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CAPTAIN CHRIS JENMAN, MNI, spoke on this very topical subject that the NI has taken very seriously, especially with respect to training. He went into detail on the concepts of DP control systems by saying that the more thrusters that a vessel has available, the more difficult it is to know what to use in any particular circumstances.
DP control is very good in calm weather and great help in marginal weather; however, the most difficult situation was during rapid change of, for example, wind speed.
DP vessels have many uses. He gave the example of the Seaways Sandpiper, a converted bulk carrier, which is used for rock dumping along pipelines where a high degree of position accuracy is required, then he mentioned the DP shuttle tankers with their 'dog house' on the fo'c'sle where the DP controls the vessel's position to reduce the wear on the mooring gear with SPM. One interesting point mentioned was that with big vessels using DP when the vessel is observed to be going off station and getting closer to another structure, it is usually already too late to save the situation.
He then went into some detail to explain an actual DP operation that he was involved in during 1986 and amongst other things mentioned the concern he had with tugs where it was found that when called the shortest time that a tug would take to give a steady pull was 1-2 minutes whereas a 30-second response time was required. Another aspect mentioned was that if, in this case a crane barge, with moorings as well as DP, was asked to move position by a few metres, after the move it would take another 30 minutes to stabilise again for DR.
Several questions then followed on such subjects as worst weather conditions for DP, insurance premiums, HMS Challenger and incidents per year. For the last item it was stated that each vessel will have an average of about two incidents a year, of these perhaps only 30 per cent are attributable to equipment failure and 70 per cent to operator error.
Contributed by P. R. Owen, FNI
Seaways February 1991