Dynamic Positioning System Evaluation

Abstract

ABSTRACT Evaluation of dynamic positioning systems consists principally of determining the holding capability against expected weather and the reliability of the DP system in operation. Efforts to quantify reliability, by estimating mean time between failure, now permit several useful risk analysis evaluations. The main considerations for DP evaluation are covered here. INTRODUCTION Dynamically positioned vessels have been in operation since 1961, and they have allowed us to do many things that anchored vessels are not able to do. Core holes have been drilled and reentered in over 18,000 ft of water, and oil exploration wells have been drilled in 7500 ft of water. Within anchor depths DP has eliminated the risk of anchor damage to subsea pipelines and facilities. A tracking mode allows a pipe or cable laying vessel to move along a pre-set corridor. Since its first use in core drilling and then exploration drilling its use has grown to include vessels for pipe laying and cable laying, diving support, heavy lift construction, and search and salvage. Most recently it has found its way to small supply and anchor handling boats which are also useful for light maintenance work on subsea facilities. For all its virtues, its record has not been unblemished. Failure to hold position in bad weather has been an occasional problem. This is not necessarily because of inadequately designed thrust capacity. More often it is lack of redundancy. Reliability, or the lack of it, is the most frequent cause of position failure. The mean time between failure (MTBF) of DP systems has ranged from less than one month to well over three years, depending very largely on the level of redundancy provided and the elimination of single point failures. Reliability cannot be measured or calculated with much accuracy, but past experience with many systems, some reasonable estimates of component failure rates, and an understanding of the role of redundancy allow useful estimates to be made. Depending on the job to be done it can be very important to know where in the broad range of MTBFs a particular system falls. Although weather plays an important role in determining DP system reliability, the ability to include weather in estimating system MTBF is fairly recent. Evaluation of holding capability and reliability along with expected weather exceedance and probable cost to the job for a DP system failure permit a fairly straightforward risk analysis to be made. This provides a rational and reasonably quantitative basis for judging the suitability of an existing DP system or the design of a new one for the particular work that will be required of it. This approach can also be used to quantify justification for additions to redundancy for either new or existing vessels. This paper does not address the suitability of a vessel regarding seaworthiness, motion characteristics, load carrying, deck space, or the many other things that must be evaluated for a particular job to be done.