Next Generation: Construction of a cantilevered bow turret for FPSO

Courtesy of MODEC

FPSO Whakaaropai

Location: Maui B Field, New Zealand

Client Name: Shell Todd Oil Services Ltd.

Installation Date: August 1996

Water Depth: 110m (361ft)

Tanker Size: 135,510 dwt

Vessel Type: MODEC Converted FPSO

Storage Capacity: 750,000 bbls

Fabrication: External Cantilevered Bow - Singapore

Vessel - Singapore

External Turret Mooring Systems provide an excellent solution for a wide range of FSO/FPSO applications.

These proven single-point mooring systems permit the vessel to freely "weathervane" 360 degrees, allowing normal operations in moderate to extreme sea conditions. External turret systems are less expensive than internal turret designs and can be delivered in a shorter period of time, making them an excellent choice for many applications. External Turret Mooring Systems can be mounted at either the bow or stern of converted tankers or new-built vessels. External turret systems are designed and built for a few risers, in shallow water, for moderate environmental conditions but pending on some customer and market requirements, turrets could be designed and extended in technology for use in harsh environments and to support a large number of risers and flow throughput. The FPSO Whakaaropai (Maui-B), offshore New Zealand, was developed to withstand 10.6m significant seas and support two large risers in 110 meters of water. The turret mooring required the design and development of a high load capacity and fatigue resistant turret and turret support system.

Design Environmental Criteria (100-year storm) :-
Significant Wave Height: 10.7m (35.1f)
Wind Velocit: 40.2m/s (74.5 knots)
Current: 1.1m/s (2.2 knots)

Mooring System :-
10-leg Chain-wire-chain asymmetric catenary:
6-in. ORQ chain, 4.375-in. ORQ +20% top chain
5.5-in. 6-Strand wire rope, 24mt and 32mt High holding power drag embedment anchors

Fluid Swivel Assembly :-
Crude Oil: 1 x 12-in. piggable toroids
(680 psi design/1,025 psi test)

Produced Gas: 1x 6-in. piggable in-line (2,550 psi design/3,850 psi test)

Riser System :- 1 x 10.5-in. Flexible riser, 1 x 4.5-in. Flexible riser

When designing turret mooring systems for ship-shaped vessels, one of the most important vessel design factors affecting the mooring line tensions is the location of the turret. The farther forward the turret is located away from the mid-ship, the easier it becomes for the vessel to weathervane into an equilibrium heading under non-collinear environments.

However, the farther away the turret is placed, the more the vertical motions at the fairleads due to the vessel pitch will increase, which could have an adverse effect on the mooring line tensions in the line dynamic mooring analysis. When considering the total impact of the turret on the hull, the bow turret has proven more cost effective in both benign and harsh environments.

The bow turret can be configured in two ways: Integral bow turret (built within tanker bow) or the Cantilevered bow turret as in the Whakaaropai FPSO.

A fully weathervaning vessel has operation expense advantages over a controlled heading limited rotation vessel, but the inherent requirements of a swivel joint for each flow path imposes practical limitations on the number of flow paths that can be provided for a fully weathervaning vessel.

Whilst all turret systems are disconnectable, the term is only used for turrets having the facility for quick connection and quick disconnection (QCQDC). Most of the turret systems that have been designed for fairly benign weather and shallow water are disconnected when typhoon is expected.

In 1995, one of my first project in the shipyard's subsidiary is the construction of the 750ton gross weight turret which involves alot of thick welding ( on the chain table where steel thickness ranges from 4 to 6 inches ) and there is need to understand where to use temporary bracing ( using 6-8inch diameter heavy schedule pipes) to hold the chain table reinforcement structures to prevent weld shrinkage and unexpected weld cracks. This crack could be heard with loud "explosion" sound when it happened. The chain table is main core structure that seats the big 6 metre diameter roller bearing ( costing turret designer few mils to order ) and this roller bearing allows the turret to freely weathervane so that the tanker can take up the position of the least resistance to the prevailing weather, at all times. I recalled that the stud bolts for holding this big bearing is sized approximately 2.5inch diameter by 3feet length. One person would have hard time trying to carry this heavy stud. Bolt tensioning by hydraulic system was done on these 100plus pieces of studbolts.This stretching/elongation is maintained by the head of the bolt and the nut on the joint thereby maintaining the joint at the desired tension (Bolt tensioning).To produce clamp load, the bolt must be placed in tension. If the bolt is not stretched then there is no clamping load. The bolt start to stretch elastically (not reaching the yield), proportional to the amount of nut advancement. As the nut is further turned by the "tommy bar" the threads of the bolt and nut are forced together under enormous pressure generating friction between the mating threads and also causing tensional twisting to the body of the bolt between the clamped surfaces. The bolt is experiencing two forces simultaneously, tension and torsion.
In a bolted connection, the bolt must be stretched sufficiently to produce static preload upon the connection that is greater than the expected external load rather than the joint assembly acting upon the bolt themselves. These external loads must be known so that the proper grade, size, diameter, thread pitch and number of fasteners can be chosen to create a safe joint or fastening.

