Motion compensation of a K-type installation vessel using suction piles

Abstract

Jumbo is a heavy lift, shipping, offshore transportation and installation contractor. Jumbo is an experienced company in ocean transportation for more than 45 years. Since 2003, building on their heavy lift capability, Jumbo is rapidly getting involved in the offshore installation market.

Jumbo normally uses dynamic positioning systems for the stabilization of the vessels during the installation process. These systems are quite expensive and there is a need for alternative options for the stabilization of the vessels. One idea is the use of spudpoles for a K-type installation vessel which are inserted into the soil and connected to the vessel via hydraulic cylinders. The spudpoles transfer the loads caused by the motions of the vessel to the soil resulting in the reduction of the movements of the vessel. The goal of this project is the reduction of the motions of the vessel in offshore operation improving its workability. A design of the compensation system along with an investigation of its performance are necessary in order to achieve this.

A survey of several types of foundation for the spudpoles is conducted in order to find the appropriate solution for this specific project. After a Multi Criteria Analysis the suction bucket solution was selected, since it is a quick and easy way for the penetration and extraction of the piles, at the start and the end of the installation procedure respectively.

To model the vessel a 6 degrees of freedom (DOF) time domain model was created. The hydrodynamic coefficients as well as the response amplitude operators (RAO) were obtained with the diffraction program WAMIT. For the calculation of the motion of the vessel the time consuming convolution integral was replaced by an extensive state space system. This specific model was validated using a Matlab model based on the convolution scheme. The two spudpoles were discretized using finite differences. Moreover, for a better representation of the real life, an irregular sea state was used. In the final state space model, the vessel, the spudpoles, the hydraulic cylinders and the environmental loads are included.

An initial design for the spudpoles was done using steel type S355. After performing the necessary stress and buckling checks for the spudpoles and taking into consideration the limitations of the motions of the vessel, it came out that the required thickness of the spudpoles was too large and so the type of steel was changed to S690. A number of simulations were performed with several sets of different diameter and thickness in order to obtain the optimum design. Furthermore, the suction buckets were checked for their pull-out and bearing capacity. A final design was selected for both the spudpoles and the suction buckets taking into account the outcome of the checks and the limitations of the vessel’s motions. The vessel moves between the allowable limits for the 6 DOF, proving that this compensation system is a realistic and feasible solution.

Based on the performed simulations, the critical wave directions were identified. The wave directions of 105º and 135º are the two critical for the stress and buckling checks. The wave direction of 90º is the critical one for the pull-out capacity check of the suction buckets. These wave directions should be avoid during the operation to assure higher safety standards for the vessel.