Abstract

With current landing string tensile loads approaching 1.5MM lbs., deepwater drilling programs are quickly eliminating comfortable design margins. Consideration of static loading alone in these cases can sometimes be insufficient. This paper provides a mathematical model to predict dynamic axial loads imposed on a deepwater landing string by a drillship's response to ocean waves. The model can be adapted to a given landing string's geometric and mechanical constraints. The model will help the user identify the correct window for pausing a landing operation when sea states are increasing, or alternatively, may allow the user to continue deepwater landing operations with increased confidence during what may be considered marginal conditions.

Introduction

Landing heavy, large diameter casing strings from dynamically positioned drillships in deep water is becomming much more common. As static tensile loads approach the capacity of today's landing strings, considering the effects of additional dynamic loads becomes critical for preventing overload. Dynamic loads caused by vessel heave are imparted to the drillstring when it is setting ‘stationary&’ in the slips and when suspended from the elevators at loads greater than what the motion compensator can absorb.

Dynamic loading of drillpipe during tripping was previously investigated by Arthur Lubinski [1]. Drillstring vibration in that study considered impulse excitation caused by adding a stand of drillpipe and then setting the slips. During tripping operations, the top of the drillstring experiences a sudden change in velocity within a short period of time (impulse) before reaching constant tripping speed. The heave-induced drillstring vibration considered in this paper is similar in the sense that both excitations originate from imposed motion (or change in velocity) originating at the top of the drillstring. However, the response of the drillstring to heave-induced excitation is expected to be significantly different in two main respects. First, the excitation caused by setting pipe in the slips during tripping is short in duration, a characteristic of a transient impulse excitation. In the extreme case where the slips are set abruptly (over an extremely short time period) the excitation spectrum could cover the majority of the resonance modes of the system. The heave-induced excitation, on the other hand, is mainly low frequency persistent excitation. This is partially due to the low frequency nature of ocean swells as well as to the sharp response of the drillship around the resonant frequency of the coupled drillship-ocean vibration system. Observed drillship oscillation periods are typically above 7 seconds. In comparison, the period of the lowest resonant state of a typical deepwater landing string is about 5 seconds. Therefore, without losing significant accuracy, only the lowest couple of modes of the combined casing & landing string assembly need to be considered. Second, the initial condition of the input excitation as an " impulse wave" can be well defined in the case of drillpipe tripping. Heave-induced drillship motion, however, is persistent and continuous and can only be estimated mathematically without processing empirically obtained heave vs. time data. As such, it must be assumed that the recorded vertical drillship displacement over a given time span is reasonably representative of the excitation shortly before and after the measurement. Likewise, " when the wave(s) began" is of little consequence when considering the time continuum in which this excitation wave form exists. Though this does not hold true for extremely large impulse-heave excitations (as may be the case in extremely adverse weather conditions with extremely large amplitude-long period waves), in practice, no deepwater casing landing operation is expected to be performed during days when the ocean presents such adverse conditions. As such, landing string response to this type of wave excitation is not considered.

It should be noted that dynamic loads in the landing string due to tripping (adding stands of drillpipe) continue to exist in addition to heave-induced dynamic loads and additional margin must be reserved for these loads in the final design. However, because the two phenomena are independent and because the former has been previously considered [1], the details associated with dynamic loads due to tripping are not discussed in this paper and for simplicity, are not considered in the results.

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