Monday, 12 August 2013

In a nutshell: Peaks and Troughs (Yemm)



The Peaks and troughs of wave energy is a treasure trove of gems awaiting discovery. There are associated papers, but there are also audio recordings of all the talks. Richard Yemm's (Pelamis) talk is particularly good. He shared his experience of "the coal-face" (oh dear), and gave his candid opinion of mistakes Pelamis had made, as well as things that they felt they'd got right. It was clear that the pitfalls experienced by Pelamis await others following in their path, and his talk was peppered with advice for other developers.


Advice for wave power developers


  • Experimental modelling is the lynch pin of development. [26:00]
  • Numerical modelling also essential as experiments are slow and expensive. [27:30]
  • Rather than optimising construction of a sub-optimal solution, go back to the drawing board. [48:35]
  • You need to get into the water to find out what you need to do. [29:40/50:10]
  • Hardest decision is "when to go full scale"? [29:20/35:00]
  • Technology development was harder than envisioned [46:45] : "engineering soak time" is required. [47:45]
  • Period to return (of capital) is a problem for conventional finance. [47:10]
  • Ability to patent principle of operation helps raise finance. [7:10]
  • O&G techniques and technology are applicable but not necessarily suitable. [50:20]
  • The industry needs to share safety information to avoid the sector becoming tarnished. [31:30]


What Pelamis got wrong


  • Went to a multi-machine farm before the technology was ready (driven by finance) – only a few 100 hours of P1 tests [33:00] when array was designed. As a result, capital was wasted making similar mistakes three times. [34:10]
  • Rush to install first array resulted in missed opportunity to learn from first prototype: P1 had 1750 hours at sea – “not enough”. [30:30]
  • Struggled at Orkney in 2004 with big boat lifting big weights – not economic; not safe. [23:10]
  • P1 gave half of predicted performance [31:15]. Installed dampers not capable of reaching required damping levels [31:10], and "the world is not linear" (numerical models). [31:00]
  • P1 didn't have enough joint angle [38:10]. Choice was driven by volumetric efficiency.
  • Joint angle problem was not picked up by extremes analysis: they'd tested in individual 100yr waves – common in O&G industry; recognise it is more correct to test in longer time series (this gives almost double joint angle). [37:35/39:10]


What Pelamis got right


  • No big installation boats were available in Portugal so this drove development of tether latch assembly for self-winching mechanical and electrical plug-in. [23:30]
  • The timing of the full-scale sea trials was valuable to the company. [29:30]
  • Full scale test rig proved in lab before going to sea. [27:00]
  • Towing trials before moored installation was enough to prove dynamic systems and catch software bugs. [41:00]
  • P1 trials allowed numerical model to be improved. The performance of the P1 was in agreement with numerical models within a few percent for the waves measured by the wave buoy. [30:55]
  • Atkins verification allowed Pelamis to be insured for 3% of capital value. [25:20]


Hydrodynamics of Pelamis


  • Key issue industry needs to understand: mechanism for absorbing waves is radiating waves. [10:50]
  • A heaving device is an omnidirectional wave maker. Only a proportion of the radiated wave can be put to good effect down-wave of machine. [11:30]
  • A line absorber (modelled as several heave absorbers): individual circular patterns interfere constructively down-wave and interfere destructively up-wave, resulting in a directional beam. [12:15]
  • Directionality: in short waves, capture width increases for waves at an angle, and in longer waves is at a maximum for head-on waves. [14:20]
  • Joints: rotate up and down (pitch) and side to side (yaw), but the axis is rolled over (so each pair of rotations contains some pitch and some yaw). If you put different constraints in these 2 directions, you induce a cross-coupled response. The relative damping controls the angle of the inclined path, which determines stiffness. [16:00]
  • Recognise the need to optimise response for a given sea state, but don't want a system that resonates in design waves – instead control parameters match response to sea state; this is similar to resonance. [15:30]


Cost of Energy considerations


  • Typically 1:10 ratio between average and peak power. However, 1:100 has been observed: have measured 2.5MW peaks in 20kW average conditions.[17:20]. Needs to resist 20MW/m in design storm. [18:40]
  • Reason for accumulators: the high ratio between average and peak power. Without accumulators, there would be more operation at part load (very inefficient). [18:40] . Also, if there is a desire to use reactive control (mass terms) [18:20], then high efficiency is vital.
  • Site selection: Ocean facing coast (in Europe Western Seaboard) 75m-150m depth, within 10-20km of shore. [1:03:00].
  • Designed for offsite maintenance: crucial for high availability [8:20]. If you can't work in 2.5-3m waves (24hr window during Dec/Jan) chances are you'll wait a month.[19:50]
  • Requirement for an offsite maintenance strategy is deployment & retrieval in 2-2.5m waves. [21:00]
  • Maintenance strategy: 1 week summer annual refit & reactive maintenance should be possible with standard hydraulic components.[1:00:00]


1 comment:

  1. Paper now available too: http://rsta.royalsocietypublishing.org/content/370/1959/365.full.pdf+html

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