6000 Ton Offshore Oil Rig Analyzed with Inexpensive Computerized Engineering Handbook

by

Herbert Roussel

Offshore oil drilling and production platforms in the Gulf of Mexico represent one of the world's most difficult structural analysis challenges. Such platforms must be capable of withstanding the 140 mile per hour winds and 70 foot high waves of a so-called "100 year storm". In any given year there is only a 1% chance of encountering a 100 year storm. During that last 20 years, a few storms likely developed the maximum design sea states. Hurricane Andrew of 1992 passed directly over several Roussel-designed platforms without damage. During a storm, the oil companies evacuated all personnel and set special valves designed to prevent an oil spill.

Because constructing the foundation for a structure offshore is much more expensive than on land, an offshore platform typically uses only from three to twelve pilings, depending on soil conditions -- far fewer than would be employed on a building of equivalent height. These pilings are large: 36 to 96 inch pipe piles that penetrate the sea floor as much as 500 feet compared to 12 to 18 inch piles 150 feet deep for a typical building of equivalent height. Loads transferred through the piling may be as high as 10,000,000 pounds. This means that analysis of such a structure -- and particularly the load path to the pilings -- is extremely critical. Basic design standards published by the American Petroleum Institute specify wind and wave loading, how pilings should resist lateral load, allowable stress for steel design, etc.

Roussel Engineering, Inc., Kenner, Louisiana, for the last 24 years has been one of a small group of companies that specialize in this demanding task. Of the 2000 offshore platforms currently operating in the Gulf, Roussel has designed about 100. The company's experience includes analysis of such structures for a list of customers that reads like a who's who list in the industry: Amoco Production Co.; Avondale Offshore; Conoco, Inc; Dravo Corporation; Dubai Petroleum; Exxon Company U.S.A.; Ingram Contractors, Inc.; Mitchell Energy Offshore Corporation; Odeco; Production Management Structural Systems; Shell Oil Company; Southern Industries, Inc.; State of Louisiana.

The companies that analyze these structures face a major obstacle. A typical platform designed to rest in 250 feet of water in the Gulf of Mexico might have 700 structural members. Performing a complete finite element analysis takes about one week per member. Thus, it would be impracticably expensive to perform finite element analysis of every structural member. It would also be unnecessary because only a few members operate anywhere close to code limits. Thus, a very important first step in the analysis of such a structure is dividing the members, which are in the critical range from the others.

In the past, this step was accomplished by reaching for a reference manual or handbook such as Roark & Young, searching through it to find a section that applies to the task at hand, selecting a series of formulas, generating equations and then solving them with a calculator. This took an average of one hour per member. The amount of time spent on this activity varies according to the specific project but can easily amount to 25% of a consulting engineer's day. Recently, Roussel Engineering has substantially reduced the amount of time required for this job with a software package called The Desktop Engineer was purchased from Desktop Engineering, Woodcliff Lake, New Jersey.

Using this software, called a computerized engineering handbook, we now simply select the type of problem and enter the required parameters in response to prompts, reducing the time required to just a minute or two. Most frequently, we use the beams module of the program and enter the span of the beam from each support, beam properties, cross sectional areas, moments of inertia and imposed loads. The program calculates resisting moments and shears of the supports. The program also generates complete documentation of all the calculations which were performed which saves additional time. The savings from this one type of usage are dramatic. The time require to analyze a typical structure is reduced from months to weeks by identifying those critical members in advance where finite element analysis is necessary and using only The Desktop Engineer program on all of the others.

The computerized engineering handbook is also used to fulfill several other functions. For members that require detailed finite element analysis, the program results serve as a valuable verification tool. Because finite element analysis can easily produce misleading or incorrect results if the problem is incorrectly formulated, we previously used hand calculations to initially understand the structure's behavior, to focus the analysis, and to ultimately validate the analysis results. In a minute or two, DE/CAASE provides basic force, stress and deflection calculations which determine whether or not the more detailed results provided by the full analysis is required.

The computerized handbook is also used to generate principle stresses in different planes, which are not generated by finite element analysis. In addition, the handbook is used to model substructures and cross-sections of complex members which substantially reduces the amount of time require to create the finite element model and also to perform the analysis itself. Finally, the computerized handbook can be used to quickly perform a parametric study of a component to get a better understanding of the effect of various dimensions and features on its mechanical properties. Often, this leads to pre-analysis design changes, which greatly reduce the number of iterations required.

The Desktop Engineer incorporates solutions to over 5000 structural/mechanical engineering equations found in over 100 reference books. It includes 37 modules grouped into the following categories: geometric and material properties; beams and columns; rings, arches and frames; plates, shells, and pressure vessels; cables and springs; natural frequencies and dynamics; stresses; user defined modules; and utilities. Modules are self-prompting to help the user find section properties, displacement, forces, stresses, etc. DE/CAASE also includes a material property database and a unit conversion utility. Once the problem is defined, the program will perform the necessary calculations and solutions to provide the required output. The program operates under MS-DOS or Microsoft Windows as well UNIX using the X-Window system and the Motif graphical user interface.