Page 101 - Practical Design Ships and Floating Structures

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1 INTRODUCTION

The operational and technical necessity to replace two existing North Sea hydrographic survey vessels
(HSV) of the Royal Netherlands Navy initiated a materiel project for two new vessels. The primary
mission of these vessels will be carrying out hydrographic surveys to comply with civil and NATO
commitments. These tasks will in particular be carried out in the Netherlands part of the Continental
Shelf and in the area of the Netherlands Antilles and Aruba. Secondary tasks consist of general
military support tasks, assistance in calamities, and support in typical coast guard operations. The
development of a more rational design method (see Wolff (2000)) as well as the operational
experience within the hydrographic service of the RNLN have resulted in a clear and complete set of
operational requirements regarding the hydrodynamic performance. The approach taken and described
has lead to a balanced design between those hydrodynamic performance aspects and cost.

2 THE REQUIREMENTS
2.1 Propulsion

A) Cruising speed at least 13 kn in calm water with 6 months fouling allowance.
B) The ship should be able to sail for prolonged periods at low speeds, e.g. 1.5 to 2 knots and at
intermediate speeds during surveying, i.e. between 4 and 9 knots.

2.2 Manoeuvrabili@
A) Good manoeuvrability and course stable in particular for typical surveying speeds (4 - 9 kn).
B) For 95% of the time within 5 m lateral distance deviation during trackkeeping for conditions up to
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BE, kn current and sea state 4 (Hsig = 1.88 m, TI = 6 s).
C) During autopilot sailing, less than 5' heading deviation, in the same environmental conditions.
D) The ship should be able to turn 1 80° and change tracks that are 100 m apart within 5 minutes.
E) Unassisted berthing or unberthing should be possible in wind speeds corresponding to up to Bf7.
F) Buoy handling in winds up to Bf7 combined with up to 2 kn current, with less than 10 m
positioning deviation relative to the sea floor.

2.3 Seakeeping
The ship has to be able to perform measurements during 128 dayslyear on the Netherlands part of the
Continental Shelf (NCS). Given per year a total of 210 sailing days and 30 days transit to and from the
measurement location, 10 days loss due to technical reasons and 16 days for secondary tasks, there is a
number of 154 survey days available. This leads to the conclusion that the maximum allowable
downtime due to ship motions on the NCS equals 17%. The seakeeping requirements for the new
hydrographic vessel are much more stringent than for the current survey vessels. The ships to be
replaced are on average capable of 103 measurement daydyear corresponding to a downtime of 33%.
2.4 Sea Conditions

Looking at available wave statistics on the operational area, a probability of exceedance of 17% for the
NetherIands coastal waters leads to a significant wave height of 2m. In the more northerly part of the
NCS this probability leads to a significant wave height of 3 m, so the requirement of 128 hydrographic
survey days can be met if the ship motion behaviour meets the criteria for seastates up to maximum sea
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