Ships – Keels – Bilge
Reexamination Certificate
1999-03-05
2001-01-23
Morano, S. Joseph (Department: 3617)
Ships
Keels
Bilge
C114S140000, C114S141000
Reexamination Certificate
active
06176191
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of submarine production of petroleum. More particularly, it is related to the adaptation of decommissioned ships for use in petroleum production in deep water, consisting in floating production systems, which are also known to specialists as FPSOs (“Floating Production, Storage and Offloading”). Even more particularly, it is related to a means of minimizing the negative effects of the transverse swaying, or roll, inherent in the ships, which are intensified when they are adapted for FPSOs.
BASIS OF THE INVENTION
There is at present a worldwide tendency to use so-called FPSOs as floating production systems in maritime oilfields. Because of their availability and economy, recently decommissioned ships have been used as floating platforms for the FPSOs, after the necessary adaptations. The main feature of the changes to these ships relates to the installation, usually in the bow, of a turret which is anchored at the bottom of the sea. By the use of bearings, the ship can rotate freely around the vertical axis of symmetry of the turret. One of the functions of the turret is to allow the transfer of fluids between the submarine production system, which is stationary, and the FPSO, which is floating and has mobility around the turret.
The decommissioned ships have an elongated shape for greater efficiency of sailing. This shape inherently provides the ability to damp longitudinal swaying movements, or pitching, i.e. rotational movements around a transverse horizontal axis which passes through the middle of the ship. The principal mechanism of this damping is connected to the ability to generate waves, in the longitudinal direction, which carry the energy of the damped swaying away from the ship.
However, the same thing does not happen with rolling or transverse swaying movement, i.e. rotational movement around a horizontal axis, now longitudinal, which passes through the central longitudinal plane of the ship. Owing to the elongated shape, when the ship rolls the generation of waves is negligible and the rolling movement, once initiated, continues for a long time, or, in other words, is not damped.
This problem is aggravated by the fact that the typical natural period of oscillation of the roll is close to the periods of oscillation associated with sea waves. This natural period of roll of the vessel is due to the distribution of the total inertia and the hydrostatic restoration, which is practically impossible to change unless the shape of the ship is radically changed.
As was to be expected, the counterproductive characteristics of the ships which show a tendency to roll, together with a low damping factor for this movement, are “inherited” by the FPSOs. In addition, the FPSOs, unlike the ships, which have manoeuvring systems, are generally passive and practically stationary. In ships which move it is always possible to choose a bearing relative to the waves such as to minimize the effect of the rolling. Similarly, in the case of the FPSOs, the articulation provided by the turret allows alignment relative to the dominant environmental force. However, when the environmental incidences are different, i.e. when the waves, wind and current have different directions, the ship may be badly positioned relative to the waves. One possible consequence would be excitation of the transverse swaying of sufficient severity to interfere with the performance of the processing plant situated on board the FPSO.
PRIOR ART
Maritime petroleum production, storage and offloading (FPSO) systems installed in adapted oil tankers fitted with permanent anchorage systems have been used for many years.
Such systems, however, are used in relatively calm sea conditions, owing to the difficulties of anchoring big ships in rougher sea conditions. The reduction in the price of petroleum, and greater knowledge of anchorage systems relevant to the project, together with greater experience and confidence in the use of such systems, has led recently to a greater tendency to use FPSQ type systems in more exposed locations. In these conditions one of the problems to be solved is the control of transverse swaying without imposing a heavy burden on the cost of the undertaking, or in other words retaining the approach of using adapted decommissioned ships as the basis for these systems.
The present trend with regard to the control of transverse swaying in FPSO type systems is simply to accept the situation existing before the decommissioning or change in the activity of the ship. The solutions available for the problem of rolling are thus the same as those used for ships which sail and are therefore generally unsuitable for FPSOs.
Basically there are two types of devices in use to counter transverse rolling. The first type, which is cheap and effective and is consequently used fairly widely, is the construction, in the bilge of the ship, of a mechanical element called a “bilge keel”. This element comprises fixed perpendicular plates extending from the bilge of the ship. An extension of this idea is the use of flaps, or, in other words, small transverse wings, which are much shorter than the bilge keels and which act as an active controller by varying the angle of attack. Two other types of devices can also be mentioned: the stabilization tank and, in the same “passive stabiliser” family, the so-called U tube. In both cases the principle of operation is that of the “dynamic vibration absorber”. In these cases, oscillatory masses are introduced in such a way that when they oscillate they make the ship practically stop.
The work presented by J. A. Pinkster and H. R. Luth in 1993, at the 25
th
Offshore Technology Conference, “The Reduction of Low-Frequency Motion of Moored FPSO's”, describes and analyzes the possibility of using an articulated blade, in the prow of the ship, creating a passive system for conversion of the energy of the incident water. In high wave conditions the blade produces thrusts of medium and low frequency which oppose the forces, produced by the waves, which tend to move the ship. However, such a system is not specifically intended for control of the transverse swaying movement, apart from being of more complex design than that of the present invention.
The conventional bilge keel consists basically of a certain number of flat plates, with sharp edges, placed at right angles to the hull, forming a kind of line on each of its sides, with separations, extending along the parallel middle body of the ship (the central region of the ship excluding the extreme bow and stern portions, presenting an approximately uniform cross-section, and which, for oil tankers, comprises about 80% of the length thereof, being less for other types of ships).
Conventionally, in order to ensure that the resistance to forward movement of the ship is not affected too much by the introduction of the bilge keels, an attempt is made to align them along the natural flow lines of the hull in motion. Moreover, the width thereof is just sufficient to guarantee the separation of the boundary layer, at the corner point of the flat plate, when the ship starts to roll. When rolling occurs, the bilge keel produces a contrary moment, of viscous origin, which is strongly influenced by the generation of vorticity.
In addition to this viscous effect there is another effect, a supporting effect, which appears when the ship has a forward speed. In this case, locally, in each bilge keel, when rolling commences, concomitantly, a relative angle of attack is created which produces a contrary dissipating moment, which is not negligible, and is essentially proportional to the forward speed. In order to increase the efficacy of this effect, the bilge keels are not usually made continuous. Thus the best construction, from the hydrodynamic point of view, consists of sectioned bilge keels, forming discontinuous stretches, along the length of the ship. The spacing between each stretch must be sufficient to increase the efficacy of the supporting effect.
The term “total bilge keel length” as us
Fernandes Antonio Carlos
Masetti Isaias Quaresma
Morano S. Joseph
Nixon & Vanderhye PC
Olson Lara A.
Petroleo Brasileiro S.A.
LandOfFree
Bilge keel and method for FPSO petroleum production systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bilge keel and method for FPSO petroleum production systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bilge keel and method for FPSO petroleum production systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2533839