Patent Application
20070264086
1. Field of the Invention
This invention relates generally to tension leg platforms for deep-sea hydrocarbon production and specifically to mooring tendons therefor.
2. Description of the Prior Art
Tension leg platforms (TLPs) are increasingly used for exploitation of deep sea hydrocarbon reserves. A TLP is a semi-submersible floating platform anchored to a foundation on the sea bed by mooring elements, often called tension legs, tethers, or tendons. The tendons are maintained in tension at all times by ensuring net positive TLP buoyancy under all environmental conditions. The tendons stiffly restrain the TLP against vertical offset, essentially preventing heave, pitch and roll, yet they compliantly restrain the TLP against lateral offset, allowing limited surge, sway and yaw.
As shown in
Each hull (14) is designed to mate with a number of tendons (12) at tendon porches located near the keel (24). The tendon porches contain connection sleeves (22) to receive and clamp tendons at the length adjustment joint (LAJ) (27), which are located at upper ends of the tendons (12). The connection sleeves (22) are often ring-shaped, requiring vertical entry of the tendons, or are slotted, allowing side entry of the tendons. The tendons (12) are usually made of hollow steel pipes. Steel pipe tendons are frequently watertight and internally sealed from the sea environment, filled with air at atmospheric pressure at sea level to reduce their weight in water and the resultant loading on the TLP. The tendons (12) terminate at their lower ends with bottom latch assemblies (50). The bottom latch assemblies form “stab” connectors that are received and locked into pilings (52) in a seabed foundation structure (54). The bottom latch assemblies (50) are usually designed to allow some tendon pivoting with respect to the foundation structure to accommodate limited lateral motion of the TLP due to wind, waves and currents.
The tendons (12) often accommodate tendon support buoys (TSBs) (30), which are temporarily secured to upper portions of the tendons to provide positive buoyancy and maintain the tendons in a vertical orientation prior to and during TLP installation. After the TLP is locked-off to the tendons and de-ballasted to tension the tendons, the TSBs are usually neither required nor desired to be carried on the tendons, as they increase wave loading on the TLP.
Tendons are subject to a number of competing design criteria, including considerations such as TLP size and design, expected environmental loads from wind and currents, the amount of allowed set-down and depth of water at the mooring location, the number of mooring tendons, tendon material, corrosion effects, and cost concerns. Aside from these considerations, tendon design usually strikes a compromise between the cross-sectional area of the pipe required for tensile strength (which can be expressed in terms of outer diameter and wall thickness), the wall thickness required to withstand bending moments, the ability to withstand external crushing force of the sea pressure at depth (which is a function of the outer diameter to wall thickness ratio, D/t), and buoyancy (which also can be expressed in terms of D/t, where greater D/t results in greater buoyancy and D/t equal to about 30 indicates a neutrally buoyant steel tendon). D/t ratios may thus vary along the length of the tendon (12) to achieve the desired overall tendon characteristics.
As deep water production progresses and the mooring depth increases, the lower portions of the tendons are subjected to increased hydrostatic pressure, which can cause buckling or crushing of the tendon. To prevent tendon failure under high seawater pressures, a strength of materials analysis shows that smaller D/t ratios are required for those portions of the tendon (12) located in deep water. In other words, without changing the tendon, material, tendons require smaller outer diameters and/or greater wall thicknesses. Unfortunately, greater wall thicknesses, coupled with longer length of the tendons, can result in tendon weights that exceed a TLP's support capacity. Larger TLPs may be required to support the heavier tendons, but larger TLPs may not be cost effective.
Alternatively, instead of increasing wall thickness, the lower portions of the tendons can be pressurized to balance the net tendon pressure at depth, as taught by U.S. Pat. No. 4,521,135 issued to Silcox and U.S. Pat. No. 6,682,266 issued to Karal et al., or the interior of the lower portion of the tendons can flooded with seawater in free communication with the exterior environment, as taught by U.S. Pat. No. 4,630,970 issued to Gunderson et al. and U.S. Pat. No. 5,683,206 issued to Copple. Composite and fiber tendons are also known in art that attempt to address the problem associated with heavier tendons as the depth increases. See, for example, U.S. Patent Publication No. 2005/0244231 for Liao et al.
