Numerical Study on Offshore Lifting Operations of a Subsea Spool

Abstract

Within the scope of marine operations, offshore lifting operations present significant challenges due to the instability and harshness of environmental conditions. Therefore, lifting operations become expensive and risky. Lifting operations call for an early planning in order to assess operative procedures, carry out logistical tasks and to determine workable weather windows. In order to contribute to the solution, accurate numerical models and methods have become essential tools for predicting the response of the lifting systems.

This thesis addresses a numerical study on offshore lifting operations of a subsea spool. The two operations studied within the subsea lift process are: the lift-off of the spool from the deck of a transportation barge (the lift-off phase), and the lowering of the spool through the wave zone (the lowering phase). Moreover, accurate numerical models of the lifting systems were developed. The process also included the hydrodynamic analysis of the construction vessel and the transportation barge supporting the operations.

The use of numerical models and methods, along with the conduction of time-domain analyses, bring relevant benefits and advantages to the assessment of allowable sea states. In this thesis work, a systematic methodology was followed for the assessment of the allowable sea states governing the lifting operations. Firstly, the corresponding critical events and limiting parameters were defined. The calculation of the characteristic responses was then carried out by means of time-domain simulations. Finally, the allowable sea states were obtained by comparing the characteristic responses and the corresponding allowable limits.

Particularly, the assessment of the allowable sea states was a comprehensive task in this thesis. The process in itself required the statistical uncertainty to be reduced, which demanded numerous time-domain simulations. In addition, a methodology was suggested to filter seeds that present a suitable scenario for the lift-off phase. The assessment delivered that the allowable sea states obtained for the lift-off phase are lower than those for the lowering phase.

Since the operability of marine operations plays an important role during the planning phase, an operability analysis was also conducted in this thesis. The methodology principally considered weather window analysis and two methods devised for the installation of a given number of spools. One of the differences between the methods resides in the allowable sea states that govern the lift-off of the spool. In the first installation method, the entire subsea lift process is governed by the allowable sea states obtained for the lowering through the wave zone. In the second method, the lift-off occurs from the deck of a transportation barge and thus the respective allowable sea states apply.

The main objective of the operability analysis was to determine which of the methods, in terms of time, provides the most efficient option for the installation of a given number of spools. It was observed that low allowable sea states governing even one single activity may virtually make a marine operation unfeasible.

Lastly, a sensitivity study to the navigation time of the construction vessel was conducted. The outcome suggests that this parameter may become decisive in determining whether transportation barges should be integrated in order to reduce the overall installation time.