Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
1998-08-25
2001-02-20
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S212000, C405S185000
Reexamination Certificate
active
06190091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates generally to floating structures used in conducting offshore petroleum operations, e.g., drilling, exploration, production, and storage. More specifically, the invention relates to an apparatus and method for controlling tension levels in tensile elements, e.g., mooring lines, marine tendons, and risers, which extend between a floating structure and the seafloor or other body.
2. Background Art
The oil industry is increasingly relying upon offshore oil deposits to meet the needs of the energy market. However, offshore operations, such as exploration, drilling, and production, are subject to a host of challenges that do not exist on dry land. These challenges become even more acute in deep water where floating structures, which are subject to irregular motions during operation, are employed. As illustrated in
FIG. 1
, a floating structure that is stationed in an open sea environment is subject to environmental forces of wind, waves, and current which may combine to induce the generally undesirable response of oscillatory motions along six degrees of freedom. Generally, displacements in the vertical, longitudinal, and transverse directions are referred to as heave, surge, and sway, respectively. Rotations about the heave, surge, and sway axes are generally referred to as yaw, roll, and pitch, respectively. For floating structures that are generally symmetric, the term lateral offset or surge may be used to refer to surge or sway motion of the floating structure and the term pitch may be used to refer to pitch or roll motion of the floating structure.
Frequently, it is desirable for a floating structure to remain relatively stationary either in relation to a fixed point on the seafloor or relative to another body. Holding a floating structure in position, or on station, and reducing lateral excursions about this station against the forces of the environment is referred to as station-keeping. Station-keeping is difficult in any offshore operation, especially when relatively rigid fluid-carrying pipes such as risers extend between the floating structure and the seafloor. In operations involving risers, stringent requirements are usually imposed on the station-keeping system to prevent damage to the risers. It is usually desirable to maintain tension in a riser to prevent the riser from buckling or collapsing under its own weight or under the action of the environmental forces. Thus, as the floating structure responds to the environmental forces, one of the challenges then becomes keeping the floating structure on station while providing appropriate tensile support to the riser. Various prior art structures have been developed to compensate for the motions of the floating structure while providing tensile support to risers. Deep water operations, however, have pushed the limits of traditional systems employed for riser tensioning and station-keeping. Nevertheless, the discovery of large, deep water oil deposits and the forces of economics continue to drive the industry into increasingly deeper water, thus making it desirable to have a station-keeping system and a riser tensioning system that is effective in even deep water.
Floating structures typically employ dynamic positioning systems or a system of tensile elements attached between the floating structure and the seafloor for station-keeping. Dynamic positioning systems use active means of monitoring position combined with thruster control to hold a fixed position. However, the use of dynamic positioning systems are generally limited to short term operations, such as drilling. For long term operations, floating structures generally employ tensile elements, such as mooring lines and marine tendons. Mooring lines are the most common tensile elements employed for station-keeping. Some floating structures use both mooring lines and marine tendons for station-keeping. Mooring lines are typically made of sections of chain, wire rope, synthetic rope, or a combination of such materials. In harbors, ropes are typically used to attach a floating structure to a dock or to hold station in open water. In open seas, catenary mooring lines are commonly used. Marine tendons are typically vertical, relatively rigid pipes that extend between the floating structure and the seafloor.
FIG. 2
illustrates a floating structure
10
which employs a catenary mooring system, e.g., catenary mooring line
12
, for station-keeping. The catenary mooring line
12
has one end attached to the floating structure
10
and another end attached to an anchor
14
on the seafloor. Typically, the length of a catenary mooring line is significantly in excess of the depth of water in which the floating structure is moored so that the mooring line forms a characteristic sagging or catenary shape between the floating structure and the seafloor. The length of the mooring line often exceeds the water depth by a factor of three to five. The mooring line
12
connects to the floating structure
10
at a connection angle &phgr;, where &phgr; is measured with respect to the vertical axis of the floating structure
10
. The larger the connection angle &phgr;, the more effective is the mooring line
12
in restraining surge motions of the floating structure
10
. However, the connection angle for a catenary mooring line is relatively low, typically less than forty-five degrees.
The connection angle &phgr; of a catenary mooring line may be made larger by increasing the pre-tension in the mooring line or by adding buoys to the mooring line. The mooring line
18
indicates the new position of the mooring line
12
when pre-tension in the mooring line
12
is increased. The mooring line
22
indicates the new position of the mooring line
12
when buoys
20
are added to the mooring line
12
. As shown, increasing the pre-tension in the mooring line
12
or adding buoys to the mooring line
12
shifts the mooring line
12
upward, thereby increasing the connection angle of the mooring line. However, as water depth increases, the connection angle of the catenary mooring line tends to diminish due to the increasing weight of the catenary mooring line, making the catenary mooring line less desirable in very deep water. The catenary mooring line may be replaced with a taut mooring line which has a much shorter length and weighs less than the catenary mooring line.
FIG. 2
shows a taut mooring line
24
having one end connected to the floating structure
10
and another end connected to a pile
26
on the seafloor. The taut mooring line
24
is pre-tensioned to achieve a desired connection angle with the floating structure
10
. The connection angle of the taut mooring line is generally larger than the connection angle &phgr; of the catenary mooring line, allowing the taut mooring system to provide better station-keeping characteristics. A taut mooring system, however, requires substantially higher pre-tensioning than a catenary mooring system.
In both taut and catenary mooring systems, the weight of the mooring line and the geometry of the mooring system configuration combine to give a generally non-linear relationship between tensions in the mooring line and lateral offsets of the floating structure.
FIG. 3
shows an example of a mooring line tension versus lateral offset curve. As shown, mooring line tension increases gradually through an initial range of lateral offsets, but beyond the initial range of lateral offsets, mooring line tension increases exponentially. As a result of this non-linear behavior, relatively small lateral offsets result in large tension variations in the mooring line in the region where mooring line tension increases exponentially. For example, a lateral offset ∂X
0-1
for a mooring line with a pre-tension To induces a tension variation ∂T
0-1
. Often, it is desirable to have a highly pre-tensioned mooring line, since this will enhance the restoring response of the mooring system. This is especially true for a taut mooring system. However, a much higher pre-tension induces a much
Bagnell David
Lagman Frederick L.
Novellent Technologies LLC
Rosenthal & Osha L.L.P.
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