Aguas profundas

J. Marine. Sci. Appl. (2009) 8: 156-162 DOI: 10.1007/s11804-009-8114-0

FEM analysis of deepwater drilling risers under the operability and hang-off working conditions
DAI Wei1*, GAO Feng1 and BAI Yong1,2
1. College of Shipbuilding Engineering, Harbin Engineering University, Harbin 150001, China 2. OPR Inc., Houston TX77079, USA Abstract: In recent years, numerous exploration activities ofoil and gas industry have been conducted in ultra deep water. The global offshore industry is building systems today for drilling in even deeper water, progressively using new technologies, and significantly extending existing technologies. This is the general trend in the offshore oil and gas industry. So the technology of ultra-deepwater risers, which is the main tool in drilling oil, is more andmore standard. This paper manly focuses on the global analysis of the drilling risers. And it is divided into two parts, operability analysis and hang-off analysis that are used to check the design of the riser. In this paper, the rotation angle and stress of the riser in the drilling mode are calculated to determine the operability envelop. The number of the buoyancy modules has been determinedand according to the API standard, all the worked out values have been checked out. From all the above, it is concluded that the operability envelop is relatively small under harsh condition and the number of the buoyancy modules is a little large. And above all, the design of this riser is successful. Keywords: drilling riser; operability; hang-off; FEM analysis CLC number: TE52 Document code: AArticle ID: 1671-9433(2009)02-0156-07

1 Introduction1
The analysis of riser mechanics can be divided into static analysis and dynamic analysis[1]. The effect of current and the offset of floater is taken into account in the static analysis stage, which is the first step of riser’s global analysis, and also the starting point of the following eigenvalue and dynamic analysis. While the dynamicanalysis can study the non-liner dynamic responses of the riser, and all the responses are usually activated by the motion of the floater and various dynamic environmental conditions, such as wind, wave and current. The static and dynamic analysis of the drilling riser in service, as well as the safety of the riser under various environmental conditions, are conducted by using the FEA software ABQUSin this paper. Under harsh environmental condition, the riser should be suspended at the LMRP or BOP, which is defined as a hang-off condition, to ensure that the ocean surface and subsea equipments are not destroyed[2]. The hang-off condition can be classified into two kinds, hard hang-off and soft hang-off, the difference between them is whether the telescopic joint is wiped off. When it isunder hard hang-off condition, there is no telescopic joint, the heave motion of the floater will be
Received date: 2009-02-27. Foundation item: Supported by the 111 Projects Foundation from State Administration of Foreign Experts Affairs of China and Ministry of Education of China under Grant No.B07019. *Corresponding author Email: daiwei@hrbeu.edu.cn

directly passed on to the riser. While, whenit is under soft hang-off condition, the telescopic joint can offset the motion the floater passed to the riser, but the stroke of the TJ is very important now, since it falls in an allowable range. Under extreme sea condition, it is difficult to meet the need of stroke, so the hard hang-off system is selected here[3]. The FEA process of the hang-off riser is similar to the operational riser,which is commonly divided as static analysis and dynamic analysis. But it needs to modify the riser model under these two conditions, and the standards of calibration are different.

2 Brief introduction to the drilling riser
Drilling riser is a special kind of riser, and its design is very important for the process of deepwater drilling. In recent years, there have been many accidents caused by...