ABS Announces Offshore Assessment Program for Semisubmersibles

With major new deepwater discoveries being made by floating mobile offshore drilling units or MODUs, semisubmersibles have become a strong choice for some operators. Class society ABS, who at the end of 2007 had received more than half of the new construction orders for semisubmersibles, has developed an offshore structural assessment program specifically designed for this MODU type. The software allows designers to efficiently check against ABS MODU Rules in order to meet classification requirements.

Evaluating dynamic loads, hull strength and site-specific fatigue assessments are key to developing appropriate design criteria for semisubmersibles. The ABS Eagle Offshore Structure Assessment Program (OSAP) consists of three main functions: design wave calculation; global strength assessment; and buckling and ultimate strength evaluation.

"This software simplifies the complex and time consuming task of creating a comprehensive assessment of wave conditions that can be expected at specific locations or are specified by the owner," says Pao-Lin Tan, Manager, ABS Corporate Research & Product Development.

The powerful yet user-friendly software was developed by ABS and is specifically designed to seamlessly work with leading engineering simulation software packages from ANSYS, Inc. The program is open and flexible meaning the software's architecture can easily interface with other commercial design software.

"The software tools from ABS are complex and demanding. As such, they require the adaptive architecture of the technology from ANSYS," said Joe Solecki, vice president at ANSYS, Inc. "The product will provide tremendous benefits to designers in the oil and gas industry."

"Multiple assessments can be carried out once the fatigue, buckling and global strength data is entered. The designers can check to make sure the structure meets ABS class requirements," Tan explained further.

A team of ABS offshore engineers and simulation technology experts from ANSYS will be demonstrating the strength assessment software at an upcoming seminar in Houston, Texas. Initial reviews have been positive with designers playing a role in the software's design. An easy to use interactive graphic user interface (GUI) allows designers to zoom, pan, clip and rotate huge finite element models and navigate to specific areas of interest and to query various structural elements, such as shell, beam or pipe, independently or collectively as pre-defined sets.

Highlights of the software's capabilities include: vessel motions and wave data entered into the program create the wave's associated critical response forces and accelerations; design waves and vessel response amplitude operators (RAOs) can be graphically depicted to facilitate the selection of rational loads for hull strength assessment; degrees of pitching, rolling and yawing can be graphically depicted; and yielding, buckling and fatigue checks in accordance with the ABS MODU Rules can be performed with the results graphically depicted.

With industry demand for semisubmersibles the tool is timely. Semisubmersibles can readily be upgraded to 10,000 feet water-depth capability if required. ABS says it is seeing higher specification units rated for 7,500 feet or greater water depth with some pushing the boundaries to the 12,000 feet mark.

Approximately 74 percent of the world's offshore fleet of exploration units, which includes jackups, semisubmersibles and drillships, are classed to ABS Rules, with 76 percent of all contracted newbuild mobile drilling units specifying ABS class.