Challenges in Wireless Data Transmission from Anchor to Surface
Authors: Ruinen Roderick Michael 
Published: October 2011

With the industry installing facilities in more complex locations, for example crowded seabed infrastructure, reliable installation data for drag embedment anchors is required. For this purpose a real time anchor data measuring system is being developed giving data on the key installation parameters (depth, drag, pitch, roll and force). During the development of the system one of the key requirements was reliable wireless (acoustic) data transfer from the anchor to the surface. To ensure that this is possible, significant research has been made on the theory and practice of wireless data transmission. The proposed paper will present the results of the research into the wireless data transmission together with the results of field tests that have been performed. Additionally the results of a fully instrumented and tested anchor will be provided. This will give an impression of the real time data that the user can receive on the installation vessel. Having the accurate anchor installation data available will make it easier for the user to meet classification societies / regulators requirements with regards to drag embedment anchor installation.

Introduction

To gain more detailed information on the installation of drag embedment anchors a real time anchor data acquisition system has been developed. This paper will present the requirements defined for the system, the challenges encountered with acoustic communication and the testing performed with the product.

System Requirements and Description

The following minimum requirements have been defined for an anchor embedment data acquisition system:Real-time measurement and display of all data measured on the anchor itself,Measuring of the applied load at the anchor,Penetration depth of the anchor below seabed,Drag of the anchor,Trajectory of the anchor in the seabed,Pitch of the anchor - this shows whether the anchor is capable of further embedment (fluke angle still sufficient) after completion of the installation,Roll of the anchor - this shows whether the anchor is stable in the seabed andAny equipment left behind on the anchor should be low cost.

All expensive equipment should be recoverable upon completion of the anchor installation.

Based on the requirements set out for the project, the system was developed to consist of a load measuring anchor shackle connecting the mooring line to the anchor, inclinometers to measure roll and pitch of the anchor and pressure sensors to measure anchor depth. The off- the-shelf sensors are housed in a watertight canister in the anchor. All of the data from the sensors are combined with a processor housed in the canister and transmitted by means of an umbilical cable to a subsea modem located above the seabed.

The subsea modem transmits the raw data from the seabed to the receiver hanging below the installation vessel. The data are then forwarded to a computer for real time processing into the information required (roll, pitch, load, depth and drag). As the system is measuring the penetration depth of the anchor, the drag length and anchor trajectory can be constructed from this data using the pitch of the anchor. To ensure correct readings, all of the sensors are calibrated beforehand to ensure the correct zero settings.

The data acquisition system uses relatively cheap sensors and electronics onboard the anchor as these parts stay in place after completion of the anchor installation. The more expensive subsea modem can be recovered after the anchor has been installed and reused at a different location.