METHOD FOR CONFIGURING THE PONTOONS OF A SEMI-SUBMERSIBLE HULL STRUCTURE

Description

FIELD AND BACKGROUND

The present invention broadly relates to the design of semi-submersible floating structures to produce oil and natural gas, with a specific focus on the pontoons of these hull structures.

A range of floating structures are utilized in offshore oil and gas production in deeper waters. Each structure offers its own set of benefits and drawbacks related to motion characteristics, which may make it appropriate for specific environmental conditions. The semi-submersible is among the most favored floating structures, as it possesses several distinct features compared to other floating structures like Spars and Tension Leg Platforms (TLPs).

Conventional semi-submersible structures, as illustrated in FIG. 1, offer several benefits, including excellent stability from a large footprint and low center of gravity, reduced steel tonnage, and potential drilling capabilities. They accommodate numerous risers due to ample pontoon space and allow quayside assembly, which cuts costs and speeds up scheduling. With a short development timeline and low initial investment, they support shallow draft for global integration. They provide greater payload capacity than Spars and operate in deeper water than TLPs, and they are easier to install than both Spars and TLPs.

Simple, regular steel catenary risers (SCRs) are favored for deep and ultra-deep waters because of their simplicity and lower cost. The “simple” shape of a regular SCR consists of a free-hanging steel catenary riser without intermediate buoys or floating devices. The decision to use SCRs relies on the floater's motion and velocity characteristics, particularly the vertical motion and velocity at hang-off locations. Depending on the environmental conditions, water depth, diameter of the SCR, and its internal pressure and fluid temperature, the permissible vertical motion and velocity under extreme environmental conditions are typically outlined in the basis of design provided by the owner.

The conventional semi-submersible also exhibits several drawbacks. The most prominent is its substantial heave (vertical) motions in extreme sea states, which can induce strength and fatigue problems in SCRs more rapidly, demanding more rigorous and expensive design measures. For platforms with large-diameter SCRs, solving these issues might become technically or economically impractical. As a result, conventional semi-submersibles are not ideal for simple SCR configurations.

The approaches to achieve low motion characteristics in semi-submersibles, as investigated by the industry, generally fall into the following categories: 1) Reduce the width of one of the four pontoons, as displayed in FIG. 2; 2) Configure complex column shapes (five-sided and six-sided columns), as depicted in FIG. 3; 3) Adopt a ring pontoon and position four columns inside the ring pontoon, as shown in FIG. 4.

The drawbacks of the first alternative mentioned are as follows: 1) it only marginally improves heave motions and is suitable for minor adjustments; 2) it results in an asymmetrical structure layout and affects load transfer among structural members; 3) it complicates construction since structural members are not interchangeable.

The shortcomings of the second alternative mentioned are as follows: 1) it greatly complicates the design of the columns; 2) it results in significantly unfavorable column compartmentation because the irregular spaces are difficult to use; 3) it considerably complicates construction due to the irregular column shapes and associated components; 4) it impedes the arrangement of marine systems due to the constrained and irregular space within the columns, potentially leading to access, egress, inspection, and safety issues.

The disadvantages of the third alternative mentioned are as follows: 1) It raises significant concerns about how the ring pontoon connects to the four columns. The ring pontoon bears substantial loads, with only eight sides of the columns (two sides per column) connecting to it and distributing loads among the columns and the ring pontoon, making this arrangement practically unachievable; 2) It leads to considerable access issues from the columns to the ring pontoon, causing problems with access, egress, inspection, and safety.

The industry is striving to significantly reduce overall project development costs, including those related to hulls and risers. A holistic approach demands not only reducing hull costs but also ensuring that its motion and velocity at hang-off locations are compatible with simple regular SCRs, which are a proven cost-effective solution. A hull that accommodates simple regular SCRs is essential to meet market demands.

This invention is designed to feature a simplified design, accelerated construction, rapid assembly and installation, and a hull compatible with simple regular SCRs. The technology described provides an alternative solution to optimize efficiency, reduce costs, and enhance the overall economic feasibility of deep and ultra-deep water development projects.

