Flexible Risers and Umbilicals

Technical

Bend stiffeners are a common sight on many offshore platforms where they are used to reduce wear and fatigue in flexible risers and umbilicals. Installing bend stiffeners, however, can be a time consuming and hazardous business, especially when divers are involved in securing the bend stiffener connection. Moreover the installation is dependent upon good weather to complete the operation.

One way round the problem is to use ROVless and diverless bend stiffener connectors developed by First Subsea Ltd, a specialist in subsea connectors, to enable a safer installation for risers and umbilicals in crowded production environments and installations in the splash zone.

Bend Stiffener Connections

Flexible risers either connected to a subsea riser base or the turret exit of a floating production  vessel are subject to dynamic environmental loads that cause the riser to flex about a fixed location. This movement, in combination with large axial loads, can cause damage to the riser structure due to overbending and fatigue. Bend stiffeners are used to prevent this by providing localised stiffness to the flexible this limiting the bending stresses and curvature to acceptable levels.

Typically a bend stiffener is a conically shaped, polyurethane moulding. Up to 39 ft (12m) in length and weighing in excess of 11 kips (5 tonne), each bend stiffener comprises a conical external profile, cylindrical tip section, and smooth bore to suit the external diameter of the riser. Connecting the bend stiffener (and riser) can be a time consuming and hazardous business, especially when divers are involved in securing the bend stiffener connection. Moreover the installation is dependent upon good weather to complete the operation. One way round the problem is to use a diverless bend stiffener connector (DBSC).

The concept of the DBSC enables a fully integrated riser connector design that maximises the engineering performance of both the stiffener and connector elements. It is optimised for each project with respect to the bending moments and load path analysis, system FEA, and fatigue analysis.

Unlike other bend stiffener connection technologies which use external locking mechanisms and hydraulic pressure to hold the bend stiffener in place, the DBSC uses a ball and taper type connection which works on the simple principle of a ball engaged in a taper. The male connector is inserted within a female receptacle or I/J tube. As the male connector's balls roll up the receptacle's wall, tapers drive the balls outwards and the tightness of the grip increases in direct proportion to the load applied. The connector is self-aligning and self-energising, enabling it to be fitted and secured in position without the need for divers, making the installation safer and quicker.

DSBC Connection Integrity

In its simplest form the DBSC can be fitted within an existing I/J tube and bellmouth during tieback to a FPSO. The Type I DBSC uses a ROV 'hot stab' hydraulic clamp to activate and lock the connector's balls in position and satisfy the DBSC's torque requirements, preventing the bend stiffener rotating in the I/J tube.

The DBSC employs the same connection principle used with ball and taper-based, subsea mooring connectors. Developed and deployed in over 300 deepwater mooring systems worldwide, the mooring connector is subjected to typical minimum breaking loads (MBL) of 4,046 kips (17,998kN).

Manufactured from precision-machined forged materials and super duplex balls, the ball and taper type connection is optimised to maximise the connector's strength and resilience. In order to protect the connector from corrosion in the splash zone, the DBSC has a thermally sprayed aluminium (TSA) coating. Independent testing has shown that TSA performs better than electronickel plating or fluoropolymer coating. Anodic protection may also be employed to complement any corrosion protection system that may be in-place within the connecting I/J tube.

Riser Installation Loads

The key engineering challenge in developing the DBSC has been the pull-in load experienced during riser connection. In-house testing is conducted to determine the pull in loads at different angles and various loads, replicating the loads likely to be experienced by the DBSC in the field.

Whilst pulling-in the DBSC into the I/J tube, side/angled, loads are induced due to the tension in the risers and the angle of the riser. Pull in loads for the DBSC vary for different inclined angles and side/angle loads. It has been shown that for an angle of 10 degrees or less the required pull in load is less than the side/angled load. However, for angles between 10 and 15 degrees the pull in load exceeds the side/angled load by a small factor. Even so the loads experienced will be well within the expected DBSC installation loads offshore. In general, the larger the riser diameter, the smaller the available pull-in angle, moreover, the pull-in angle is also dependent upon the type of DBSC used.

New-build Riser Connections

For new build projects, a two part (male / female) connection or Type II DBSC has been developed enabling a full ball and taper connection with a smaller connector footprint where space is limited. As both elements of the connection are manufactured to designed tolerances, this allows for a simpler and more compact connector. This type of DBSC is intended for installations where the pre-machined, female receptacle is fixed to the 'I' or 'J' tube during a FPSO or buoys structural fabrication stage. The Type II DBSC is designed to release the end fitting once the male DBSC has engaged within the female receptacle. And, unlike the Type I, the Type II connection is only used during installation, once installed it does not require a hydraulic locking mechanism and/or clamp to hold it in position. This type of connection was used on the installation of four diverless bend stiffeners for offloading risers and umbilicals carrying electrics and hydraulics on submerged turret loading (STL) buoys for the Neptune Deep Water Project North and South, off the coast of Gloucester, Massachusetts.

ROV-less DBSC Installation

Both the Type 1 and Type II release DBSC require some level of ROV intervention. The latest DBSC development is a ROV-less Type II for 'crowded' turrets or buoys on FPSOs where space is either limited or shallow water, splash zone, environments make a ROV impractical. The new Type II DBSC uses an Automatic Release Clamp (ARC) self-latching disconnection system attached to the riser end fitting which, once engaged with the female receptacle, uses a system of rubber springs to maintain the pre-load on the male connector's ball configuration, and dogs to release the end fitting from the DBSC.

The ARC design address the problem of the 'weak link' often experienced with some bend stiffener connectors where the shear-pin breaks ahead of complete installation, or is too strong resulting in connector being 'over-pulled' leading to installation problems. Instead, the ARC DBSC connector uses an integrated pull-in head locking mechanism, automatically de-latching when it is fully connected and eliminating the need for a ROV and diver during termination head connection .

Fatigue test rig for DBSC testing

Design and Testing

Diverless bend stiffener connectors undergo extensive design analysis and testing including:

•Field life connection integrity
•Interface suitability
•Design longevity – Fatigue
•Design longevity – Corrosion
•Connection/disconnection procedure.