Pioneering Spirit Jacket Lift System: A dynamic analysis of the jacket mating loads

Abstract

Allseas Engineering B.V. is an offshore contractor, focussed on offshore pipeline installation. Since the late 80's, Allseas has been working on the design and construction of the Pioneering Spirit, a heavy lift vessel designed for the single-lift installation and removal of offshore topsides. From 2019, it is scheduled to be able to single-lift install or remove jackets with the jacket lift system.

The jacket lift system is located at the aft of the vessel and consists of two 170 meter tilting lift beams, hinged around the stern. During a jacket removal, the jacket is cut-off at the bottom, hoisted from the tip of the beams, partially rotated, and then tilted inboard. In between the hoisting and tilting is the transition phase. At the interface between jacket and the tilting lift beams, jacket support structures are located. In this thesis, the behaviour of the free-hanging of the jacket and the loads on these support structures due to the mating with the jacket are researched.

Jacket removal operations will take place in open sea and are thus subjected to environmental actions. The jacket, suspended from the tip of the beams, is excited due to fluid acceleration and velocity causing inertia forces and predominantly drag forces on the slender members of the jacket, as well as the excitation of the vessel through the suspension point. These motions are calculated with two different approaches. A direct-time domain model is developed of the jacket as submerged pendulum. To accelerate model simulations, the jacket is modelled as one single cylinder with different equivalent diameters to account for the total of drag and inertia. The second model is a full geometrical time-domain model simulation in AQWA.

In the design sea state, simulations showed that the most probable maximum momentum of the jacket due to environmental excitation is 4.5 106 kg m/s, and the motions of the centre of gravity of the jacket are all within a range of 1 meter, for the single and double pendulum and the full model. The influence of the environmental actions on motions of the structure is concluded to be small. This suggest that the jacket mating loads are governed by the tilting velocity of the tilting lift beams and the motion of its tip. The jacket mating simulations are done in the full AQWA model approach.

Fenders are modelled to absorb energy during the jacket mating. A sensitivity analysis showed that the parameters that have most influence on the mating phase characteristics are the tilting velocity and the stiffness of the fender. From still water conditions it seems that by increasing or decreasing the tilting velocity, this can affect the number of re-bounces and the maximum deflection. With increasing sea state, the duration and intensity of the mating phase increases. The fender deflection will increase for increasing significant wave height. The wave period does not have a significant influence on the maximum fender force. The amount of re-bounces reduces significantly for higher damping coefficients, while its influence on maximum deflection decreases.

The maximum force on the jacket support structures caused by the static gravitational force by the jacket once it is fully tilted is approximately 10 times higher than the maximum occurring force exerted on one of the fenders. Therefore, this static gravitational force is critical for the design of the jacket support structure, not the force caused by the mating.