SUMMARY OF THE INVENTION

In FIG. 5 and FIG. 6, the present embodiment features a design that adheres to a simple philosophy by retaining the traditional column configuration of a semi-submersible, as shown in FIG. 1, while modifying the pontoons to optimize vertical motion characteristics for simple regular SCR applications and minimize hull steel weight.

The columns rise from the keel plate, pass through the mean water level (MWL), and extend above it to a designated height. Each base pontoon is center-hollowed and aligned with its corresponding column along a central vertical axis, with two connection pontoons forming a 90-degree angle. Four columns are arranged in a square formation, each coupled to a base pontoon, and the four base pontoons connect to the four connection pontoons. One end of each connection pontoon connects to the middle of a base pontoon, while the other connects to the adjacent base pontoon, forming a closed-loop pontoon structure. In FIG. 6, the central axis of the connection pontoon aligns with the vertical axes of both the column and the base pontoon.

In FIG. 5, “cs” stands for column central to central spacing; “cw” represents column width; “pl” means connection pontoon length; “pw” is connection pontoon width; “ph” stands for pontoon height; “ch” represents column height above the pontoon; “d” means the platform draft; “bw” is the base pontoon width extending outward from the column. By neglecting the rounded corners of the columns and base pontoons, the volume of four columns is Vc=4*(ch+ph)*cw{circumflex over ( )}2, the volume of four base pontoons is Vb=4*bw*(bw+cw)*ph and the volume of four connection pontoons is Vcp=4*pl*pw*ph. Total pontoon volume Vp=Vb+Vcp. The optimal vertical motion characteristics depend on the hydrodynamic interactions of the columns, base pontoons, and connection pontoons. For simplicity in the present embodiment, the column dimensions and column spacing are held constant, with only the base pontoon width, connection pontoon width, and pontoon height being adjusted.

In FIG. 5 and FIG. 6, from a pure hydrodynamics point of view, a smaller connection pontoon width results in better vertical motion of the platform. However, for the given pontoon height (ph), the connection pontoon width (pw) can't be too small due to minimum structural strength requirements for the connection pontoons during extreme environmental conditions. For simplicity, the minimum allowable connection pontoon width can be set at 0.3*cw. Since the present design philosophy is to keep features of traditional semi-submersibles, such as the capability of quayside integration of the hull and topsides, the minimum volume of the platform at the specified draft at quayside is another constraint for the parameter study. The minimum allowable pontoon height can be derived accordingly. For simplicity, the minimum allowable pontoon height can be set at 0.35*cw. Since the topsides payloads are given based on design and holding column configuration, and the column span remains unchanged, allocating Vb and Vcp explores optimizing the vertical motion of the platform. There are at least two ways to allocate Vb and Vcp: 1) If fixing pontoon height, systematically increase/decrease base pontoon width and decrease/increase connection pontoon width—no less than the minimum allowable connection pontoon width; 2) If varying pontoon heights—no less than the minimum allowable connection pontoon height, systematically vary base pontoon width and connection pontoon width—no less than the minimum allowable connection pontoon width. By comparing the resultant vertical motion and velocity, as well as the resultant platform volume of all parameter cases, the optimal solution can be derived accordingly.

In FIG. 7 and FIG. 8, the first alternative embodiment of the base pontoons and connection pontoons is illustrated. Unlike the arrangement in FIG. 5 and FIG. 6, the base pontoons remain in place while the connection pontoons are shifted outward to align with the external walls of adjacent base pontoons. This creates a continuous ring pontoon from the external walls of the four base and connection pontoons.

In FIG. 7b and the bottom view of FIG. 8, virtual dashed lines indicate the boundaries between the base pontoons and connection pontoons. The central axial lines of the connection pontoons are shown misaligned with the central horizontal axes of the corresponding columns and base pontoons.

To address this misalignment and facilitate load sharing, a support frame is introduced, connecting the base and connection pontoons at their midpoint. This support frame enhances the overall effectiveness and reliability of the hull structure, ensuring structural integrity and performance under various conditions.

