Riser Bend Stiffener Connections Prepare To Go ROV-less

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 can be a time-consuming and hazardous business, especially when divers are involved in securing the bend stiffener connection.

One way around the problem is to use the type of diverless bend stiffener connector (DBSC) developed by First Subsea Ltd. The company’s latest development is an ROV-less bend stiffener connector designed specifically for risers 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, for example, are subject to dynamic environmental loads that cause the riser to flex around 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 mitigate this damage by providing localized stiffness to the flowline and limiting bending stresses and curvature to acceptable levels.

Typically a bend stiffener is a conically shaped polyurethane molding. Up to 39 ft (12 m) in length and weighing in excess of five metric tons (11 kips), each bend stiffener comprises a conical external profile, cylindrical tip section, and smooth bore to suit the external diameter of the riser.

First Subsea’s DBSC technology, developed in collaboration with bend stiffener manufacturer Trelleborg Offshore, enables a fully integrated DBSC design that maximizes the engineering performance of both the stiffener and connector elements.

Connector Technology

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. 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 outward. The tightness of the grip increases in direct proportion to the load applied. The connector is self-aligning and self-energizing, enabling it to be fitted and secured in position without the need for divers. This approach makes the installation safer and quicker.

The Type II DBSC is being installed on the Neptune LNG project approximately 10 miles (16 km) off the coast of Gloucester, Mass. (Images courtesy of First Subsea Ltd.)

The DBSC employs the same connection principle used with First Subsea’s ball and taper-based subsea mooring connectors. Developed and deployed in more than 200 deepwater mooring systems worldwide, the mooring connector is subjected to typical minimum breaking loads of 17,998 kN (4,046 kips).

The DBSC’s ball and taper connection is manufactured from precision-machined forged materials and uses super duplex balls to optimize the connector’s strength and resilience. To protect the connector from corrosion in the splash zone, the DBSC has a thermally sprayed aluminum (TSA) coating. Independent testing has shown that TSA performs better than electronickel plating or fluoropolymer coating. Anodic protection also can be used to complement any corrosion protection system in place within the connecting I/J tube.

Although the concept of the DBSC’s connection is relatively simple, the connector is optimized for each project with respect to the bending moments and load path analysis, system finite element analysis, and fatigue analysis. In addition, 3-D modeling of the complete connector enables the production of bending moment and shear stress studies.

Installation Loads

The key engineering challenge in developing the DBSC has been the pull-in load experienced during connection. In-house testing has been 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.

While pulling 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/angled 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. For angles between 10 and 15 degrees, however, the pull-in load exceeds the side/angled load by a small factor. Even so, the loads experienced are well within the range of expected installation loads for these operations offshore.

In general, the larger the riser diameter, the smaller the available pull-in angle. The pull-in angle also is dependent upon the type of DBSC used.

DBSC Connections

The first DBSCs were designed to allow the connector to be fitted within an existing I/J tube and bellmouth during tieback to an existing facility. The Type I used an ROV hot stab hydraulic clamp to activate and lock the connector’s balls in position and meet the DBSC’s torque requirements, preventing the bend stiffener from rotating through the I/J tube.

The new Type II DBSC features an automated release clamp. Recent testing of the ARC has demonstrated the connector’s viability as a practical solution to ROV-less installation.

For newbuild projects, a two part (male/female) connection or Type II DBSC was developed, enabling a full ball and taper connection with a smaller footprint where space is limited. As both elements of the connection are manufactured to designed tolerances, this allows for a simpler compact connector. This DBSC is intended for installations where the pre-machined female receptacle is fixed to the “I” or “J” tube during the structural fabrication stage of a floating production, storage, and offloading (FPSO) vessel or buoy. The Type II DBSC is designed to release the end fitting once the male DBSC has engaged within the female receptacle. Unlike the Type I, the Type II connection is used only during installation. Once installed, it does not require a hydraulic locking mechanism or clamp to hold it in position.

ROV-less DBSC Installation

Both the Type I and Type II release DBSC require some level of ROV intervention.

The latest DBSC development is an ROV-less Type II for “crowded” turrets or buoys on platforms and FPSOs where space is either limited or in shallow-water, splash zone environments where ROV installation is 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 addresses the problem of the “weak link” experienced with some bend stiffener connectors where the shear-pin breaks ahead of complete installation. This design also addresses situations where the shear pin is too strong, which can cause the connector to be over-pulled, leading to installation problems. The ARC DBSC connector uses an integrated pull-in head locking mechanism that automatically de-latches when it is fully connected, which eliminates the need for ROV intervention during the installation.

Recent testing of the ARC has demonstrated the connector’s viability as a practical solution to the ROV-less installation of risers.