Riser tensioner systems constitute critical components on offshore platforms and floating production storage and offloading vessels (FPSOs), as these systems compensate for the wave motions of such vessels or platforms.
The most important requirements for steel wire ropes used in Riser tensioners are high breaking strength as well as long service live under flexural stress, as safety requirements and cost-effectiveness are key criteria on offshore platforms. Meanwhile, eight-strand high performance ropes with plasticised cores and compacted strands are already being marketed by various manufacturers. Compared with six-strand ropes that are being widely used these days, eight-strand ropes offer advantages in terms of life span, safety and value.
The reason for recent developments regarding eight-strand high performance ropes in this segment is that riser tensioner systems have undergone profound modifications, as will be explained below. Older riser tensioner systems rely on the so-called slip and cut method. In this case, the steel wire rope is wound in one large length onto a reel. Only a part of the length is used in the tensioning system.
After a defined number of bending cycles the steel wire rope is displaced by a defined length and shortened. This serves to ensure that the zone subjected to the highest continuous flexural stress gets shifted at regular intervals.
In order to save space and reduce weight, newer systems no longer use the slip and cut method, but rely on steel wire ropes that are of a fixed length. It is especially in these new systems - but also in the slip and cut systems - that plasticised eight-strand rope designs such as Teufelberger's QS 814 VG has their advantages.
Eight-strand high performance ropes, compacted by using Superfill technology, feature superior minimum breaking loads as compared to conventional six-strand ropes of the same diameter. As a result, the specific loads individual wires are subjected to will be lower, and the safety factor of the riser tensioner system will increase accordingly. The benefit of plasticised, lubricated ropes for riser tensioner systems is their enhanced durability obtained through the cushioning characteristics of the plastic-coated core.
Plasticisation (in the form of the Plastfill core) causes punctiform pressures to be distributed across larger areas so as to ease the contact pressure per unit area exerted on the wire. This in turn alleviates the stress concentration and, as a result, enhances the rope's service life.
Furthermore, the plastic cover envelops the steel core and thus the special grease contained in it, which is of paramount importance for ropes used in riser tensioner systems: Since flexural loading is concentrated in certain locations (natural even wave motion), the continuous flexural stress generates elevated temperatures in these zones. Special grease reduces fretting corrosion in the core and minimises wear. Teufelberger employs its Pro Active Core Lubrication on all ropes that are exposed to localised high alternating bending loads, for example, when used for active heave compensation or riser tensioner systems.
When analysing the cost-effectiveness of riser tensioner systems, wear on sheaves must not be ignored. Compared to six-strand ropes, eight-strand rope designs such as Teufelberger QS 814 VG entail reduced sheave wear. This is due to the fact that a compacted, eight-strand rope has a greater contact surface than a compacted six-strand rope. This reduces the contact pressure on the sheave and minimises wear.
Each of the above factors contributes to the fact that eight-strand ropes such as QS 814 VG, in combination with technologies such as Superfill, Plastfill and Pro Active Core Lubrication, significantly improves the cost-effectiveness and safety on riser tensioner systems.
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Dr Roland Konrad, Managing Director, Teufelberger Seil Ges.m.b.H., Wels, Austria. www.teufelberger.com