A preferred method for reducing tendon weight is to increase the buoyancy of the tendon, usually by increasing the volume of displaced water. This is often accomplished by strapping permanent buoyancy modules near the tops of the tendons. Alternatively, buoyancy of the tendon may be increased by increasing the tendon outer diameter, usually along the upper portion of the tendon, and in particular, at the of the tops of the tendons. However, it is usually not desirable to increase the diameter of the tendon near the sea floor, where the tendon is subjected to increased hydrostatic pressure. Thus, hollow, sealed, and stepped-diameter tendons, having sea level atmospheric pressure air therein, are known in the art. These stepped tendons seek to achieve neutral or slightly positive tendon buoyancy, by having an uppermost section with a large outer diameter and positive buoyancy that compensates for the negatively buoyant lower section that has a smaller diameter and thicker wall.
For example, U.S. Pat. No. 6,851,894 issued to Perret et al. discloses a tendon having an upper section of large diameter, an intermediate section of smaller diameter, and a lower section of smallest diameter. The upper section attaches to the TLP. Due to the large diameter of the upper section, that section is positively buoyant. The buoyancy of the upper section compensates for the weight of the heavy lower section so that the overall buoyancy of the tendon is close to neutral.
A drawback of having larger diameter tendons at the TLP is that the TSBs used during installation must in turn be enlarged to fit around the upper tendon section, and the larger tendons have increased surface area subjected to waves and current. Furthermore, if vortex fairings are to be used, they must also be larger.
3. Identification of Features Provided by Some Embodiments of the Invention
A primary object of the invention is to provide a tendon with an uppermost section that has a reduced diameter that allows smaller tendon support buoys and/or vortex fairings to be used, resulting in lower cost.
Another object of the invention is to provide a tendon with an uppermost section that has a reduced diameter that is results in reduced drag due to waves and currents.
Another object of the invention is to provide a tendon with reduced weight in water to reduce loading on the TLP.
Another object of the invention is to provide a stepped tendon, with the benefits thereof, but that retains advantages of having a reduced diameter uppermost section.
The objects identified above, as well as other features of the invention are incorporated, in a preferred embodiment, in a mooring system for TLPs including tendons having uppermost sections with reduced diameters. Introducing reductions in the diameter of the uppermost sections of the tendons provides reduced drag due to waves and currents and accommodates smaller tendon support buoys and vortex fairings.
The entire tendon is preferably watertight and sealed from the ocean environment. The interior of the tendon is preferably filled with dry air at sea level pressure to reduce weight and increase buoyancy. One or more interior watertight bulkheads are preferably included in the tendon to maintain substantial watertight integrity in the event of a flooding casualty to one or more interior compartments.
Each tendon preferably has an uppermost pipe segment, an intermediate pipe segment axially connected to the bottom of the uppermost segment, and a lower pipe segment axially connected to the bottom of the intermediate pipe segment. The top of uppermost pipe segment is terminated with a length adjustment joint connector assembly that is adapted to connect to the TLP hull. The bottom of the lower pipe segment is terminated with a bottom latch connector assembly that is adapted to be received and locked into a foundation structure on the seabed.
The uppermost pipe segment has an outer diameter that is smaller than the outer diameter of the intermediate pipe segment. The reduced diameter of the uppermost segment provides reduced drag due to waves and currents and accommodates smaller tendon support buoys and vortex fairings, while the larger outer diameter of the intermediate pipe segment is used to increase overall tendon buoyancy. The lower pipe segment has an outer diameter that is smaller than the outer diameter of the intermediate pipe segment. The smaller outer diameter of lower pipe segment provides greater crush resistance to withstand larger hydrostatic pressures at depth.
