Light-weight topsides for heavy-weight projects
The weight of an offshore platform's topsides affects the overall economics of the project.
The weight of an offshore platform's topsides affects the overall economics of the project. Designing and fabricating topsides that minimize the number of modules required – thus reducing weight – is proven to minimize costly offshore hookup and commissioning of a new installation. Accurate prediction and management of the topsides' weight and center of gravity during the early phases of design leads to the successful completion and delivery of light-weight topsides for heavy-weight projects.
Project life-cycle analysis
KBR has established a work process and weight management system to deliver light-weight topsides, which can eliminate schedule delays and cost increases. The weight management process starts during the early stages of the project life-cycle. In the early conceptual phase, the design is not fixed so various options can be evaluated quickly. As the project progresses, the ability to influence weight starts to diminish and the cost of reducing weight starts to increase.
One key component in the execution of offshore technology is the ability to provide accurate weight prediction in a short period of time. This is done using historical data and a 3D model, which is ideal for screening design options. The historical data includes weights and dimensions of topsides components. If a proper topsides weight and center of gravity estimate has not been done in the front-end engineering and design (FEED), the risks of schedule delays and cost escalation during detailed design increase.
By using the 3D model, piping, and structural weight reports can be generated along with an equipment list.
Furthermore, a well-defined scope improves the definition of the equipment count, and therefore the weight, particularly in areas where design for future growth and decommissioning are involved. A poor scope definition often leads to ambiguity and increased estimating uncertainty.
One important aspect of weight management is use of reliable historical benchmark data. This information can be used in early project phases to ensure that the proposed design is within the historical range.
Integrated approach in weight management
Once the FEED phase is completed and the overall scope of the project is defined, a project will move forward to Detailed Engineering and the Procurement and Construction (EPC) phase. It is well known that structural, equipment, and piping are the main components of the total dry weight of a topsides facility.
Emphasis is often placed on minimizing the structural weight. However, a proper weight management starts with the driver of the facility's physical layout – equipment.
|Typical 3D design models for topsides equipment. Left: a module from an FPSO. Right: multiple modules on a compliant tower.|
From the big picture perspective, once the process design is determined, the topsides weight is affected most directly by the equipment number and sizes, layout, piping, structural, and riser (and other topsides) loads.
Layout improvement is a centerpiece of weight management. To minimize topsides weight, the total deck area is minimized by employing the following reviews:
• Optimize the number of platform cranes by evaluating capacities, boom lengths, and required crane lifts
• Optimize electrical switchgear/motor control center building configurations
• Optimize equipment locations by "stacking" equipment and use of individual equipment access platforms for more than one piece of equipment
• Minimize piping runs by consolidating equipment in central locations and optimizing piperack locations
• Use portable lifting devices instead of fixed ones
• Reduce heights between decks while considering maintainability
• Optimize equipment layout by mirror imaging to reduce pipe runs.
|An example of the benchmark data from KBR's weight management database. The x-axis is the total deck area and the y-axis is the total topsides weight. The graph is based on over 50 platforms that can be filtered to better match each project and the type of facilities such as FPSOs, spars, TLPs, and semisubmersibles. It can also be compared to public domain data for platforms designed by others.|
Equipment optimization is another important aspect to minimize offshore weight. Topsides modules are comprised of many vendor packages. In projects where the engineering company is the overall EPC contractor, it is necessary to engage the equipment package suppliers early to ensure each supplier's weight reporting method will lead to predictable results, and to ensure that lighter weight options are fully explored. In addition, the following equipment issues are routinely examined:
• Provide fit for purpose designs
• Design for current capacity and avoiding over designing for the future
• Select lighter weight equipment designs when multiple offerings are available from proprietary equipment suppliers
• Minimize living quarters size and use aluminum structure members
• Reduce resident time in vessels.
