Get daily industry updates in your inbox. Free.
Your account already exists. Please login first to continue managing your settings.
The state-owned Brazilian oil operator Petrobras has unveiled details of further advances in its Procap 3000 program, which is designed to develop technological innovation in ultradeepwater exploitation systems.
Earlier Procap projects focused on developing technology to enable production in up to 1,000m and 2,000m of water, respectively. In line with this progression Procap 3000 concentrates on mooring systems for deep and ultradeep waters up to 3,000m.
Since 1986 Petrobras has foreseen that fiber ropes would play a key role in extending the depth capability of drilling and production vessels. Initial studies focused on substitutes for steel wire rope in catenary systems with grounding chain and drag embedment anchors.
The introduction of fiber ropes in spread moorings required a systematic approach to design and materials selection. A pilot study indicated that to attain the standard requirements for the maximum riser offset, polyester (PET) fiber ropes would be the most cost-effective solution.
Detailed studies characterized the relevant properties of these ropes, appraised their use in catenary and taut-leg moored semisubs in water depths up to 1,500m and developed basic design procedures and criteria.
Between 1992 to 1996 basic installation techniques were developed and tested together with trials on the performance of catenary moored vessels.
The development of the giant Marlim field in the early 1990s with floating production units posed a considerable technical challenge to Petrobras.
A congested seafloor layout and provision for workover demanded a short-radius mooring system for these floaters. Development of existing PET fiber rope technology led to the use of taut-leg systems for three semisubs in the field. Furthermore, the challenge of installing FPSOs in 1,420m of water in the Marlim South field with the deepest anchor set in more than 1,600m resulted in the selection of a catenary system with long PET fiber rope inserts for the mooring lines.
In a paper to delegates at IBC's Floating Production Systems conference in London, Luis Costa, a metallurgical engineer with Petrobras, outlined three alternative solutions for the Procap 3000 program: spread catenary, taut-leg and differential compliance (DICAS) mooring systems.
For each system, four main aspects are being investigated:
design procedures and criteria;
installation (including anchor handling boats); and
inspection and maintenance.
Safety management in moorings incorporating fiber ropes
Petrobras has developed a combined approach to design safety, inspection and maintenance and retirement criteria.
Systems incorporating fiber ropes are designed with a safety factor 20% to 25% higher for the fiber component than for the steel components of the line.
The rope manufacturer conducts six tensile tests to failure in samples from the rope. To assess the endurance properties of the rope, two samples are cycled 200,000 times between 20% and 40% of MBL (maximum breaking load), again with the same terminations.
After installation a ROV survey checks the ropes for damage.
Short samples of fiber rope are installed in selected mooring lines. These are removed regularly to evaluate retained strength. The number of samples removed depends on the retained strength measured. If strength reaches the MBL of the steel components of the line, replacement is scheduled, or design of the mooring system is refined to check if the reduced strength is acceptable.
Recently, short samples were removed from the units P-19, P-26 and P-27, after one year of operation. The breaking test results have shown that the PET ropes did not suffer any loss of breaking strength due to deterioration of the PET fibers.
Parametric studies of taut-leg and spread mooring systems
This project evaluated tankers from 30,000 to 280,000 dwt, and a production and drilling semisub in water depths of 1,000m, 2,000m and 3,000m.
Basic assumptions were:
the tankers have eight mooring lines and a turret system;
the drilling semisub has eight mooring lines;
the production semi has 12 or 16 mooring lines;
wind speed is 37.22m/s, and maximum offset is 5% of water depth;
current is 1.02m/s, and maximum offset is7.5% of water depth; and
all environmental loads are coming from the southwest.
The study concluded that taut-leg systems are simple and cost-effective solutions to moor this range of floaters. However, the research also found that for the spread mooring systems designed with chain and steel wire rope in more than 2,000m water depth, there was no combination of pretension and offset to meet Petrobras' operational requirements. Those operational requirements were reached only after the substitution of the steel wire ropes by PET ropes.
Development of improved terminations
So far all the ropes used by Petrobras have spliced terminations. However, the accessories used with splices are heavy and sometimes difficult to handle.
There is currently an interest in developing terminations that are more efficient, easier to handle and less sensitive to workmanship than splices.
UK-based Bridon International has developed improved socket terminations for PET fiber ropes. Petrobras provided the specification for the rope. It also helped steer the development and performed a fatigue test on a 5,000kN breaking load rope sample. Rope construction has seven subropes of braided construction in parallel enclosed in a braided jacket.
Fatigue for 198,000 cycles between 20% and 40% of the MBL of the rope was accomplished without problems. According to Costa, there were no external signs of damage, and the rope has been taken to 70% of its MBL without failure.
Torque and twist
Petrobras has also been carrying out research into the torsional characteristics of mooring components.
Two phenomena were observed in deep and ultradeepwater operations in the Campos Basin. The first was twist accumulated iron grounding chain in chain-wire chain systems with or without a fiber rope in the mooring line. The second was cyclic twist in catenary systems with wire rope in series with fiber rope.
Excessive twist in grounding chain may be difficult to remove and can impair its strength. Cyclic twist in wire rope as caused by tension fluctuation when one of the freely rotating ends has the potential to cause premature failure of the wire rope.
Costa believes that for ultradeep floaters, proper design of the mooring system in relation to torque and twist should be performed. A further research program has been set up in collaboration with the University of Reading (UK) and the Technological Research Institute in Sao Paulo.
For each mooring component, torque and twist characteristics are being evaluated. A model is being assembled to analyze the static and dynamic properties of a mooring line with mixed components.