In the second alternative embodiment shown in FIG. 9 and FIG. 10, the configuration of the base and connection pontoons has been updated. The previous support frame has been removed, and the inner corners of the base pontoons are symmetrically cut off and replaced with an inner vertical wall that intersects with two adjacent connection pontoons. This change transforms the base pontoons' external contour into a trapezoidal shape, while the connection pontoons are shortened at both ends to fit this new design. The four columns and their spacing remain unchanged.

In FIG. 9b and the bottom view of FIG. 10, virtual dashed lines indicate the boundaries between the base and connection pontoons. Central axial lines of the connection pontoons are depicted along their lengths, misaligned with the central horizontal axes of the corresponding columns.

To address this misalignment, the local inner corners at the intersections of the base and connection pontoons will be reinforced to enhance structural strength and fatigue performance. This modification offers an alternative configuration for the semi-submersible hull structure.

In FIG. 11 and FIG. 12, the third alternative embodiment of the base and connection pontoons is illustrated. In this configuration, the inner wall of the base pontoon is shifted further toward the center of the corresponding column, exposing the column's inner corner. The intersection lines between the inner wall and the column corners are positioned slightly away from the rounded edges to prevent overstressing, resulting in separation between the inner corners of the base pontoons and the column corners. This reduction in the base pontoons'volume allows for an increase in both the length and width of the connection pontoons, while the four columns and their spacing remain unchanged.

In FIG. 11b, virtual dashed lines indicate the boundaries between the base and connection pontoons, with similar lines in the bottom view of FIG. 12. Central axial lines of the connection pontoons are depicted, though they are misaligned with the central horizontal axes of the corresponding columns. The degree of misalignment in this design is less severe than in previous figures, and the vertical cross-section of the connection pontoons is significantly larger. To address this misalignment, the local inner corners at the intersections of the base and connection pontoons will be reinforced for structural strength and fatigue performance. This reinforcement will be less extensive than that in FIG. 9 and FIG. 10, reflecting the reduced misalignment severity. This design modification provides an alternative option for the semi-submersible hull structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a traditional four-column semi-submersible hull structure (prior art), a) an elevation view; b) a top view and c) a perspective view.

FIG. 2 illustrates an alternative four-column semi-submersible hull structure (prior art) that features a design with one of the pontoons narrowed, a) an elevation view; b) a top view and c) a perspective view.

FIG. 3 displays an alternative four-column semi-submersible hull structure (prior art) that incorporates a combination of five-sided and six-sided columns, a) an elevation view; b) a top view and c) a perspective view.

FIG. 4 shows an alternative four-column semi-submersible hull structure (prior art) features a continuous external ring pontoon that links all four columns, positioned within the ring, a) an elevation view; b) a top view and c) a perspective view.

FIG. 5 depicts the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure includes a) an elevation view; b) a top view and c) a perspective view.

FIG. 6 illustrates the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure, featuring a perspective view of the pontoons only (upper) and a bottom view (lower).

FIG. 7 displays the first alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure includes a) an elevation view; b) a top view and c) a perspective view.

FIG. 8 shows the first alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure features a perspective view of the pontoons only (upper) and a bottom view (lower).

FIG. 9 depicts the second alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure includes a) an elevation view; b) a top view and c) a perspective view.

FIG. 10 illustrates the second alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure features a perspective view of the pontoons only (upper) and a bottom view (lower).

FIG. 11 displays the third alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure includes a) an elevation view; b) a top view and c) a perspective view.

FIG. 12 shows the third alternative of the present embodiment of the pontoon configuration for a four-column semi-submersible hull structure features a perspective view of the pontoons only (upper) and a bottom view (lower).

DETAILED DESCRIPTION

In FIG. 5, the method of the present embodiment adheres to a straightforward design philosophy, preserving the traditional semi-submersible column configuration depicted in FIG. 1 while modifying only the pontoons of the traditional semi-submersible hull structure. This modification aims to optimize vertical motion characteristics for simple regular SCR applications with minimizing hull steel weights. Column 101 extends from the keel plate, rises upward, passes the mean water level (MWL), and terminates at a sufficient height above the MWL.

