Hydraulic and earth engineering – Marine structure or fabrication thereof – Tension leg platform
Patent
1997-12-08
2000-02-08
Bagnell, David
Hydraulic and earth engineering
Marine structure or fabrication thereof
Tension leg platform
4051951, 4052244, E02D 700
Patent
active
060221747
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for installing an offshore tension leg platform, comprising the steps of bringing a freely floating platform structure to a temporary draft which is somewhat larger than normal draft in operating condition, bringing the platform structure into a predetermined position with respect to substantially vertically arranged tension legs, which in advance have been attached to one ore more foundations on the sea floor and which at their upper ends have been provided with a coupling element, guiding the tension legs in place with respect to the platform structure so that their coupling elements assume a position a distance above the corresponding connecting means on the platform structure, and causing a relative movement between the coupling elements and the platform structure in order to bring the coupling elements to attachment in the corresponding connecting means, whereupon further tensioning of the tension legs takes place by reducing the ballast of the platform structure.
Floating tension leg platforms (TLP) are tethered to the sea floor by means of vertical prestressed tension legs or tendons. The prestressing occurs as a result of the buoyancy being larger than the weight of the platform. Since the tendons have a substantial axial stiffness, the vertical movements of the platform due to waves are almost completely suppressed. The prestressing of the tendons are set so that the downwardly directed wave forces acting on the platform cannot make the tendons go slack. On the other hand, the tendons must possess sufficient strength to withstand the corresponding upwardly directed wave forces.
A substantial cost element is related to the use of temporary arrangements in connection with the attachment of the tendons and for moderating the transient dynamic behaviour of the platform going from freely floating to fixed tensioned condition. At the start of the installation, the platform will be floating freely, possibly with the exception of the interaction from tug boats and a catenary tethering system. In this phase, typical resonance periods will be 15-25 seconds for heave motion and 30-70 seconds for rolling and pitching. In the final installed condition, the stiffness of the tendons will reduce the heave/rolling/pitching resonance periods to 2-4 seconds. Under the external influence of direct wave forces, slowly varying second order wave forces and dynamics due to wind gusts, the restoring properties of the platform are gradually changed by the activation of the stiffness of the tendons. It is generally known that dynamic systems going from one dynamic steady-state condition to another, will do this through a transient dynamic transition. Such transient transitions may be violent also where the increasing restoring has a linear behaviour. For the installation of a TLP, this must normally be done in such a way that the increase in the restoring is not linear. It is known that the dynamic behaviour of non-linear systems may be worse than for linear systems and, furthermore, it is much more difficult to describe this behaviour through calculations. Up to now, this transient effect has been regarded as such a great problem for the tendons that substantial efforts have been spent in order to reduce its magnitude. This is clearly illustrated by the five TLP platforms built up to now and the same is done by the patent literature.
A method of the type mentioned in the introductory paragraph is known from U.S. Pat. No. 5,054,963. Here, four hydraulic cylinders having a stroke of 1,5 m are used for each tension leg in order to take up the transient movements. The platform is deballasted until it is approaching its normal functioning level and concurrently the stroke of the pistons of the hydraulic cylinders is gradually reduced. When a corner of the platform finds itself in a wave trough the pistons for this corner are locked. This locking operation is repeated for the other corners. Thereafter the pistons are adjusted until the tension is the same in all the tension legs, and fin
REFERENCES:
patent: 4938632 (1990-07-01), Eie
patent: 5054963 (1991-10-01), Williamsson
patent: 5174687 (1992-12-01), Dunlop et al.
Husvik J.o slashed.rgen
Muren Jan
Natvig Birger
Schamaun Paul
Vogel Horst
Aker Engineering AS
Bagnell David
Singh Sunil
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