For piping, an engineering contractor can investigate the use of non-metallic materials such as reinforced thermosetting resin pipe (RTRP) for selected utility systems to reduce the total weight. However, rigorous numerical analysis often is required to ensure that the non-metallic materials can meet the stress requirement for all the foreseeable operating conditions. Hydraulic transient analysis also must be conducted properly to minimize the impact of surge pressure on the non-metallic piping. Generally speaking, RTRP is limited to utilities.
Structural steel is the largest single contributor to topsides weight of offshore facilities. The structural weight primarily depends on the topsides equipment weight, deck area, and loads. The loads include lift, transportation and handling, in-place motions, riser, drilling, and safety considerations (e.g. blast, fire). With a rigorous weight management methodology, proper selection of contingencies, and change control during the early stages of projects, topsides structural weight usually stays relatively constant during project phases. Hence, once the topsides weight has been properly estimated, the total topsides dry weight is kept within a manageable range throughout the entire design process. The result is the final weight and center of gravity are within the targets established during initial design.
An example of a platform in which this practice was conducted is Chevron's Tombua- Landana compliant piled tower, installed offshore Angola. Here, topsides weight was kept within the design limit while sensitive design constraints were satisfied.
Another example of reducing topsides weight is a recent project where the piperack on a newbuild FPSO was fully integrated into the adjacent portside modules and thus fully supported from the existing portside module support stools. Normally, piperacks are separate modules supported by their own module support stools and located in the center of the vessel between the port and starboard modules. In order to eliminate the stool supports for the piperack modules, eliminate the piperack module lifts, and minimize the integration hook-up between the piperack modules and the adjacent portside set of modules, the piperacks on this FPSO were integrated with the portside modules by locating them along the inside of these modules running down the length of the ship. Though additional main and cantilevered structure was added to the portside modules to support these piperacks, there was considerable overall weight, cost, and schedule savings accrued by eliminating hull support stools, installation lifts, and pipe hook-up and integration involving eight significant piperack modules.
|The Tombua-Landana platform. Photo courtesy of Chevron.|
In addition to piperacks, well riser loads can be significant in determining the total weight of the topsides for floating facilities that accommodate dry trees, especially in deeper water. In order to reduce these loads, consideration should be given to:
• Single versus dual casing risers. Dual casing directly and significantly increases the riser load compared to single casing. A risk assessment should assess the benefit of dual casing over single casing, depending on the client's policies and relevant codes
• Buoyancy to reduce both vertical and lateral loading on the risers. As always, capital costs and maintenance expense must be weighed against the material and weight savings
• The span between supports for the structure the carries the TTR (Top Tensioned Riser) loads. This significantly impacts the structural steel required and thus the facility weight.
Anything that can be done to reduce the TTR loads and/or the supporting span length will make a notable difference in the topsides weight. Furthermore, the way these loads are carried into the hull is also a key consideration. Integrated analyses of the topsides and hull can be used early in the project to determine the potential reduction in topsides weight achievable by using knee braces or other means to transfer the load into the hull.
Other execution techniques used to ensure lightweight topsides design are to:
1. Use the Value Improvement Process (VIP)
2. Enroll clients in the weight savings process
3. Establish target weights for each discipline by benchmarking
4. Implement weight reduction challenges during the FEED phase
5. Use PDMS and proprietary weight management software coupled with the equipment weight provided by the suppliers to provide weight and center of gravity (COG) of the design
6. Establish weight control philosophy, weight estimate, and contingency to mitigate weight variation during the design phase
7. Bring awareness to specific lifting requirements for installation or hull sensitivities to control the COG.
The process outlined above must be implemented throughout the execution of project phases via project life-cycle analysis, with continuous input from all engineering disciplines and stakeholders.
The authors thank the many individuals within KBR whose contributions in various offshore projects made this paper possible. Specifically, the authors acknowledge KBR's weight management team for its input to this paper: Marlowe Bentley, Jeffrey Feng, Steve Gunzelman, Bob Hilton, Ricky Leblanc, Loriana Morris, David Sagastegui, Hoss Shariat, Mike Simpson, Ralph Thrasher, and Hieu Tran.