Flexible Risers and Umbilicals

Technical

From the outset, the approach taken to developing the diverless bend stiffener connector (DBSC) has been to use the respective expertise of Trelleborg Offshore in bend stiffener design and First Subsea in connector optimisation.

This provides customers with a single interface responsible for the design and manufacture of the bend stiffener and connector components, and any associated design change requirements (interfacing details) driven by real time loadcase project revisions. As a result a fully integrated DBSC design that maximises the engineering performance of both the stiffener and connector elements, is possible.

Bend Stiffener Engineering

Typically a bend stiffener is a conically shaped, polyurethane moulding. Polyurethane elastomer is used because of its low modulus and high elongation at break compared with other materials. 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 pipeline.

The stiffener is custom-designed for either dynamic or static applications. This ensures it meets the pre-defined loadcase (tension vs angle) of each application, protecting the pipeline's minimum bend radius (MBR) under the defined tension and angle combinations. Other factors taken into account during design include fatigue resistance, creep resistance, tensile strength, tear resistance and temperature dependency.

Connector Technology

The diverless bend stiffener connector (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 – it is self-energising and self-aligning. 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 objectives in developing the diverless connection for bend stiffeners were:

• a connector that was self-energising, enabling it to be fitted and secured in position without the need for divers, making the installation safer and quicker,
   
• a connector that could be fitted within an existing I/J tube and bellmouth, if necessary,
   
• a two part (male / female) connection on new projects providing a smaller footprint for bend stiffener connections where space is limited, and with ROV use limited to inspection only,
   
• a two part (male / female) connection on new projects, providing a smaller footprint for bend stiffener connections where space is limited and without the use of an ROV.
   

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.
 

Field Life Connection Integrity

A ball and taper type connection used in the DBSC is manufactured from precision-machined forged materials and employs the same connection principle used on ball and taper-based, subsea mooring systems. Developed and deployed in over 200 mooring applications in numerous projects worldwide where typically the connector is subjected to minimum breaking load (MBL) of 4,046 kips (17,998kN), the ball and taper is a proven and reliable connection technology.
Interface Suitability

For each DBSC, the ball and taper connector design is optimised with respect to the bending moments and load path analysis, system FEA, and fatigue analysis. 3D modelling of the complete tool enables the production of bending moment and shear stress studies.

Where an existing I/J tube and bellmouth is used, the DBSC connector design draws on the First Subsea experience of the male connector as a pipeline recovery tool technology where the male connector is inserted in the flexible or rigid pipe (which acts as the female receptacle) the balls engage and the pipe winched to the surface. For the DBSC, the male connector uses slightly larger balls to overcome any pipe tolerance and marine growth issues and, in so doing, provide an effective multi-point grip within the tube. Modelling of the installed system also allows for analysis of the installation requirements and access in the areas local to the connection.

For a two part male / female connection, a full ball and taper connection is established. As both elements of the connection are manufactured to designed tolerances this allows for a more simple and compact connector. In addition, the pre-installation of a female receptacle to the I/J tube can enable the geometry to be optimised for each riser/umbilical combination.

Design Longevity – Fatigue

The ball and taper connector is designed with very simple load paths. This makes fatigue analysis (to BS 7608), using mechanical and finite element analysis, easier and more reliable. Moreover the male connector's ball configuration provides multiple contact points within the female connector or existing I-tube giving a high degree of in-built redundancy.
Loading capability - The DBSC design for existing I/J tube, with a male connector only, has been qualified through a series of fatigue tests, qualified with DNV consultation, using an in-house test rig. The connector tool was mounted vertically in a test I-tube and weighted to simulate the bend stiffener. The tool was inserted with very little pre-load to re-create the worst case scenario in field. With a 1,000,000-cycle test providing the loading to match the fatigue data requirements, based on bend stiffener analysis, the interaction between the connector balls and the internal face of the I-tube is fully tested and validated.

Testing of the DBSCs based on a male and female connection had been conducted using the existing testing regime for ball and taper connector systems.
 

Design Longevity – Corrosion

The DBSC has super duplex, stainless steel balls in the connector. The choice of high strength alloys is driven by the need to optimise 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 showed the 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 structure.

DBSC Connection/Disconnection Procedure

The pull-in load experienced by the DBSC during connection to an existing I/J tube was identified as a critical design factor. First Subsea carried out in-house testing to determine what the pull in loads would be for pulling in DBSC's into 'I' or 'J' tubes at different angles and various loads. The main reason for testing was to replicate the loads required to pull in DBSC's in the field.

Side loads that are present when the DBSC is inserted, are created by installing the connectors into 'J' tubes which are typically inclined at 5-15 degrees. In this scenario the connector will be pulled vertically upwards into a bell mouth which will self align the connector to the angle of the 'J' tube while tension is still acting vertically downwards due to the weight of the riser.

Whilst installing the DBSC into an 'I' tube, side/angled loads are induced due to the tension in the risers acting down the departure angle of the riser, commonly the use of a spool piece is placed at the bottom of the DBSC to reduce the tension seen in the connector but even so, there is still a side load acting on the connector which creates the need for a greater pull in load.

The test demonstrated that pull in loads for DBSC's varied for different inclined angles and side/angle loads. Moreover it can be assumed that for an angle of 10 degrees or less the required pull in load is less than the side/angled load. In addition, for angles between 10 and 15 degrees the pull in load exceeds the side/angled load by a small factor. The loads experienced throughout the testing were found to be well within the expected installation loads for these operations offshore.