28 November 2018
Contracted by the Spanish construction group Cobra, BOURBON completed the installation of the first floating wind turbine at the Kincardine site, 15 kilometers off the coast of Aberdeen. Vryhof are their mooring solutions partner.
Regarding floating offshore wind energy Vryhof expects rapid future technological and commercial progress towards pre-commercial and the first (modest) commercial-scale windfarms. Most probably already within two to four years.
Devotion to floating offshore systems
The past decade our Project Director Senol Ozmutlu devoted significant time and effort in development floating offshore systems. And as part of that dedication Vryhof supported many early developers as well.
We commenced with mooring systems supplied to Statoil’s Hywind Demo 2.3MW spar prototype in 2009, and Principle Power’s 2MW semi-submersible prototype a year later. The cumulative track record also includes the prestigious Fukushima Forward demonstration project in Japan, comprising two Hitachi turbines of 2MW and 5MW each, one 7MW hydraulic Mitsubishi turbine, and a floating 66kV substation. In addition, we mention the first floating turbine in the US (VolturnUS), Japan’s Skwid wind/wave project, and several floating wave and tidal energy devices.
Senol Ozmutlu: “Prior to the first phase of Fukushima Forward we were called in by the developers. Their soil analysis had unexpectedly revealed much more difficult soil conditions at the various platform installation sites. These soils would be hard to penetrate with local anchoring technology, creating fear for projects costs. After in-house evaluation, we offered our drag embedded STEVSHARK® anchor for the project.” Initially, one anchor was purchased and subjected to an extensive full scale testing program. Vryhof was selected to become the first and only non-Japanese hardware supplier for Fukushima Forward.
Over the years, a range of anchoring solutions from classic ship anchors have evolved into hinge-type anchors which are still widely used in shipping. Vryhof, however, designed various completely differently-looking anchor models. The oil & gas industry from the 1970s moved towards deeper water and larger installations, which led to a gradual switch from fixed bottom to floating production platforms, floating refinery and (temporary) storage facilities. Main problem of classic anchors in one-piece, as well as later developed hinged ship anchors is that they become unstable from above certain line tension and penetration depth mainly due to their unsuitable design geometry. That in turn created opportunities for new-generation anchors.
Since 1980s Vryhof started developing prismatic geometries, and introduced many different anchor models, characterised by wide flukes and V-shaped wide shanks. Most important is that the new-generation anchors (STEVPRIS®/STEVSHARK® range) offer a favourable high ratio between ultimate holding capacity (UHC) versus anchor weight. Also high structural strength, and quick deep anchor penetration performance within very short drag length.
The development of these new-generation anchors was based on two elementary philosophies. The first is reducing anchor resistance during penetration and consequently penetrate deep to obtain high holding capacity. Secondly, enlarge flukes to the maximum for mobilizing maximized soil resistance.
These efforts resulted in STEVPRIS® and STEVSHARK® anchors with characteristic wide flukes. For reducing fluke bending moments the shank is composed of two parts. Each widely spaced at the fluke side and gradually narrows towards the shackle connection.
Another benefit of the shank’s V-shape is that soil resistance is not only mobilised in the cable pulling direction, but also perpendicular to the large shank surfaces. And soil is compressed in forward and sideward directions, causing the shank to mobilize higher soil resistance and adding a downward directed load component to the anchor for enhanced penetration.
Installation and recovery of Vryhof range of anchors is claimed easy and straight-forward. The anchors can be installed by using direct ‘bollard pull’ from an anchor handling vessel.
Alternatively, surface or subsea tensioning devices can be operated from tugs, offshore construction vessels or barges. Depending on installed anchor intended lifetime, permanent mooring systems must meet storm-related loads during 20 – 30-year design life, or days to several months with temporary mooring systems.
Senol Ozmutlu: “Some anchor types are designed for efficient penetration in softer soils, like STEVPRIS® and STEVMANTA®, while STEVSHARK® performs best in extreme hard and difficult to penetrate soils. We spent a lot of time and effort in optimising shank types and shapes. The shark-teeth shaped shank bottom sections of STEVSHARK® for instance reinforces the breaking-up of soil layers and/or rock during penetration, whereas the open shank interconnecting structure promotes efficient ‘broken soil’ transport. The latest STEVSHARK®REX for extremely challenging complex hard soils and rocky seabed conditions was introduced in May 2017.”
