Floating production systems (FPSs) are increasingly becoming the offshore industry’s most popular development concept around the world, with floating production, storage, and offloading (FPSO) vessels taking the lead, in part because of their inherent flexibility.

With more FPSOs being deployed in harsh environments, there is a growing need for the vessels to be dis-connectable.

“The advantage of such a system is that in case a large hurricane or typhoon is moving towards the FPSO, it can be disconnected from its risers and mooring quickly to avoid any damage to the facility,” said Olaf Waals of Maritime Research Institute Netherland (MARIN). “In this disconnection phase, the turret buoy is detached from the vessel and drops to a safe depth of 100 to 200 m (328 ft to 656 ft) below the surface. The FPSO is then moved to a safe area until the storm has passed by. Afterward , the buoy can be picked up again and reconnected to the FPSO in a short period of time.”

A number of turret buoy systems have been deployed, but because the need to disconnect in severe weather conditions remains a rare event, there is limited verification data available for their design.

A three-year joint industry project (JIP) is being set up to gain a better understanding of the hydrodynamic behavior of a disconnected turret buoy in close proximity to an FPSO during the critical disconnection and reconnection phase.

The Disco (short for “disconnectables”) JIP, scheduled to begin in 2Q 2012, will be managed by offshore research and testing specialists at MARIN. It will involve close cooperation with oil companies, operators, and marine system suppliers, with a JIP steering group expected to meet every six months.

The JIP members will select two of four generic buoy shapes for study and analysis (Image courtesy of MARIN)

Buoy design

In general, the vertical pretension on a buoy is designed so the risk of collision between the buoy and the FPSO during disconnection is reduced to a minimum. Due to the large vertical pretension, the buoy generally moves away quickly from the FPSO after disconnection. The speed at which this happens, of course, depends on the size and shape of the buoy and the vertical pretension of the mooring system, Waals explained.

During the reconnection phase, the speed at which the buoy approaches the FPSO depends on the maximum capacity of the winch. The speed at which the winch can pull the buoy into position can be limited by the fact that the chains can be covered with soil after pulling them from the ocean floor. Generally the speed of reconnecting the buoy is much slower than the speed of disconnection.

Although the reconnection will be done in much milder sea states, the loading on the buoy and reconnection wire may be considerable.

The Disco JIP will focus on the hydrodynamic design aspects of the buoy in disconnection and reconnection phase to deliver a methodology to predict behavior in the design phase of future projects.

For hurricane areas, buoy disconnection generally will be carried out in high sea states. It is vital, therefore, to design the mooring system and disconnectable turret buoy in such a way that the buoy separates and departs quickly enough from the vessel to avoid collision with the buoy.

The acceleration of the buoy and final drop speed depend on factors such as:

Vertical pretension of the mooring system;

Horizontal offset of the FPSO;

Geometry of the turret buoy;

Underwater weight of the turret buoy;

Initial gap between the buoy and the FPSO (water

entrance);

Presence of current; and

Presence of orbital wave motions.

The hydrodynamic behavior of the buoy generally is dominated by waves, current, the motions of the FPSO, and the shape of the buoy and turret.

The objective of MARIN research is to predict dropping behavior and the motions and mooring forces on the buoy during the disconnected phase in a given sea state and current. For this objective, a time domain simulation tool will be made in which user-defined sea states and mooring systems can be analyzed.

Typically, full mooring systems have been tested at a model scale of 1:50 to 1:70. Tests at various scales have shown buoy acceleration in the early stage of the disconnection can depend on the tested scale and is therefore difficult to predict. Experts at MARIN now are applying computational fluid dynamics (CFD) as a technique to explain these scaling effects and use them for better predictions of the actual system. The first step will be for tests to be carried out at a larger model scale.

Project scope

The JIP will be divided into five main work phases, or “packages.” In an effort to limit the scope of work to a manageable amount, only two buoy shapes will be analyzed. Analysis of these two shapes will provide better understanding of the problem and will serve as validation material for the methodology.

The research institute is planning a kick-off meeting for May 2012 to review a series of buoy designs and to select two designs for the JIP. As part of the preparation for this decision, preliminary open-water CFD analysis on this buoy series will be performed and the results shown at the kick-off meeting to help in the selection of the two buoy shapes.

The work packages will include items such as carrying out detailed 3-D CFD analysis to compute the buoy loading under varying conditions. To understand the interaction effects between the FPSO and the buoy, calculations will be performed twice – once with the buoy in open water and once with the buoy in the proximity of the FPSO.

Engineers expect to perform calculations for:

Current loads;

Drop resistance;

Reconnection resistance;

Combined current load and drop resistance (oblique flow);

Imposed oscillatory loads; and

Oscillatory loads due to regular waves. Experiments, both with and without an FPSO, will include captive tow tests to investigate current loads, imposed vertical trajectory tests, oscillation tests to investigate added mass and damping, and dynamic drop tests in calm water. Two tests proposed in regular waves are a dynamic drop test with fixed and moving FPSOs and a dynamic motion test with the buoy positioned just below the hull immediately before reconnection. The Disco JIP hopes to succeed with an optional proposal in which a full-scale measurement is carried out during a real disconnection procedure. “This will be done during a test phase of an actual turret buoy,” Waals said. “The details of this measurement will be discussed with the operator, and the plan for these measurements will be presented to the JIP participant meeting after the operator has agreed to the plan.”

A final phase of the JIP work scope would see the development of a numerical time domain module to predict the loads on the buoy. This will be used in an engineering environment to make a realistic prediction of the relative motions between the FPSO and the buoy in time domain.

Editor’s thanks go to Dr. Bas Buchner, Olaf Waals, and William Otto of MARIN for assistance in producing this article.