Mooring System Instrumentation for Floating Drilling and Offshore Construction Vessels

ABSTRACT For many years instrumenting the anchor lines, wire rope or chain, on drilling vessels and offshore construction vessels was left to be done after the basic vessel design work had been completed. In almost every instance, construction was underway or even completed before consideration was given to monitoring tension in the anchor lines. Early attempts at hanging an appendage or modifying a component actually designed for another function barely proved that anchor line tension could be monitored. Only in the most recent years has instrumentation been included at the design stage. All contracts now include a requirement for tension measurement with some calling for speed and footage instrumentation too. This paper will higthlight the history of the mooring system relative to monitoring line tension, speed and footage and the development of the winch, windlass or fairlead for integrated tension, speed and footage sensors. INTRODUCTION No formal exchange of information has been generated by the various winch or windlass manufacturers, wire rope or chain manufacturers, drilling and construction vessel designers or even the instrumentation manufacturers to this date. Though a portion of this paper is a compilation of historical data this method of presentation is required to develop the background necessary to describe and critique the current systems being used and those that will be introduced within the next few years. Design concepts are deep rooted and may appear to be the best solution, however, too many times a compromise is necessary to accomplish the required results with the trade-off leaving something desired. It is the hope of the author that this paper will stimulate an interest in the development of a basic standard or recommended practice for mooring systems applicable to the needs of offshore' operations. EARLY DRILLING VESSEL MOORING SYSTEMS AND THEIR INSTRUMENTATION One of the first mooring systems developed for a floating drilling vessel was put into service in the late 1950's. This system, known as the "Spring Buoy", included a small rectangular float (buoy) with a sufficient length of chain and an anchor for mooring in the relatively shallow depths of the Santa Barbara channel or offshore Louisiana. These buoys were attached to the drilling vessel with a two-part line from the vessel. The anchor line extended from the winch aboard the vessel through a sheave on the buoy and back to a dead end at the vessel. A hydraulic tension load cell was placed between the wire end and the deck dead-end pad with the load cell connected to bourdon tube gage calibrated to read the anchor chain tension. The "Spring Buoy" system required many hours to set the anchors and presented considerable danger to the crew when being handled in bad weather. In addition the buoy would sometimes submerge, lengthening the anchor line without an appreciable change in the tension reading but allowing the vessel to move off location.