In FIG. 6, base pontoon 102 is center-hollowed and coupled with the corresponding column 101, positioned at the center of the hollowed base pontoon 102 along the same central vertical axis. The column starts at the keel plate level and ends at a designated height above MWL. The segment of column 101 from the keel plate level to the pontoon height is considered part of column 101, not part of base pontoon 102. Additionally, base pontoon 102 connects to two connection pontoons 103, positioned at a 90-degree angle, starting from the keel level and maintaining the same pontoon height.

The distance from the center of base pontoon 102 to the center of the adjacent base pontoon 102 matches the distance from the center of corresponding column 101 to the center of the adjacent corresponding column 101. To meet stability requirements, the center-to-center spacing and dimensions of column 101 are established, which also define the center-to-center distance of base pontoon 102 and the dimensions of the center-hollowed horizontal cross-section.

Four columns 101 are arranged in a square formation around the hull origin, as shown in the bottom view of FIG. 6, each coupled with its corresponding base pontoon 102. Horizontally oriented connection pontoons 103 link the base pontoons, forming a cohesive semi-submersible hull structure, as illustrated in FIG. 5c. One end of each connection pontoon 103 connects to the midpoint of its corresponding base pontoon 102's side wall, while the other end connects to the midpoint of an adjacent base pontoon 102's opposite side wall. As illustrated in the bottom view of FIG. 6, the central axis of each connection pontoon 103 aligns with the central horizontal axis of both the corresponding column 101 and base pontoon 102 along its length.

For the specified topsides payloads, metocean criteria, and stability requirements of a semi-submersible hull, the width of column 101 (cw), the central spacing between columns 101 (cs), and the underwater height of column 101 (ch+ph or d) can be initially estimated, as illustrated in FIG. 5. The total pontoon volume, which comprises the volume of base pontoon 102 plus the volume of connection pontoon 103, can also be derived based on the optimal volumetric ratio of column 101 volume to pontoon volume. Pontoon volume is calculated as the product of pontoon height (ph) and the horizontal cross-sectional area of the pontoons, which includes the areas of both base pontoon 102 and connection pontoon 103.

Two methods can be employed to conduct parametric studies: 1) keeping the pontoon horizontal cross-sectional area constant while varying the pontoon heights, and 2) keeping the pontoon height constant while varying the pontoon horizontal cross-sectional areas. Given that the pontoon horizontal cross-sectional area encompasses both base pontoon 102 and connection pontoon 103 areas, how these areas are allocated is crucial for achieving optimal low vertical motion and minimizing hull steel weights.

By neglecting the rounded corners of base pontoons 102, the horizontal cross-sectional area of the four base pontoons 102 can be calculated as 4*bw*(bw+cw), where bw represents the base pontoon 102 width extending outward from column 101. The horizontal cross-sectional area of the four connection pontoons 103 is calculated as 4*pl*pw where pl denotes connection pontoon 103 length and pw represents connection pontoon 103 width.

The optimal vertical motion characteristics depend on the hydrodynamic interactions among columns 101, base pontoons 102, and connection pontoons 103. For simplicity in the present embodiment, the dimensions and spacing of columns 101 are held constant, with adjustments made only to the widths of base pontoon 102 and connection pontoon 103, as well as the pontoon height.

By considering the first option of the parametric studies—keeping the pontoon horizontal cross-section area constant while varying the pontoon heights. The present embodiment design philosophy emphasizes maintaining features of traditional semi-submersibles, such as the capability for quayside integration of the hull and topsides. A key constraint for the parameter study is the minimum volume of the platform at the specified draft at quayside. Given the assumed horizontal cross-section area of the pontoon, the minimum allowable pontoon height can be derived accordingly.

Additionally, for the specified width of the connection pontoon 103, the connection pontoon height (ph) cannot be too small due to structural strength requirements during extreme environmental conditions. By comparing the minimum pontoon heights based on the requirements for both minimum volume at quayside and minimum structural strength, a resultant minimum pontoon height can be established. For simplicity, the minimum allowable pontoon height can be set at 0.35*cw.

