New Technology For Mooring Offshore Structures
Authors: Nixon R.A. , Elliot G. 
Published: November 2019

Mooring of oil production platforms during construction in operation imposes problems not encountered in ship mooring systems. Ships are generally moored in shallow water where there is good ground, and considerable freedom of movement is allowable. Floating production platforms are moored in deep waters. Virtually no movement of the platforms are moored in deep waters. Virtually no movement of the platforms will be allowable, and the bottom material will depend platforms will be allowable, and the bottom material will depend arbitrarily on where oil is found. The paper describes developments currently being pursued by NEL, to solve these problems, including mooring monitoring systems and self-burying anchors of high holding power at short scope. power at short scope

Introduction

The historical development of mooring has been concerned with the shipping business, and therefore reflects the operational practices of that industry. Mooring of offshore structures has no real historical precedent, and must meet different operational needs. So a new technical must grow out of the old. The common linking factor is the marine environment.

SHIPS MOORINGS

Even the largest ships would normally anchor only in relatively calm conditions in a sheltered haven lying to one or two bower anchors and being free to swing about these anchors to lie in the position of least resistance of the combined effects of both wind and current. In the situation where the weather deteriorates then the ship can reduce the forces on her moorings by using her main engine or by lifting her anchors and proceeding to sea until conditions improve.

MOORING OFFSHORE

Moored offshore structures on the other hand using basically the same equipment have to maintain a constant position within relatively close tolerances irrespective of weather. They cannot be allowed to align themselves in the position of least resistance although due to position of least resistance although due to their symmetry this factor is not likely to be greatly significant. The rig must be restrained almost equally to all points of the compass, and therefore for any given prevailing direction the mooring system is largely redundant to the order of 75 per cent. This in practical terms will mean 4–6 anchor's cables being usefully employed at any one time each showing the forces to some varying degree dependent on their direction. The magnitude of these forces remains for the moment one of the great mysteries, since not a great deal is known of the ultimate or worst weather conditions that can be expected for any given location. On a statistical basis predictions are based on the strongest winds that could be expected to occur on one occasion in 25 years, 50 years or 100 years, each becoming progressively worse the longer the time sample. progressively worse the longer the time sample. On this basis one can only logically assume that this progression continues to infinity with 200 year, 500 year and 700 year conditions continuing to increase in severity. For practical purposes it is normally accepted as a basis for purposes it is normally accepted as a basis for design that the 100 year storm is the worst that can be expected. Even this seems to increase in magnitude at regular intervals almost monthly; this can only be due to the relatively short term over which reliable data exists.