The Positioning Of Offshore Constructions, Research And Training By Simulation
Authors: Witt Franz G.J. , Klaas Meurs 
Published: December 2007

In 1976, two research program concerning the positioning of offshore gravity platforms were carried out on the ship manoeuvring simulator of the N.S.M.B. for Smit International Marine Services B.V., which company is specialized in the towage and positioning of offshore constructions like the Ekofisk 1 and Mobil Beryl 'A' units.

The results and the consequent recommendations have meanwhile been applied by Smit International in the organization and execution of several positioning operations.

The 'tug order display', designed by the Netherlands Ship Model Basin to give the towmaster a view of all standing tug orders has also been used for instance in towing the ANDOC-built Dunlin-platform out of Europoort.

Introduction

Putting a gravity structure down somewhere on the North Sea bottom is a job in which a variety of specialisms of different nature in involved. Already in the design and construction stage, knowledge, ideas and convictions of many experts have to converge into a total agreement on the planned effort.

It is obvious that differences in opinion on decisions to be taken during such an operation are very likely to result in grounding or loss of the platform. To avoid this possibility and to ensure a smooth handling of such a tow, a very strict organization is of crucial importance, especially for the last and most critical phase of the transport: the positioning. In a number if cases this positioning involved bringing the platform to a predetermined location and keeping it there during the sinking of the structure in its definite position. The position as well as the heading of the structure had to be correct within very narrow accuracy limits. The accuracy of positioning, which in first instance was a target area with a diameter of 15 m within which the centre of gravity of the structure was to be kept long enough to allow the platform to sink by ballasting its tanks.

The accuracy of its heading was 5 degrees on either side of the intended heading. Rightly, the question was asked whether such an accuracy would be achievable in the manner planned for the operation. The way in which the positioning was to be carried out involved the manipulation of six tugs which were grouped around the platform in a star-shaped pattern.

The total tug power was intended to be 70,000 HP. (Fig. 1). A towmaster was charged with the actual command of the positioning. This towmaster usually is a man with a long and extensive towing experience. It is his task to direct the tugs, with the available data and the aid of a staff of navigation specialists, in such a way that the ultimate position is reached and kept during the sinking.

Fig. 2 gives a block-diagram of the working scheme during positioning. The positioning of large objects like these with a weight in excess of 300,000 tons with an accuracy of less than 15 m, during which a great number of strong tugs have to be directed, is an extremely difficult job.

Therefore, both research programs reviewed in this article, served first to answer the question: 'Is it anyhow possible to carry out such an operation with sufficient accuracy while taking into account disturbances like tidal stress etc.' and secondly: 'Which accuracy can be reached?'. A favourable circumstance of the 'real-time' simulation of these operations is the fact that the towmaster who has to do the job can get a specific training for this complicated task by active participation in the research program.