Experience from hydrodynamic analysis indicates that as pontoon height increases, vertical motion performance deteriorates. Therefore, the maximum pontoon height is typically kept below half of the platform draft “d”. By varying the pontoon heights from the minimum to the maximum allowable heights, the vertical motion response amplitude operators (RAOs) can be calculated accordingly.

By considering the second option of the parametric studies—keeping the pontoon height constant while varying the pontoon horizontal cross-sectional areas—it is crucial to effectively allocate the horizontal cross-sectional areas between base pontoon 102 and connection pontoon 103 to achieve optimal vertical motion and minimize hull steel weights. With the width of column 101 (cw) already determined, the base pontoon 102 width (bw) can vary from a minimum of 0.2*cw to maximum of 1.0*cw. The connection pontoon 103 width (pw) can be derived based on the total pontoon volume and the known pontoon height (ph).

In FIG. 5 and FIG. 6, for the specified pontoon height (ph), the minimum allowable connection pontoon width (pw) is established to meet structural strength requirements during extreme environmental conditions. For simplicity, this minimum allowable width can be set at 0.3*cw. If the derived connection pontoon 103 width is less than this minimum, the minimum allowable width will be used, prompting a recalculation of the corresponding base pontoon 102 width. The maximum allowable connection pontoon 103 width is defined as not exceeding 2*cw.

For a selected pontoon height, if a specific horizontal cross-sectional area is chosen, the allocation of cross-sectional areas between base pontoon 102 and connection pontoon 103 can vary. If the horizontal cross-sectional area of base pontoon 102 is assumed, the base pontoon width, the cross-sectional area of connection pontoon 103, and the corresponding connection pontoon width (pw) can be calculated. Conversely, if the cross-sectional area of connection pontoon 103 is assumed, the connection pontoon width (pw) and the corresponding base pontoon area and width can be derived. The calculated connection pontoon 103 width must be checked against the minimum and maximum allowable widths. If the derived width falls within these boundaries, it is retained; otherwise, the closer of the minimum or maximum allowable widths is adopted. Similarly, for base pontoon 102, if the derived width is within its allowable range, it is kept; otherwise, the closer of the minimum or maximum is used.

A parametric study case matrix can be structured as follows: for the given pontoon height, 1) vary base pontoon 102 width (bw) from 0.2*cw to 1.0*cw and calculate the corresponding connection pontoon 103 width; 2) vary connection pontoon 103 width from 0.3*cw to 2.0*cw and derive the corresponding base pontoon 102 width. Cases falling outside the minimum and maximum allowable widths will be excluded, resulting in a final parametric study case matrix for hydrodynamic analyses.

By comparing the resultant vertical motion response amplitude operators (RAOs) from all cases, the optimal parametric combination can be identified, leading to an optimal low vertical motion while minimizing hull steel weights.

In FIG. 7 and FIG. 8, the first alternative embodiment configuration of the base pontoons 102 and connection pontoons 103 is illustrated. Unlike the arrangement depicted in FIG. 5 and FIG. 6, the positions of the base pontoons 102 remain unchanged, while the connection pontoons 103 have been shifted outward to align with the external walls of two corresponding adjacent base pontoons 102. This configuration allows the external walls of the four base pontoons 102 and four connection pontoons 103 to form a continuous ring pontoon, thereby enhancing the structural integrity of the design and hydrodynamic performance of hull motion response characteristics.

Additionally, a support frame consisting of four long structural members 104, four short structural members 105, and four slanted structural members 106 connects the four base pontoons 102 and four connection pontoons 103 at the midpoint of the pontoon height, providing essential structural support. The long structural members 104 connect two base pontoons near their inner corners at the midpoint of the pontoon height. The short structural members connect the center of the long structural members 104 to the center of the connection pontoons 103. The slanted structural members 106 connect corresponding adjacent long structural members 104 at one-quarter of the long structural member length on both sides.

This support frame arrangement significantly enhances the overall effectiveness and reliability of the pontoon structure, ensuring structural integrity and performance under various conditions.

In FIG. 7b, virtual dashed lines are added between the base pontoons 102 and connection pontoons 103 to indicate their virtual boundaries. Similarly, in the bottom view of FIG. 8, virtual dashed lines are also included between the base pontoons 102 and connection pontoons 103.