As a rule, the joint will have been designed with sufficient fastener to apply the required clamp load at 65% of the fastener proof load stress figure i.e. well below the fastener’s yield point. There is also an auto-greasing system connected by stainless steel tubing with over hundred grease outlet points around the perimeter of the roller bearing and the timing of autogrease is done through the small auto programming pack.

Courtesy of Bolt tight

The chain-table forms the connection point for the anchor lines of the turret mooring system to the FPSO vessel. Mooring loads are transferred from the chainhawse connection points through the chain-table structure to the turret and the main weathervaning bearing into the ship’s bow structure. The chain-table also provides the entry points for the product risers and houses the foundations for the bend restrictors (as required). The chain table after fabrication weighs about 250tons and would required to be post-weld heat treated due to the welding sizes of exceeding more than 2 inches. A temporary shed was moved into the workshop where the chain table was completed and prepared for gas fired to heat the 250ton steel to 650degree centigrade and holding at this temperature for two hours and then letting it cool down to ambient temperature. About 200bottle of LPG gas with 3 to 4 gas burners were mobilised to do this PWHT treatment. Spent about $70k internal insulating the shed enclosure to keep the temperature inside and preventing much heat loss to the outside. After this heat treatment process, weld stresses relieved, an Australian machining company Furmanite was called in to carry out the laser level chain table bearing surface machining.

The Chainstoppers and Chainhawses :-

Ten chain hawse supports, each consisting of two cast steel pieces having a hook shape, are welded to the chain-table deck. The chain hawse assemblies complete with chainstoppers are fitted into the hooks via a self lubricated bearing which allows the chain hawse to rotate and align with the chains. A tubular guide trumpet ensures proper alignment of the chain during tensioning and helps the articulation of the chainhawses.

The turret also consist of a patented design Swivel Stack Assembly. The swivel stack assembly consists of an inner non-rotating ring and an outer rotating ring that encloses a toroidal shaped chamber. The individual swivel rings are stacked atop of each other to provide multiple independent flow paths. The inner and outer ring concentricity is maintained by triple race roller bearings. Seals around the periphery of the interface between the inner and the outer rings prevent leakage of the working fluid.

The swivel design is based upon the following guiding principles:

-Swivel redundancy

-Flow re-routing as back mode of operation

-Ease of maintenance with minimum intervention
-Maximum use of field proven designs

Some design the driving arrangement of the swivel stack as integral part of the overhead frame-work structure. An individual driving ring is supported at two opposite sides by a fork arrangement. Each swivel unit is therefore driven on both sides of the outer ring simultaneously resulting in a pure driving torque with no net shear being applied on the piping flanges. The swivel sealing system incorporates double seal sets in each module. A leak detection system and recuperation system is built-in between the seals which collects any product fluid and returns it to the production flow.

After completion of post weld heat treatment and special laser level machining done by Furmanite, the 250ton chain table is being moved out for the main hull assembly and installation of the main 6metre diameter slew roller bearing.

Installing the main turret hull to the chain table ( left hand side )

Above is sample setup - not actual one as I could not find from my archives machining video of chain table

Furmanite Australia was engaged to carry out machining of the approx. 6m diameter chain table. At that time, the mobilisation and machining cost for the job was almost more than quarter mil. They supply and use a range of specialized self-levelling rotary milling machine fixed to a rotating beam arm. These are ideal for use in machining very large circular or irregular shaped surfaces. Circular faces up to 45m (150 feet) or rectangular faces up to 11.5m by 11.5m (38ft x 38ft) are able to be machined in a single set up. The machines follow a reference plane electronically, utilising the strength of the item being machined for support. This allows the use of lightweight machines. The machines are highly adaptable, allowing for machining of all faces from vertical to inverted.

The machine uses a laser controlled actuating system to raise and lower the cutting head which in turn maintains the flatness and “best fit” for the service.