In a second embodiment, the tendon has only an uppermost pipe segment and a lower pipe segment axially connected below it. The top of the uppermost pipe segment is terminated with a length adjustment joint connector assembly that is adapted to connect to the TLP hull. The bottom of the lower pipe segment is terminated with a bottom latch connector assembly that is adapted to be received and locked into a foundation structure on the seabed.
The uppermost pipe segment has an outer diameter that is smaller than the outer diameter of the lower pipe segment. The reduced diameter of the uppermost pipe segment provides reduced drag due to waves and connects and accommodates smaller tendon support buoys and vortex fairings, while the larger outer diameter of the lower pipe segment is used to increase overall tendon buoyancy.
In a third embodiment, the tendon has an uppermost pipe segment, an upper pipe segment, an intermediate pipe segment, and a lower pipe segment. The top of the uppermost pipe segment is terminated with a length adjustment joint connector assembly that is adapted to connect to the TLP hull. The upper pipe segment is axially connected to the bottom of the uppermost pipe segment, the intermediate pipe segment is axially connected to the bottom of the upper pipe segment, and the lower pipe segment is axially connected to the bottom of the intermediate pipe segment. The bottom of the lower pipe segment is terminated with a bottom latch connector assembly that is adapted to be received and locked into a foundation structure on the seabed.
The uppermost pipe segment has an outer diameter that is smaller than the outer diameter of the adjacent upper pipe segment. The intermediate pipe segment has an outer diameter that is smaller than the outer diameter of the upper pipe segment, and the lower pipe segment has an outer diameter that is smaller than the outer diameter of the intermediate pipe segment. The reduced diameter of the uppermost pipe segment provides reduced drag due to waves and currents smaller tendon support buoys and vortex fairings, while the larger outer diameter of the upper pipe segment is used to increase overall tendon buoyancy. The consecutively smaller outer diameters of the intermediate and lower pipe segments provide greater crush resistance as depth increases.
Although single, double, and triple stepped tendons are described herein, the invention covers tendons with even greater numbers of steps, provided the uppermost pipe segment (defined by its top connection to a length adjustment joint or other TLP connector) has a smaller outer diameter than at least one of the lower pipe segments and more preferably, the next lower adjacent pipe segment.
The invention is described in detail hereinafter on the basis of the embodiments represented in the accompanying figures, in which:
“Pipe segment” as used herein refers to a generally continuous portion of the tendon of a given outer diameter. A pipe segment may, however, have varying wall thicknesses and may be constructed of shorter lengths of pipe fastened together, as is typical in tendon construction. “Pipe Segment” is not intended to include either the upper or lower connector assemblies 27, 50.
The entire tendon 120 is preferably watertight and sealed from the ocean environment. The interior of tendon 120 is preferably filled with dry air at sea level pressure to reduce weight and increase buoyancy. One or more interior watertight bulkheads 140 are preferably included in tendon 120 to prevent flooding of the entire tendon 120 should a leak occur.
The outer diameter and wall thickness is selected for each point along the length of tendon 120 to carry tension from the buoyant and partially submerged TLP (which consists of a nominal tension plus tension variations due to functional and environmental loads), to maintain a necessary tendon stiffness, to achieve a desired buoyancy, and to withstand the crushing forces of the surrounding sea. Crushing force becomes more significant as the depth and hydrostatic pressure increases. At depths greater than around 1,000 meters, D/t of tendon 120 are preferably less than 30.
