Anasuria Winch Destruct Project

Case Study

Project Description and Objectives

An anchor chain winch on the Anasuria FPSO was deemed to be in such a poor condition that it could not be recertified and therefore needed to be replaced. The winch was located in the turret of the FPSO and could not be removed in one piece due to access constraints and load limits on the turret supporting structure. Sigma 3 therefore tasked Proserv Offshore to sever the winch into several sections that could be safely removed from the turret. This included severing the winch drum in half.

Winch details

The Anchor winch was approximately 3.0m high with a drum diameter of 1,500mm. The skin of the winch drum was 160mm thick and had a ribbed skin for the anchor chain to sit in. The winch was located in a Zone 1 area and was surrounded by process pipework. The process pipework not only limited access it also meant that extra safety features had to be put in place to protect it from the cutting operation.

Cutting Sequence

Even when severed in half the winch drum was too heavy to be lifted out of the turret. Therefore the first stage of cutting involved the removal of the top and bottom sections of the winch flanges. This involved a linear cut along the flange and the severing of the gearing that is wrapped around the circumference of the flange

Four linear passes were completed cutting through 50 mm thick steel for 2,500 mm along the winch flanges. Eight gear cuts were also completed through 75 mm thick steel.

In order to gain access to the drum shaft an access hatch was required to be cut into the winch drum. This involved completing two horizontal and two vertical cuts in order for a 500 mm² section to be removed from the skin of the drum.

The drum skin was over 160 mm thick in places due to the ribbing on the drum skin. As well as completing the four cuts for the square two lifting pockets 100 mm x 100 mm were cut into the square section so that rigging could be attached.

With access into the drum now gained the next stage involved the severing of the drum shaft. The drum shaft was hollow in the centre with a diameter of 310mm and a wall thickness of 50mm

With the drum shaft severed in half the final stage of the cut sequence involved the severing of the skin of the drum in half. Again the cuts were through over 160mm thick steel with a cut length of 4055mm.

Equipment Used

Ultra-High Pressure (UHP) Cutting System

In order to sever the Anasuria winch a UHP cutting system was utilised. The UHP system pumps water at pressures up to 36,000 psi through a cutting nozzle. At the cutting nozzle garnet is induced into the water stream creating the cutting ability of the system.

CCM - Circumferential Chain Manipulator

neopreen_rubber_coating_removal_002_220 The CCM was used to sever the drum shaft, drum skin and complete the vertical cuts for the access hatch. The unit is driven by a hydraulic motor and a chain which is wrapped around the circumference of the drum or shaft.

LCM - Linear Cutting Manipulator.

The linear cutting manipulator was used for the longitudinal cuts along the winch flanges and for the horizontal cuts for the access hatch in the winch drum. The unit is held in place by magnets with the cutting nozzle again driven hydraulically.

Gear Cutter

The gear cutter was used to cut the gearing around the circumference of the winch flanges. The cutting nozzle is mounted on a lead screw and is driven by a hydraulic motor producing a linear cut. The cutter is held in place by a G-clamp arrangement which sits over the gearing.

Hazards

The largest risk during the project was the risk of damaging the process pipe work surrounding the winch with the UHP cutting jet. A protective habitat was constructed around the winch, which consisted of scaffolding, hard wearing tarpaulin and metal plates mounted along the cutting lines. Wooden boarding was also mounted around the process pipe work to add extra protection. As well as the protective habitat a shut off pedal was connected into the UHP system. The shut off pedal had to be compressed by the UHP operator when cuts were being conducted. Should the cutting system need to be shut down due to any unforeseen circumstances the operator only had to release the shut-off pedal and the system would shut down almost immediately.

Another hazard was that the UHP system could not be located on the turret. This meant that the UHP pump, hydraulic power pack and equipment containers had to be located on the main section of the FPSO and an umbilical run between the system and the turret. The Anasuria FPSO is configured to move freely around the turret therefore there was the risk that the umbilical would become strained and burst should the vessel move significantly around the turret. To reduce this risk a break in the umbilical was located at the crossover between the turret and the vessel. This break was only connected when cutting operations were taking place and a turret watch was also put in place checking the umbilical and the vessel movement.

Results

The project was successfully completed between the 26^th December 2007 and the 18^th January 2008 with no lost time incidents or damage to process pipe work. All cuts were successfully completed with minimum downtime on equipment. The project proved that UHP cutting can be successfully used in Zone 1 areas and can sever steel upwards of 160 mm thick.