For oil and gas exploration in shallow to deep water (~ 400m), catenary mooring is the cheapest most common solution. This application involves a combination of ‘free-hanging’ chains, chain + wire-rope, or chain + fibre-rope combinations, with an orientation that gradually changes from a predefined pretension angle at the floater into horizontal at the anchoring point.
With this solution, the anchor is subjected to horizontal loads only. The number of mooring lines depends on floater type, size and application, but in practice 3, 6, 8, or 9, or 16 lines and 3 – 16 anchors.
Highest track record experience
In floating wind and current water depths, catenary mooring is deployed for semi-submersible floaters (i.e. WindFloat, Fukushima Mirai, Fukushima Shimpuu, VolturnUS), spar-type ( i.e. Hywind, and Fukushima Kizuna), and barge-type (i.e. Floatgen), with the first topologies boasting the highest track record experience.
Dedicated anchoring solutions
Many other floater concepts are in development. In generic terms, these can be grouped into semi-submersible, spar-type, bargetype, hybrid, and tension-leg designs. Their specific mooring layouts can vary from multi-line-spread-moored to turret-moored to single-point mooring.
Vryhof has dedicated anchoring solutions for all these variants depending on selected mooring line profile and from catenary to taut-leg type.
Horizontal and vertical loads
With taut leg mooring, typically composed of fibre ropes like polyester or HMPE, or spiral-strand ropes with top and bottom chain or wire-rope segments, the anchor point is subjected to both high vertical and horizontal loads.
For such specific tension-leg type applications Vryhof developed the STEVMANTA® (a Vertical Load Anchor), an ingenious-looking device with a system of wires connected to a plate. This anchor is designed to accept horizontal and vertical loads and installed like a drag-embedment anchor with a tug applying a horizontal load to the mudline for obtaining the deepest penetration possible.
By changing the anchor pulling point after installation completion, additional maximized vertical loading capacity of the plate is obtained. Other potentially suitable anchor types are dynamically embedded pile or plate anchors, driven piles and suction piles, and all with their own price tag.
The third alternative is tension-leg mooring, initially developed to moor extremely large oil & gas platforms in very deep water, with a key characteristic the vertical cable arrangement requiring vertical-load anchors.
Several floating turbine pioneers have opted for this solution, including Gicon SOF, SBM Offshore Wind Floater, Glosten Pelastar and Blue H Engineering. A typical mooring point solution here are suction buckets, but with alternatives gravity-based ‘dead-weight’, gravity installed drop anchors, driven piles, and STEVMANTA®.
On a realistic costing between the various anchoring options, it is very important that such comparison must consider material cost and offshore installation costs. Otherwise, misleading outcomes may result in nasty surprises during the offshore installation phase. Numerous such cost comparisons were done by Vryhof experts and third parties.
These cost comparisons were conducted for four main anchoring solutions, driven piles, suction piles, drag-embedment fluke anchors (i.e. STEVPRIS®), and drag-embedment plate anchors like STEVMANTA®.
The below comparisons include anchor costs including materials, and offshore installation costs of the same design including load capacity criteria, with main outcomes:
- A driven pile (material + installation cost) costs 6 to 7 times more than drag-embedment fluke anchors (i.e. STEVPRIS®) and is 5 - 6 times more expensive compared to drag-embedment plate-anchors (i.e. STEVMANTA®);
- A suction pile (material + installation cost) is 3 - 4 times more expensive than drag-embedment fluke anchors, again like STEVPRIS®) and costs 2 - 3 times more set against drag-embedment plate anchors (i.e. STEVMANTA®).
Vryhof’s future vision on floating wind focuses at locations with water depths exceeding 50 – 60m - a likely maximum for bottom-fixed - and at specific shallow water locations. Here, bottom-fixed designs typically face technical and commercial challenges due to difficult soil conditions.
On the potential for anchoring cost reductions when entering commercial-scale windfarm developments with 6 – 10MW+ turbines, Ozmutlu adds: “Increasing floater number, each with three mooring legs, from 1 to 250 could result in 30 – 40 per cent anchoring cost reduction. Doubling this with similar floaters and up to 500m water depths could increase these savings to 40 – 50 per cent.”
Additional cost reductions in the 5 – 20 per cent range are estimated possible for other mooring elements plus their cumulative offshore installation expenses and again from single floaters to 250 units.
Vryhof participates in several ongoing studies on optimization and industrialization of mooring systems through national and international joint industry and research projects like the EU Horizon 2020 programme.