In a preferred embodiment, uppermost pipe segment 122 has an outer diameter that is smaller than the outer diameter of intermediate pipe segment 124. Uppermost pipe segment 122 is connected to intermediate pipe segment 124 by a short transition piece 130, although a longer transition piece with more a gradual change in diameter may be used. The reduced diameter of the uppermost segment 122 of the tendon 120 provides reduced drag due to waves and currents and accommodates smaller tendon support buoys and vortex fairings. The larger outer diameter of the intermediate pipe segment is used to increase overall tendon buoyancy. Lower pipe segment 126 has an outer diameter that is smaller than the outer diameter of intermediate pipe segment 124. The smaller outer diameter of lower pipe segment 126 provides greater crush resistance to withstand larger hydrostatic pressures at depth. Lower pipe segment 126 may have a larger, equal, or smaller outer diameter compared to the uppermost pipe segment 122 outer diameter. Lower and intermediate pipe segments 126, 124 are joined by a transition piece 132. The length adjustment joint 27 may have a smaller diameter than uppermost pipe segment 122, but it is not considered to be part of the uppermost pipe segment 122.
For example, a steel tendon for use at a depth of 4375 feet and 144 foot TLP hull draft may have an uppermost pipe segment 122 about 200 feet tall with an outer diameter of 36 inches and a wall thickness of 1.70 inches (D/t≈21), an intermediate pipe segment 124 about 1970 feet tall with an outer diameter of 44 inches and wall thickness along the upper third of 1.33 inches (D/t≈33) and the lower two-thirds of 1.44 inches (D/t≈30.5), and a lower pipe segment 126 about 2025 feet tall with an outer diameter of 36 inches and a wall thickness along the upper half of 1.50 inches (D/t=24) and the lower half of 1.55 inches (D/t≈23.2). Transition pieces 130, 132 are preferably about 3.5 feet tall each.
The entire tendon 220 is preferably watertight and sealed from the ocean environment. The interior of tendon 220 is preferably filled with dry air at sea level pressure to reduce weight and increase buoyancy. One or more interior watertight bulkheads 240 are preferably included in tendon 220 to prevent flooding of the entire tendon 220 should a leak occur.
The outer diameter and wall thickness is selected for each point along the length of tendon 220 to carry tension from the buoyant and partially submerged TLP (which consists of a nominal tension plus tension variations due to functional and environmental loads), to maintain a necessary tendon stiffness, to achieve a desired buoyancy, and to withstand the crushing forces of the surrounding sea. Crushing force becomes more significant as the depth and hydrostatic pressure increases. At depths greater than around 1,000 meters, D/t of tendon 220 are preferably less than 30.
In the embodiment of
The entire tendon 320 is preferably watertight and sealed from the ocean environment. The interior of tendon 320 is preferably filled with dry air at sea level pressure to reduce weight and increase buoyancy. One or more interior watertight bulkheads 340 are preferably included in tendon 320 to prevent flooding of the entire tendon 320 should a leak occur.
The outer diameter and wall thickness is selected for each point along the length of tendon 320 to carry tension from the buoyant and partially submerged TLP (which consists of a nominal tension plus tension variations due to functional and environmental loads), to maintain a necessary tendon stiffness, to achieve a desired buoyancy, and to withstand the crushing forces of the surrounding sea. Crushing force becomes more significant as the depth and hydrostatic pressure increases. At depths greater than around 1,000 meters, D/t of tendon 320 are preferably less than 30.
In the embodiment of
Although single, double and triple stepped tendons are described herein, the invention covers tendons with even greater numbers of steps, provided the uppermost pipe segment (defined by its top connection to a length adjustment joint or other TLP connector) has a smaller outer diameter than at least one of the lower pipe segments and more preferably, the next lower adjacent pipe segment.
The Abstract of the Disclosure is written solely for providing the United States Patent and Trademark Office and the public at large with a means by which to determine quickly from a cursory inspection the nature and gist of the technical disclosure, and it represents solely a preferred embodiment and is not indicative of the nature of the invention as a whole.
While some embodiments of the invention have been illustrated in detail, the invention is not limited to the embodiments shown; modifications and adaptations of the above embodiment may occur to those skilled in the art. Such modifications and adaptations are in the spirit and scope of the invention claimed herein:
