Dynamic Positioning 



Dynamic positioning is an automatic metlnod for positioning a floating 

 vessel without the use of anchors and anchor lines. Dynamic positioning 

 utilizes on-board power units which maintain the vessel in a fixed location 

 against wind, wave, and current forces. 



Dynamic positioning has proven to be a fairly successful technique 

 for maintaining position. Operators of vessels using the method claim it is 

 relatively easy to maintain position within 5% of the water depth; percentages 

 of 1 or 2% are not uncommon. More sophisticated systems controlled by 

 computers are fully automated, providing almost instantaneous response to 

 the constantly changing sea surface. ^^ 



There are three types of position sensing systems used for dynamic 

 positioning: (1 ) taut wire, (2) sonar, and (3) radar. The taut wire system 

 has been successful in water up to 2,000 feet deep. It consists of a heavy 

 sinker placed on the ocean floor which is connected to the vessel by a wire 

 line. Any drift in the ship will change the orientation of the wire and this 

 change is measured by a sensing device which, in turn, feeds a signal to the 

 control system which executes the steps necessary to correct for the drift. 

 This system is the quickest, easiest, and most practical to use.^^ 



Radar and sonar sensing devices measure the changes in the relation- 

 ships between the surface vessel and fixed points. The latter can be points on 

 a nearby land mass, buoys, or points on the ocean floor. Radar apparently 

 has an advantage over sonar, primarily because sonar sensing devices usually 

 require submerged buoys for satisfactory operation. 



Probably the most successful and sophisticated dynamic positioning 

 system is that on the GLOMAR CHALLENGER, a drilling vessel recently 

 commissioned by its owner, the Global Marine Corporation. On its maiden 

 voyage, the unanchored ship was held in position by its main propellers and 

 four side thrusters, with great precision. During one operation, the ship was 

 drilling in 17,600 feet of water but did not drift more than 125 feet from its 

 set point, even though there were winds of up to 30 knots. It is claimed that 

 the ship can maintain position in over 2,000 fathoms in a sea state 6 and 

 40 knot winds.^ 



The system on the CHALLENGER is relatively simple. The lateral 

 distance and direction of the ship from either of two reference sonar beacons 

 on the ocean floor are determined by use of a digital computer. The system 

 is automatic and requires little power. Apparently the reference beacons are 

 expendable; they have a life of about 10 days.^ 



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