The Management of Long, Suspended Strings of Tubulars from Floating Drilling Vessels
Authors: Steddum Riddle 
Published: October 2008

The management of suspended strings of tubulars from floating drilling vessels becomes more critical as operations move into deeper waters, harsher wave, and current environments. Examples of such strings are marine drilling risers during deployment or recovery operations and the running strings for casing or other subsea equipment. An especially important example is the riser and LMRP suspended from a drifting vessel resulting from a forced disconnect event of a dynamically positioned vessel.

The analysis of the vertical and lateral response of such strings will be discussed with emphasis on the influence of important parameters such as added mass and damping in the axial direction. The establishment of allowable sea states during deployment and recovery operations will be addressed. A related issue is the length of riser that can be safely left suspended in heavy weather such as hurricanes or North Atlantic storms.

Various means of support of the upper end of the string will be examined, especially "soft" vs. "hard" hang-off.

The response of the disconnected riser when suspended from a drifting vessel will be discussed with particular emphasis on management of the deflected shape and the reaction of shear loads into the vessel.

Introduction

It is necessary, during well construction operations from floating vessels, to suspend various strings of tubulars from the vessel. The management of these strings of tubulars becomes more difficult as operations move into deeper waters and more severe environments. Such strings are subject to axial excitation from the heave motion of the vessel and from lateral excitation arising from vessel surge and sway and from wave and current loading. These strings often consist of drill pipe, casing and casing landing strings, marine drilling riser, and landing strings for subsea equipment such as trees.

In many cases, these strings are deployed under controlled conditions. The maximum environmental conditions for which the string can be safely suspended are established at the utset of operations and the string deployed only if an adequate weather window exists. In other situations, the strings should be designed to survive the maximum environmental conditions under which well construction operations are conducted. For example, should a dynamically sitioned drill ship lose stationkeeping ability, the marine drilling riser must be disconnected immediately and suspended below the ship. In this case, it would seem prudent that the suspended riser be able to withstand the maximum environment in which it is designed to remain connected.

Analysis

A thorough analysis of a suspended tubular string would include an investigation of both lateral and axial static and dynamic response of the string including an assessment of vortex inducted vibration (VIV). However, in a large number of cases, a one-dimensional axial analysis may be adequate. Practically all deployment and recoveries are conducted in benign environmental conditions. Although there is usually insignificant lateral response in these cases, the axial response can be substantial.

In such cases, a normal mode approach is often used to estimate the axial response of the string. A finite element representation of the riser is used to develop the axial stiffness and consistent mass matrices.