Riser protection nets and other marine systems

TTI provides technical expertise relating to ropes and chains in marine applications. As an acknowledged leader in the technology of synthetic fibre ropes, TTI offers independent advice, specification and design services for all types of marine applications.

See also:

Riser Protection Nets

Riser Protection Nets (RPN) are sometimes necessary to protect vulnerable risers from impact by an errant supply boat.

Only fibre ropes have the properties that allow RPNs to be successfully engineered, Elastic Modulus being the key property. This allows a net to be designed so that the energy of an impact can be absorbed (through conversion to strain energy) within the strength capability of the ropes.

RPNs have been deployed on seven major TLP and FPS projects. Four of these systems have been designed by TTI.

Riser Protection Nets using Synthetic Ropes
Offshore Facility	Installed	Material	Main Span Ropes
Hutton TLP (Conoco)	1984	Polyester	200 tonne MBS
Snorre TLP (Saga)	1991	Nylon	200 tonne MBS
Auger TLP (Shell)	1993	Polyester	360 tonne MBS *
Heidrun TLP (Conoco)	1995	Polyester	400 tonne MBS *
Asgard B FPS (Statoil)	2000	Polyester	600 tonne MBS *
Roncador FPS (Petrobras)	2000	Polyester	??
Caspian Sea DQ Jacket	2003	Aramid	320 tonne MBS *
* TTI designed

Typical features of a riser protection net:

Main Ropes spanning between the columns are the main components of the net (there may be 3 or more).

Main ropes have a “span” section and two “wing” sections.

Vertical ropes link the main ropes together to prevent a boat slipping between them and to activate some sharing of the work between the main ropes.

Engineering an RPN for optimal performance, durability and cost is a complex but familiar task for TTI. Some of the key design considerations are:

• Net Configuration, Arrangement & Dimensions

• Rope Sizes, Materials & Construction (specs.)

• Design Pretension Level and Tolerances

• Boat Impact Performance (energy & penetration)

• Environmental Loading (ice, marine Growth, waves, current, etc.)

• Strength, Fatigue & Durability Assessments

• Installation and Tensioning Requirements

• Attachment Hardware and Ancillary Structures

A boat impact analysis is done to determine the energy absorbed by the net at progressive stages of penetration by the impact point.

The Riser Protection Net will be subjected to wave action which will also need to be considered in the design. A hydrodynamic analysis must therefore be carried out to determine the response of the net to wave loads in the extreme environmental conditions and also to provide data for conducting a fatigue analysis.

Riser Tethers

The usual role of the tethers is in restraining the geometry of the flexible riser system. The most typical riser tether applications are in the restraint of submerged riser arch buoys and in controlling the touch-down geometry of “lazy” flexible riser configurations.

Design considerations focus mainly on the strength and durability of the tethers, with other important aspects being length and length tolerance (including the effect of creep), rope construction, terminations and encapsulation. The full range of design aspects considered by TTI include:

• Configuration, Arrangement & Dimensions

• Rope Sizes, Materials & Construction (specs.)

• Environmental Loading (marine growth, waves, current, VIV etc.)

• Performance in “Near” and “Far” Design Cases

• Rope Strength and Fatigue Assessments

• Wear, Creep and other Durability Issues

• Handling and Installation Requirements

Riser buoy tethers are often engineered using chain, however, polyester rope tethers have been found to be considerably more effective in this application. Advantages in using synthetic fibre ropes over steel wire or chain include:

• Very low weight – so no additional buoyancy is needed to support the tethers.

• Durability – better than steel for resistance to corrosion and fatigue.

• Ease of handling – simpler installation procedures using divers or ROVs.

Some of the more notable riser tether projects using synthetic fibre ropes are:

Synthetic Ropes used in Riser Tether Applications
Offshore Facility	Installed	Material	Main Span Ropes
AHL - Ivanhoe/Rob-Roy	1989	Polyester	4 x 100 tonne MBS *
Shell - Teal & Guillemot	1996	Polyester	4 x 200 tonne MBS *
Chevron – Benchamas	1999	Polyester	2 x 112 tonne MBS *
Saga – Vigdis	1996	Aramid	1 x 100 tonne MBS
Norsk Hydro – Njord	1997	Aramid	28 x 105 tonne MBS
BP - Foinaven	1997	Aramid	14 x 53 tonne MBS
Statoil – Lufeng	1997	Aramid	8 x 70 tonne MBS
Talisman – Buchan	1999	Aramid	2 x 405 tonne MBS *
* Tethers used for configurations with buoyant riser arch support(s).

Guideline & Practice Development

TTI advises and works with professional bodies, classification societies, regulatory and other organisations developing guideline and recommended practice documents for moorings. Such contributions include:

• OCIMF Single Point Mooring Hawser Guidelines.

• Engineer’s Design Guide for Deepwater Fibre Moorings

• ABS Guidance Notes on the Application of Synthetic Ropes for Offshore Mooring

• API RP2SM Design and analysis of Synthetic Rope Mooring Systems.

• Minerals Management Service