Additionally, central axial lines of connection pontoons 103 are depicted along their lengths, which are misaligned with the central horizontal axes of the corresponding columns 101/base pontoons 102.

To address this misalignment and facilitate load sharing and transfer, a support frame is introduced, as shown in both FIG. 7 and FIG. 8. This support frame helps mitigate the negative effects arising from the misaligned central axes of columns 101/base pontoons 102, and connection pontoons 103, thereby enhancing the structural integrity and stability of the semi-submersible pontoon structure.

In the second alternative embodiment depicted in FIG. 9 and FIG. 10, the configuration of the base pontoons 102 and connection pontoons 103 has been revised. The previously established support frame connecting the base and connection pontoons, shown in FIG. 7 and FIG. 8, has been eliminated. Instead, the inner corners of base pontoons 102 have been symmetrically truncated and replaced with an inner vertical wall that intersects with the adjacent connection pontoons 103. This alteration transforms the original square external contour of base pontoons 102, as shown in FIG. 5 to FIG. 8, into trapezoidal-shaped external contour base pontoons 107.

As a result, the lengths of the original connection pontoons 103, illustrated in FIG. 5 to FIG. 8, have been reduced at both ends to accommodate this new design, leading to shorter connection pontoons 108. Throughout this modification process, the positions, spacing, and dimensions of all four columns remain unchanged. This design change provides an alternative option for the pontoon configuration of a semi-submersible hull structure, eliminating the need for any support structural components while maintaining structural integrity and performance.

In FIG. 9b, virtual dashed lines are added between the base pontoons 107 and connection pontoons 108 to indicate their virtual boundaries. Similarly, in the bottom view of FIG. 10, virtual dashed lines are also included between the base pontoons 107 and connection pontoons 108.

Additionally, central axial lines of connection pontoons 108 are depicted along their lengths, which are misaligned with the central horizontal axes of the corresponding columns 101.

To address this misalignment, the local inner corners at the intersections of the base pontoons 107 and connection pontoons 108 will be enhanced to ensure the structural strength and fatigue performance of the semi-submersible pontoon structure. This reinforcement will help maintain the integrity of the overall design while accommodating the unique configuration of the pontoons.

In FIG. 11 and FIG. 12, the third alternative embodiment of the original base and connection pontoons (102 and 103) has been modified and illustrated. In this configuration, the inner vertical wall of base pontoon 107, as depicted in FIG. 9 and FIG. 10, is shifted further toward the center of the corresponding column 101, exposing the inner corner of column 101 externally. The intersection lines where the inner wall meets the inner corners of columns 101 are positioned slightly away from the rounded corner lines to avoid overstressing at those locations. This alteration transforms the trapezoidal-shaped base pontoons 107 shown in FIG. 9 and FIG. 10 into trapezoidal-shaped base pontoons with open inner vertical walls 109 since the inner corners of the base pontoons 109 are separated by the corresponding corners of the columns 101. Additionally, the reduction in the volume of the modified base pontoons 109 leads to an increase in both the length and width of the original connection pontoons 103 to the adjusted connection pontoons 110, while the dimensions of four columns 101 and their spacing remain unchanged. This design modification provides an alternative option for the pontoon configuration of a semi-submersible hull structure.

In FIG. 11b, virtual dashed lines are added between the base pontoons 109 and connection pontoons 110 to indicate their virtual boundaries. Similarly, the bottom view in FIG. 12 features virtual dashed lines between these pontoons. Central axial lines of connection pontoons 110 are also depicted along their lengths, though they are misaligned with the central horizontal axes of the corresponding columns 101.

The degree of misalignment in FIG. 11 and FIG. 12 is less severe than in the earlier figures, and the vertical cross-section of connection pontoon 110 is significantly larger than that of connection pontoon 108. To address this misalignment, the local inner corners at the intersections of base pontoons 109 and connection pontoons 110 will be reinforced to ensure structural strength and fatigue performance. This reinforcement will be less extensive than that in FIG. 9 and FIG. 10, reflecting the reduced severity of misalignment in this design.