetic energy made available through freefall, The other advantages include precise horizontal positioning, applicability for taut leg mooring and low sensitivity to increasing water depth. As has been proven in practice, this kind of anchoring can be utilized for both drilling and offshore production activities; fit has enough holding power for large production units, while it is easily recoverable for drilling. The holding capacity of the anchor is mainly determined by the penetration depth, which would depend on the impact velocity, penetration angle into the seabed, and the seabed soil properties. In order to precisely predict the holding capacity, the impact velocity has to be known in advance. Nevertheless, to calculate the impact velocity, the values of drag coefficient (CD) have to be precise. The torpedo anchor is generally launched from a high enough elevation above the seabed to acquire sufficient kinetic energy and reach a designated velocity so that it can penetrate into the target burial depth.

Advantages and disadvantages of torpedo anchors

Advantages

  • Advantages of torpedo anchorscost effective and low cost fabrication
  • can be used in ultra-deep water
  • shorter installation duration
  • largely independent of water depth
  • easy installed anchor for riser flowline restraint
  • large holding capacity
  • no hammering required for installation
  • small footprint

Disadvantages

  • pile may tilt in soil
  • due to horizontal drift, may go out of target