Feature 1 | RISERS AND MOORINGS
the connection stronger, and with higher engineering integrity, than current plate links and thimbles connectors using a single large eye splice. In addition, the fibre rope connectors are
Fibre-rope clam connector rope to chain (Credit: First Subsea Ltd).
and anchor. On a 12-line mooring system this means 24 polyester rope segments. Each rope segment connection is facilitated by splicing the rope ends to produce an eye-shaped connection, as shown in Figure 3. Te quality of the splice is vital to ensuring the integrity of the mooring line. Current deepwater rope splice designs
are the result of many years of continuous development. Tey involve multiple sub-ropes which are terminated by hand splicing. Te eye splice is engineered for high strength efficiency. To achieve this, each sub-rope is allocated a preset position around the eye so that the load is shared equally by all the sub-ropes. In the ideal splice each sub-rope is exactly the same length. In reality, however, there will always be some variation. Te effect of length variation between sub
ropes is an unequal loading on the shortest sub-rope in the eye, resulting in a shorter fatigue life. Failure of the shortest rope initiates a domino effect as the next shortest rope in turn takes an increasing load leading to catastrophic rope failure. It follows that the maximum break load of the assembled rope is governed by the length variability of each sub-rope within the rope. Terefore, the smaller the eye, and thus minimal length variation between sub-ropes, the more efficient the splice.
Reduced deployment times As the mooring lines go deeper, so more spliced fibre rope connections are needed. Reducing the number of connectors by
30
increasing the length of continuous rope between connections is the preferred solution, however, there are not enough vessels capable of handling very long lengths of rope and these vessels tend to command high day rates. To use more readily available vessels with lower day rates typical restricts mooring lines to approximately 1000m (3280ſt). Recent developments in deepwater
mooring have, therefore, concentrated on reducing the time needed to make up the fibre rope connections. H-link, Pearlink and Platelink (Figure 4) connections are labour-intensive, and time consuming to assemble offshore. Moreover, they are prone to being weakened by the bending moments experienced when deployed over the mooring boat’s stern roller. Tis is less of a problem for short-term mobile offshore drilling units (MODU) drilling moorings, but is an area of concern for long-term, 30-year, deepwater moorings. New fibre rope connectors for rope-rope,
rope-chain and rope-wire connections hold out the prospect of significantly reducing the time needed to make rope connections offshore during deployment (Figure 5). The connectors are designed to enable quicker, safer and stronger connections than traditional thimble connectors. To produce a stronger connector, the
single large eye splice has been replaced with smaller, more compact eye splices (using the same number of sub-ropes) such that each splice is closer to the ‘ideal splice’ design with sub-ropes the same length. Tis makes
smaller and lighter than current connectors for the same MBL. Tis is important from a health and safety perspective. Making up traditional offshore mooring line connections can be difficult and sometimes a hazardous task as the large eye splice is manoeuvred and held in position in the thimble. With the new connector designs, the rope is spliced into a forged steel, sub-connector during rope manufacture. Tis sub-connector, positioned at the end of the rope, is unwound from the rope reel and secured within the main fibre rope connector’s body during make-up offshore. Tis has clear practical benefits in terms of the ease, speed and safety of mooring line deployment. Dependent upon the type of connection:
rope-rope, rope-chain and rope-wire, the connectors offer a range of make-up options, vertical or horizontal, to improve connector handling during line make-up. Te cylindrical shape of the connectors also makes them easier to run and retrieve across stern rollers and on anchor handling vessels. Typically, the rope-to-rope connector has a maximum breaking load up to 2100tonnes. Offshore testing of the fibre rope-rope connector is planned for later this year.
Pre-laying deepwater lines Beyond reducing mooring line deployment times, further efforts to reduce the cost of deepwater mooring line deployment may take longer to realise. Pre-laying long-term, deepwater mooring lines on the sea bed many months ahead of installation will change the mooring process scenario significantly. By pre-installing the anchors, mooring connector and rope mooring line, mooring installations can be handled by smaller, lower-cost anchor handling vessels months ahead of connection and at a time best suited to the local weather conditions. Te practice is widely used for MODU moorings, but concerns by the US MMS over the risk of sand penetration of the rope and subsequent abrasion effects on fatigue life mean it will be some time before the pre-laying of long term mooring lines is permitted. OMT
Offshore Marine Technology 3rd Quarter 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52